U.S. patent application number 14/154981 was filed with the patent office on 2014-05-08 for structure for growth of nitride semiconductor layer, stacked structure, nitride-based semiconductor element, light source, and manufacturing method for same.
This patent application is currently assigned to Panasonic Corporation. The applicant listed for this patent is Panasonic Corporation. Invention is credited to Songbaek CHOE, Akira INOUE, Atsushi YAMADA, Toshiya YOKOGAWA.
Application Number | 20140124816 14/154981 |
Document ID | / |
Family ID | 47668143 |
Filed Date | 2014-05-08 |
United States Patent
Application |
20140124816 |
Kind Code |
A1 |
CHOE; Songbaek ; et
al. |
May 8, 2014 |
STRUCTURE FOR GROWTH OF NITRIDE SEMICONDUCTOR LAYER, STACKED
STRUCTURE, NITRIDE-BASED SEMICONDUCTOR ELEMENT, LIGHT SOURCE, AND
MANUFACTURING METHOD FOR SAME
Abstract
A structure for growth of a nitride semiconductor layer which is
disclosed in this application includes: a sapphire substrate of
which growing plane is an m-plane; and a plurality of ridge-shaped
nitride semiconductor layers provided on the growing plane of the
sapphire substrate, wherein a bottom surface of a recessed portion
provided between respective ones of the plurality of ridge-shaped
nitride semiconductor layers is the m-plane of the sapphire
substrate, the growing plane of the plurality of ridge-shaped
nitride semiconductor layers is an m-plane, and an absolute value
of an angle between an extending direction of the plurality of
ridge-shaped nitride semiconductor layers and a c-axis of the
sapphire substrate is not less than 0.degree. and not more than
35.degree..
Inventors: |
CHOE; Songbaek; (Osaka,
JP) ; YOKOGAWA; Toshiya; (Nara, JP) ; INOUE;
Akira; (Osaka, JP) ; YAMADA; Atsushi; (Osaka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Corporation |
Osaka |
|
JP |
|
|
Assignee: |
Panasonic Corporation
Osaka
JP
|
Family ID: |
47668143 |
Appl. No.: |
14/154981 |
Filed: |
January 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14085523 |
Nov 20, 2013 |
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14154981 |
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PCT/JP2012/004958 |
Aug 3, 2012 |
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14085523 |
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Current U.S.
Class: |
257/98 |
Current CPC
Class: |
C30B 29/403 20130101;
H01L 33/20 20130101; H01L 33/58 20130101; H01L 21/20 20130101; H01L
21/02433 20130101; H01L 2224/16225 20130101; H01L 21/0254 20130101;
H01L 21/0262 20130101; H01L 21/02609 20130101; H01L 2224/14
20130101; H01L 21/0265 20130101; H01L 33/18 20130101; H01L 33/32
20130101; H01L 21/02639 20130101; C30B 25/04 20130101; H01L 21/0242
20130101; H01L 33/12 20130101; H01L 33/007 20130101; H01L 21/02642
20130101; H01L 21/02458 20130101; C30B 25/18 20130101; H01L 21/0243
20130101; H01L 33/16 20130101 |
Class at
Publication: |
257/98 |
International
Class: |
H01L 33/18 20060101
H01L033/18; H01L 33/58 20060101 H01L033/58; H01L 33/32 20060101
H01L033/32 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2011 |
JP |
2011-174300 |
Nov 25, 2011 |
JP |
2011-257924 |
Feb 1, 2012 |
JP |
2012-019641 |
Claims
1. A semiconductor light-emitting device, comprising a
nitride-based semiconductor multilayer structure that includes an
active layer having a principal surface which is a non-polar plane
or semi-polar plane and emitting polarized light, wherein the
device includes a plurality of stripe structures provided at
positions traversed by the polarized light with intervals
therebetween, and an absolute value of an angle between an
extending direction of the stripe structures and a polarization
direction of the polarized light is not less than 3.degree. and not
more than 45.degree..
2. The semiconductor light-emitting device of claim 1, wherein the
principal surface is an m-plane, the polarization direction is an
a-axis direction, and an absolute value of an angle between the
extending direction of the stripe structures and the a-axis
direction is not less than 3.degree. and not more than
35.degree..
3. The semiconductor light-emitting device of claim 1, wherein an
absolute value of an angle between the extending direction of the
stripe structures and the polarization direction is not less than
3.degree. and not more than 10.degree..
4. The semiconductor light-emitting device of claim 1, wherein the
semiconductor light-emitting device has a light emission surface
from which light is emitted to an outside, and the plurality of
stripe structures are provided in the light emission surface.
5. The semiconductor light-emitting device of claim 1, wherein the
plurality of stripe structures are provided inside the
nitride-based semiconductor multilayer structure.
6. The semiconductor light-emitting device of claim 1, further
comprising a substrate which is in contact with the nitride-based
semiconductor multilayer structure, wherein the plurality of stripe
structures are provided between the nitride-based semiconductor
multilayer structure and the substrate.
7. The semiconductor light-emitting device of claim 6, wherein the
substrate is formed of a material which is different from a nitride
semiconductor.
8. The semiconductor light-emitting device of claim 6, wherein the
substrate is a sapphire substrate having a principal surface which
is an m-plane.
9. The semiconductor light-emitting device of claim 1, wherein a
gap is provided between adjacent ones of the stripe structures.
10. The semiconductor light-emitting device of claim 1, wherein the
stripe structures are gaps.
11. The semiconductor light-emitting device of claim 9, wherein a
width of the gap increases as it is more distant from the active
layer.
12. The semiconductor light-emitting device of claim 1, wherein the
principal surface is an m-plane, and the polarization direction is
an a-axis direction.
13. The semiconductor light-emitting device of claim 12, wherein
the polarized light has such a light distribution characteristic
that it has a wider emission angle in a c-axis direction than in an
a-axis direction of the active layer.
14. The semiconductor light-emitting device of claim 1, wherein the
stripe structures include a material having a lower refractive
index than a nitride semiconductor has.
Description
[0001] This is a continuation of International Application No.
PCT/JP2012/004958, with an international filing date of Aug. 3,
2012, which claims priority of Japanese Patent Applications No.
2011-174300, filed on Aug. 9, 2011, No. 2011-257924, filed on Nov.
25, 2011, and No. 2012-019641, filed on Feb. 1, 2012, the contents
of which are hereby incorporated by reference.
BACKGROUND
[0002] 1. Technical Field
[0003] The present disclosure relates to a structure for growth of
a nitride semiconductor layer, a multilayer structure including
that structure, a nitride-based semiconductor device including that
multilayer structure, and a light source including that
nitride-based semiconductor device, and manufacturing methods
thereof.
[0004] 2. Description of the Related Art
[0005] A nitride semiconductor containing nitrogen (N) as a Group V
element is a prime candidate for a material to make a short-wave
light-emitting device because its bandgap is sufficiently wide.
Among other things, gallium nitride-based compound semiconductors
(GaN-based semiconductors) have been researched and developed
particularly extensively. As a result, blue light-emitting diodes
(LEDs), green LEDs, and semiconductor laser diodes in which
GaN-based semiconductors are used as materials have already been
used in actual products (see Japanese Laid-Open Patent Publications
No. 2001-308462 and No. 2003-332697, for example).
[0006] The GaN-based semiconductor includes an
Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x<1, 0.ltoreq.z<1,
0<y.ltoreq.1, x+y+z=1) semiconductor and has a wurtzite crystal
structure. FIG. 1 schematically illustrates a unit cell of GaN. In
an Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x<1, 0.ltoreq.z<1,
0<y.ltoreq.1, x+y+z=1) semiconductor crystal, some of the Ga
atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
[0007] FIG. 2 shows four primitive vectors a.sub.1, a.sub.2,
a.sub.3 and c, which are generally used to represent planes of a
wurtzite crystal structure with four indices (i.e., hexagonal
indices). The primitive vector c runs in the [0001] direction,
which is called a "c-axis". A plane that intersects with the c-axis
at right angles is called either a "c-plane" or a "(0001) plane".
It should be noted that the "c-axis" and the "c-plane" are
sometimes referred to as "C-axis" and "C-plane".
[0008] In fabricating a semiconductor device using GaN-based
semiconductors, a c-plane substrate, i.e., a substrate of which
principal surface is a (0001) plane, is used as a substrate on
which GaN semiconductor crystals will be grown. In a c-plane,
however, there is a slight shift in the c-axis direction between a
Ga atom layer and a nitrogen atom layer, thus producing electrical
polarization there. That is why the c-plane is also called a "polar
plane". As a result of the electrical polarization, an internal
electric field is generated due to spontaneous electrical
polarization or piezoelectric polarization along the c-axis
direction in the InGaN quantum well in the active layer. Once such
an internal electric field has been generated in the active layer,
some positional deviation occurs in the distributions of electrons
and holes in the active layer due to the quantum confinement Stark
effect of carriers. Consequently, the internal quantum efficiency
decreases. Thus, in the case of a semiconductor laser diode, the
threshold current increases. In the case of an LED, the power
dissipation increases, and the luminous efficacy decreases.
Meanwhile, as the density of injected carriers increases, the
internal electric field is screened, thus varying the emission
wavelength, too.
[0009] Thus, to overcome these problems, it has been proposed that
a substrate of which the principal surface is a non-polar plane
such as a (1-100) plane that is perpendicular to the [1-100]
direction and that is called an "m-plane" be used. As used herein,
"-" attached on the left-hand side of a Miller-Bravais index in the
parentheses means a "bar" (a negative direction index). As shown in
FIG. 2, the m-plane is parallel to the c-axis (primitive vector c)
and intersects with the c-plane at right angles. On the m-plane, Ga
atoms and nitrogen atoms are on the same atomic-plane. For that
reason, no electrical polarization will be produced perpendicularly
to the m-plane. That is why if a semiconductor multilayer structure
is formed perpendicularly to the m-plane, no piezoelectric field
will be generated in the active layer, thus overcoming the problems
described above. The "m-plane" is a generic term that collectively
refers to a family of planes including (1-100), (-1010), (10-10),
(-1100), (01-10) and (0-110) planes. As used herein, the "X-plane
growth" means epitaxial growth that is produced perpendicularly to
the X plane (where X=c, m, etc.) of a hexagonal wurtzite structure.
As for the X-plane growth, the X plane will be sometimes referred
to herein as "principal surface" or "growing plane". A layer of
semiconductor crystals that have been formed as a result of the
X-plane growth will be sometimes referred to herein as an "X-plane
semiconductor layer".
[0010] Thus, for example, an LED which has such a non-polar plane
as the principal surface can have improved emission efficiency as
compared with a conventional device which is manufactured on a
c-plane.
[0011] As of now, LEDs and laser diodes that employ a nitride
semiconductor structure of which principal surface is an m-plane
that is a non-polar plane have been realized in laboratories.
Almost all of these laboratory studies employ, as the substrate for
growth, a GaN bulk substrate of which principal surface is the
m-plane. Therefore, the problems of lattice mismatch and thermal
expansion coefficient difference between a growing film and the
substrate would not occur, so that growth of a nitride
semiconductor device structure with high crystal quality is
possible, and a high efficiency LED and laser oscillation are
realized.
[0012] However, this GaN bulk substrate which is presently used for
crystal growth of a nitride-based semiconductor device of which
principal surface is the m-plane is expensive as compared with a
sapphire substrate which has been conventionally used for a c-plane
GaN-based LED, and also, it is difficult to realize a large
diameter.
[0013] For example, in the current market, the price of a GaN bulk
substrate is higher than that of a sapphire substrate of the same
size by two orders of magnitude or more. As for its size, a
substrate of m-plane GaN has a square size of about 1-2 cm on each
side, and even in the case of a bulk substrate of c-plane GaN, a
large diameter of two inches or more is difficult to realize as of
now. On the other hand, the sapphire substrate is presently
inexpensive, e.g., several thousands of Japanese yen, so long as
its size is two inches, large diameters of four inches, six inches,
and greater, have already been realized.
[0014] Thus, it can be said that, even in nitride semiconductor
growth on an m-plane that is a non-polar plane, using sapphire as
the substrate is particularly advantageous from the viewpoint of
cost reduction.
[0015] On a sapphire substrate of which principal surface is the
m-plane (hereinafter, "m-plane sapphire"), an m-plane nitride
semiconductor can be grown (PCT INTERNATIONAL APPLICATION
PUBLICATION NO. 2008/047907). Further, on the m-plane sapphire
substrate, (11-22) plane and (10-1-3) plane, which are semi-polar
planes, can be grown under predetermined conditions (Japanese
Journal of Applied Physics 45, No. 6, L154-L157 (2006)).
[0016] In general, a nitride semiconductor crystal grown on a
heterogeneous substrate which has a different crystalline
structure, lattice constant, or thermal expansion coefficient, such
as a sapphire substrate, (i.e., a so-called "hetero-grown" nitride
semiconductor crystal) includes threading dislocations with high
density (that mean edge dislocations, screw dislocations, and mixed
dislocations, which are generically and simply referred to as
"dislocations") and stacking faults. This is mainly attributed to a
large lattice mismatch degree and a difference in crystalline
structure between the nitride semiconductor and the different type
of substrate. Dislocations and defects caused at the interface
between the different type of substrate and the nitride
semiconductor reach the active layer or device surface,
significantly deteriorating the device characteristics, such as
decrease in efficiency of the LED, decrease in device life. Since
stacking faults are usually produced in the c-plane, in a
nitride-based semiconductor device grown on a conventional c-plane
sapphire, the stacking faults would not extend in the growing
direction. Therefore, in the conventional c-plane growth, stacking
faults do not reach the active layer. However, in growth of a
non-polar plane, there is a c-plane lateral surface, and therefore,
there is a probability that stacking faults produced in the c-plane
reach the active layer or device surface, and this can be a major
cause of deterioration in device characteristics. Thus, in a
nitride-based semiconductor device of which principal surface is a
non-polar plane, in order to realize a highly-efficient LED or
laser diode, it is necessary to reduce the stacking fault density
in addition to the threading dislocation density.
[0017] One known technique for reducing these dislocations and
stacking faults is a selective growth method in which a mask
pattern is employed. Such a growth method is commonly referred to
as "Epitaxy Lateral Over Growth (ELOG)".
[0018] Epitaxial lateral overgrowth with the use of a semi-polar or
non-polar GaN is already reported in Japanese Laid-Open Patent
Publication No. 2009-295994 and other documents. In Japanese
Laid-Open Patent Publication No. 2009-295994, a GaN film of which
principal surface is an A-plane is grown on an R-plane sapphire
substrate, and a SiO.sub.2 mask is formed on that GaN surface for
achieving epitaxial lateral overgrowth. In Japanese Laid-Open
Patent Publication No. 2009-295994, the plane orientation
dependence of the lateral growth was examined with varying
stripe-shaped SiO.sub.2 mask orientations in the plane.
[0019] One of the other epitaxial lateral overgrowth methods is a
Pendeo growth method. This method is similar to the
previously-described ELOG method but different in that lateral
growth is carried out using a substrate which has an uneven
structure. Since regrowth of a nitride semiconductor starts only
from raised portions of the uneven structure, a regrown film is in
a hung state, which is why the method is named Pendeo ("hang" in
Latin).
[0020] There are some similar epitaxial lateral overgrowth methods,
and also, there are some similar names for the growth methods. In
this specification, methods wherein a substrate that has an uneven
structure including nitride semiconductor regions which serve as
growth cores or starting points for regrowth and heterogeneous
substrate surface regions which are exposed by processing, such as
etching, is provided, and a nitride semiconductor film is
selectively regrown from the nitride semiconductor regions of the
raised portions of the uneven structure, are generically referred
to as "Pendeo growth". A manner of Pendeo growth in which the
regrowth is carried out with the mask remaining at the crest
portions of the raised portions is simply referred to as "Pendeo
growth" or "masked Pendeo growth", whereas a manner of Pendeo
growth in which the regrowth is carried out without the mask at the
crest portions of the raised portions is referred to as "maskless
Pendeo growth".
[0021] There are also lateral selective methods in which, in Pendeo
growth, the surface of the recessed portions is covered with a
dielectric (Applied Physics Letters 76, 3768 (2000) and Applied
Physics Letters 75, 2062 (1999)). These methods are commonly called
"air-bridged ELO" or "LOFT (lateral overgrowth from trenches)". In
the air-bridged ELO, etching is not continued till the
heterogeneous substrate is exposed. Rather, the nitride
semiconductor film is etched to some extent, and thereafter, that
nitride semiconductor surface is masked with a dielectric material.
In the LOFT, the surface of the heterogeneous substrate which has
been exposed by etching is masked with a dielectric material. These
methods would not cause regrowth on the dielectric mask as in the
previously-described ELOG method.
SUMMARY
[0022] In the above-described conventional techniques, further cost
reduction and improved quality for formation of the m-plane nitride
semiconductor layer have been demanded. Cost reduction and improved
quality have also been demanded in nitride semiconductor layers
with non-polar and semi-polar planes which are different from the
m-plane.
[0023] A nonlimiting exemplary embodiment of the present
application reduces the cost for manufacture of a nitride
semiconductor layer, nitride-based semiconductor device, and light
source, while the qualities thereof are improved.
[0024] In one general aspect, a structure for growth of a nitride
semiconductor layer includes: a sapphire substrate of which growing
plane is an m-plane; and a plurality of ridge-shaped nitride
semiconductor layers provided on the growing plane of the sapphire
substrate, wherein a bottom surface of a recessed portion provided
between respective ones of the plurality of ridge-shaped nitride
semiconductor layers is the m-plane of the sapphire substrate, the
growing plane of the plurality of ridge-shaped nitride
semiconductor layers is an m-plane, and an absolute value of an
angle between an extending direction of the plurality of
ridge-shaped nitride semiconductor layers and a c-axis of the
sapphire substrate is not less than 0.degree. and not more than
35.degree..
[0025] According to the above aspect, crystal growth from the
lateral surfaces of the substrate which are exposed inside recessed
portions and from the bottom surfaces of the recessed portions is
prevented. Therefore, a nitride semiconductor layer with high
surface flatness can be formed. This enables cost reduction of a
multilayer structure, nitride-based semiconductor device, and light
source which have a nitride semiconductor layer, while the
qualities thereof are improved.
[0026] These general and specific aspects may be implemented using
a method. Additional benefits and advantages of the disclosed
embodiments will be apparent from the specification and Figures.
The benefits and/or advantages may be individually provided by the
various embodiments and features of the specification and drawings
disclosure, and need not all be provided in order to obtain one or
more of the same.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a perspective view schematically illustrating a
unit cell of GaN.
[0028] FIG. 2 is a perspective view showing primitive vectors
a.sub.1, a.sub.2, a.sub.3, and c representing a wurtzite crystal
structure.
[0029] FIGS. 3(a) to 3(d) are schematic diagrams showing the
process of epitaxial lateral overgrowth according to a masked
Pendeo method.
[0030] FIGS. 4(a) to 4(d) are schematic diagrams showing the
process of epitaxial lateral overgrowth according to a maskless
Pendeo method.
[0031] FIGS. 5(a) and 5(b) are a plan view and a cross-sectional
view showing the configuration of an unevenly-processed substrate
910 (structure for growth of a nitride semiconductor layer) of
exemplary Embodiment 1.
[0032] FIGS. 6(a) and 6(b) are plan views showing a variation of a
ridge-shaped nitride semiconductor layers 830 of Embodiment 1.
[0033] FIGS. 7(a) and 7(b) are plan views showing a variation of
the ridge-shaped nitride semiconductor layers 830 of Embodiment
1.
[0034] FIGS. 8(a) and 8(b) are diagrams showing a variation of the
unevenly-processed substrate 910 (structure for growth of a nitride
semiconductor layer) of Embodiment 1.
[0035] FIGS. 9(a) to 9(e) are diagrams showing a formation method
of an unevenly-processed substrate 910 (structure for growth of a
nitride semiconductor layer) and a heterogeneous m-plane nitride
semiconductor substrate 920 according to Embodiment 2.
[0036] FIGS. 10(a) and 10(b) are diagrams for illustrating the
definition of the in-plane mask tilt angle (.theta.=0.degree.).
[0037] FIGS. 11(a) and 11(b) are diagrams for illustrating the
definition in the case where the in-plane mask tilt angle .theta.
is not 0.degree. (.theta..noteq.0.degree.).
[0038] FIGS. 12(a) and 12(b) are diagrams for illustrating the
definition of the in-plane mask tilt angle in the case of using an
m-plane sapphire substrate which is made off by a degree in the
a-axis direction.
[0039] FIGS. 13(a) and 13(b) are diagrams for illustrating the
definition of the in-plane mask tilt angle in the case of using an
m-plane sapphire substrate which is made off by .beta. degrees in
the c-axis direction.
[0040] FIG. 14(a) is a schematic cross-sectional view in the
extending direction (longitudinal direction) of stripes in the
unevenly-processed substrate 910 in the case where the tilt angle
of the lateral surface, .gamma., is greater than 0.degree. and
smaller than 90.degree.. FIG. 14(b) is a schematic cross-sectional
view in the extending direction (longitudinal direction) of stripes
in the unevenly-processed substrate 910 in the case where the tilt
angle, .gamma., is 90.degree..
[0041] FIG. 15(a) is a schematic diagram showing the ridge-shaped
nitride semiconductor layer 830 and recessed portions 850 in the
case where the in-plane mask tilt angle .theta. is 0.degree.
(.theta.=0.degree.). FIG. 15(b) is a schematic diagram showing the
ridge-shaped nitride semiconductor layer 830 and recessed portions
850 in the case where the in-plane mask tilt angle .theta. is
90.degree. (.theta.=90.degree.).
[0042] FIG. 16(a) is a diagram showing the relationship between the
(11-22) plane and the a-plane facet. FIG. 16(b) is a diagram
showing the relationship between the m-plane and the r-plane
facet.
[0043] FIG. 17(a) is a diagram showing stacking faults in a c-plane
GaN provided on a c-plane sapphire substrate. FIG. 17(b) is a
diagram showing stacking faults in an m-plane GaN provided on an
m-plane sapphire substrate.
[0044] FIG. 18 is a graph showing the result of an X-ray
2.theta.-.omega. measurement carried out on a seed crystal m-plane
nitride semiconductor film 812 grown on an m-plane sapphire
substrate.
[0045] FIG. 19(a) is a graph showing the result of an X-ray
2.theta.-.omega. measurement carried out on a sample in which a
nitride semiconductor film with a semi-polar plane ((10-1-3) plane)
principal surface was provided on an m-plane sapphire substrate.
FIG. 19(b) is a graph showing the result of an X-ray
2.theta.-.omega. measurement carried out on a sample in which a
nitride semiconductor film with a semi-polar plane ((11-22) plane)
principal surface was provided on an m-plane sapphire
substrate.
[0046] FIG. 20 is a graph showing the result of an X-ray rocking
curve measurement carried out on a seed crystal m-plane nitride
semiconductor film 812 grown on an m-plane sapphire substrate.
[0047] FIGS. 21(a) and 21(b) are SEM images of the
unevenly-processed substrate 910 in which a stripe-shaped mask was
used. FIG. 21(a) shows a cross-sectional image (trapezoidal
structure) in the extending direction (longitudinal direction) of
the mask and a bird's-eye view. FIG. 21(b) shows a cross-sectional
image (triangular structure) in the extending direction
(longitudinal direction) of the mask and a bird's-eye view.
[0048] FIG. 22 is a diagram showing a surface morphology of a
heterogeneous m-plane GaN substrate (heterogeneous m-plane nitride
semiconductor substrate 920) after regrowth, which was obtained by
a laser microscope.
[0049] FIGS. 23(a) to 23(c) show SEM images of a heterogeneous
m-plane GaN substrate (heterogeneous m-plane nitride semiconductor
substrate 920) after regrowth. FIG. 23(a) is a bird's-eye view.
FIG. 23(b) is a cross-sectional view of a raised-portion nitride
semiconductor region. FIG. 23(c) is a cross-sectional view in the
vicinity of the interface between the recessed portion 850 and the
sapphire substrate.
[0050] FIGS. 24(a) to 24(h) show the in-plane mask tilt angle
dependence of the surface morphology. FIG. 24(a) shows the
observation result for .theta.=0.degree.. FIG. 24(b) shows the
observation result for .theta.=17.degree.. FIG. 24(c) shows the
observation result for .theta.=21.degree.. FIG. 24(d) shows the
observation result for .theta.=25.degree.. FIG. 24(e) shows the
observation result for .theta.=35.degree.. FIG. 24(f) shows the
observation result for .theta.=39.degree.. FIG. 24(g) shows the
observation result for .theta.=47.degree.. FIG. 24(h) shows the
observation result for .theta.=80.degree..
[0051] FIG. 25 is a graph showing the in-plane mask tilt angle
dependence of the surface rms roughness of a heterogeneous m-plane
GaN substrate (the heterogeneous m-plane nitride semiconductor
substrate 920) after regrowth.
[0052] FIGS. 26(a) to 26(c) show the surface morphology in the case
where regrowth is carried out on an unevenly-processed substrate
which is formed of only a GaN layer. FIG. 26(a) shows the
observation result for .theta.=0.degree.. FIG. 26(b) shows the
observation result for .theta.=45.degree.. FIG. 26(c) shows the
observation result for .theta.=90.degree..
[0053] FIGS. 27(a), 27(b), and 27(c) are graphs showing the results
of X-ray 2.theta.-.omega. measurements carried out on a regrown
heterogeneous m-plane GaN substrate (heterogeneous m-plane nitride
semiconductor substrate 920) in the cases where the in-plane mask
tilt angles were 0.degree. (FIG. 27(a)), 43.degree. (FIG. 27(b)),
and 90.degree. (FIG. 27(c)).
[0054] FIG. 28 is a graph showing the in-plane mask tilt angle
dependence of the X-ray integrated intensity ratio between the
(11-22) plane and the m-plane (2-200) plane.
[0055] FIGS. 29(a) and 29(b) are graphs showing the in-plane mask
tilt angle dependence of the (11-22) plane/m-plane (2-200) plane
X-ray integrated intensity ratio in the case where the etching
lateral surface depth of the sapphire substrate was 250 nm (FIG.
29(a)) and in the case where the etching lateral surface depth of
the sapphire substrate was not more than 150 nm (FIG. 29(b)).
[0056] FIGS. 30(a) and 30(b) are diagrams for illustrating the
direction of the crystalline axis in the cases where the growing
planes are the m-plane and the off-plane, respectively.
[0057] FIG. 31 shows the epitaxy relationship of an M-plane GaN
film formed on an M-plane sapphire substrate. FIG. 31(a) is a
lattice diagram of the M-plane GaN. FIG. 31(b) is a lattice diagram
of the M-plane sapphire.
[0058] FIG. 32 is a graph showing the in-plane mask tilt angle
dependence of the XRC full width at half maximum of a regrown
heterogeneous m-plane GaN substrate.
[0059] FIGS. 33(a) and 33(b) are graphs showing the results of room
temperature PL measurement carried out on a regrown heterogeneous
m-plane GaN substrate (heterogeneous m-plane nitride semiconductor
substrate 920) in the case where the in-plane mask tilt angle was
5.degree. (FIG. 33(a)) and in the case where the in-plane mask tilt
angle was 14.degree. (FIG. 33(b)).
[0060] FIGS. 34(a) to 34(c) are graphs showing the results of low
temperature (10K) PL measurements carried out on a regrown
heterogeneous m-plane GaN substrate (heterogeneous m-plane nitride
semiconductor substrate 920) in the cases where the in-plane mask
tilt angles were 0.degree., 5.degree. and 21.degree., respectively.
FIG. 34(d) is a graph showing the result of a low temperature (10K)
PL measurement of a seed crystal m-plane GaN before selective
growth was carried out.
[0061] FIG. 35 is a graph showing the relationship between the
ratio between the Deep level emission intensity and the band edge
emission intensity and the in-plane mask tilt angle in a regrown
heterogeneous m-plane GaN substrate (heterogeneous m-plane nitride
semiconductor substrate 920).
[0062] FIGS. 36(a) to 36(c) are graphs showing the surface
morphology, the surface RMS roughness, and the value of Ra in the
cases where the in-plane mask tilt angles were 0.degree.,
5.degree., and 10.degree., respectively.
[0063] FIGS. 37(a) to 37(f) are graphs showing the results of PL
measurements carried out on a regrown heterogeneous m-plane GaN
substrate (heterogeneous m-plane nitride semiconductor substrate
920) at low temperature 10K with varying stripe width ratios, S
width/(L width+S width). The values of S width/(L width+S width)
were (a) 0.29, (b) 0.38, (c) 0.50, (d) 0.58, (e) 0.64, and (f)
0.99.
[0064] FIG. 38 is a graph showing the relationship between the
intensity ratio between the emission that is attributed to the
donor bound exciton (D0, X) near 3.48 eV and the emission that is
attributed to the stacking faults near 3.42 eV and the stripe width
ratio, S width/(L width+S width), in the PL spectrum at low
temperature 10K of a regrown heterogeneous m-plane GaN substrate
(heterogeneous m-plane nitride semiconductor substrate 920).
[0065] FIGS. 39(a) to 39(c) are graphs showing the PL spectrum at
low temperature (10K) of an InGaN-based quantum well structure
grown on a regrown heterogeneous m-plane GaN substrate
(heterogeneous m-plane nitride semiconductor substrate 920). FIG.
39(a) shows the result for the case where the structure was
directly grown on an m-plane GaN bulk substrate for the sake of
comparison. FIGS. 39(b) and 39(c) are graphs showing the results of
measurements for the cases where a quantum well structure was grown
on a Pendeo epitaxial lateral overgrowth m-plane GaN with the
values of S width/(L width+S width) were 0.67 and 0.29,
respectively.
[0066] FIGS. 40(a) to 40(e) are laser microscope images obtained
from the front surface side (i.e., m-axis side) of a regrown
heterogeneous m-plane GaN substrate (heterogeneous m-plane nitride
semiconductor substrate 920) with the L width being constant at 5
.mu.m and varying S widths, (a) 10 .mu.m, (b) 50 .mu.m, (c) 100
.mu.m, (d) 200 .mu.m, and (e) 300 .mu.m.
[0067] FIG. 41 is a schematic cross-sectional view of a
nitride-based semiconductor light-emitting device 801 of an
embodiment.
[0068] FIG. 42 is a cross-sectional view showing an embodiment of a
white light source.
[0069] FIG. 43 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the fourth embodiment.
[0070] FIG. 44(a) is a cross-sectional view showing a heterogeneous
nitride semiconductor substrate which includes stripe-shaped gaps
according to the fourth embodiment. FIG. 44(b) is a plan view where
the stripe-shaped gaps are seen in the m-axis direction.
[0071] FIGS. 45(a) and 45(b) are enlarged schematic cross-sectional
views of heterogeneous nitride semiconductor substrates which show
variations of the gaps.
[0072] FIGS. 46(a) to 46(c) are enlarged schematic cross-sectional
views of heterogeneous nitride semiconductor substrates which show
other variations of the gaps.
[0073] FIG. 47 is a schematic plan view of a heterogeneous nitride
semiconductor substrate which shows a variation of the stripe
structure.
[0074] FIGS. 48(a) and 48(b) are schematic plan views of
heterogeneous nitride semiconductor substrates which show other
variations of the stripe structure.
[0075] FIGS. 49(a) and 49(b) are schematic plan views of
heterogeneous nitride semiconductor substrates which show other
variations of the stripe structure.
[0076] FIG. 50(a) is a schematic diagram showing a propagation
vector of light polarized in the a-axis direction in a GaN. FIG.
50(b) is a schematic diagram showing the light distribution
characteristics in the a-axis direction and the c-axis direction
when seen along the m-axis in the GaN.
[0077] FIG. 51 is a schematic cross-sectional view showing an
example of polarized light which is incident on a slope surface of
a gap provided in a heterogeneous nitride semiconductor substrate
in the case where the in-plane tilt angle .beta. of the gap with
respect to the a-axis direction is 0.degree..
[0078] FIG. 52 is a schematic diagram showing an example of
polarized light which is incident on a gap formed in a direction
perpendicular to the polarization direction)
(.beta.=90.degree.).
[0079] FIG. 53(a) is a schematic diagram showing an example of
incoming light from the a-axis direction and transmitted light with
respect to an emission surface in a semiconductor light-emitting
device which has a flat emission surface. FIG. 53(b) is a schematic
diagram showing an example of incoming light from the a-axis
direction to a gap and transmitted light according to the fourth
embodiment.
[0080] FIG. 54 is a diagram showing the relationship between the
incidence angles of p-wave and s-wave and the reflectance and
transmittance.
[0081] FIGS. 55(a) to 55(c) are graphs showing the calculation
results of the incidence angle dependence of the energy
reflectances Rp and Rs of the p-wave and the s-wave at interfaces
which have different refractive indices.
[0082] FIGS. 56(a) to 56(d) are schematic cross-sectional views
showing the sequential steps of a method for forming gaps according
to the epitaxial lateral overgrowth method of the fourth
embodiment.
[0083] FIGS. 57(a) to 57(d) are schematic cross-sectional views
showing the sequential steps of a variation of a method for forming
gaps according to the epitaxial lateral overgrowth method of the
fourth embodiment.
[0084] FIGS. 58(a) and 58(b) show an unevenly-processed substrate
according to the fourth embodiment and the fifth embodiment. FIG.
58(a) is a schematic cross-sectional view. FIG. 58(b) is a
schematic plan view.
[0085] FIGS. 59(a) and 59(b) are schematic cross-sectional views
showing a variation of the unevenly-processed substrate according
to the fourth embodiment and the fifth embodiment.
[0086] FIG. 60(a) is a schematic cross-sectional view showing a
heterogeneous nitride semiconductor substrate which includes
stripe-shaped gaps according to the fifth embodiment. FIG. 60(b) is
a schematic plan view where a heterogeneous nitride semiconductor
substrate of the fifth embodiment is seen from the m-axis
direction.
[0087] FIG. 61 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the fifth embodiment.
[0088] FIG. 62 is a schematic cross-sectional view which is seen
from the a-axis direction in a semiconductor light-emitting device
according to a variation of the fifth embodiment.
[0089] FIGS. 63(a) and 63(b) show a semiconductor light-emitting
device according to a variation of the fifth embodiment. FIG. 63(a)
is a schematic cross-sectional view which is seen from the c-axis
direction. FIG. 63(b) is a schematic cross-sectional view which is
seen from the a-axis direction in a n-electrode.
[0090] FIGS. 64(a) and 64(b) are schematic cross-sectional views
showing the sequential steps of a manufacturing method of a
heterogeneous nitride semiconductor substrate for use in a
semiconductor light-emitting device according to the sixth
embodiment.
[0091] FIG. 65 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the sixth embodiment.
[0092] FIG. 66 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the first variation of the
sixth embodiment.
[0093] FIG. 67 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the second variation of the
sixth embodiment.
[0094] FIG. 68(a) is a schematic cross-sectional view showing a
semiconductor light-emitting device of the seventh embodiment. FIG.
68(b) is a schematic cross-sectional view showing a semiconductor
light-emitting device of a variation of the seventh embodiment.
[0095] FIG. 69 is a schematic cross-sectional view showing a
semiconductor light-emitting device of the eighth embodiment.
[0096] FIG. 70 is a schematic cross-sectional view showing a
semiconductor light-emitting device of EXAMPLE A.
[0097] FIG. 71 is a schematic diagram showing a measurement system
for the polarization degree of emitted light in a semiconductor
light-emitting device.
[0098] FIGS. 72(a) and 72(b) are schematic diagrams showing a
measurement system for the light distribution characteristics.
[0099] FIG. 73 is a graph showing the light distribution
characteristics in a semiconductor light-emitting device of
Comparative Example 4.
[0100] FIGS. 74(a) and 74(b) are graphs showing the light
distribution characteristics in semiconductor light-emitting
devices of Inventive Example 6, Reference Example 1, and
Comparative Example 1.
[0101] FIG. 75 is a graph showing the relationship between the
in-plane tilt angle .beta. of a stripe structure and the specific
polarization degree in semiconductor light-emitting devices of
Inventive Example 8, Reference Example 3, and Comparative Example
3.
[0102] FIG. 76 is a graph showing the relationship between the
angle formed between the stripe structure and the a-axis and the
specific light extraction efficiency in semiconductor
light-emitting devices of Inventive Example 6, Reference Example 1,
and Comparative Example 1.
[0103] FIGS. 77(a) and 77(b) are cross-sectional scanning electron
microscopic (SEM) images showing an unevenly-processed substrate of
EXAMPLE B.
[0104] FIGS. 78(a) and 78(b) are examples of cross-sectional SEM
images showing heterogeneous nitride semiconductor substrates which
have gaps.
[0105] FIGS. 79(a) and 79(b) are transmission electron microscopic
images showing a semiconductor light-emitting device of EXAMPLE B
which was manufactured on a heterogeneous nitride semiconductor
substrate.
[0106] FIGS. 80(a) and 80(b) show the light distribution
characteristics of EXAMPLE B. FIG. 80(a) is the evaluation result
of Comparative Example 5. FIG. 80(b) is the evaluation result of
Inventive Example 9.
[0107] FIGS. 81(a) and 81(b) are examples of cross-sectional SEM
images showing a heterogeneous nitride semiconductor substrate
which has gaps according to EXAMPLE C.
[0108] FIGS. 82(a) to 82(h) are surface microscopic images showing
the in-plane tilt angle .beta. dependence of the surface morphology
of a heterogeneous nitride semiconductor substrate.
[0109] FIG. 83 is a graph showing the in-plane tilt angle .beta.
dependence of the surface rms roughness in a heterogeneous m-plane
GaN substrate which is a regrown heterogeneous nitride
semiconductor substrate.
[0110] FIGS. 84(a) to 84(c) are graphs showing the results of X-ray
2.theta.-.omega. measurements carried out on a heterogeneous
nitride semiconductor substrate which has a stripe structure of the
in-plane tilt angle .beta..
[0111] FIG. 85 is a graph showing the in-plane tilt angle .beta.
dependence of the value of the X-ray integrated intensity ratio of
the (11-22) plane and the m-plane (2-200) plane.
[0112] FIGS. 86(a) and 86(b) are schematic diagrams showing the
relationship of the crystal orientation of a (11-22) semi-polar
plane grown on an m-plane sapphire.
[0113] FIGS. 87(a) and 87(b) show a stripe-shaped nitride
semiconductor layer 110a. FIG. 87(a) is a schematic perspective
view in the case of the in-plane tilt angle .beta.=0.degree.. FIG.
87(b) is a schematic perspective view in the case of the in-plane
tilt angle .beta.=90.degree..
[0114] FIGS. 88(a) and 88(b) are graphs showing the in-plane tilt
angle .beta. dependence of the value of the (11-22) plane/m-plane
(2-200) plane X-ray integrated intensity ratio in the case where
the depth of the etching lateral surface of the sapphire substrate
was varied.
[0115] FIG. 89 is a graph showing the in-plane tilt angle .beta.
dependence of the XRC full width at half maximum in a heterogeneous
nitride semiconductor substrate.
[0116] FIGS. 90(a) and 90(b) are graphs showing the results of room
temperature PL measurements in heterogeneous nitride semiconductor
substrates in which the in-plane tilt angles .beta. were 5.degree.
and 14.degree., respectively.
[0117] FIGS. 91(a) to 91(c) are graphs showing the results of low
temperature PL measurements carried out on heterogeneous nitride
semiconductor substrates in the case where the in-plane tilt angles
.beta. were 0.degree., 5.degree., and 21.degree., respectively.
FIG. 91(d) is a graph showing the result of a low temperature PL
measurement carried out on a seed crystal m-plane GaN film which
was directly grown on an m-plane sapphire substrate for the sake of
comparison.
[0118] FIG. 92 is a graph showing the in-plane tilt angle .beta.
dependence of the value of the ratio between the Deep level
emission intensity and the band edge emission intensity in a
heterogeneous nitride semiconductor substrate.
DETAILED DESCRIPTION
[0119] The summary of one embodiment of the present disclosure is
as follows.
[0120] A structure for growth of a nitride semiconductor layer
which is one embodiment of the present disclosure includes: a
sapphire substrate of which growing plane is an m-plane; and a
plurality of ridge-shaped nitride semiconductor layers provided on
the growing plane of the sapphire substrate, wherein a bottom
surface of a recessed portion provided between respective ones of
the plurality of ridge-shaped nitride semiconductor layers is the
m-plane of the sapphire substrate, the growing plane of the
plurality of ridge-shaped nitride semiconductor layers is an
m-plane, and an absolute value of an angle between an extending
direction of the plurality of ridge-shaped nitride semiconductor
layers and a c-axis of the sapphire substrate is not less than
0.degree. and not more than 35.degree..
[0121] The absolute value of the angle may be greater than
0.degree..
[0122] An angle inside the plurality of ridge-shaped nitride
semiconductor layers between a lateral surface which is parallel to
the extending direction of the plurality of ridge-shaped nitride
semiconductor layers and the m-plane may be greater than 0.degree.
and smaller than 150.degree..
[0123] A depth of the bottom surface relative to an interface
between the sapphire substrate and the nitride semiconductor layers
may be more than 0 nm and not more than 150 nm.
[0124] A structure for growth of a nitride semiconductor layer
which is another embodiment of the present disclosure includes: a
sapphire substrate of which growing plane is an m-plane; and a
plurality of ridge-shaped nitride semiconductor layers provided on
the growing plane of the sapphire substrate, wherein a bottom
surface of a recessed portion provided between respective ones of
the plurality of ridge-shaped nitride semiconductor layers is the
m-plane of the sapphire substrate, the growing plane of the
plurality of ridge-shaped nitride semiconductor layers is an
m-plane, and a depth of the bottom surface relative to an interface
between the sapphire substrate and the nitride semiconductor layers
is not less than 0 nm and not more than 150 nm.
[0125] An angle between the extending direction of the plurality of
ridge-shaped nitride semiconductor layers and the c-axis of the
sapphire substrate may be not less than 0.degree. and not more than
10.degree..
[0126] A structure for growth of a nitride semiconductor layer
which is still another embodiment of the present disclosure
includes: a substrate which has a growing plane; and a plurality of
ridge-shaped nitride semiconductor layers which have a different
crystal structure from that of the substrate and which are provided
on the growing plane, wherein the substrate is exposed at a bottom
surface of a recessed portion provided between respective ones of
the plurality of ridge-shaped nitride semiconductor layers, a
growing plane of the plurality of ridge-shaped nitride
semiconductor layers is a non-polar plane or semi-polar plane, a
lattice mismatch degree between the substrate and the plurality of
ridge-shaped nitride semiconductor layers is not less than 2% in a
first direction which is defined by orthogonal projection of a
c-axis of the plurality of ridge-shaped nitride semiconductor
layers onto the growing plane of the substrate, and the lattice
mismatch degree between the substrate and the plurality of
ridge-shaped nitride semiconductor layers is not less than 10% in a
second direction which is perpendicular to the first direction in
the growing plane, and a depth of the bottom surface relative to an
interface between the substrate and the plurality of ridge-shaped
nitride semiconductor layers is not less than 0 nm and not more
than 150 nm.
[0127] The lattice mismatch degree in the first direction may be
less than 10%.
[0128] Where an interplanar spacing of the substrate is ds, an
interplanar spacing of the plurality of ridge-shaped nitride
semiconductor layers is dg, and the lattice mismatch degree is
M(%), the lattice mismatch degree M(%) may be represented by
Formula I as follows:
M(%)=100(dg-ds)/ds (Formula 1)
[0129] The substrate may be a sapphire substrate of which growing
plane is an m-plane, and the growing plane of the plurality of
ridge-shaped nitride semiconductor layers may be a (11-22)
plane.
[0130] Where a width of the plurality of ridge-shaped nitride
semiconductor layers at a base is L width and a width of the
recessed portions at a bottom surface is S width, a value of S
width/(L width+S width) may be not less than 0.6 and less than
1.
[0131] An upper surface of the plurality of ridge-shaped nitride
semiconductor layers may not be provided with a mask.
[0132] The L width may be not less than 0.1 .mu.m and not more than
10 .mu.m, the S width may be not less than 0.15 .mu.m and not more
than 30 .mu.m, and the value of S width/(L width+S width) may be
not less than 0.6 and not more than 0.996.
[0133] The L width may be not less than 0.1 .mu.m and not more than
10 .mu.m, the S width may be not less than 30 .mu.m and not more
than 300 .mu.m, and the value of S width/(L width+S width) may be
not less than 0.75 and less than 1.
[0134] The L width may be not less than 1 .mu.m and not more than 5
.mu.m, the S width may be not less than 30 .mu.m and not more than
300 .mu.m, and the value of S width/(L width+S width) may be not
less than 0.857 and less than 1.
[0135] An angle inside the plurality of ridge-shaped nitride
semiconductor layers between a lateral surface which is parallel to
the extending direction of the plurality of ridge-shaped nitride
semiconductor layers and the growing plane of the substrate or the
sapphire substrate may be greater than 0.degree. and smaller than
150.degree..
[0136] The structure may further includes a buffer layer provided
between the growing plane of the substrate or the sapphire
substrate and the ridge-shaped nitride semiconductor layers, the
buffer layer being made of Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x,
y, z.ltoreq.1, x+y+z=1).
[0137] The buffer layer may not be present on a bottom surface or
lateral surface of the recessed portions.
[0138] The buffer layer may be made of AlN.
[0139] A multilayer structure which is still another embodiment of
the present disclosure includes: any of the above-described
structures for growth of a nitride semiconductor layer; and a
nitride semiconductor layer which is in contact with the plurality
of ridge-shaped nitride semiconductor layers in the structure for
growth of a nitride semiconductor layer.
[0140] A multilayer structure which is still another embodiment of
the present disclosure includes: any of the above-described
structures for growth of a nitride semiconductor layer; and a
nitride semiconductor layer which is in contact with an upper
surface of the plurality of ridge-shaped nitride semiconductor
layers in the structure for growth of a nitride semiconductor
layer.
[0141] A nitride-based semiconductor light-emitting device which is
still another embodiment of the present disclosure includes any of
the above-described multilayer structures.
[0142] A light source which is still another embodiment of the
present disclosure includes: the above-described nitride-based
semiconductor light-emitting device; and a wavelength converter
including a phosphoric material which is capable of converting a
wavelength of light radiated from the nitride-based semiconductor
light-emitting device.
[0143] A method for fabricating a multilayer structure which is
still another embodiment of the present disclosure includes the
steps of: (a) providing a substrate which has a growing plane; (b)
growing a nitride semiconductor film on the growing plane; (c)
forming a plurality of recessed portions so as to penetrate through
the nitride semiconductor film, thereby forming a plurality of
ridge-shaped nitride semiconductor layers; and (d) growing a
nitride semiconductor layer such that the growth starts from the
plurality of ridge-shaped nitride semiconductor layers, wherein a
growing plane of the plurality of ridge-shaped nitride
semiconductor layers is a non-polar plane or semi-polar plane, a
lattice mismatch degree between the substrate and the plurality of
ridge-shaped nitride semiconductor layers is not less than 2% in a
first direction which is defined by orthogonal projection of a
c-axis of the plurality of ridge-shaped nitride semiconductor
layers onto the growing plane of the substrate, and the lattice
mismatch degree between the substrate and the plurality of
ridge-shaped nitride semiconductor layers is not less than 10% in a
second direction which is perpendicular to the first direction in
the growing plane, and in step (c), the plurality of recessed
portions are formed such the substrate is exposed at a bottom
surface of the plurality of recessed portions, a depth of the
bottom surface of the recessed portions relative to an interface
between the substrate and the plurality of ridge-shaped nitride
semiconductor layers is not less than 0 nm and not more than 150
nm.
[0144] A method for fabricating a multilayer structure which is
still another embodiment of the present disclosure includes the
steps of: (a) providing a sapphire substrate of which growing plane
is an m-plane; (b) growing a nitride semiconductor film on the
growing plane; (c) forming a plurality of recessed portions so as
to penetrate through the nitride semiconductor film, thereby
forming a plurality of ridge-shaped nitride semiconductor layers;
and (d) growing a nitride semiconductor layer such that the growth
starts from the plurality of ridge-shaped nitride semiconductor
layers, wherein a bottom surface of the recessed portions provided
between respective ones of the plurality of ridge-shaped nitride
semiconductor layers is the m-plane of the sapphire substrate, the
growing plane of the plurality of ridge-shaped nitride
semiconductor layers is an m-plane, and in step (c), the plurality
of recessed portions are formed such that an absolute value of an
angle between an extending direction of the plurality of
ridge-shaped nitride semiconductor layers and a c-axis of the
sapphire substrate is not less than 0.degree. and not more than
35.degree. and that where a width of the plurality of ridge-shaped
nitride semiconductor layers at a base is L width and a width of
the recessed portions at a bottom surface is S width, a value of S
width/(L width+S width) is not less than 0.6 and less than 1.
[0145] A method for fabricating a multilayer structure which is
still another embodiment of the present disclosure includes the
steps of: (a) providing a sapphire substrate of which growing plane
is an m-plane; (b) growing a nitride semiconductor film on the
growing plane; (c) forming a plurality of recessed portions so as
to penetrate through the nitride semiconductor film, thereby
forming a plurality of ridge-shaped nitride semiconductor layers;
and (d) growing a nitride semiconductor layer such that the growth
starts from the plurality of ridge-shaped nitride semiconductor
layers, wherein a bottom surface of the recessed portions provided
between respective ones of the plurality of ridge-shaped nitride
semiconductor layers is the m-plane of the sapphire substrate, the
growing plane of the plurality of ridge-shaped nitride
semiconductor layers is an m-plane, and in step (c), the plurality
of recessed portions are formed such that the substrate is exposed
at a bottom surface of the plurality of recessed portions, that a
depth of the bottom surface of the recessed portions relative to an
interface between the substrate and the plurality of ridge-shaped
nitride semiconductor layers is not less than 0 nm and not more
than 150 nm, and that where a width of the plurality of
ridge-shaped nitride semiconductor layers at a base is L width and
a width of the recessed portions at a bottom surface is S width, a
value of S width/(L width+S width) is not less than 0.6 and less
than 1.
[0146] A method for fabricating a multilayer structure which is
still another embodiment of the present disclosure includes the
steps of: (a) providing a sapphire substrate of which growing plane
is an m-plane; (b) growing a nitride semiconductor film on the
growing plane of the sapphire substrate; (c) forming a plurality of
recessed portions so as to penetrate through the nitride
semiconductor film, thereby forming a plurality of ridge-shaped
nitride semiconductor layers; and (d) growing a nitride
semiconductor layer such that the growth starts from the plurality
of ridge-shaped nitride semiconductor layers, wherein in step (c),
the plurality of recessed portions are formed such that a bottom
surface of the plurality of recessed portions is the m-plane of the
sapphire substrate, the growing plane of the plurality of
ridge-shaped nitride semiconductor layers is an m-plane, and an
absolute value of an angle between an extending direction of the
plurality of ridge-shaped nitride semiconductor layers and a c-axis
of the sapphire substrate is not less than 0.degree. and not more
than 35.degree..
[0147] In step (c), in the plurality of ridge-shaped nitride
semiconductor layers, an angle inside the plurality of ridge-shaped
nitride semiconductor layers between a lateral surface which is
parallel to the extending direction of the plurality of
ridge-shaped nitride semiconductor layers and the m-plane of the
sapphire substrate may be greater than 0.degree. and smaller than
150.degree..
[0148] In step (c), the plurality of recessed portions may be
formed such that a minimum value of a width of the bottom surface
of the plurality of recessed portions is not less than 0.1 .mu.m
and not more than 30 .mu.m.
[0149] In step (c), the plurality of recessed portions may be
formed such that a depth of the bottom surface of the plurality of
recessed portions relative to an interface between the sapphire
substrate and the plurality of ridge-shaped nitride semiconductor
layers is more than 0 nm and not more than 500 nm.
[0150] In step (c), the plurality of recessed portions may be
formed such that a depth of the bottom surface of the plurality of
recessed portions relative to an interface between the sapphire
substrate and the plurality of ridge-shaped nitride semiconductor
layers is more than 0 nm and not more than 150 nm.
[0151] The plurality of recessed portions may be formed such that
an angle between the extending direction of the plurality of
ridge-shaped nitride semiconductor layers and the c-axis of the
sapphire substrate is more than 0.degree. and not more than
10.degree..
[0152] In step (c), the plurality of recessed portions may be
formed using a photolithography technique.
[0153] A nitride-based semiconductor device which is still another
embodiment of the present disclosure employs the nitride
semiconductor layer formed according to any of the above-described
methods as a substrate.
[0154] The substrate or the sapphire substrate may be removed.
[0155] A method for fabricating a multilayer structure which is
still another embodiment of the present disclosure includes the
steps of: (a) providing a sapphire substrate of which growing plane
is an m-plane; (b) growing a nitride semiconductor film on the
growing plane of the sapphire substrate; (c) forming a plurality of
recessed portions so as to penetrate through the nitride
semiconductor film, thereby forming a plurality of ridge-shaped
nitride semiconductor layers; and (d) growing a nitride
semiconductor layer such that the growth starts from the plurality
of ridge-shaped nitride semiconductor layers, wherein in step (c),
the plurality of recessed portions are formed such that a bottom
surface of the plurality of recessed portions is the m-plane of the
sapphire substrate, the growing plane of the plurality of
ridge-shaped nitride semiconductor layers is an m-plane, and a
depth of the bottom surface of the recessed portions relative to an
interface between the sapphire substrate and the plurality of
ridge-shaped nitride semiconductor layers is not less than 0 nm and
not more than 150 nm.
[0156] A method for manufacturing a nitride-based semiconductor
device which is still another embodiment of the present disclosure
includes any of the above-described multilayer structure
fabrication methods.
[0157] A semiconductor light-emitting device which is still another
embodiment of the present disclosure includes a nitride-based
semiconductor multilayer structure that includes an active layer of
which principal surface is a non-polar plane or semi-polar plane
and which is configured to emit polarized light, wherein the device
includes a plurality of stripe structures provided at positions
traversed by the polarized light with intervals therebetween, and
an absolute value of an angle between an extending direction of the
stripe structures and a polarization direction of the polarized
light is not less than 3.degree. and not more than 45.degree..
[0158] The principal surface may be an m-plane, the polarization
direction may be an a-axis direction, and an absolute value of an
angle between the extending direction of the stripe structures and
the a-axis direction may be not less than 3.degree. and not more
than 35.degree..
[0159] An absolute value of an angle between the extending
direction of the stripe structures and the polarization direction
may be not less than 3.degree. and not more than 10.degree..
[0160] A semiconductor light-emitting device which is still another
embodiment of the present disclosure includes a nitride-based
semiconductor multilayer structure that includes an active layer of
which principal surface is a non-polar plane or semi-polar plane
and which is configured to emit polarized light, wherein the device
includes a plurality of stripe structures provided at positions
traversed by the polarized light with intervals therebetween, and
an absolute value of an angle between an extending direction of the
stripe structures and a polarization direction of the polarized
light is not less than 0.degree. and less than 3.degree..
[0161] The semiconductor light-emitting device may have a light
emission surface from which light is emitted to an outside, and the
plurality of stripe structures may be provided in the light
emission surface.
[0162] The plurality of stripe structures may be provided inside
the nitride-based semiconductor multilayer structure.
[0163] The semiconductor light-emitting device may further include
a substrate which is in contact with the nitride-based
semiconductor multilayer structure, wherein the plurality of stripe
structures may be provided between the nitride-based semiconductor
multilayer structure and the substrate.
[0164] The substrate may be made of a material which is different
from a nitride semiconductor.
[0165] The substrate may be a sapphire substrate of which principal
surface is an m-plane.
[0166] A gap may be provided between adjacent ones of the stripe
structures.
[0167] The stripe structures may be gaps.
[0168] A width of the gap may increase as it is more distant from
the active layer.
[0169] The principal surface may be an m-plane, and the
polarization direction may be an a-axis direction.
[0170] The polarized light may have such a light distribution
characteristic that it has a wider radiation angle in a c-axis
direction than in an a-axis direction of the active layer.
[0171] The stripe structures may include a material of which
refractive index is lower than that of a nitride semiconductor.
[0172] A light source device which is still another embodiment of
the present disclosure includes: any of the above-described
semiconductor light-emitting devices; and a wavelength converter
including a phosphoric material which is capable of converting a
wavelength of light radiated from the semiconductor light-emitting
device.
[0173] To realize cost reduction of a nitride-based semiconductor
device of which principal surface is a non-polar m-plane, it is
effective to replace an expensive GaN bulk substrate which is
usually used in the conventional devices with a different type of
inexpensive substrate.
[0174] As a substrate for growth of a nitride semiconductor crystal
of which principal surface is the m-plane, a SiC substrate or
sapphire substrate of which principal surface is the m-plane may be
used. Alternatively, a LiAlO.sub.2 substrate of which principal
surface is the (100) plane may also be used. The sapphire substrate
is advantageous because it is inexpensive, a large diameter
substrate can readily be realized, and it is thermally and
chemically stable. The sapphire substrate has been used in many
conventional c-plane GaN-based light-emitting devices.
[0175] However, the present inventors found that, when one attempts
to realize epitaxial lateral overgrowth using an m-plane nitride
semiconductor film grown on an m-plane sapphire substrate, it is
difficult to reduce the density of dislocations and defects is
difficult, and therefore there are problems in improvement of the
quality, which is not the case with c-plane nitride semiconductor
growth on a conventional c-plane sapphire substrate and epitaxial
lateral overgrowth in a-plane nitride semiconductor growth on an
r-plane sapphire substrate.
[0176] According to the researches conducted by the present
inventors, when an m-plane nitride semiconductor film is formed on
an m-plane sapphire substrate to carry out Pendeo growth, a
semi-polar nitride semiconductor film of the (11-22) plane can grow
from the m-plane sapphire substrate exposed by etching. That is,
when regrowth is carried out according to a Pendeo method, the
m-plane and the semi-polar plane of (11-22) plane coexist, which
may deteriorate the crystallinity and the surface roughness.
[0177] The same problems can also occur when a nitride
semiconductor of which growing plane is a non-polar plane other
than the m-plane or a polar plane is formed as a film on a sapphire
substrate or non-sapphire substrate. The present inventors found
that, even when in the first direction which is defined by
orthogonal projection, on the growing plane of the substrate, of
the c-axis of a plurality of ridge-shaped nitride semiconductor
layers for regrowth by a Pendeo method, the lattice mismatch degree
between the substrate and the plurality of ridge-shaped nitride
semiconductor layers is not less than 2%, and in the second
direction which is perpendicular to the first direction in the
growing plane, the lattice mismatch degree between the substrate
and the plurality of ridge-shaped nitride semiconductor layers is
not less than 10%, crystals which have different plane orientations
are likely to grow concurrently.
[0178] Under such circumstances, the present inventors found means
for preventing semi-polar abnormal growth, which is an intrinsic
problem that occurs when an m-plane nitride semiconductor is grown
on an m-plane sapphire substrate, and abnormal growth which occurs
when a nitride semiconductor of which growing plane is a non-polar
plane other than the m-plane or a polar plane is formed as a film
on a sapphire substrate or non-sapphire substrate, thereby
achieving cost reduction and improved quality.
[0179] Next, the Pendeo growth method is described. FIG. 3 is a
schematic diagram of the Pendeo growth method. As shown in FIG.
3(a), firstly, a nitride semiconductor film 812 is grown on a
heterogeneous substrate such as a sapphire substrate 811, and
thereafter, a mask 820 is formed of a dielectric. For the
dielectric mask, for example, SiO.sub.2, SiN, SiON, or ZrO may be
used. Thereafter, maskless space portions 840 are etched away,
whereby the nitride semiconductor film 812 at opening portions is
removed as shown in FIG. 3(b). In this way, new recessed portions
850 in which the heterogeneous substrate is exposed are formed,
whereby an unevenly-processed substrate 900 is provided. Then, a
nitride semiconductor film is regrown on this unevenly-processed
substrate 900. This is shown in FIG. 3(c). In this phase, a nitride
semiconductor film does not grow on the mask 820. The mask 820
functions as a regrowth prevention layer. Thus, in the
unevenly-processed substrate 900, a nitride semiconductor film can
be grown only from the lateral surfaces among the nitride
semiconductor surfaces, so that a nitride semiconductor film 860 is
regrown in lateral directions.
[0180] In this Pendeo growth method, as shown in FIG. 3(b), the
ridge-shaped nitride semiconductor layers 830 (portions remaining
after the etching), from which the regrowth is to start, and the
recessed portions 850 in which the heterogeneous substrate is
exposed by the etching are formed.
[0181] The recessed portions 850 formed by the etching are
periodic. Although this uneven shape usually has a stripe shape
which is thin and elongated along a direction which is based on the
crystal orientation in one plane, it is not necessarily limited to
a stripe shape so long as dislocations and defects laterally
extend, and as a result, the density of dislocations and defects in
the vicinity of the growing surface is reduced. It can be processed
into various forms, including polygonal forms and circular
forms.
[0182] In regrowth of a nitride semiconductor film on the substrate
of FIG. 3(b), regrowth occurs preferentially from the nitride
semiconductor layers 830 of the raised portions, and the regrown
nitride semiconductor film grows in lateral directions from the
lateral surfaces of the raised portion regions. The growth advances
so as to cover the recessed portions 850 in which the heterogeneous
substrate regions are exposed. When the growth is continued in this
way, the laterally-growing nitride semiconductor films 860 connect
to each other to form a connecting portion 890, so that the exposed
surface of the sapphire substrate 811 (the bottom surface 851 of
the recessed portions 850) is covered with the regrown film. When
the growth is further continued, in this turn, a regrown nitride
semiconductor film grows in a direction perpendicular to the
substrate (i.e., the m-axis direction) so as to entirely cover the
mask 820 and form connecting portions above the mask 820 as shown
in FIG. 3(d), and finally, regrowth of a flat nitride semiconductor
is possible. In this process, there is a probability that a space
in which the epitaxial film is not present is produced between the
recessed portions 850 and the laterally-grown nitride semiconductor
film 860 (see FIG. 3(d)). Note that, however, this space gap is not
always produced, and under the conditions that the source materials
are sufficiently supplied, the space between the recessed portions
850 and the nitride semiconductor film 860 can be substantially not
formed. Here, in the case of Pendeo growth, connecting portions are
formed by the lateral growth such that two types of connecting
portions, i.e., the connecting portions 880 above the mask 820 and
the connecting portions 890 above the recessed portions 850, are
periodically formed. Since regrowth of the nitride semiconductor
film starts from the raised-portion nitride semiconductor layers
830 and advances in lateral directions, some of the dislocations
bend in lateral directions rather than the m-axis direction that is
the vertical direction, so that the density of dislocations and
defects can be greatly reduced in recessed portions. Therefore, the
quality of the surface region of the nitride semiconductor film 860
can be improved.
[0183] In general, in Pendeo growth, it is preferred that the depth
of recessed portions formed by etching is as large as possible
relative to the nitride semiconductor regions of the raised
portions. This is because there is a probability that a nitride
semiconductor film grows from the heterogeneous substrate surface
of the recessed portions which has been exposed by etching in the
phase of regrowth. In this Pendeo growth, regrowth is allowed to
occur only from the nitride semiconductor regions of the raised
portions, whereby reduction of the density of dislocations and
defects is realized. Thus, it is important to prevent occurrence of
regrowth from the heterogeneous substrate surface of recessed
portions formed by etching, and even when such regrowth occurs, it
is important to prevent the regrowth from affecting the lateral
regrowth. As the etching depth increases, the source materials
supplied during the regrowth become more difficult to reach the
bottom of the recessed portions, so that the growth preferentially
occurs only from the nitride semiconductor regions of the raised
portions. Thus, the epitaxial lateral overgrowth is enhanced. Even
if growth occurs in the recessed portions, the influence or
interference on the regrown film is small so long as the etching
depth is large.
[0184] The Pendeo growth method can realize the epitaxial lateral
overgrowth without the mask 820 shown in FIG. 3 which is made of a
dielectric. This point is different from the ELOG method. This
method is called a maskless Pendeo growth. Removing a dielectric
mask is advantageous in that contamination by impurities from the
mask material itself is prevented and formation of the connecting
portions 880 above the mask 820 would not occur. The maskless
Pendeo method is shown in FIG. 4. In the step shown in FIG. 4(a),
as in masked Pendeo growth, unevenness is formed by etching, and
thereafter, the unevenly-processed substrate 910 from which the
mask has been removed as shown in FIG. 4(b) is used as a substrate
for regrowth, and regrowth is started from the raised-portion
nitride semiconductor layers 830. As shown in FIG. 4(c), lateral
growth occurs so as to cover the recessed portions 850 while,
concurrently, growth occurs at the upper surfaces of the raised
portions. That is, the upper surfaces of the raised portions and a
regrown film are in contact with each other. By continuing the
regrowth, a flat nitride semiconductor film 870 can be finally
obtained as shown in FIG. 4(d).
[0185] In maskless Pendeo growth, the mask is removed so that
contamination by impurities from the dielectric film, such as
SiO.sub.2, would not occur. Therefore, there is an advantage that a
high quality regrown nitride semiconductor film is obtained.
Further, the step of forming a dielectric mask can be omitted, and
accordingly, the manufacturing cost can advantageously be
reduced.
[0186] Hereinafter, a nitride-based semiconductor device of an
exemplary embodiment is described with reference to the drawings.
An embodiment of the present disclosure relates to epitaxial
lateral overgrowth by a Pendeo method of an m-plane nitride
semiconductor on a sapphire substrate of which principal surface is
the m-plane. In the drawings mentioned below, for the sake of
simple description, elements which perform substantially the same
functions are denoted by the same reference numerals. The present
disclosure is not limited to the embodiments which will be
described below.
[0187] Note that, in the present embodiment, nitride gallium layers
(hereinafter, GaN layers) are mainly described as a seed crystal
film and a regrown film, although these layers may contain Al, In,
or B. Also, the seed crystal and the regrown film are not
necessarily be formed by only a GaN layer. For example, it may
include one Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x, y, z.ltoreq.1,
x+y+z=1) layer. Alternatively, it may include a plurality of
alternately-stacked Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x, y,
z.ltoreq.1, x+y+z=1) layers which have different compositions, and
furthermore, the B element may be contained in these layers.
Embodiment 1
[0188] FIGS. 5(a) and 5(b) are a cross-sectional view and plan view
showing a structure for growth of a nitride semiconductor layer
(unevenly-processed substrate 910) according to exemplary
Embodiment 1.
[0189] As shown in FIG. 5(a), the unevenly-processed substrate 910
includes a sapphire substrate 811 which has an m-plane as a growing
plane 811a and a plurality of ridge-shaped nitride semiconductor
layers 830 provided on the growing plane 811a of the sapphire
substrate 811. Between respective ones of the plurality of
ridge-shaped nitride semiconductor layers 830, there are recessed
portions 850. In the example shown in FIG. 5(b), the ridge-shaped
nitride semiconductor layers 830 extend in a direction which is
inclined by angle .theta. with respect to the c-axis direction of
the sapphire substrate 811. Note that, however, in the present
embodiment, the extending direction of the ridge-shaped nitride
semiconductor layers 830 is not necessarily inclined with respect
to the c-axis direction of the sapphire substrate 811. The angle
.theta. between the extending direction of the ridge-shaped nitride
semiconductor layers 830 and the c-axis of the sapphire substrate
811 may be not less than 0.degree. and not more than 35.degree..
Thanks to this arrangement, when source materials are supplied for
growing a nitride semiconductor layer by using the ridge-shaped
nitride semiconductor layers 830 as seed crystals, growth of a
semi-polar nitride semiconductor from lateral surfaces 852 of the
sapphire substrate 811 which are exposed in the recessed portions
850 is prevented. Thus, the crystallinity and surface flatness of a
nitride semiconductor layer which is to be grown improve.
[0190] At the bottom surface 851 of the recessed portions 850, the
m-plane of the sapphire substrate 811 is exposed. The source
material particles for the nitride semiconductor layer are unlikely
to adhere onto the m-plane of the sapphire. Therefore, in the case
where the nitride semiconductor layer is grown with the
ridge-shaped nitride semiconductor layers 830 being used as seed
crystals, source material particles are more likely to adhere onto
the ridge-shaped nitride semiconductor layers 830 of the lateral
surfaces than onto the bottom surface 851 of the recessed portions
850. As a result, growth of a low-crystallinity nitride
semiconductor from the bottom surface 851 is prevented.
[0191] The growing plane (upper surface) of the ridge-shaped
nitride semiconductor layers 830 is the m-plane of the nitride
semiconductor. FIGS. 5(a) and 5(b) show an example where the
growing plane of the ridge-shaped nitride semiconductor layers 830
is not provided with a mask, although in the present embodiment the
mask 820 such as shown in FIGS. 3(a) to 3(d) may be provided.
[0192] The unevenly-processed substrate 910 of the present
embodiment is formed using the sapphire substrate 811 which is in a
wafer form, for example. In the drawings, only part of the wafer is
shown. Respective elements are shown in consideration of
visibility, and the actual scale of the respective elements is not
limited to the scale of the elements shown in the drawings.
[0193] In the present embodiment, the growing plane of the sapphire
substrate 811 may be inclined by an angle of not more than
5.degree. with respect to the m-plane. A plane which is exposed at
the bottom surface 851 of the recessed portions 850 may also be
inclined by an angle of not more than 5.degree. with respect to the
m-plane. The growing plane of the ridge-shaped nitride
semiconductor layers 830 may also be inclined by an angle of not
more than 5.degree. with respect to the m-plane.
[0194] A plane which is inclined by not more than 5.degree. with
respect to the m-plane has the same characteristics as those of the
m-plane. Therefore, "m-plane" of the present disclosure includes a
plane which is inclined by not more than 5.degree. with respect to
the m-plane.
[0195] In the example shown in FIG. 5(b), "the extending direction
of the ridge-shaped nitride semiconductor layers 830" is identical
with the extending direction of the long side of the planar shape
of the ridge-shaped nitride semiconductor layers 830. These may not
necessarily be identical with each other.
[0196] Further, the absolute value of the angle .theta. may be not
less than 0.degree. and not more than 10.degree.. In this case, the
stacking fault density can be particularly reduced. The details
will be described later with reference to measurement results.
[0197] The plurality of ridge-shaped nitride semiconductor layers
830 of the present embodiment are not limited to the arrangement
shown in FIG. 5(b). Note that the ridge-shaped nitride
semiconductor layers 830 may be interrupted on the wafer and may
not be oriented in the same direction. For example, the plurality
of ridge-shaped nitride semiconductor layers 830 shown in FIG. 6(a)
have a rectangular planar shape. In FIG. 6(a), the length of the
ridge-shaped nitride semiconductor layers 830 in the m-plane is
smaller than the length of the wafer. Each of the ridge-shaped
nitride semiconductor layers 830 is inclined by angle .theta. with
respect to the c-axis direction of the sapphire. The absolute value
of the angle .theta. is, for example, not less than 0.degree. and
not more than 35.degree.. The plurality of ridge-shaped nitride
semiconductor layers 830 shown in FIG. 6(b) includes ridge-shaped
nitride semiconductor layers 830a which are inclined clockwise by
angle .theta. with respect to the c-axis direction of the sapphire
and ridge-shaped nitride semiconductor layers 830b which are
inclined counterclockwise by angle .theta. with respect to the
c-axis direction.
[0198] As shown in FIG. 7(a), in the present embodiment, the widths
of the ridge-shaped nitride semiconductor layers 830 and the
recessed portions 850 (the length along the a-axis direction of the
sapphire) may not be constant. The plurality of ridge-shaped
nitride semiconductor layers 830 may have different planar shapes.
For example, as shown in FIG. 7(b), ridge-shaped nitride
semiconductor layers 830c, 830d which have different lengths may be
arranged.
[0199] As shown in FIG. 8(a), in the present embodiment, lateral
surfaces 830A of the ridge-shaped nitride semiconductor layers 830
may be sloped. In this case, angle .gamma. inside the ridges which
is formed between the lateral surfaces 830A that are parallel to
the extending direction of the ridge-shaped nitride semiconductor
layers 830 and the m-plane may be greater than 0.degree. and
smaller than 150.degree..
[0200] The cross section of the ridge-shaped nitride semiconductor
layers 830 is not limited to a quadrangular or trapezoidal shape
but may be a triangular shape or any other polygonal shape.
Alternatively, it may include a curved surface.
[0201] In the present embodiment, the inclination with respect to
the c-axis direction of the sapphire of the extending direction of
some of the plurality of ridge-shaped nitride semiconductor layers
830, or part of one ridge-shaped nitride semiconductor layer 830,
may not meet the condition of not less than 0.degree. and not more
than 35.degree. In this case, the angle formed between the
extending direction of at least 50% of the plurality of
ridge-shaped nitride semiconductor layers 830 and the c-axis of the
sapphire substrate 811 may be not less than 0.degree. and not more
than 35.degree..
[0202] The sapphire substrate 811 may be made of a sapphire crystal
of which principal surface is the m-plane. The thickness of the
sapphire substrate 811 is, for example, not less than 0.1 mm and
not more than 1 mm. The diameter of the sapphire substrate 811
(wafer) is, for example, not less than 1 inch and not more than 8
inches. The thickness of the ridge-shaped nitride semiconductor
layers 830 is, for example, not less than 10 nm and not more than
10 .mu.m. The width of the ridge-shaped nitride semiconductor
layers 830 (the length in a cross section which is parallel to the
c-axis of the nitride semiconductor) is, for example, not less than
0.1 .mu.m and not more than 30 .mu.m. The width of the recessed
portions 850 is not less than 1 .mu.m and not more than 100
.mu.m.
[0203] In general, the sapphire substrate has high thermal
stability even under the growing condition for the nitride
semiconductor, i.e., even at a high temperature which is equal to
or higher than 1000.degree. C. Also, the sapphire substrate is
chemically stable, and a large diameter substrate can be realized
relatively inexpensively. Thus, it is a suitable candidate for a
heterogeneous substrate for use in nitride semiconductor growth,
i.e., a hetero-substrate.
[0204] The nitride semiconductor layers 830 are realized by forming
the recessed portions 850 in the nitride semiconductor film which
is formed by m-plane growth over the entire sapphire substrate 811.
The nitride semiconductor layers 830 are remaining portions after
the nitride semiconductor film has been partially removed by
photolithography, for example.
[0205] To avoid the nitride semiconductor from remaining on the
bottom surface 851 of the recessed portions 850, the etching for
formation of the recessed portions 850 can be advanced deeper. In
this case, the sapphire substrate 811 is also partially removed so
that the lateral surfaces 852 of the sapphire substrate 811 are
exposed at the lower part of recessed portions 850 as shown in FIG.
5(a). It is not necessary to partially remove the sapphire
substrate 811 for the purpose of forming the recessed portions 850.
In this case, as shown in FIG. 8(b), the bottom surface 851 of the
recessed portions 850 is at the same level as the interface between
the sapphire substrate 811 and the nitride semiconductor layers
830.
[0206] The depth of the bottom surface 851 of the recessed portions
850 relative to the interface between the sapphire substrate 811
and the nitride semiconductor layers 830 may be, for example, not
less than 0 nm and not more than 500 nm, or not less than 0 nm and
not more than 150 nm.
[0207] In the present embodiment, the area of the sapphire which is
exposed at the lateral surfaces of the ridge-shaped nitride
semiconductor layers 830 may be reduced instead of making the
absolute value of the angle formed between the extending direction
of the ridge-shaped nitride semiconductor layers 830 and the c-axis
of the sapphire substrate 811 fall within the range of not less
than 0.degree. and not more than 35.degree.. That is, the depth of
the bottom surface 851 of the recessed portions 850 relative to the
interface between the sapphire substrate 811 and the nitride
semiconductor layers 830 may be not less than 0 nm and not more
than 150 nm. In this structure also, the amount of the semi-polar
nitride semiconductor growing from the sapphire substrate 811 which
is exposed at the lateral surfaces of the ridge-shaped nitride
semiconductor layers 830 decreases, and therefore, the
crystallinity and surface flatness of the nitride semiconductor
layer can be improved. Note that, in the case where a plane which
is different from the m-plane is exposed at the bottom surface of
the recessed portions between the ridges, growth of the nitride
semiconductor from the bottom surface of the recessed portions
causes a problem. Therefore, it is necessary to form the recessed
portions so as to have a deep bottom surface. In the present
embodiment, the bottom surface 851 of the recessed portions 850 is
m-plane sapphire, and therefore, the nitride semiconductor is
unlikely to grow from the bottom surface 851. Thus, the depth of
the recessed portions 850 can be decreased without considering the
growth from the bottom surface 851.
[0208] Further, the structure may be configured such that the
absolute value of the angle formed between the extending direction
of the ridge-shaped nitride semiconductor layers 830 and the c-axis
of the sapphire substrate 811 is not less than 0.degree. and not
more than 35.degree. while the depth of the bottom surface 851 of
the recessed portions 850 relative to the interface between the
sapphire substrate 811 and the nitride semiconductor layers 830 may
be more than 0 nm and not more than 150 nm. With this arrangement,
even when the depth of the bottom surface 851 is more than 0 nm,
the amount of the semi-polar nitride semiconductor growing from the
sapphire substrate 811 which is exposed at the lateral surfaces of
the ridge-shaped nitride semiconductor layers 830 can be further
reduced.
Embodiment 2
[0209] Next, a nitride semiconductor growing method of exemplary
Embodiment 2 is described with reference to FIG. 9. According to
this method, the unevenly-processed substrate 910 of Embodiment 1
and the nitride semiconductor layer which is grown using the
unevenly-processed substrate 910 can be formed.
[0210] [Preparation of Sapphire Substrate and Preparation of Seed
Crystal Nitride Semiconductor Film 812]
[0211] According to the nitride semiconductor growing method of
Embodiment 2, firstly, as shown in FIG. 9(a), an m-plane sapphire
substrate 811 is provided. The sapphire substrate 811 used may have
a size of 1 inch to 8 inches in diameter, for example. The
thickness of the sapphire substrate 811 is, for example, from 0.1
mm to 1 mm. Also, an m-plane sapphire substrate of which substrate
surface has an tilt angle (hereinafter, referred to as "off-angle")
may be suitably used. So long as the off-angle is in the range of
0.degree. to 5.degree., an embodiment of the present disclosure can
be carried out without causing any trouble. The direction of that
inclination may be a direction perpendicular to the m-axis. For
example, it may be the c-axis, a-axis, or [11-22] axis
direction.
[0212] Then, a surface treatment (washing or the like) is carried
out on the m-plane sapphire substrate. Examples of the step of the
surface treatment include organic cleaning, surface treatment with
an acidic solution, such as sulfuric acid, phosphoric acid,
hydrofluoric acid, or the like, and water washing. These steps may
be used in combination. A thermal treatment may be employed as the
surface treatment on the sapphire substrate. The sapphire substrate
is thermally treated at a high temperature near 1000.degree. C. to
1400.degree. C., whereby a surface which has atomic layer steps can
be obtained. Here, the gas atmosphere may be an atmosphere which
contains nitrogen, oxygen, hydrogen, chlorine, etc. The thermal
treatment method may employ an electric furnace. Alternatively,
this thermal treatment step itself may be carried out in a growth
furnace. This thermal treatment step may be combined with the
previously-described organic cleaning or acidic cleaning. These
solution-based cleanings and the surface treatment realized by a
thermal treatment may be omitted. So long as a
commercially-available substrate is sufficiently carefully handled,
growth of a nitride semiconductor can be carried out without
cleaning.
[0213] Then, as shown in FIG. 9(b), the nitride semiconductor film
812 for seed crystal is grown on the m-plane sapphire substrate
811. For growth of the nitride semiconductor, for example, the
MOCVD (metal organic chemical vapor deposition) method or HVPE
(hydreide vapor phase epitaxy) method may be used. The MOCVD method
and HVPE method are advantageous as the nitride semiconductor
growing method in that growth is possible at a high temperature,
and the growth furnace is suitable to growth of a large diameter
substrate. For growth of the nitride semiconductor, the MBE
(molecular beam epitaxy) method may be employed. In the present
embodiment, a growing method which is based on the MOCVD method is
described.
[0214] In the MOCVD method, firstly, thermal cleaning of the
m-plane sapphire substrate is carried out. This step is carried out
mainly for the purpose of decomposition and removal of moisture and
organic substances adhered on the sapphire surface but may be
carried out for the purpose of obtaining the previously-described
atomic layer steps in the sapphire substrate surface. The
conditions are, for example, the temperature of 800-1200.degree. C.
and the duration of 10-60 minutes. Here, the pressure inside the
growth furnace is 10-100 kPa. The carrier gas used may be H.sub.2
or N.sub.2, or a mixture gas thereof.
[0215] Then, a nitride semiconductor film is grown on the m-plane
sapphire substrate. The carrier gas used may be H.sub.2 or N.sub.2,
or a mixture gas thereof. Examples of the Group III source
materials include: trimethyl gallium (TMG) and triethyl gallium
(TEG) as the Ga source material; trimethyl indium (TMG) as the In
source material; trimethyl aluminum (TMA) as the Al source
material; and triethyl boron (TEB) as the B source material. As the
nitrogen source material, for example, ammonium (NH.sub.3) may be
used.
[0216] The growth conditions for the nitride semiconductor film may
be determined such that the plane orientation of the principal
surface of the nitride semiconductor is controlled to be identical
with the m-plane orientation. As described above, in growth of the
nitride semiconductor on the m-plane sapphire substrate, a
semi-polar plane nitride semiconductor of which principal surface
is the (10-1-3) or (11-22) plane can sometimes grow according to
the growth conditions. To grow the m-plane nitride semiconductor on
the m-plane sapphire substrate, the conditions after thermal
cleaning of the sapphire substrate, such as (1) presence or absence
of the Group III source materials or Group V source materials, (2)
buffer layer, (3) epi-layer on the buffer layer, and (4) growth
temperature, irradiation and growth durations, etc., in respective
steps, may be appropriately selected.
[0217] For example, in the present embodiment, the substrate
temperature is decreased to 400.degree. C.-800.degree. C. after the
thermal cleaning, and then, as the above step (1), TMA irradiation
is carried out for 2-30 seconds under the supply rate condition of
0.1-100 .mu.mol/min. Thereafter, an Al.sub.xGa.sub.yIn.sub.zN
(0.ltoreq.x, y, z.ltoreq.1, x+y+z=1) buffer of step (2) is grown at
the same temperature. Here, an AlN layer may be used as the buffer
layer. The growth conditions are such that, for example, the V/III
ratio is not less than 10 and not more than 5000, and the thickness
is in the range of 20 nm to 500 nm. After the growth of the buffer
layer, as the epi-film of step (3), growth of an
Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x, y, z.ltoreq.1, x+y+z=1) film
is carried out. The epi-film may be GaN, for example. In this case,
the respective conditions are such that, for example, the growth
temperature is from 800.degree. C. to 1100.degree. C., the TMG
supply rate is 1-200 .mu.mol/min, the V/III ratio is not less than
10 and not more than 10000, and the pressure is 10-100 kPa.
Carrying out the growth under such conditions enables growth of the
nitride semiconductor of which principal surface is the m-plane.
This nitride semiconductor layer can be used as the nitride
semiconductor film 812 for seed crystal.
[0218] The thickness of the seed crystal nitride semiconductor may
be appropriately selected. However, considering that a mask is
formed on this seed crystal film in a subsequent step and etching
is carried out till the sapphire substrate is exposed in some
regions, these processes are difficult if the thickness of the seed
crystal nitride semiconductor is excessively large. From this
viewpoint, a desired seed crystal thickness is, for example, from
10 nm to 10 .mu.m.
[0219] This nitride semiconductor layer may be provided with
conductivity control. The n-type conductivity can be realized by,
for example, doping with Si and Ge as the dopants, using SiH.sub.4
and GeH.sub.4 as the source material gases. The p-type conductivity
can be realized by, for example, doping with Mg as the dopant,
using Cp.sub.2Mg (BIS CYCLOPENTADIENYL MAGNESIUM) as the source
material gas.
[0220] [Preparation of Unevenly-Processed Substrate 910]
[0221] Next, a method for manufacturing an uneven substrate for
Pendeo growth is described. In the present embodiment, a method
which is based on the maskless Pendeo growth shown in FIG. 4 is
mainly described.
[0222] First, as shown in FIG. 9(c), a resist is applied over the
seed crystal surface, and a mask pattern is formed according to a
common photolithography technique. As the mask pattern, for
example, a typical line & space (L&S) pattern, i.e., a
pattern of thin and elongated stripes, may be used. The line
portions of this mask 820 (resist portions remaining after
exposure) and the space portions 840 (portions where the resist
does not remain after exposure so that the surface of the nitride
semiconductor film 812 for seed crystal which is the underlayer is
exposed) determine the widths of the nitride semiconductor layers
830 of the raised portions and the recessed portions 850 after the
processes. That is, the line portions are regions in which the seed
crystals remain as they are and are therefore low crystal quality
regions in which the dislocation density and the defect density are
high. The space portions are regions in which a film is formed from
the seed crystal portion by lateral regrowth and are high crystal
quality regions. As for their widths, for example, the line width
is from 0.1 .mu.m to 30 .mu.m, and the space width is from 1 .mu.m
to 100 .mu.m. Note that the dislocations can be observed by, for
example, a transmission microscope, or the like.
[0223] Then, the nitride semiconductor film 812 for seed crystal is
removed from the space portions 840 by etching till a sapphire
substrate surface portion is exposed. As a result, recessed
portions 850 are formed so as to penetrate through the nitride
semiconductor film 812 for seed crystal. The ridge-shaped nitride
semiconductor layers 830 are formed by portions of the nitride
semiconductor film 812 for seed crystal which are covered with the
mask 820.
[0224] The etching method includes various methods including, for
example, wet etching, sputter etching, plasma etching, sputter ion
beam etching, and reactive ion beam etching. These methods may be
appropriately employed.
[0225] In forming the recessed portions 850 by etching, part of the
sapphire substrate 811 may be etched away together with the nitride
semiconductor film 812 for seed crystal. If there is a remaining
nitride semiconductor film in the recessed portions 850 that are
openings without being etched away, regrowth occurs from the
remaining film during regrowth of the nitride semiconductor film,
so that a high quality regrown film cannot be obtained in some
cases. Thus, from the viewpoint of entirely removing such a nitride
semiconductor film that can remain in the recessed portions 850, it
is preferred that part of the sapphire substrate is etched away. In
this case, the lateral surfaces of the recessed portions 850
include not only the lateral surfaces of the ridge-shaped nitride
semiconductor layers 830 but also the lateral surfaces 852 of the
sapphire substrate 811. At the bottom surface of the recessed
portions 850, the m-plane is exposed.
[0226] The formation method of the previously-described recessed
portions 850 may be realized by a method which is different from
the etching. For example, it may be a mechanical processing method,
such as common scribing, or scribing with the use of laser, or may
be a method which is combined with the previously-described etching
method.
[0227] In the present embodiment, the etching depth of the sapphire
substrate of the recessed portions 850 which is formed when the
unevenly-processed substrate 910 is manufactured by etching the
nitride semiconductor film 812 for seed crystal is preferably as
small as possible. This etching depth of the sapphire substrate is
equal to the depth (height) of the lateral surfaces 852 of the
sapphire substrate 811. As described above, in common Pendeo
growth, this etching depth is preferably as deep as possible. This
is for the purpose of decreasing the probability that the source
materials reach the sapphire substrate surface of the recessed
portions 850, so that crystal growth at the opening portions can be
prevented. However, it was found that, in the present embodiment,
at some etching depths of the recessed portions 850, a semi-polar
plane nitride semiconductor film of which crystal orientation is
different from the m-plane disadvantageously grows in a subsequent
regrowth step. It was found that such semi-polar plane growth
occurs from the lateral surfaces 852 of the sapphire substrate 811
which have been formed in the etching. To prevent this semi-polar
plane growth, the etching depth of the sapphire substrate of the
recessed portions 850 (i.e., the height of the lateral surfaces 852
of the sapphire substrate 811) may be controlled so as to be not
less than 0 nm and not more than 500 nm, within the range of not
less than 0 nm and not more than 150 nm.
[0228] Next, the in-plane tilt angle of the stripe mask is
described. The definition of the in-plane tilt angle of the stripe
mask in the present embodiment is illustrated in FIG. 10 and FIG.
11. FIG. 10 is a diagram showing the case where an m-plane GaN is
grown on an m-plane sapphire substrate of which off-angle is
0.degree.. FIG. 10(a) is a diagram which is seen from the principal
surface side. FIG. 10(b) is a cross-sectional view taken along the
broken line of FIG. 10(a). Note that, in this specification and
drawings, the case where the extending direction of the stripe mask
is parallel to the c-axis direction of the sapphire (the a-axis
direction of the GaN) is assumed as .theta.=0.degree. (FIG. 10). In
this specification, the in-plane mask tilt angle
.theta..noteq.0.degree. means a state where the stripe mask is
rotated in the plane from the c-axis direction of the sapphire (the
a-axis direction of the GaN) as shown in FIG. 11. For example, FIG.
11 shows a case where 0.degree.<.theta.<90.degree., and
.theta.=90.degree. means that the extending direction of the stripe
mask is parallel to the a-axis direction of the sapphire (the
c-axis direction of the GaN film).
[0229] FIG. 12 shows an example of the in-plane mask tilt angle
where the m-plane sapphire substrate has an off-angle in the a-axis
direction. For example, in the case of FIG. 12, the m-plane
sapphire substrate is made off by a degree in the a-axis direction.
In this case also, considering the in-plane mask tilt angle
relative to the a-axis direction of the GaN, the in-plane mask tilt
angle is assumed as .theta.=0.degree. when the a-axis direction of
the GaN (in the case of the m-plane sapphire, the c-axis direction)
is parallel to the extending direction of the stripe mask. In this
case, the direction of .theta.=90.degree. is parallel to a
projected component on the growth principal surface of the a-axis
direction of the sapphire (the c-axis of the GaN).
[0230] FIG. 13 is an example of the in-plane mask tilt angle where
the m-plane sapphire substrate has an off-angle in the c-axis
direction. For example, in the case of FIG. 13, the m-plane
sapphire substrate is made off by p degrees in the c-axis
direction. In this case, considering the in-plane mask tilt angle
relative to the direction of a projected component on the growth
principal surface of the a-axis of the GaN, the in-plane mask tilt
angle is assumed as .theta.=0.degree. when the direction of the
projected component is parallel to the extending direction of the
stripe mask. In this case, the direction of .theta.=90.degree. is
parallel to the a-axis direction of the sapphire (the c-axis of the
GaN).
[0231] In this case, an embodiment of the present disclosure can be
carried out without causing any trouble so long as the range of the
off-angle of the m-plane sapphire substrate is the range of
0-5.degree. as previously described.
[0232] After the recessed portions 850 have been formed, the resist
mask remaining on the surface is removed, whereby the
unevenly-processed substrate 910 shown in FIG. 9(d) can be
manufactured.
[0233] FIG. 14 shows a schematic cross-sectional view of the
unevenly-processed substrate 910 of the present embodiment. In this
drawing, the extending direction of the stripes is the c-axis
direction of the GaN (i.e., the in-plane mask tilt angle
.theta.=90.degree.: the a-axis direction of the m-plane sapphire
substrate). In FIGS. 14(a) and 14(b), the tilt angles of the
lateral surfaces, y, are different. FIG. 14(a) shows a case of
0.degree.<.gamma.<90.degree.. FIG. 14(b) shows a case of
.gamma.=90.degree.. The tilt angle .gamma. refers to an angle
between the m-plane of the sapphire substrate 811 and the lateral
surface of the ridge-shaped nitride semiconductor layer 830. The
tilt angle .gamma. of the lateral surface can be controlled by the
shape of the resist or the etching method, and as a result, the
cross-sectional shape can be controlled so as to be trapezoidal,
triangular, or polygonal. For example, in the case of forming the
nitride semiconductor layers 830 which have a rectangular
cross-sectional shape in which the upper base and the lower base
have substantially the same lengths (.gamma.=90.degree.), a resist
mask which has also a near-rectangular cross-sectional shape may be
formed, and the etching conditions may be appropriately selected.
In the case of forming the ridge-shaped nitride semiconductor
layers 830 which have a trapezoidal cross-sectional shape in which
the lower base is longer than the upper base, for example, the
cross-sectional shape of the resist mask may have a configuration
which is inclined in the lateral surface direction, such as a
triangular shape, and the etching conditions may be appropriately
selected. In the case of forming the ridge-shaped nitride
semiconductor layers 830 which have a trapezoidal cross-sectional
shape in which the lower base is shorter than the upper base on the
contrary (.gamma.>90.degree.), the etching may be carried out in
a (side etch) state where the etching rate from the lateral
surfaces is increased by wet etching, or the like.
[0234] The lateral surface tilt angle .gamma. of the lateral
surfaces of the nitride semiconductor layers 830 that are seed
crystals and the lateral surface tilt angle .gamma. of the lateral
surfaces 852 of the sapphire substrate 811 have generally equal
values, although they do not necessarily need to be equal values.
In the present embodiment, the lateral surface tilt angle .gamma.
can be selected from a wide range and, for example, the range of
0.degree.<.gamma.<150.degree. is desirable.
[0235] In the present embodiment, the thickness of the ridge-shaped
nitride semiconductor layers 830 is, for example, 10 nm to 10
.mu.m. However, this thickness may be appropriately selected. The
epitaxial lateral overgrowth of the m-plane nitride semiconductor
of the present disclosure can be realized so long as the seed
crystal portions of the same m-plane nitride semiconductor from
which m-plane nitride semiconductor regrowth starts and the
recessed portions 850 from which the nitride semiconductor film
portions are removed such that the m-plane sapphire substrate is
exposed are formed. Therefore, the ridge-shaped nitride
semiconductor layers 830 may be realized by formation of nitride
semiconductor layers of which principal surface is the m-plane. For
example, the ridge-shaped nitride semiconductor layers 830 may be
formed by only the previously-described buffer layers.
[0236] [Regrowth of the Nitride Semiconductor Film 870 on the
Unevenly-Processed Substrate 910]
[0237] Then, as shown in FIG. 9(e), regrowth of the m-plane nitride
semiconductor film 870 on the unevenly-processed substrate 910 is
carried out. For regrowth of the m-plane nitride semiconductor, a
method used for crystal growth of the nitride semiconductor, such
as MOCVD, HVPE, MBE, or the like, may be appropriately used. The
growing methods of the previously-described nitride semiconductor
film 812 for seed crystal and the nitride semiconductor film 870 do
not necessarily need to be the same. However, considering that
growth is possible at a high temperature and it is suitable to
increase in diameter, it can be said that MOCVD and HVPE are
suitable regrowth methods of the nitride semiconductor. In the
present embodiment, an embodiment which employs MOCVD as the
regrowth method is described.
[0238] After the unevenly-processed substrate 910 is carried into
an MOCVD apparatus, regrowth of the m-plane nitride semiconductor
layer is performed. The carrier gas used may be H.sub.2 or N.sub.2,
or a mixture gas thereof. Examples of the Group III source
materials used include TMG, TEG, TMI, TMA, and TEB. As the nitrogen
source material, for example, NH.sub.3 may be used.
[0239] In the present embodiment, the pressure inside the growth
furnace is 10-100 kPa. A mixture gas of H.sub.2 and N.sub.2 is used
as the carrier gas. After the substrate is carried into the growth
furnace, the substrate is increased to the growth temperature. In
the middle of the temperature increase period, when the substrate
temperature reaches 400-1000.degree. C., the NH.sub.3 gas is
supplied into the growth furnace while the increase of the
temperature is continued. In this phase, the flow rate of the
NH.sub.3 gas is 0.1-5 slm (standard liter/min).
[0240] After the substrate temperature reaches the regrowth
temperature, the Group III source material is supplied, and
regrowth is started. The regrowth temperature is 800-1100.degree.
C. When a GaN film is grown, for example, the TMG flow rate is
1-200 .mu.mol/min, the V/III ratio is not less than 10 and not more
than 10000, and the pressure is 10-100 kPa. The carrier gas used
may be a H.sub.2 gas or may be switched to N.sub.2. Alternatively,
a mixture of these gases may be used.
[0241] One of the important points in the regrowth is that films
regrown from the respective stripe-shaped seed crystals are allowed
to bond with each other so as to obtain a flat film. In general, in
non-polar plane growth of a nitride semiconductor, a relatively
flat film can be obtained under reduced pressure, low V/III ratio
conditions. Therefore, the conditions for obtaining a flat regrown
film are such that, for example, in the case of growing a GaN film,
the regrowth temperature is 800-1100.degree. C., the TMG flow rate
is 1-200 .mu.mol/min, and the V/III ratio is not less than 10 and
not more than 250, and the pressure is 10-33 kPa. The carrier gas
used may be a H.sub.2 gas or may be switched to N.sub.2.
Alternatively, a mixture of these gases may be used.
[0242] The thickness of the regrown film can be selected from a
wide range and, for example, it is from 1 .mu.m to 30 .mu.m. When
MOCVD is employed, it is from 1 .mu.m to 10 .mu.m, for example.
[0243] [In-Plane Mask Tilt Angle Dependence]
[0244] The unevenly-processed substrate 910 is prepared as
described above, and regrowth of a nitride semiconductor film is
carried out under the above-described conditions, whereby a nitride
semiconductor film 870 which has reduced dislocation density and
defect density thanks to the Pendeo method can be obtained.
However, in the case of the unevenly-processed substrate 910 which
is formed by an m-plane nitride semiconductor film formed on an
m-plane sapphire substrate, (1) the surface flatness, (2)
presence/absence of semi-polar abnormal growth, and (3) the
crystallinity vary depending on the in-plane tilt angle of the
stripe-shaped raised portions. It can be said that such a
phenomenon is specific to an m-plane nitride semiconductor provided
on m-plane sapphire. Next, the reasons for this will be
described.
[0245] FIGS. 15(a) and 15(b) are schematic diagrams of one raised
portion (ridge structure) in the unevenly-processed substrate 910.
FIGS. 15(a) and 15(b) show the most simple cases where the in-plane
mask tilt angles are .theta.=0.degree. and .theta.=90.degree.,
respectively. As for the epitaxy relationship between the m-plane
sapphire substrate and the m-plane nitride semiconductor, the
m-axes that are the normal directions of the principal surfaces are
parallel, but the a-axes and the c-axes in the plan are deviated by
90.degree.. Therefore, when the unevenly-processed substrate 910 is
provided as shown in FIG. 15(a), the principal surface of the GaN
at which regrowth occurs is the m-plane, while the lateral surface
is the c-plane. On the other hand, the principal surface of the
sapphire substrate which is formed in the same process
(corresponding to the recessed portion 850) is the m-plane, while
the lateral surface 852 is the a-plane of the sapphire. That is, in
the case of FIG. 15(a), although the principal surfaces are the
same m-plane, the lateral surfaces are different. Specifically, the
lateral surface of the GaN is the c-plane while the lateral surface
of the sapphire is the a-plane. On the other hand, in the case
where the in-plane mask tilt angle shown in FIG. 15(b) is
.theta.=90.degree., the lateral surface of the GaN is the a-plane,
and the lateral surface of the sapphire is the c-plane. As a
result, when the tilt angle of the stripe-shaped mask is varied in
the plane from .theta.=0.degree. to .theta.=90.degree., in the GaN,
regrowth in which the a-plane facet serves as the starting point
gradually transitions to a regrowth mode in which the c-plane facet
serves as the starting point. On the other hand, at the lateral
surface of the sapphire, it gradually transitions from the a-plane
facet to the c-plane facet.
[0246] In the unevenly-processed substrate 910 in which the m-plane
nitride semiconductor on the m-plane sapphire substrate serves as
the basic body, the facet surface which serves as the starting
point of lateral regrowth complicatedly varies depending on its
in-plane mask tilt angle, and therefore, (1) the surface flatness,
(2) presence/absence of semi-polar abnormal growth, and (3) the
crystallinity of a film which can be obtained by regrowth vary.
Next, the respective in-plane mask tilt angle dependences will be
described.
[0247] [(1) Relationship Between the In-Plane Mask Tilt Angle and
the Surface Flatness]
[0248] Firstly, the surface flatness of the regrown nitride
semiconductor film 870 is described. Depending on the in-plane tilt
angle of the mask, the facet surface which serves as a starting
point of the epitaxial lateral overgrowth varies, and accordingly,
the surface flatness greatly varies. Examples of the facet surface
which serves as a starting point of the lateral growth include the
a-plane, c-plane, (10-11) plane, and (11-22) plane of the GaN, and
r-planes obtained by making these planes inclined in the m-axis
direction. It is known that, in the lateral growth of the GaN
layer, the growth speed is faster in the a-axis direction than in
the c-axis direction. This is probably because the c-plane facet is
more susceptible to heat than the a-plane facet, so that
decomposition and omission of the GaN layer are more likely to
occur, and therefore, the effective growth speed decreases. Thus,
when the in-plane mask tilt angle .theta. is 0.degree. (the
extending direction of the stripe mask is the a-axis direction of
the GaN, and the facet which serves as a starting point of the
lateral growth is the c-axis direction), the growth speed is the
slowest so that the effect of surface flattening is unlikely to be
obtained. On the other hand, as .theta. increases, the surface
flatness also improves (gaps are more likely to be filled). This
effect is the greatest when .theta.=90.degree. (i.e., the extending
direction of the stripe mask is the c-axis direction of the GaN,
and the facet which serves as a starting point of the lateral
growth is the a-axis direction).
[0249] Performing the regrowth under the conditions that can
maximize this surface flattening effect is important from the
viewpoint of improving the crystallinity of the regrown nitride
semiconductor film. According to the maskless Pendeo growth of the
present embodiment, a film which is regrown on the nitride
semiconductor layers 830 (line portions) that are seed crystal
portions has remaining dislocations and defects as they are and
therefore has poor crystallinity, and in a film formed by lateral
regrowth in the recessed portions 850 (space portions), the effect
of reducing the dislocation density and the defect density can be
obtained. From this viewpoint, the ratio between the line portions
and the space portions in the period of mask formation is desirably
determined such that the space portions are larger. However, if the
space portions are excessively large, the nitride semiconductor
films 870 regrown from the nitride semiconductor layers 830 cannot
sufficiently bond with each other, resulting in a regrown film
which has gaps. Therefore, to form the space portions (i.e.,
recessed portions 850) which have a larger area ratio and obtain a
flat regrown film surface without gaps, it is desired to select the
in-plane mask tilt angle such that the surface flattening effect is
maximized.
[0250] However, the surface flattening effect in the
previously-described lateral regrowth is the result of the case
where only the m-plane nitride semiconductor is considered.
According to the examinations carried out by the present inventors,
in the case where before the regrowth is carried out as in the
present embodiment the substrate surface has a region in which the
m-plane sapphire surface is exposed, such as at the bottom surface
of the recessed portions 850, the relationship between the surface
flatness and the in-plane mask tilt angle varies.
[0251] It was found from the results of experiments carried out by
the present inventors that, in the process of providing the
unevenly-processed substrate 910 and growing a regrown nitride
semiconductor film 870, when the in-plane mask tilt angle .theta.
increases from 0.degree. to 35.degree., the surface flatness
improves according to the same principle as the
previously-described characteristics of the GaN film. However, when
.theta.>35.degree., the surface flatness begins to deteriorate.
In the range of 35.degree.<.theta.<90.degree., formation of a
regrown nitride semiconductor film with excellent surface flatness
is difficult.
[0252] It was found that such a surface morphology variation which
occurs when the in-plane mask tilt angle .theta. is not less than
35.degree. has a relation with the semi-polar plane abnormal growth
which will be described below.
[0253] [(2) Semi-Polar Plane Abnormal Growth from the m-Plane
Sapphire Substrate]
[0254] According to the examinations carried out by the present
inventors, it was found that, when the in-plane mask tilt angle is
in the range of 35.degree.<.theta.<90.degree., the surface of
the regrown film deteriorates because in this angle range the
(11-22) plane that is the semi-polar plane grows from the m-plane
sapphire regions exposed at the recessed portions 850. Precisely,
it is probably attributed to the lateral surfaces 852 of the
m-plane sapphire substrate 811 which are formed in the etching of
the space portions 840.
[0255] When the in-plane mask tilt angle .theta. is 0.degree., the
normal line component of the lateral surface facet of the GaN is
oriented in the c-axis direction, while the lateral surfaces 852 of
the sapphire substrate 811 are oriented in the a-axis direction
(FIG. 15(a)). As the in-plane mask tilt angle .theta. increases,
the lateral surfaces 852 of the sapphire substrate 811 gradually
transition from the a-axis direction to the c-axis direction.
[0256] That is, in the lateral surfaces 852 of the sapphire
substrate 811, when .theta..noteq.0.degree., there are facets which
have the a-plane and the c-plane. As the value of .theta.
increases, the proportion of the c-plane facet in the lateral
surfaces becomes larger than that of the a-plane facet. It is
inferred that this c-plane facet of the sapphire has a relation
with the (11-22) plane semi-polar plane growth. It is also inferred
that the proportion of this c-plane facet increases according to
the increase of .theta., and that when .theta.>35.degree. growth
of the semi-polar plane nitride semiconductor film remarkably
occurred from the lateral surfaces 852 of the sapphire substrate
811 exclusive of the nitride semiconductor layers 830.
[0257] It is known that, when a nitride semiconductor film is grown
on an m-plane sapphire substrate, some growth conditions cause
growth of a nitride semiconductor film of which principal surface
is the (11-22) semi-polar plane rather than the m-plane (Japanese
Journal of Applied Physics 45, No. 6, L154-L157 (2006)). The
relationship of the plane orientation of this (11-22) semi-polar
plane is shown in FIG. 16(a). It is inferred that growth of the
(11-22) semi-polar nitride semiconductor on the m-plane sapphire
substrate has a relation with the r-plane facet of the m-plane
sapphire surface (see FIG. 16(b)). In the GaN shown in FIG. 16(a),
the angle between the (11-22) plane and the (11-20) plane that is
the a-plane is 31.6.degree.. In the sapphire shown in FIG. 16(b),
the angle between the (1-100) plane that is the m-plane and the
r-plane (1-102) plane is 32.4.degree.. The difference of these
angles is not more than 1.degree.. It is well known in the art that
a nitride semiconductor of which principal surface is the a-plane
is grown on an r-plane sapphire substrate. Therefore, as shown in
FIG. 16, one of the possible causes of (11-22) semi-polar nitride
semiconductor growth on an m-plane sapphire substrate is a
mechanism which allows an a-plane nitride semiconductor to grow
from an r-plane facet which is present on the growing plane of the
m-plane sapphire. It is inferred that, as a result of the growth of
the a-plane nitride semiconductor on the r-plane facet, a
semi-polar nitride semiconductor of which principal surface is the
(11-22) plane grows.
[0258] Considering the above circumstances, the r-plane facet of
the sapphire faces in the c-axis direction. Therefore, assuming
that part of the m-plane sapphire substrate is etched away together
with the nitride semiconductor regions in the process of preparing
the unevenly-processed substrate 910 as in the present embodiment,
r-plane steps are more readily formed in the case where the c-plane
facet is formed than in the case where the a-plane facet is formed,
and as a result, (11-22) semi-polar plane growth is more likely to
occur.
[0259] As described above, the area proportion of the c-plane facet
of the sapphire included in the lateral surface varies depending on
the in-plane mask tilt angle .theta.. Therefore, it is inferred
that, as a result, the probability of occurrence of (11-22)
semi-polar plane growth also varied depending on .theta.. According
to the results of the examinations carried out by the present
inventors, the range of the in-plane mask tilt angle .theta. which
enables reduction of the effects of the semi-polar plane abnormal
growth and regrowth of a nitride semiconductor crystal which has
only the m-plane orientation was from 0.degree. to 35.degree..
[0260] That is, by controlling the in-plane mask tilt angle of the
stripe structure of the unevenly-processed substrate 910 so as to
be within the range of 0.degree. to 35.degree., the semi-polar
plane abnormal growth can be prevented, and the nitride
semiconductor film 870 which has excellent surface flatness can be
obtained.
[0261] The (11-22) semi-polar abnormal growth that occurs from the
recessed portions 850 at which the m-plane sapphire substrate
surface is exposed depends not only on the in-plane mask tilt angle
but also on the etching depth of the recessed portions 850 (i.e.,
the depth of the lateral surfaces 852 of the sapphire substrate
811). Even when the in-plane mask tilt angle was in the range of
0-35.degree., if this etching depth was excessively deep, the
semi-polar plane abnormal growth occurred in some cases. This is
probably because, even when the process is carried out so as to
achieve .theta.=0.degree., shaping the lateral surfaces 852 of the
sapphire substrate 811 so as to be flat at the atomic level is
difficult in actuality, so that facets which have the c-plane and
the r-plane are partially formed. It is considered that this
phenomenon is more likely to occur as the area of the etched
lateral surfaces of the sapphire substrate increases. Such
semi-polar plane abnormal growth depends on the in-plane mask tilt
angle, as a matter of course. However, the abnormal growth can be
prevented more effectively by controlling the etching depth of the
sapphire substrate of the recessed portions 850 (i.e., the height
of the lateral surfaces 852 of the sapphire substrate 811) so as to
be more than 0 nm and not more than 500 nm, more desirably more
than 0 nm and not more than 150 nm.
[0262] [(3) Relationship Between the In-Plane Mask Tilt Angle and
the Crystallinity and Dislocation Density/Stacking Fault
Density]
[0263] Next, the in-plane mask tilt angle of the nitride
semiconductor film and the effect of reducing the dislocation
density and the defect density are described. First, as for the
dislocation density, it is known that the dislocation density can
be reduced by epitaxial lateral overgrowth over a wide range of the
in-plane mask tilt angle. For example, when the
previously-described surface flatness is high and the in-plane mask
tilt angle .theta. which can prevent the semi-polar plane abnormal
growth is from 0.degree. to 35.degree., the crystal quality can be
improved thanks to the dislocation density reducing effect.
However, in the non-polar plane nitride semiconductor, reducing the
stacking fault density is important as well as reduction of the
dislocation density.
[0264] FIG. 17 shows a schematic diagram of stacking faults in a
device structure. FIG. 17(a) is a schematic diagram of a common
c-plane GaN of which principal surface is a polar plane. FIG. 17(b)
is a schematic diagram of an m-plane GaN in the present embodiment.
A stacking fault is usually formed along the c-plane (Such a
stacking fault is commonly referred to as "Basal Stacking Fault
(BSF)"). Therefore, in a conventional device structure of which
principal surface is the c-plane such as an LED or the like, even
if a stacking fault should occur, the probability that the fault
reaches the active layer region is low, and the fault would not
constitute a major cause of deterioration of the emission
efficiency. On the other hand, in a non-polar plane structure and a
semi-polar plane structure such as in the present embodiment, the
c-plane is present as the lateral surface and the slope surface.
Therefore, if a stacking fault occurs, this defect is also present
in the active layer region and can be a cause of deterioration of
the emission efficiency and variation of the emission wavelength.
Thus, in the non-polar plane semi-polar plane growth, it is
necessary to reduce not only the dislocation density but also the
stacking fault density.
[0265] The effect of reducing the stacking fault density greatly
varies depending on the plane orientation of a facet at which
selective growth occurs. As shown in FIG. 17, stacking faults
mainly occur along the c-plane, and therefore, in the case where
the in-plane mask tilt angle .theta. is 0.degree. as shown in FIG.
15(a), even if there are stacking faults in the seed crystal, the
stacking faults would not extend in lateral directions. On the
other hand, in the case of .theta.=90.degree. as shown in FIG.
15(b), the facet plane of the GaN is the a-plane, and therefore,
stacking faults which are present in the c-plane extend in the
a-axis direction with the progress of the lateral growth. Thus, the
stacking faults are also present in a film obtained by the lateral
growth. Hence, in order to reduce the stacking fault density by the
epitaxial lateral overgrowth, it is desired that the normal line of
the lateral growth facet plane is nearer to the c-axis. Thus, from
the viewpoint of the stacking fault density reducing effect, the
in-plane mask tilt angle .theta. is ideally closer to
0.degree..
[0266] On the other hand, from the viewpoint of the
previously-described surface flatness, it is desired that the
in-plane mask tilt angle is not less than 0.degree..
[0267] It was found that the previously-described stacking fault
density can be reduced even when the in-plane mask tilt angle is
greater than 0.degree.. According to the examinations carried out
by the present inventors, a desirable range of the in-plane mask
tilt angle which enables reduction of the stacking fault density is
from 0.degree. to 10.degree.. Further, so long as the in-plane mask
tilt angle is within this range, the surface flatness can also be
improved at the same time by appropriately selecting the mask
pattern.
[0268] As for the above-described limitation on the in-plane mask
tilt angle, the in-plane mask tilt angle does not need to be
limited to the range of 0.degree. to 10.degree. when the stacking
fault density in the m-plane nitride semiconductor film provided on
the m-plane sapphire substrate that serves as the seed crystal is
low (for example, not more than 10.sup.3 cm.sup.-1) because it is
not necessary to further reduce the stacking fault density. In this
case, in the regrown film, it is only necessary to consider the
surface flatness and prevention of the semi-polar abnormal growth,
and therefore, the in-plane mask tilt angle may be controlled so as
to be within a wide range from 0.degree. to 35.degree..
[0269] Even when the in-plane mask tilt angle .theta. is in the
range of 0.degree. to 35.degree., some regrowth conditions for the
nitride semiconductor (growth conditions, such as thickness), or
some widths of the lines and spaces of the mask pattern, lead to
failure of sufficient bonding of regrown films, resulting in a
surface in which there are gaps. However, so long as the in-plane
mask tilt angle is in this range, the semi-polar abnormal growth
would not occur, and a regrown film of an m-plane nitride
semiconductor which has high quality crystallinity is obtained.
[0270] As for the surface flatness, even when .theta. is 0.degree.,
a gapless surface can be obtained by, for example, decreasing the
space width or increasing the growth duration. From the viewpoint
of device application, gaps do not necessarily need to be removed.
In the case where a device structure is designed on the assumption
that there are gaps, it is only necessary to prevent the semi-polar
plane abnormal growth. Therefore, the in-plane mask tilt angle may
be appropriately selected from the range of 0.degree. to
35.degree..
[0271] In the present embodiment, the maskless Pendeo epitaxial
lateral overgrowth method has been mainly described. The same
effects can also be achieved in a Pendeo growth method with a mask,
in which the nitride semiconductor film 870 is formed with a
dielectric mask of SiO.sub.2, SiN, or the like, a metal mask which
is made of Ti, Ni, Ta, Al, W, Mo, or the like, or a nitride metal
mask, such as TiN or the like, remaining as the mask 820 of the
raised-portion nitride semiconductor regions as shown in FIG. 3. In
this case, in the mask 820 of FIG. 3, dislocations which are
present in the nitride semiconductor layers 830 can be prevented
from extending to a portion overlying the mask 820, so that there
is a probability that the crystallinity can be improved.
[0272] The raised-portion nitride semiconductor layers 830 and
nitride semiconductor films 870 grown over the m-plane sapphire
substrate may be provided with n-type dopant or p-type dopant
additives for conductivity control. Preferred examples of the
n-type dopant include Si and Ge. Preferred examples of the p-type
dopant include Be, Zn, and Mg.
[0273] In the present embodiment, as the mask used in manufacture
of the unevenly-processed substrate 910, thin and elongated
stripe-shaped structures which are periodically arranged have been
mainly described, although the shape of the mask is not limited to
this example. Masks of various shapes may be used. For example,
mask shapes which have a polygonal shape, such as a quadrangular
shape, or a circular shape, such as an elliptical shape, may be
periodically arranged. Alternatively, the mask structure may be
configured such that structures which are bent in a zigzag fashion
are elongated along one direction in the plane and are periodically
arranged in a direction orthogonal to the direction of the
elongation, rather than a linear stripe-shaped mask. Each of these
patterns does not necessarily need to be formed periodically.
However, in the case of reducing not only the dislocation density
but also the stacking fault density by the epitaxial lateral
overgrowth as described above, it is desirable that the normal
lines of the lateral surfaces which are provided in directions
perpendicular to the extending direction of the raised-portion
nitride semiconductor regions which are formed by the processes are
closer to the c-axis direction of the nitride semiconductor. Since
a desirable range of the in-plane mask tilt angle in which the
stacking fault density reducing effect can be obtained is from
0.degree. to 10.degree., a heterogeneous m-plane nitride
semiconductor substrate 920 which has low dislocation density and
low stacking fault density can be manufactured irrespective of the
shape of the mask so long as etched lateral surfaces which satisfy
such a condition are formed.
[0274] Pendeo growth is carried out using the m-plane nitride
semiconductor film on the m-plane sapphire substrate as the seed
crystal under the above-described conditions, whereby the
heterogeneous m-plane nitride semiconductor substrate 920 in which
the semi-polar plane abnormal growth which is a specific problem in
the case of using an m-plane sapphire substrate is prevented, and
which has excellent surface flatness and excellent crystallinity,
can be obtained.
Inventive Example 1
[0275] Hereinafter, a method for manufacturing a high quality
m-plane heterogeneous GaN substrate of the present disclosure is
described based on a specific example.
[0276] However, embodiments of the present disclosure are not
limited to examples which will be described below.
[0277] [Growth of an m-Plane Nitride Semiconductor on an m-Plane
Sapphire Substrate that Serves as the Seed Crystal]
[0278] Firstly, a method for manufacturing an m-plane nitride
semiconductor on an m-plane sapphire substrate that is used as the
seed crystal of this example is described. This manufacturing
method includes the following steps: [0279] (1) Surface treatment
on the m-plane sapphire substrate; [0280] (2) Thermal cleaning on
the m-plane sapphire substrate; [0281] (3) Trimethyl aluminum (TMA)
source material irradiation on the m-plane sapphire substrate and
low temperature Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x, y,
z.ltoreq.1, x+y+z=1) buffer layer growth; and [0282] (4) Growth of
an m-plane GaN film. Hereinafter, the respective steps will be
described.
[0283] [Step (1): Surface Treatment on the m-Plane Sapphire
Substrate]
[0284] In this example, the thickness of the m-plane sapphire
substrate was 430 .mu.m, the diameter was 2 inches, and the angle
between the normal line of the principal surface of the m-plane
sapphire substrate and the normal line of the m-plane was
0.degree..+-.0.1.degree.. Washing of the substrate before growth
was carried out through the following procedure. The substrate was
washed with an organic solvent. Thereafter, the substrate was
washed in a solution containing sulfuric acid and phosphoric acid
which were mixed in the ratio of 1:1 at 130.degree. C. for 15
minutes, and rinsed with pure water. Even when this substrate
washing process is omitted, it did not largely affect the
crystallinity or the surface flatness of the m-plane GaN which will
be described later.
[0285] [Step (2): Thermal Cleaning of the m-Plane Sapphire
Substrate]
[0286] In this example, a MOCVD (metal organic chemical vapor
deposition) method was used for the growth of the m-plane nitride
semiconductor. The carrier gas used was a mixture gas of H.sub.2
and N.sub.2.
[0287] The m-plane sapphire substrate was carried into a MOCVD
apparatus, and thereafter, the temperature was increased for
thermal cleaning. The thermal cleaning temperature was
1000-1200.degree. C., and the duration was 10-60 minutes.
[0288] [Step (3): TMA Irradiation on the m-Plane Sapphire Substrate
and Growth of a Buffer Layer]
[0289] After the thermal cleaning was finished, the substrate
temperature was decreased to 400-800.degree. C., and then, the
substrate was irradiated with TMA. Thereafter, a buffer layer was
grown at a temperature continuously maintained at the same
level.
[0290] The TMA irradiation duration was 2-30 seconds. Thereafter,
the substrate was irradiated with NH.sub.3 as the nitrogen source
so as to grow a buffer layer at the same temperature. The buffer
layer used herein was an AlN layer. The growth conditions were such
that the V/III ratio was not less than 10 and not more than 5000,
and the thickness was in the range of 20 nm to 500 nm.
[0291] [Step (4): Growth of m-Plane GaN Film]
[0292] After the buffer layer was grown, the substrate temperature
was increased to a temperature in the range of 900.degree. C. to
1100.degree. C. with the substrate being irradiated with NH.sub.3,
and then, the procedure waited for the temperature stability for
1-5 minutes before growth of a GaN film. In this example, a 1-3
.mu.m thick GaN film was grown under the conditions that the
substrate temperature was 950.degree. C., the flow rate of
trimethyl gallium that was the Ga source material was 40 sccm, the
flow rate of NH.sub.3 was 500 sccm, and the pressure was 13
kPa.
[0293] The plane orientation of the grown GaN film can be confirmed
by X-ray diffraction measurement. FIG. 18 is the 2.theta.-.omega.
measurement result of an m-plane GaN seed crystal on an m-plane
sapphire substrate prepared in this example. In this measurement,
the incoming X-ray was parallel to the a-axis direction of the
m-plane GaN.
[0294] In FIG. 18, the peak at 2.theta.=68.7.degree. is the peak of
the m-plane sapphire substrate, which is the diffraction peak of
the (3-300) plane. On the lower angle side of this peak, there is a
diffraction peak from the (2-200) plane of the m-plane GaN, which
was observed near 2.theta.=67.9.degree.. In the m-plane GaN film on
the m-plane sapphire substrate prepared in this example, no peak
from other semi-polar plane GaN films was observed.
[0295] In the case of film formation under growth conditions which
were greatly deviated from those of this example, peaks from the
semi-polar plane GaN such as shown in FIGS. 19(a) and 19(b) were
observed. FIG. 19(a) is the 2.theta.-.omega. measurement result of
a semi-polar plane GaN which has the (10-1-3) plane. The
diffraction peak of the (10-1-3) plane was observed near
2.theta.=63.5.degree.. FIG. 19(b) is the 2.theta.-.omega.
measurement result of a semi-polar plane GaN which has the (11-22)
plane. The diffraction peak of the (11-22) plane was observed near
2.theta.=69.2.degree..
[0296] It has been described that, by appropriately selecting the
growth conditions for the GaN film on the m-plane sapphire
substrate as described above, crystal growth of only a nitride
semiconductor of which principal surface is the m-plane (i.e., no
coexistence of a semi-polar plane crystal) can be realized.
However, the crystallinity of the thus-obtained m-plane nitride
semiconductor is generally low.
[0297] FIG. 20 shows an example of the (1-100) plane X-ray .omega.
rocking curve (XRC) measurement result of an m-plane GaN film on an
m-plane sapphire substrate which was obtained by the
above-described growth method. The solid line and the dotted line
represent the XRC measurement results in the case where the
incoming X-rays were respectively parallel to the a-axis and c-axis
directions of the GaN. The crystal quality and the density of
dislocations and defects can be estimated from the half-value
widths of the XRC. For example, in a conventional c-plane GaN on a
c-plane sapphire substrate, the half-value width of the symmetrical
plane (0002) plane that is the principal surface is generally about
several hundreds of seconds. However, the (1-100) plane XRC full
width at half maximum of the m-plane GaN obtained in this example
was about 1000 seconds, which was a very large value, when the
incoming X-ray came along the a-axis direction of the GaN. This
means that the crystal quality of the m-plane GaN film obtained in
this example is worse than that of the conventional c-plane GaN
film, and the m-plane GaN film has a very high dislocation density.
The dislocation density estimated by a transmission electron
microscope is on the order of about 10.sup.10 cm.sup.-2, which is
higher than that of the conventional nitride semiconductor film on
the c-plane sapphire by one or more orders of magnitude,
considering that the dislocation density of the conventional
nitride semiconductor film on the c-plane sapphire is 10.sup.8 to
10.sup.9 cm.sup.-2 or smaller.
[0298] The XRC full width at half maximum obtained when the
incoming X-ray came along the c-axis direction of the m-plane GaN
film was worse than the above, which was about 2000 seconds. A
heterogeneous m-plane GaN film which was thus obtained in the
present embodiment exhibited a result such that the XRC full width
at half maximum greatly varied when the X-ray incidence direction
in the plane was changed. This means that the crystallinity of the
heterogeneous m-plane GaN film has asymmetry in the plane.
Occurrence of such asymmetry is attributed to stacking faults,
which are plane defects, in the heterogeneous m-plane GaN film.
[0299] That is, in heterogeneous growth of an m-plane nitride
semiconductor on an m-plane sapphire substrate according to the
present embodiment, there are dislocations at high density due to a
large degree of lattice mismatch with the sapphire substrate. In
addition, as can be confirmed from asymmetry of the XRC, there are
stacking faults. Thus, to improve the crystal quality of the
m-plane GaN formed by heterogeneous growth and improve the device
characteristics, it is necessary to reduce both the density of the
dislocations and the density of the stacking faults, which is not
the case with the conventional heterogeneous growth of the c-plane
GaN.
[0300] In this example, the m-plane GaN film on this m-plane
sapphire substrate is used as the seed crystal in order to
manufacture a high quality regrown m-plane nitride semiconductor
substrate which has reduced dislocation density and reduced
stacking fault density according to a epitaxial lateral overgrowth
method which will be described below.
[0301] [Manufacture of Unevenly-Processed Substrate 910]
[0302] In this example, a method for manufacturing the
unevenly-processed substrate 910 for maskless Pendeo growth shown
in FIG. 4 is described. Firstly, an m-plane nitride semiconductor
film grown on an m-plane sapphire substrate was prepared through
the previously-described growth procedure, and a mask pattern was
formed by a common photolithography technique (FIG. 4(a)). As the
mask pattern, a typical line & space (L&S) pattern, i.e., a
pattern of thin and elongated stripes, was used. In the L&S
pattern used in this example, the width of the line portions of the
mask 820 was L=5 .mu.m, and the width of the space portions 840 was
S=10 .mu.m. The thickness of the resist after the photolithography
step was finished was about 2-3 .mu.m. In this example, the
in-plane mask tilt angle .theta. was 0.degree..
[0303] Then, part of the nitride semiconductor film 812 for seed
crystal was removed from the space portions 840 using an
inductively coupled plasma etching (ICP etching) apparatus such
that m-plane sapphire substrate surface portions were exposed,
whereby ridge-shaped nitride semiconductor layers 830 and recessed
portions 850 were formed. In forming the recessed portions 850 by
etching, part of the sapphire substrate 811 was also etched away
such that part of the nitride semiconductor film 812 for seed
crystal would not remain.
[0304] The etching was carried out such that part of the GaN layer
which was present in the regions of the space portions 840 was
entirely removed, and the m-plane sapphire substrate surface was
exposed, whereby the recessed portions 850 were formed. Thereafter,
the resist mask remaining on the surface was removed, whereby the
unevenly-processed substrate 910 shown in FIG. 4(b) was
completed.
[0305] Examples of the unevenly-processed substrate 910 of the
present embodiment are shown in FIGS. 21(a) and 21(b). FIGS. 21(a)
and 21(b) are scanning electron microscopic images (SEM images)
obtained after raised portion GaN films and recessed portions 850
in which the sapphire surface was exposed by etching were formed
using a stripe-shaped L&S pattern mask. Here, a cross-sectional
view taken along the extending direction of the raised portion GaN
(left) and a bird's-eye view (right) are shown. The shape of the
ridge-shaped nitride semiconductor layers 830 can be controlled by
appropriately selecting the mask formation conditions or etching
conditions. As shown in FIG. 21, the cross-sectional shape of the
ridge-shaped nitride semiconductor layers 830 can be controlled so
as to be (a) trapezoidal or (b) triangular. In this example, a GaN
film of which cross-sectional shape was trapezoidal was used as the
ridge-shaped nitride semiconductor layers 830. Also, as shown in
FIGS. 21(a) and 21(b), in this example, in the recessed portions
850, the sapphire substrate was also partially etched away, and the
depth was about 250 nm.
[0306] When the in-plane mask tilt angle is 0.degree. or an angle
near 0.degree. as in this example, the opposite lateral surfaces of
the ridge-shaped nitride semiconductor layer 830 are GaN facets of
the +c plane and the -c plane (or planes inclined with respect to
.+-.c planes). In general, the +c plane and the -c plane have
different etching tolerances and therefore have different etching
speeds. Thus, normally, when etching is carried out with the
in-plane mask tilt angle being near 0.degree., the cross-sectional
shape can be asymmetry in some cases.
[0307] In this example, cross sections with relatively good
symmetry was successfully obtained as shown in FIGS. 21(a) and
21(b) by appropriately selecting the previously-described ICP dry
etching conditions. However, in the present embodiment, the
regrowth starts from the raised-portion nitride semiconductor
layers 830, and therefore, it is inferred that the effect of
asymmetry of the shapes of the stripe-shaped opposite lateral
surfaces on the regrown film is small. Thus, the opposite lateral
surfaces formed in the raised portion nitride semiconductor layer
do not necessarily need to have symmetrical tilt angles.
[0308] In this example, the thickness of the raised-portion nitride
semiconductor layers 830 is about 1-3 .mu.m, although this
thickness may be appropriately selected. The epitaxial lateral
overgrowth of the m-plane nitride semiconductor of the present
disclosure can be realized so long as the seed crystal portions of
the same m-plane nitride semiconductor from which m-plane nitride
semiconductor regrowth starts and the recessed portions 850 from
which the nitride semiconductor film portions are removed such that
the m-plane sapphire substrate is exposed are formed. As described
above, the TMA irradiation on the m-plane sapphire substrate and
the buffer layer are processes which are necessary for obtaining a
nitride semiconductor film of which principal surface is the
m-plane (i.e., for preventing the semi-polar plane growth) and are
therefore indispensable steps in this example. Thus, in one
embodiment, the raised-portion nitride semiconductor layers 830 may
be formed by only the previously-described buffer layer.
[0309] However, as shown in FIG. 4, the crystal quality of the
m-plane nitride semiconductor film 870 of this example which is
obtained by lateral selective regrowth largely depends on the
crystal quality of the ridge-shaped nitride semiconductor layers
830 that serve as the seed crystal. In the case where the
previously-described raised-portion nitride semiconductor layers
830 are formed by only the buffer layer, it is difficult to obtain
a high quality regrown nitride semiconductor film. In view of such,
in this example, the nitride semiconductor film 812 for seed
crystal, of which growing plane was the m-plane, was formed on the
buffer layer, and this layer was processed to obtain the
raised-portion nitride semiconductor layers 830. In general, as the
thickness of the raised-portion nitride semiconductor layers 830
increases, the crystallinity is more likely to improve, and it is
more advantageous in improving the quality of the nitride
semiconductor film 870. However, disadvantageously, the cost
increases due to increase of the seed crystal growth duration,
increase of the etching step duration, etc. In this example, the
thickness of the nitride semiconductor film 812 for seed crystal
was 1-3 .mu.m in consideration of a trade-off between the
crystallinity of the seed crystal and the growth duration.
[0310] [Regrowth of the Nitride Semiconductor Film 870 on the
Unevenly-Processed Substrate 910]
[0311] Then, regrowth of an m-plane nitride semiconductor film 870
was carried out on the unevenly-processed substrate 910.
[0312] The unevenly-processed substrate 910 was carried into a
MOCVD apparatus, and thereafter, the temperature was increased to
the regrowth temperature. The carrier gas used was a mixture gas of
H.sub.2 and N.sub.2. In this example, a NH.sub.3 gas was supplied
in the middle of the increase of the temperature. This is for the
purpose of preventing thermal decomposition of the raised-portion
nitride semiconductor layers 830. At the timing when the substrate
temperature reached 500.degree. C., 0.5 slm NH.sub.3 gas was
supplied into the growth furnace, and the supply was configured
till the regrowth temperature was reached, and then, regrowth of
the GaN film was continued. In this example, the growth temperature
was 950.degree. C.
[0313] The other growth conditions for the nitride semiconductor
film 870 were set as follows: the V/III ratio=160, the growth
pressure 13.3 kPa, the growth speed about 4 .mu.m/hour. The growth
conditions for the nitride semiconductor film 870 are not limited
to these conditions but may be appropriately selected. However, to
obtain a flat film by allowing nitride semiconductor films 870
regrown from the respective raised-portion nitride semiconductor
regions to bond with each other as shown in FIG. 4(d), it is
desired that the regrowth is carried out under the conditions of
suitable growth temperature, V/III ratio, and growth pressure. The
regrowth conditions for the m-plane nitride semiconductor film may
be such that the growth temperature is 850-1100.degree. C., the
V/III ratio is 50-2000, and the growth pressure is 1-100 kPa. More
specifically, the conditions may be such that the growth
temperature is 950-1100.degree. C., the V/III ratio is 50-200, and
the growth pressure is 1-30 kPa.
[0314] FIG. 22 shows a microscopic image of a surface of a sample
after the regrowth. In this example, the in-plane mask tilt angle
.theta.=0.degree., and therefore, the extending direction of the
stripes is parallel to the a-axis direction of the GaN.
[0315] It can be seen that, since the growth conditions used
enhanced the previously-described lateral growth, the uneven shape
shown in FIG. 21 was not seen, the unevenness was filled by the
regrowth, and a relatively flat m-plane GaN regrown film was
realized. On the other hand, some pits can be seen along the
extending direction of the stripe-shaped uneven structures. These
correspond to the connecting portion 890 of FIG. 4(d). Due to these
pits, the surface roughness increases, and the surface rms (root
mean square) roughness of the sample of FIG. 22 was about 133
nm.
[0316] As shown in FIG. 15(a), in the case of the in-plane mask
tilt angle .theta.=0.degree., the lateral surface facet which
serves as a starting point of the lateral growth has the c-axis
direction component in the GaN. As previously described, the
c-plane facet has a slow growth speed and a short migration length
as compared with the a-plane facet, and the surface flatness
improving effect is small as compared with the a-plane facet. Thus,
by increasing the in-plane mask tilt angle so as to incline this
lateral surface facet in the a-axis direction, the migration effect
is enhanced, and generation of pits can be prevented. The in-plane
mask tilt angle dependence will be described later in Inventive
Example 2.
[0317] In this example, the space interval is 10 .mu.m. However, by
shortening this space interval, a flat regrown film surface which
does not have pits or gaps can be obtained even when the in-plane
mask tilt angle is 0.degree.. For example, it was confirmed from
the experimental results that, when the space interval is 7 .mu.m,
the pits that are seen in FIG. 22 would not occur, and a flat
regrown nitride semiconductor surface can be obtained. That is, by
narrowing the interval of the raised-portion nitride semiconductor
regions, the problem of short migration length can be overcome, and
the surface flatness can also be improved.
[0318] However, high-quality low-dislocation/defect density regions
obtained by the maskless Pendeo epitaxial lateral overgrowth of
this example are the regions of the recessed portions 850. In
regrown films which are regrown on the raised-portion nitride
semiconductor layers 830 that are the seed crystals, dislocations
and defects that are present in the seed crystals remain as they
are, so that the effect of improving the crystallinity of these
ridge-shaped nitride semiconductor layers 830 is low. Therefore,
from the viewpoint of improving the characteristics of the device
structure that is to be formed on this film, optimizing the width
of the recessed portions 850 such that, at the surface of the
nitride semiconductor film 870, the dislocation density and the
stacking fault density can be reduced and higher crystal quality
can be obtained is important.
[0319] For example, the method for improving the crystallinity of
the nitride semiconductor film 870 may be decreasing the width of
the raised-portion nitride semiconductor layers 830, i.e., L of the
L&S pattern. This width of L may be suitably selected in the
present embodiment. The range of the width may be from 1 nm to 100
.mu.m or may be from 1 nm to 10 .mu.m.
[0320] To reduce pits and gaps so as to obtain a flatter regrown
surface, the regrowth duration may be increased in addition to
shortening the S (space) interval of the L&S pattern, such that
a regrown film can be deposited till pits and gaps are filled
up.
[0321] FIGS. 23(a), 23(b), and 23(c) show SEM images of a regrown
GaN film (heterogeneous m-plane nitride semiconductor substrate
920) obtained in Inventive Example 1. FIG. 23(a) is a bird's-eye
view. FIG. 23(b) is a cross-sectional view of the nitride
semiconductor film 870 regrown from the raised-portion nitride
semiconductor layer 830. FIG. 23(c) is a cross-sectional SEM image
in the vicinity of an exposed sapphire substrate surface of the
recessed portion 850. The thickness of the regrown nitride
semiconductor film was about 8 .mu.m. It can be appreciated from
the cross-sectional SEM image that regrowth occurred from the
raised-portion nitride semiconductor layers 830 that were the seed
crystals, and films regrown from adjacent raised portions bonded
with each other so that a flat surface was finally formed.
[0322] In FIG. 23(c), regrowth of a nitride semiconductor from the
exposed sapphire surface of the recessed portions 850 is not found.
It can be appreciated that a source material arriving at the uneven
substrate surface underwent migration, without being adsorbed by
the sapphire surface of the recessed portions 850, and reached the
raised-portion nitride semiconductor regions, so that regrowth
preferentially occurred only from this nitride semiconductor film.
That is, it can be appreciated that the source material is unlikely
to be adsorbed by the m-plane sapphire surface, and epitaxial
growth of the arriving source material is unlikely to occur at the
m-plane sapphire surface.
[0323] In this example, a mask with a space width of 10 .mu.m was
used. It was also confirmed that, in an unevenly-processed
substrate with a space width of 30-250 .mu.m, by optimizing the
growth conditions, the source material is preferentially supplied
to the seed crystal portions, rather than the exposed sapphire
substrate surface of the recessed portions 850, so that epitaxial
lateral overgrowth occurs. Thus, it was found that the source
material supplied in the regrowth is unlikely to be adsorbed by the
sapphire surface and reaches the seed crystal portions to
contribute to growth of the nitride semiconductor film. Details of
the examinations on the space width will be described in detail in
the section of Inventive Example 5.
[0324] In this example, the height of the lateral surfaces 852 of
the sapphire substrate was about 250 nm, but even with such a
relatively small etching depth, regrowth from the sapphire surface
which has the m-plane would not occur, so that epitaxial lateral
overgrowth can be realized. This is attributed to the
previously-described features. The dependence of the height of the
lateral surfaces 852 of the sapphire substrate 811 (etching depth)
will be described later in detail in the section of Inventive
Example 3.
[0325] An X-ray 2.theta.-.omega. measurement was carried out on
this sample. Only the (2-200) peak from the GaN layer regrown on
the unevenly-processed substrate 910 and the (3-300) peak from the
m-plane sapphire substrate were observed as in the result of FIG.
18, and peaks which are attributed to the semi-polar planes
((10-1-3) plane and (11-22) plane) were not observed. That is, in a
GaN film which was regrown using the unevenly-processed substrate
910 that was manufactured under the conditions of Inventive Example
1, only diffraction peaks which are attributed to the m-plane were
observed. It was found that the semi-polar plane abnormal growth
did not occur.
[0326] The results of the (1-100) plane X-ray co rocking curve
(XRC) half-value width of the regrown m-plane GaN film obtained in
this example are shown in Table 1. Here, the incoming X-ray was
parallel to the a-axis and c-axis directions of the GaN. For the
sake of comparison, the values of the half-value width of the
m-plane GaN film used as the seed crystal are shown together in the
same table. The half-value width of the m-plane GaN film that was
grown on the m-plane sapphire substrate as described above is a
high value which is not less than 1000 seconds. When the X-rays are
incident in the a-axis and c-axis directions of the GaN, the XRC
full width at half maximum in the case of incidence in the c-axis
direction of the GaN is about twice as large as that in the case of
incidence in the a-axis direction. This is because, as previously
described, information of stacking faults is reflected in when the
X-ray is incident in the c-axis direction. That is, it can be seen
that the m-plane GaN film that is the seed crystal of this example
is a crystal which has asymmetry in the XRC measurement results for
the X-ray incidence in the a-axis and c-axis directions and which
includes many stacking faults.
TABLE-US-00001 TABLE 1 GaN a-axis GaN c-axis direction incidence
direction incidence Seed crystal m-plane GaN 1326 seconds 2325
seconds Regrown m-plane GaN 537 seconds 639 seconds
[0327] On the other hand, when the m-plane GaN film was regrown
after the unevenly-processed substrate 910 was formed using the
same m-plane GaN as the seed crystal, the XRC full width at half
maximums in the a-axis and c-axis directions decreased to 537
seconds and 639 seconds, respectively. The value obtained when the
X-ray was incident in the a-axis direction of the GaN decreased to
about a half. This means that the dislocation density was greatly
reduced by regrowth. The values of the half-value width of the
regrown film for the a-axis and c-axis incidence are similar to the
results of the seed crystal, and the symmetry improved. This means
that, in the regrown m-plane GaN film of this example, not only the
dislocation density but also the stacking fault density
decreased.
[0328] One of the reasons of the decrease of the stacking fault
density is that, in this example, the stripe-shaped L&S pattern
was formed in the a-axis direction of the GaN (mask tilt angle
0.degree.). Since stacking faults are present in the c-plane of the
nitride semiconductor film as shown in FIG. 17, in order to
effectively reduce the stacking fault density, it is preferred that
lateral surfaces are formed in the c-axis direction of the nitride
semiconductor, and epitaxial lateral overgrowth is caused from the
lateral surfaces.
[0329] The thicknesses of the two samples of Table 1 are different
by a factor of about four, the thickness of the regrown film being
greater than that of the seed crystal film. That is, it can be
inferred that the results of Table 1 were attributed to the
difference in thickness. However, according to the results of the
examinations carried out by the present inventors, when only the
thickness was increased to about 8 .mu.m in the growth of the seed
crystal m-plane GaN, for example, no extensive improvement was
achieved in the XRC full width at half maximum, and the values
obtained when X-rays were incident in the a-axis and c-axis
directions of the GaN only improved to about 1100 seconds and 1900
seconds, respectively. That is, it can be appreciated that the
results of Table 1 show the improving effect which was obviously
obtained by the lateral selective regrowth.
[0330] In this Inventive Example 1, the results of the regrown
m-plane nitride semiconductor film on the unevenly-processed
substrate in which the cross-sectional shape of the raised-portion
nitride semiconductor regions has a triangular structure as shown
in FIG. 21(b) are shown, although also in the case where the raised
portion nitride semiconductor which has the trapezoidal structure
shown in FIG. 21(a) was used as the seed crystal, the effect of
reducing the dislocation density and the stacking fault density was
similarly achieved. That is, the crystallinity of the regrown
nitride semiconductor film 870 is greatly affected by the
crystallinity of the raised-portion nitride semiconductor layers
830 that are the seed crystals, and therefore, it is inferred that
the dependence on the shape of the raised-portion nitride
semiconductor regions is small. Thus, the cross-sectional shape of
the extending direction of the raised-portion nitride semiconductor
layers 830 that have a shape of thin and elongated stripes may be
appropriately selected. It may have a polygonal structure, such as
a quadrangular (rectangular), trapezoidal, or triangular structure,
or may have a cross-sectional structure which includes a curve.
Inventive Example 2
In-Plane Mask Tilt Angle Dependence of the m-Plane Nitride
Semiconductor Maskless Pendeo Growth
[0331] In Inventive Example 1, the method of preparing the
unevenly-processed substrate 910 such that the in-plane mask tilt
angle is 0.degree., i.e., the extending direction of the
stripe-shaped mask is parallel to the a-axis direction of the
m-plane nitride semiconductor, and carrying out the epitaxial
lateral overgrowth thereon, and the characteristics of that film
have been described. By performing the process in such a way, the
normal lines of the opposite lateral surfaces of the stripe-shaped
raised-portion nitride semiconductor layers 830 have .+-.c axis
direction components of the nitride semiconductor. Since those
lateral surfaces were employed as starting points of the lateral
growth, not only the dislocation density but also the stacking
fault density were reduced. However, on the other hand, when the
in-plane mask tilt angle .theta. was 0.degree., the migration
length was not sufficient, and many pits were found in the surface.
The flatness of the surface was insufficient under the conditions
of Inventive Example 1.
[0332] In view of the above circumstances, in this Inventive
Example 2, for the purpose of improving the surface flatness, an
experiment was carried out with the in-plane mask tilt angle of
raised-portion nitride semiconductor regions which were formed by
stripe-shaped mask processing being varied from 0.degree. to
90.degree. with intervals of 1.degree. at the minimum.
[0333] In this Inventive Example 2, washing of the m-plane sapphire
substrate, the growing step of the nitride semiconductor film 812
for seed crystal, and the fabrication step of the
unevenly-processed substrate 910 were carried out under basically
the same conditions as those in the steps explained in Inventive
Example 1 except that the in-plane tilt angle of the stripe-shaped
mask was varied in the range of 0.degree. to 90.degree.. Thus,
description of the details of the steps is herein omitted.
[0334] Samples of the unevenly-processed substrate 910 were
prepared such that the extending direction of elongated
raised-portion nitride semiconductor regions in the shape of
stripes had different tilt angles in the m-plane that is the
principal surface. Regrowth of m-plane nitride semiconductor films
was carried out under generally the same conditions as those of
Inventive Example 1.
[0335] FIG. 24 shows examples of the surface morphology of the GaN
substrates (heterogeneous m-plane nitride semiconductor substrates
920) after the Pendeo regrowth with varying in-plane mask tilt
angles, which were obtained by a laser microscope. It can be seen
that the surface flatness greatly varied depending on the in-plane
mask tilt angle. In this Inventive Example 2, even when the
in-plane mask tilt angle was 0.degree., gaps were produced, so that
the surface roughness was worse than that of Inventive Example 1.
In this Inventive Example 2, in order to check the difference in
surface flatness and migration effect, the inventors dared to
adjust the growth temperature of the regrown nitride semiconductor
film so as to be lower than the condition of Inventive Example 1 by
30.degree. C., such that recessed portions 850 were produced even
when the in-plane mask tilt angle was 0.degree.. It is inferred
that decreasing the other conditions than the growth temperature,
such as the growth pressure and the V/III ratio, enhances the
migration length and hence improves the surface flatness.
[0336] When the in-plane mask tilt angle .theta. increased to
17.degree., the gaps which were found in the case of 0.degree. were
filled so that the surface flatness greatly improved. This is
probably because, as previously described, the lateral growth in
the c-axis direction of the GaN shifted to the lateral growth in
the a-axis direction so that the angle .theta. increased, and as a
result, the migration length increased, and the surface flatness
improved.
[0337] On the other hand, when the in-plane mask tilt angle .theta.
was near 35.degree., the surface flatness started to deteriorate.
Some unconnected regions are seen in the plane, and when 0 exceeded
40.degree., this tendency was conspicuously observed.
[0338] FIG. 25 shows the relationship between the tilt angle and
the surface roughness. This graph shows the surface rms roughness
estimated from the laser microscopic images of FIG. 24 and the
in-plane mask tilt angle. The surface flatness improved as the
in-plane mask tilt angle .theta. increased from 0.degree. and
exhibited the best values near the range of 5.degree. to
35.degree., but the surface roughness increased when 35.degree. was
exceeded. The variation of the surface flatness in a low angle
range was attributed to the difference in plane orientation of the
facet plane that served as the starting point of the lateral growth
and the difference in migration length. On the other hand, it is
inferred that the variation in the range of not less than
35.degree. was attributed to different causes. According to the
examinations carried out by the present inventors, it was found
that this variation has a relation with the semi-polar plane
abnormal growth.
[0339] As seen from the surface morphologies of FIG. 24 for the
cases where the in-plane mask tilt angles were (g) 47.degree. and
(h) 80.degree., there are protrusions that grew between the gaps of
which crystal planes were apparently different from the m-plane.
The number of such protrusions increased as the angle shifted to
the higher angle side. Particularly in the sample of
.theta.=80.degree., a large number of such protrusions were
found.
[0340] FIG. 26 shows the results of the cases where regrowth was
carried out on uneven substrates which were made of only GaN for
the sake of comparison. In these samples, the etching of the space
portions 840 was ended in the middle of the nitride semiconductor
film 812 for seed crystal, rather than carrying out the etching
till the m-plane sapphire substrate was exposed. Therefore, the
effect of the sapphire substrate was avoided, so that the
dependence on the in-plane mask tilt angle of the uneven substrates
that were made of only GaN was examined. In the drawing, the
in-plane mask tilt angles were varying angles of 0.degree.,
45.degree., and 90.degree.. In the case of 0.degree., the uneven
shape was remaining after the regrowth. This was attributed to the
short migration length in the c-axis direction of the GaN. On the
other hand, it can be seen that, when the angle was 45.degree. or
90.degree., the uneven shape was filled so that flatness was
achieved. The differences from FIG. 24 are obvious. It can be seen
that, as in this example, deterioration of the surface flatness and
generation of protrusions which occurred when the in-plane mask
tilt angle .theta. was not less than 35.degree. were apparently
attributed to the exposed m-plane sapphire substrate surface of the
recessed portions 850.
[0341] It was clarified from the XRD 2.theta.-.omega. measurements
that the protrusions were actually the (11-22) semi-polar plane.
FIG. 27 shows the 2.theta.-.omega. measurement results for the
cases where the in-plane mask tilt angles .theta. were 0.degree.,
43.degree., and 90.degree.. When the in-plane mask tilt angle was
0.degree., only the peaks of the m-plane sapphire (3-300) and the
m-plane GaN (2-200) were observed. When the in-plane mask tilt
angle was 43.degree., the diffraction peak of the (11-22) plane
emerged on the higher angle side. When the in-plane mask tilt angle
was 90.degree., the intensity of this peak was still higher.
[0342] FIG. 28 shows the in-plane mask tilt angle dependence of the
integrated intensity ratio between the (11-22) plane and the
m-plane (2-200) plane, which was estimated from the above XRD
2.theta.-.omega. measurement results. It can be seen that the
integrated intensity of the (11-22) plane started to increase near
35.degree. at which the surface roughness also started to increase.
Such a variation of the XRD measurement result accords with the
variation of the surface morphology (FIG. 25). It is inferred from
the above result that deterioration of the surface flatness which
would occur at the in-plane mask tilt angle of not less than
35.degree. is attributed to the semi-polar plane abnormal growth,
and it was found that the mentioned semi-polar plane refers to the
growth with the (11-22) plane principal surface.
[0343] The present inventors proved, by a method which will be
described below, that the (11-22) plane semi-polar abnormal growth
in this Inventive Example 2 is direct growth from the m-plane
sapphire substrate.
[0344] In regrowing an m-plane GaN film on the unevenly-processed
substrate 910, an unprocessed m-plane sapphire substrate was placed
in the MOCVD apparatus at the same time. That is, in this m-plane
sapphire substrate, none of the previously-described TMA
irradiation and buffer layer was used, and the process of the
regrown nitride semiconductor growth which has been described in
Inventive Example 1 was directly carried out.
[0345] According to the XRD 2.theta.-.omega. measurement result for
the case where a nitride semiconductor film was directly grown on
an m-plane sapphire substrate through the previously-described
process, only the (3-300) peak from the m-plane sapphire and the
diffraction peak from the (11-22) plane were observed as in FIG.
19(b), while the peak of (2-200) which was attributed to the
m-plane GaN was not observed. That is, it was found that, when the
process of the regrown nitride semiconductor is directly carried
out on an m-plane sapphire substrate, a nitride semiconductor of
which principal surface was the m-plane does not grow, while a
semi-polar plane nitride semiconductor of which principal surface
is the (11-22) plane grows.
[0346] Thus, it was found that, in the Pendeo epitaxial lateral
overgrowth of an m-plane nitride semiconductor with the use of the
m-plane sapphire substrate of the present embodiment, there is a
problem that a semi-polar plane nitride semiconductor grows from
m-plane sapphire substrate regions which are exposed in the uneven
process.
[0347] The above-described problem of undesirable growth of the
semi-polar plane is inherent in the Pendeo growth with the use of
the m-plane sapphire substrate. For example, an m-plane SiC
substrate may be used as the heterogeneous substrate of the m-plane
GaN, although in this case, in the first place, the semi-polar
plane nitride semiconductor growth would not occur. According to
methods which are different from the Pendeo growth, such as ELOG,
LOFT, air-bridged ELO, regions in which regrowth is to occur are
formed by only nitride semiconductor regions, while the other
regions are masked with a dielectric material, or the like, such
that the heterogeneous substrate surface is not exposed. Therefore,
the semi-polar nitride semiconductor growth from the heterogeneous
substrate surface, such as seen in this example, can be prevented.
However, each of the aforementioned methods is disadvantageous
because there are concerns about the problem of the substrate cost
and deterioration of the crystal quality due to contamination with
impurities from the mask materials, such as a dielectric. Thus,
realizing a Pendeo growth method of an m-plane nitride
semiconductor on an m-plane sapphire substrate at a low cost,
without using a mask, as in the present embodiment is of great
importance.
[0348] As previously described in Inventive Example 1, it is
inferred that the (11-22) plane semi-polar plane abnormal growth
was attributed to the lateral surfaces 852 of the sapphire
substrate 811 which was formed in formation of the
unevenly-processed substrate 910. It is inferred that semi-polar
plane abnormal growth occurred such that the growth started from
r-plane facets which were present in the lateral surfaces 852 of
the sapphire substrate 811 and which were oriented in the c-axis
direction. It is inferred from the experimental results that, when
the in-plane mask tilt angle is smaller than 35.degree., the number
of r-plane facet regions which serve as starting points of the
semi-polar plane growth is small, and only a small number of
semi-polar plane regions are formed so that they cannot be detected
by the XRD measurement. Therefore, its effect is at a negligible
level as compared with the regrown m-plane regions. On the other
hand, when the angle exceeds 35.degree., growth of the semi-polar
plane remarkably occurs, and the source materials are supplied not
only to the GaN layer but also to the lateral surface 852 regions
of the sapphire substrate 811, at which the growth occurs. Thus, it
is inferred that the surface morphology also changed, so that a
regrown film was obtained in which the m-plane and the (11-22)
plane coexist.
[0349] It was found from the above results that, in the Pendeo
growth in which the m-plane nitride semiconductor film on the
m-plane sapphire substrate is employed as the seed crystal,
occurrence of the semi-polar plane abnormal growth depends on the
in-plane mask tilt angle. It was found that avoiding this effect
and obtaining a regrown film which is excellent in surface flatness
and controlled so as to be in the m-plane orientation, controlling
the in-plane mask tilt angle so as to be in a range of 0.degree. to
35.degree. is necessary.
Inventive Example 3
[0350] In the descriptions of Inventive Example 2 of the present
application, the semi-polar plane abnormal growth starts from the
lateral surfaces 852 of the sapphire substrate 811 in the recessed
portions 850, and this phenomenon remarkably occurs in an
unevenly-processed substrate in which the in-plane mask tilt angle
is greater than 35.degree.. The reason why such a tendency can be
seen is that, as the in-plane mask tilt angle increases, the facet
plane of the lateral surfaces 852 of the sapphire substrate 811
changes from the a-axis direction to the c-axis direction. It is
inferred that, when the normal line of the lateral surfaces 852 of
the sapphire substrate 811 is close to the c-axis direction, the
r-plane facet is likely to be formed, so that semi-polar plane
growth is likely to occur (see FIG. 16). That is, in the present
embodiment, if there is a facet which includes a c-axis or r-axis
direction component at least partially in the lateral surfaces 852
of the sapphire substrate 811, there is a probability that
semi-polar plane abnormal growth starts from that facet.
[0351] In consideration of the above, it can be said that, for
example, even when the in-plane mask tilt angle is 0.degree., the
probability that the semi-polar plane abnormal growth occurs is not
zero. In forming lateral surfaces by etching, making that etched
surfaces flat at the atomic level is almost impossible. Therefore,
the etched lateral surfaces have some fluctuations. That is, even
in the case of 0.degree., the facet plane of the lateral surfaces
cannot be perfectly controlled, and a facet which has a crystal
plane that is different from the a-plane facet (e.g., r-plane) can
be present in the lateral surfaces 852 of the sapphire substrate
811.
[0352] Actually, in Japanese Laid-Open Patent Publication No.
2009-203151, direct growth of a nitride semiconductor on an
unevenly-processed m-plane sapphire substrate is examined, and
there is a report that even when, in that case, the extending
direction of the raised portions was the c-axis direction and the
normal line of the lateral surfaces of sapphire was oriented in the
a-axis direction (corresponding to the case where the in-plane mask
tilt angle was 0.degree. in the present embodiment), the (11-22)
plane growth was confirmed.
[0353] To prevent the semi-polar plane abnormal growth, controlling
the in-plane mask tilt angle as described in Inventive Example 2 so
as to reduce the number of facets which have normal line components
in the c-axis or r-axis direction is effective and, it is inferred
that, reducing the area of the lateral surfaces 852 of the sapphire
substrate 811 in which the semi-polar plane growth can occur is
also effective. In principle, when the depth of the lateral
surfaces 852 of the sapphire substrate 811 is close to 0, the
semi-polar plane growth is unlikely to occur, and the in-plane mask
tilt angle dependence is small. Further, even when the semi-polar
plane growth occurs, it must occur only in a very small area as
compared with the original m-plane nitride semiconductor regions.
Therefore, the effect on the entire regrown film must be extremely
small.
[0354] In this Inventive Example 3, the depth dependence of the
lateral surfaces 852 of the sapphire substrate 811 was
examined.
[0355] In this Inventive Example 3, washing of the m-plane sapphire
substrate, the growing step of the nitride semiconductor film 812
for seed crystal, stripe-shaped L&S patterns which had varying
in-plane mask tilt angles from 0.degree. to 90.degree., the step of
preparing the unevenly-processed substrate 910, and the step of
growing the nitride semiconductor film 870 were carried out under
basically the same conditions as those employed in Inventive
Examples 1 and 2. Note that, however, to examine the effect of the
depth of the lateral surfaces 852 of the sapphire substrate 811,
the etching duration in the process of the uneven structure was
varied.
[0356] FIGS. 29(a) and 29(b) show the in-plane mask tilt angle
dependence of the X-ray diffraction peak integrated intensity ratio
between the (11-22) plane and the m-plane (2-200) plane of nitride
semiconductor films 870 which were fabricated with varying etching
depths of the lateral surfaces 852 of the sapphire substrate 811.
FIG. 29(a) employs the same result as Inventive Example 2 shown in
FIG. 28 and, in this case, the depth of the lateral surfaces 852 of
the sapphire substrate 811 was approximately 250 nm. On the other
hand, FIG. 29(b) is the sample for which the etching duration in
the process of the uneven structure was short, and the etching
depth was approximately 150 nm.
[0357] As seen from FIG. 29(a), as previously described, when the
in-plane mask tilt angle .theta. is not less than 35.degree., the
diffraction intensity of the (11-22) plane is high, and it is
appreciated that a semi-polar plane is also present together in the
regrown film. On the other hand, in the sample of FIG. 29(a) where
the etching depth was 150 nm, such a tendency was not observed that
the diffraction intensity of the (11-22) plane sharply increases as
the value of 0 increases.
[0358] These experimental results demonstrate that the semi-polar
plane abnormal growth can be prevented by reducing the depth of the
lateral surfaces 852 of the sapphire substrate 811 and reducing the
area of these lateral surface regions. This is probably because, as
previously described, the number of r-plane facets that could serve
as the starting points of the semi-polar plane abnormal growth was
decreased by reducing the depth of the lateral surfaces 852 of the
sapphire substrate 811.
[0359] From the foregoing results, it was found that, in the Pendeo
growth where the m-plane nitride semiconductor film on the m-plane
sapphire substrate serves as the seed crystal, the semi-polar plane
abnormal growth can be prevented by controlling not only the
in-plane mask tilt angle but also the etching depth of the sapphire
substrate so as to be within predetermined ranges. By controlling
the in-plane mask tilt angle, the semi-polar plane growth can be
prevented even when the etching depth of the lateral surfaces 852
of the sapphire substrate 811 is in the range of 0 nm to 500 nm. By
controlling the etching depth of the lateral surfaces 852 of the
sapphire substrate 811 so as to be within the range of 0 nm to 150
nm, the semi-polar plane abnormal growth can be prevented more
effectively.
[0360] The results of the examinations carried out by the present
inventors demonstrated that, as for the problem of abnormal growth
of a nitride semiconductor with an unintended plane orientation in
the Pendeo regrowth, regions in which facets having different plane
orientations are present can be relatively reduced by controlling
the height of the substrate lateral surfaces 852 formed in the
etching step so as to be within the range of not less than 0 nm and
not more than 500 nm, or within the range of not less than 0 nm and
not more than 150 nm, the abnormal growth from the substrate
lateral surfaces 852 can be effectively prevented, without
depending on the in-plane mask tilt angle. Thus, a Pendeo regrown
film of a non-polar plane nitride semiconductor which has high
quality and excellent flatness can be obtained.
[0361] In the step of preparing the unevenly-processed substrate
910, the height of the substrate lateral surfaces 852 is controlled
so as to be within the range of not less than 0 nm and not more
than 500 nm or within the range of not less than 0 nm and not more
than 150 nm, and the in-plane mask tilt angle is within the range
of 0.degree. to 35.degree. at which abnormal growth of a nitride
semiconductor with an unintended plane orientation is unlikely to
occur, whereby abnormal growth from the regions of the substrate
lateral surfaces 852 can be prevented more effectively.
[0362] Here, the dependence of the tilt angle .gamma. of the
opposite lateral surfaces which are perpendicular to the extending
direction of the stripes in FIG. 14 is also described. The r-plane
facet of sapphire is a crystal plane which is inclined from the
c-axis to the m-axis as shown in FIG. 16. As previously described,
the semi-polar plane abnormal growth which starts from the r-plane
facet is more likely to occur when the normal lines of the lateral
surfaces 852 of the sapphire substrate 811 are oriented in the
c-axis direction (e.g., FIG. 15) rather than the a-axis direction.
Likewise, as shown in FIG. 14, when the tilt angle .gamma. of the
lateral surfaces of the sapphire substrate is controlled so as to
be 90.degree., the r-plane facet would not occur in principle, and
there is a probability that the semi-polar plane abnormal growth
can be prevented even when the in-plane mask tilt angle .theta. is
90.degree.. However, in view of the existing technology, etching
processing with steep lateral surfaces at the atomic level is
difficult. It is inferred that, even when the lateral surface tilt
angle is designed to be .gamma.=90.degree., r-plane facets occur in
some regions, and regions of .gamma.<90.degree. occur, and there
is a high probability that the semi-polar plane growth occurs from
those regions. In view of the existing technology, by controlling
the in-plane mask tilt angle .theta. and the depth of the lateral
surfaces 852 of the sapphire substrate 811, the semi-polar plane
abnormal growth can be sufficiently prevented. The lateral surface
tilt angle .gamma. in FIG. 14 can be selected from a wide range and
is, desirably, controlled so as to be within the range of
0.degree.<.gamma.<150.degree., for example.
[0363] <Conditions Under which the Semi-Polar Abnormal Growth is
Likely to Occur>
[0364] The cause of the semi-polar abnormal growth that occurs in
Pendeo regrowth which employs an m-plane sapphire substrate resides
in that, when a nitride semiconductor film is grown on an m-plane
sapphire substrate, the plane orientation (crystal orientation)
that the nitride semiconductor film can have includes a plurality
of types of plane orientations rather than only one type of plane
orientation.
[0365] Specifically, as previously described, when a nitride
semiconductor film is grown on an m-plane sapphire substrate,
growth of a nitride semiconductor of which principal surface is the
(1-100) plane or (10-1-3) plane, which is the m-plane, or the
(11-22) plane, occurs. If one of these plane orientation undergoes
film formation according to the growth conditions or the state of
the substrate surface before the growth, film formation can
sometimes occurs such that those plane orientations coexist.
[0366] For example, as shown in FIGS. 14(a) and 14(b), at the
lateral surfaces of the recessed portions 850, different plane
orientations such as, for example, the a-plane of the nitride
semiconductor and the c-plane of the sapphire substrate, are
exposed. The reasons why a mismatch occurs between the plane
orientation of the substrate and the plane orientation of the grown
film is that, in the first place, the substrate and the grown film
have different crystal structures (the sapphire has a corundum
structure while the nitride semiconductor has a wurtzite
structure), and there is a complicated epitaxy relationship between
the substrate and the grown film.
[0367] In the Pendeo lateral selective regrowth of an m-plane
nitride semiconductor that is a non-polar plane in the present
embodiment, the problem that growth of a crystal of which plane
orientation is different from that of the m-plane nitride
semiconductor concurrently occurs can arise when the substrate used
concurrently meets the following two conditions: (1) the substrate
has a crystal structure which is different from that of the nitride
semiconductor; and (2) there is a probability that a nitride
semiconductor film which has a plurality of types of plane
orientations, rather than only one type of plane orientation, is
formed.
[0368] Firstly, the condition (1) is described. For example, when a
semiconductor or oxide which has the same wurtzite structure as
that of the nitride semiconductor, such as a GaN bulk substrate or
ZnO, is used as the substrate, even if the recessed portions 850
such as shown in FIGS. 14(a) and 14(b) are formed and regions where
the substrate surface is exposed or the lateral surfaces 852 of the
substrate are formed, a crystal of which plane orientation is
different from the principal surface would not be formed from these
regions in the regrowth process. Although, strictly speaking, the
crystal structures are different, it is inferred that, when using a
SiC substrate of which crystal structure is very close to that of
the nitride semiconductor, the problem of concurrent growth of
different plane orientations would not occur.
[0369] This is because the grown film and the substrate have the
same crystal structures, and not only the growth principal surface
but also the lateral surfaces 852 of the substrate formed by
etching have the same plane orientation as that of the lateral
surfaces formed by etching of the grown film.
[0370] Next, the condition (2) is described. As previously
described, growth of a nitride semiconductor of which principal
surface is the plane orientation of the (1-100) plane or (10-1-3)
plane, which is the m-plane, or the (11-22) plane, can occur on an
m-plane sapphire substrate.
[0371] FIGS. 30(a) and 30(b) are diagrams for illustrating the
direction of the crystalline axis in the cases where the growing
planes are the m-plane and the off-plane, respectively. In FIGS.
30(a) and 30(b), the "growing plane" refers to a growing plane of
the substrate.
[0372] When the growing plane of the sapphire substrate is not
inclined from the m-plane (i.e., not made off) as shown in FIG.
30(a), the c-axis direction and the a-axis direction are parallel
to the growing plane. In this case, the present inventors found
that crystals which have different plane orientations are likely to
concurrently grow when the lattice mismatch degree between the
m-plane sapphire substrate and the nitride semiconductor film in
the c-axis direction (first direction) of the nitride semiconductor
film is not less than 2% and, in the growing plane, the lattice
mismatch degree between the m-plane sapphire substrate and the
nitride semiconductor film in the second direction (a-axis
direction) that is perpendicular to the first direction is not less
than 10%.
[0373] When the growing plane of the sapphire substrate is inclined
with respect to the m-plane (i.e., made off) as shown in FIG.
30(b), the c-axis direction is not parallel to the growing plane in
some cases. In such a case, the lattice mismatch degree is derived
relative to a direction which is defined by orthogonal projection
of the c-axis of the nitride semiconductor onto the growing plane
of the sapphire substrate (first direction: the c-axis direction
component in the growing plane). That is, in the first direction,
the lattice mismatch degree between the substrate and the nitride
semiconductor film is not less than 2%. In the second direction
that is perpendicular to the first direction in the growing plane
of the substrate, the lattice mismatch degree between the substrate
and the nitride semiconductor layer is not less than 10%.
[0374] Here, the lattice mismatch degree (strain amount) M (%) is
calculated by the following formula 1:
M(%)=100(dg-ds)/ds (Formula 1)
where ds is the interplanar spacing of the substrate, and dg is the
interplanar spacing of a film grown on the substrate.
[0375] The lattice mismatch degree is defined from the value of the
difference between the interplanar spacing of the grown film and
the interplanar spacing of the substrate at the interface between
the grown film and the substrate, and is basically different from
the residual strain which is obtained when the thickness of the
grown film exceeds the critical thickness and is sufficiently
large. In this specification, the lattice mismatch degree refers to
the strain amount which can be theoretically estimated from the
epitaxy relationship between the grown film and the substrate and
the difference in interplanar spacing, and is different from the
experimentally-determined residual strain amount.
[0376] The growth mode (epitaxy mode) of the grown film varies
depending on the largeness of the value of the difference in
interplanar spacing between the substrate and the grown film. When
the value of the difference in interplanar spacing between the
grown film and the substrate is not so large (e.g., less than 10%),
the growth occurs in the lattice match mode. On the other hand,
when the value of the difference in interplanar spacing is
extremely large, e.g., not less than 10%, the epitaxy occurs in the
domain match mode rather than the lattice match mode.
[0377] In the lattice match mode, the following relationship holds
true at the interface between the grown film and the substrate:
ads=adg(a is an integer equal to or greater than 1)
where, for example, ds is the interplanar spacing in the growing
plane of the substrate, and dg is the interplanar spacing in the
growing plane of a grown film which has an epitaxy relationship
with the lattice plane of the substrate. In this case, the lattice
plane of the substrate and the lattice plane of the grown film are
in a one-to-one relationship.
[0378] On the other hand, in the domain match mode, the following
relationship holds true at the interface between the grown film and
the substrate:
(a.+-.1)ds=adg(a is an integer equal to or greater than 1).
The growth in the domain match mode is premised on a very large
lattice mismatch. Therefore, to reduce this mismatch, a lattice
plane is added (or removed) such that the strain is decreased.
Since as described herein an extremely large strain is relieved in
the domain match mode, matching of lattice planes is achieved such
that plural ones of the lattice planes of the grown film and plural
ones of the lattice planes of the substrates form pairs
(domains).
[0379] The interface grown in the domain match mode adjusts the
number of a plurality of lattice planes of the grown film and the
substrate so as to relieve the strain. However, in the first place,
the grown film and the substrate have different numbers of lattice
planes, and therefore, at the interface, there is a misfit
dislocation in every period of that domain.
[0380] Note that, in the above-described domain match mode, the
difference in the number of interplanar spacings between the grown
film and the substrate is assumed as 1. However, this number may be
greater than 1 and may vary depending on the type of the material.
However, in epitaxial growth of a nitride semiconductor on a
heterogeneous substrate, it is 1 in almost all the cases.
[0381] As shown in above Formula I, the lattice mismatch degree in
the present embodiment is calculated on the assumption that the
lattice match mode is employed rather than the domain match mode.
That is, the calculation is carried out on the assumption that the
lattice planes (interplanar spacings) correspond on a 1:1
fashion.
[0382] FIG. 31 shows the relationship of the respective crystal
axes and the lattice constants in the case where an m-plane GaN
film is formed on an m-plane sapphire. FIG. 31 shows the atomic
arrangements which are seen from the m-axis side. Note that, in
FIG. 31, illustration of oxygen atoms is omitted.
[0383] As shown in FIG. 31, in the case of growing an m-plane
nitride semiconductor, the difference in lattice constant
(asymmetry) between the a-axis ([11-20] direction) and the c-axis
direction ([0001] direction) in the growing plane is large. Such a
tendency also applies to a case where a nitride semiconductor of a
non-polar plane which is different from the m-plane, or a nitride
semiconductor of a semi-polar plane, is grown. This asymmetry is
very large unless, as described above, the crystal structure of the
substrate material is the same as, or similar to, that of the
nitride semiconductor.
[0384] For the sake of comparison, a case where, for example, a GaN
bulk substrate or ZnO substrate is used is considered. In the case
where an m-plane GaN is grown on these substrates, the c-axis and
a-axis of the grown film are identical with the c-axis and a-axis
of the substrate. Therefore, the lattice mismatch degree which is
achieved when the GaN bulk substrate is used is 0% in both
directions. The lattice mismatch degree which is achieved when the
ZnO substrate is used is 0.4% in the c-axis direction and 1.9% in
the a-axis direction. Thus, the asymmetry is very small.
[0385] However, as shown in FIG. 31, the crystal structure of the
m-plane sapphire substrate is different from that of the nitride
semiconductor, and in the growing plane, the c-axis and a-axis of
the m-plane sapphire substrate and the c-axis and a-axis of the
nitride semiconductor are in the relationship of 90.degree.
rotation. That is, the growth occurs in a state where the a-axis of
the GaN corresponds to the c-axis of the sapphire, and the c-axis
of the GaN corresponds to the a-axis of the sapphire. The lattice
mismatch degree in the a-axis direction of the sapphire and the
c-axis direction of the GaN is 8.3% (which is calculated by
assigning the interplanar spacing d(11-20) of the sapphire and the
interplanar spacing d(0002) of the GaN in Formula I), and is in the
epitaxy relationship of the lattice match mode. On the other hand,
the lattice constant of the c-axis direction of the sapphire and
the lattice constant of the a-axis direction of the GaN are 12.99
.ANG. and 3.189 .ANG., respectively, which are different from each
other by a factor of about 3. This lattice mismatch degree in the
crystal axis direction is obtained using the interplanar spacing in
the c-axis direction of the sapphire, d(0006)=2.165 .ANG. (which is
the quotient of 12.99 .ANG. divided by 6), and the interplanar
spacing in the a-axis direction of the GaN, d(11-20)=1.595 .ANG.
(which is equal to the quotient of 3.189 .ANG. divided by 2), and
is 26%.
[0386] When there is such a large lattice mismatch degree, an
epitaxy relationship in the domain match mode where matching of
lattices is achieved with a plurality of interplanar spacings,
three sapphire c-axis direction interplanar spacings, i.e.,
d(0006).times.3, and four GaN a-axis direction interplanar
spacings, i.e., d(11-20).times.4, being a single unit holds true.
In this case, the lattice mismatch degree in the domain match mode
decreases to 1.8%. However, in this crystal axis direction, in the
first place, the number of interplanar spacings for the sapphire is
three while the number of interplanar spacings for the GaN is four,
i.e., there is a gap between the sapphire and the GaN. Therefore,
it leads to a structure where a misfit dislocation is included in
every period of the interplanar spacings. This is an example of the
previously-described epitaxy relationship in the domain match
mode.
[0387] In general, in heteroepitaxy of the non-polar plane nitride
semiconductor, there is no substrate which has a small lattice
constant difference, and the asymmetry of the lattice mismatch
degree in the plane is large. It can be said that such asymmetry of
the lattice mismatch degree is one of the factors that discourage
improvement in quality of a non-polar plane nitride semiconductor
heteroepitaxy film.
[0388] In heteroepitaxy which has such large in-plane lattice
mismatch degree asymmetry, the crystal growth mode varies depending
on the direction in the growing plane as previously described. In
the epitaxy of the m-plane GaN on the m-plane sapphire substrate,
in the c-axis direction of the GaN (the a-axis direction of the
sapphire) in which the lattice constant difference is relatively
small, crystal growth progresses in the lattice match mode, while
in the a-axis direction of the GaN (the c-axis direction of the
sapphire) in which the lattice constant difference is very large,
crystal growth progresses in the domain match mode.
[0389] In heteroepitaxy of a nitride semiconductor which has a
non-polar plane, the crystal growth mode varies in the plane as
described above in almost all the cases. Up to now, similar results
have been reported for the a-plane nitride semiconductor growth on
the r-plane sapphire substrate, the m-plane nitride semiconductor
growth on the .gamma.-LiAlO.sub.2 substrate, the m-plane ZnO growth
on the m-plane sapphire substrate, the m-plane nitride
semiconductor growth, and the like. For example, in growth of the
a-plane GaN on the r-plane sapphire substrate, the relationship of
its in-plane lattice constant is such that the a-axis of the
sapphire and the m-axis of the GaN are parallel to each other, and
the [-1101] direction of the sapphire and the c-axis of the GaN are
parallel to each other. The respective lattice mismatch degrees are
about 16.1% and about 1.2%.
[0390] In this case, the epitaxy in the a-axis of the sapphire and
the m-axis direction of the GaN, in which the lattice mismatch
degree is very large, is in the domain match mode, and dislocations
are present periodically so that the strain is relieved. Growth in
the domain match mode occurs with six d(10-10) planes of the GaN
for seven d(11-20) planes of the sapphire. In this case, the
mismatch degree is reduced to 0.5%. On the other hand, the epitaxy
in the [-1101] direction of the sapphire and the c-axis direction
of the GaN, in which the lattice mismatch degree is small, occurs
in the lattice match mode.
[0391] The above-described asymmetry of the in-plane strain and
growth mode in the heteroepitaxy of the non-polar plane nitride
semiconductor was confirmed in some studies. Such a characteristic
that a nitride semiconductor which has a plurality of plane
orientations is likely to grow such as seen in the present
embodiment is notably displayed in the case where an m-plane
nitride semiconductor is grown on an m-plane sapphire.
[0392] As described above, in the heteroepitaxy of the m-plane
nitride semiconductor on the m-plane sapphire, the lattice mismatch
degree between the c-axis of the sapphire and the a-axis of the GaN
is 26%, and the lattice mismatch degree between the a-axis of the
sapphire and the c-axis of the GaN is 8.2%. In the c-axis of the
sapphire and the a-axis direction of the GaN, growth occurs in the
domain match mode. Basically, the strain is positively relieved
with periodic dislocations being included every four d(11-20)
planes of the GaN (on the sapphire side, every three d(0006)
planes). In this crystal axis direction, the domain match mode
contributes to reduction of the strain.
[0393] On the other hand, in the a-axis of the sapphire and the
c-axis direction of the GaN which are deviated from the above
direction by 90.degree., growth occurs in the lattice match mode.
The strain amount in this direction is about 8%, which is a large
value. The present inventors speculate that this large strain
amount in the c-axis direction of the GaN is a cause of growth of a
nitride semiconductor which has a plurality of different plane
orientations, which is not seen in other non-polar
heteroepitaxy.
[0394] As a matter of fact, growth of a plurality of plane
orientations was not confirmed. In heteroepitaxy in which plane
orientation control of a non-polar plane nitride semiconductor is
relatively easy, the crystal axis direction in which growth occurs
in the lattice match mode includes a c-axis component, and its
strain amount is about 1%, which is a small value. For example, in
the a-plane GaN on the r-plane sapphire substrate, the lattice
mismatch degree between the [-1101] direction of the sapphire and
the c-axis direction of the GaN is 1.2%. Further, in the m-plane
GaN on the (100) plane .gamma.-LiAlO.sub.2, epitaxy occurs such
that the [010] direction of the .gamma.-LiAlO.sub.2 is parallel to
the c-axis of the GaN. The lattice mismatch degree of this
direction is 0.3%.
[0395] As described above, in the m-plane GaN growth on the m-plane
sapphire substrate, the strain amount of the c-axis direction
component of the GaN in the plane is extremely large as compared
with the other non-polar nitride semiconductor heteroepitaxies. It
is inferred that this large strain amount serves as a starting
point of unintended growth of a nitride semiconductor of which
plane orientation is different from the m-plane.
[0396] In the case where the m-plane nitride semiconductor is grown
on the m-plane sapphire substrate, using an AlN layer as the buffer
layer enables control of the plane orientation with high
reproducibility. From this fact, it can be confirmed that the
above-described model is correct. Since the c-axis length of the
m-plane AlN is shorter than that of the GaN, the strain amount of
the sapphire a-axis/AlN c-axis is about 4%, which is about a half
of that of the GaN. It is inferred that this is one of the reasons
that a plane orientation which is different from the m-plane, i.e.,
semi-polar plane abnormal growth, was prevented.
[0397] However, in the Pendeo epitaxial lateral overgrowth of the
present embodiment, a buffer layer of AlN is not used in regrowth.
Therefore, the bottom surface 851 of the recessed portions 850 at
which the sapphire substrate 811 is exposed and the lateral
surfaces 852 have different plane orientations, and accordingly,
the relationship of the strain between the regrown GaN and the
sapphire surface is also different. Particularly, as previously
described, when the lateral surfaces 852 include a sapphire r-plane
facet, the (11-22) plane GaN grows through the same mechanism as
that through which the a-plane GaN grows from this r-plane facet.
This is probably attributed to the fact that, in the m-plane GaN on
the m-plane sapphire, the lattice mismatch degree in the c-axis
direction of the GaN (about 8%) is dramatically reduced in the
(11-22) plane GaN growth in which this r-plane facet serves as a
starting point. (In this case, the lattice mismatch degree is
reduced to 1.2% through the same mechanism as that of the a-plane
GaN on the r-plane sapphire.)
[0398] In epitaxy which occurs in the lattice match mode in the
crystal axis which includes an in-plane c-axis direction component
of the nitride semiconductor film, it is inferred that, when its
strain amount is about 1%, abnormal growth of a nitride
semiconductor of which plane orientation is different from that of
the principal surface is unlikely to occur, in view of the
aforementioned examples of the a-plane GaN on the r-plane sapphire
substrate (the lattice mismatch degree in the lattice match
mode=1.2%) and the m-plane GaN on the .gamma.-LiAlO.sub.2 substrate
(the lattice mismatch degree in the lattice match mode=0.3%).
Therefore, in the in-plane c-axis direction component of the
nitride semiconductor film in which epitaxy occurs in the lattice
match mode, it is inferred that growth of a nitride semiconductor
which has a plurality of plane orientations is likely to occur when
its strain amount exceeds 2%.
[0399] In summary of the foregoing, it is inferred that, in the
heteroepitaxy of a non-polar plane nitride semiconductor, when of
two directions (in-plane growth axes) defined by directions in the
growing plane which are different from each other by 90.degree.,
the lattice mismatch degree in a direction defined by orthogonal
projection of the c-axis of the nitride semiconductor onto the
growing plane (first direction) is not less than 2% and the lattice
mismatch degree in a direction in the growing plane which is
perpendicular to the first direction (second direction) is not less
than 10%, abnormal growth of a nitride semiconductor which has a
plurality of plane orientations is likely to occur.
[0400] The conditions under which abnormal growth of the nitride
semiconductor is more likely to occur are such that, of two
directions defined by directions in the growing plane which are
different from each other by 90.degree., the lattice mismatch
degree in a direction defined by orthogonal projection of the
c-axis of the nitride semiconductor onto the growing plane (first
direction) is not less than 2% and less than 10%, and the lattice
mismatch degree in a direction in the growing plane which is
perpendicular to the first direction (second direction) is not less
than 10%.
[0401] There are some other materials which can cause the above
problem, in addition to the combination of the m-plane sapphire
substrate and the m-plane GaN. Such materials are combinations
which can enhance heterogeneous growth of a semi-polar plane
nitride semiconductor, for example.
[0402] One possible option is the combination of an m-plane
sapphire substrate and a (11-22) semi-polar nitride semiconductor.
In this combination, when growth of a (11-22) semi-polar plane is
attempted, there is a high probability that an m-plane or (10-1-3)
plane grows unintendedly.
[0403] According to Physica Status Solidi B 248, No. 3, 583 (2011),
the [11-20] direction of the sapphire and the [1-100] direction of
the GaN are parallel to each other, and the direction of the
sapphire and the [-1-123] direction of the GaN are parallel to each
other. Further, it is reported that the respective lattice mismatch
degrees are 16.1% and -6.3%. Since the [-1-123] direction of the
GaN is a direction defined by orthogonal projection of the c-axis
direction of the GaN onto the growing plane, in Physica Status
Solidi B 248, No. 3, 583 (2011), the strain amount of the in-plane
c-axis direction component of the semi-polar (11-22) plane GaN
(lattice mismatch degree) is -6.3%. In this Physica Status Solidi B
248, No. 3, 583 (2011), the strain amount is calculated in the
domain match mode. In this example, estimation of the lattice
mismatch degree in the lattice match mode leads to a further
increased lattice mismatch degree, which has a value of not less
than 10%.
[0404] That is, in the epitaxy for growing a semi-polar plane
(11-22) plane GaN on an m-plane sapphire substrate, the in-plane
lattice mismatch degree has a large value not less than 10% in both
the c-axis direction component of the GaN (the first direction that
is a direction defined by orthogonal projection of the c-axis onto
the growing plane of the substrate) and a direction in the growing
plane which is perpendicular to the first direction.
[0405] In the epitaxy of a (11-22) semi-polar nitride semiconductor
on an m-plane sapphire substrate, an unevenly-processed substrate
910 is prepared, a ridge-shaped nitride semiconductor layer is
formed, and Pendeo regrowth is carried out through the process
illustrated in FIG. 4, whereby the stacking fault density and the
dislocation density are reduced, so that the crystal quality of the
(11-22) semi-polar nitride semiconductor layer can be improved.
[0406] However, to dramatically reduce the stacking faults which
are present at high density in the non-polar plane or semi-polar
plane nitride semiconductor layer and effectively improve the
crystal quality, the angle between the extending direction of the
ridge-shaped nitride semiconductor layers and the c-axis of the
m-plane sapphire substrate may be 90.degree..
[0407] In the case of the epitaxy of the m-plane nitride
semiconductor on the m-plane sapphire substrate, the c-axis
direction ([0001]) in the plane of the nitride semiconductor is
parallel to the a-axis direction ([11-20]) of the sapphire. To
effectively reduce the stacking fault density in addition to the
dislocation density, the extending direction of the ridge-shaped
nitride semiconductor layers may be approximately parallel to the
c-axis of the m-plane sapphire substrate (details of the in-plane
mask tilt angle and the stacking fault density will be described in
detail in the section of Inventive Example 4).
[0408] However, in the case of a (11-22) semi-polar plane nitride
semiconductor on an m-plane sapphire substrate, the c-axis
direction component of the nitride semiconductor (a direction
defined by orthogonal projection of the c-axis direction onto the
growing plane, i.e., [-1-123]) is parallel to the c-axis direction
([0001]) of the sapphire.
[0409] From the above reasons, in the case of carrying out Pendeo
regrowth of a (11-22) semi-polar plane nitride semiconductor layer
on an m-plane sapphire substrate, the form of the
unevenly-processed substrate 910 may undergo photolithography or
etching processing such that the angle between the extending
direction of the ridge-shaped nitride semiconductor layers and the
c-axis of the m-plane sapphire substrate is 90.degree..
[0410] Further, in the step of preparing this unevenly-processed
substrate 910, when the height of the lateral surfaces 852 of the
sapphire substrate which are formed in etching is controlled so as
to be within the range of not less than 0 nm and not more than 500
nm, or within the range of not less than 0 nm and not more than 150
nm, abnormal growth from these lateral surfaces of a nitride
semiconductor layer other than the (11-22) plane can be effectively
prevented, and a Pendeo regrown film of a semi-polar plane nitride
semiconductor which has high quality and excellent flatness can be
obtained.
[0411] Note that, in this case, the angle between the extending
direction of the ridge-shaped nitride semiconductor layers and the
c-axis of the m-plane sapphire substrate does not necessarily need
to be perfectly parallel to 90.degree.. According to the results of
examinations carried out by the inventors, when this angular range
is controlled so as to be 90.degree..+-.10.degree., the
unevenly-processed substrate 910 is prepared, and Pendeo regrowth
is carried out, a (11-22) plane semi-polar nitride semiconductor
layer can be obtained in which not only the dislocation density but
also the stacking fault density are effectively reduced. Details of
these features will be described in the section of Inventive
Example 4.
Inventive Example 4
[0412] As previously described in the section of Embodiment 1, in
the non-polar plane nitride semiconductor growth, reduction of the
stacking fault density is important as well as reduction of the
dislocation density. In this example, the in-plane mask tilt angle
dependence of the crystallinity of a regrown m-plane GaN film was
examined. The effect of reducing the dislocation density was
evaluated based on the XRC full width at half maximum. Here, the
X-ray incidence direction was the a-axis direction of the GaN. The
effect of reducing the stacking fault density was evaluated by
photoluminescence (PL) measurement. This is because the PL
evaluation can more precisely examine the effect of the stacking
fault density.
[0413] In this Inventive Example 4, washing of the m-plane sapphire
substrate, the growing step of the nitride semiconductor film 812
for seed crystal, stripe-shaped L&S patterns which had varying
in-plane mask tilt angles from 0.degree. to 90.degree., the step of
preparing the unevenly-processed substrate 910, and the step of
growing the nitride semiconductor film 870 were carried out under
basically the same conditions as those employed in Inventive
Examples 1 and 2.
[0414] FIG. 32 shows the XRC full width at half maximum of the
nitride semiconductor film 870 with the in-plane mask tilt angle
.theta. varied from 0.degree. to 35.degree.. The X-ray incidence
direction was parallel to the a-axis of the GaN. In this
experiment, two m-plane GaN films on an m-plane sapphire substrate
were used as the nitride semiconductor film 812 for seed crystal,
and they were different between the sample where .theta. was 0 to
15.degree. and the sample where .theta. was 17.degree. to
35.degree.. The dotted line in the graph represents the value of
the XRC full width at half maximum of a typical seed crystal GaN
film shown in Table 1 (1326 seconds, 0.37 degree). The two seed
crystal GaNs had generally the same XRC full width at half
maximums. The XRC full width at half maximum was approximately half
of the value of the seed crystal over a wide range of the in-plane
mask tilt angle. It is inferred that, thanks to the Pendeo regrowth
of the present disclosure, the dislocation density was reduced.
Note that it seems that the XRC full width at half maximum in the
graph gradually deteriorates as the in-plane mask tilt angle
increases. This is attributed to the difference of the seed crystal
m-plane GaN films and is not an indication of the in-plane mask
tilt angle dependence.
[0415] FIG. 33 shows the PL spectrum at room temperature. In the PL
evaluation, a He--Cd laser (continuous wave, intensity: up to 30
mW) was used as the excitation source.
[0416] FIG. 33 shows, as examples, the results of samples in which
the in-plane mask tilt angle was .theta.=5.degree. and 14.degree..
The emission peak near the band edge of the GaN was seen at around
3.4 eV, while the other emissions were resulted from Deep Level.
Both samples did not have a large difference in the value of the
XRC full width at half maximum of FIG. 32. However, the emission
intensity at the band edge was high in the sample with a small
in-plane mask tilt angle .theta.=5.degree., while in the sample of
.theta.=14.degree., the Deep Level emission was dominant.
[0417] Considering that there is no difference in the XRC full
width at half maximum shown in FIG. 32, there is a small
probability that the cause of the difference in emission spectrum
between the two samples is the dislocation density. Therefore, it
is probably because of the effect of the stacking faults which was
not reflected in the results of the XRC full width at half maximum
in FIG. 32.
[0418] The effect of the stacking faults was examined by low
temperature (10K) PL measurement. FIGS. 34(a) to 34(c) shows the
results in the cases where the in-plane mask tilt angles .theta.
were 0.degree., 5.degree., and 21.degree., respectively. For the
sake of comparison, the spectrum of the nitride semiconductor film
812 for seed crystal is shown in FIG. 34(d). In the samples of
.theta.=21.degree. and seed crystal, three peaks were mainly
observed. Although the values of the respective emission peaks have
some deviations depending on, for example, the strain amount of a
grown film in some cases, it is inferred from the systematic
analysis of the experimental results obtained herein and the
comparison and analysis with other document results that the
emission at 3.42 eV was attributed to the stacking faults, and the
peak at 3.48 eV was the emission that was attributed to the donor
bound exciton (D0, X). First, comparing the results of (d) seed
crystal and (c) .theta.=21.degree., in the sample of
.theta.=21.degree., the intensity of (D0, X) increases with respect
to the peak which was attributed to the stacking faults, and it can
be seen that the stacking fault density decreased as compared with
the seed crystal. However, the intensity of (D0, X) was generally
equivalent to that of the stacking faults, and the overall emission
intensity was weak.
[0419] In comparison to the above results, in the samples of (a)
0.degree. and (b) 5.degree. in which the in-plane mask tilt angle
.theta. was small, the peak intensity which was attributed to the
stacking faults was weak, and the emission which was attributed to
(D0, X) was dominant. Further, the emission intensity also
increased by one or more orders of magnitude as compared with the
sample of .theta.=21.degree..
[0420] As described above, in the room temperature PL measurement,
in a regrown GaN film in which the emission intensity near the band
edge was strong, great reduction of the emission intensity that was
attributed to the stacking faults and increase of the (D0, X)
intensity were confirmed even in the low temperature PL
measurement. It was found that, by appropriately selecting the
range of the in-plane mask tilt angle in this way, not only the
dislocation reducing effect but also the effect of reducing the
stacking fault density can be obtained.
[0421] FIG. 35 shows the in-plane mask tilt angle dependence of the
ratio between the emission intensity from the Deep Level and the
emission intensity near the band edge which were obtained from the
room temperature PL measurement. It is inferred that, in the range
of the in-plane mask tilt angle from 0.degree. to 10.degree., the
intensity ratio was low, and the emission near the band edge was
strong even at the room temperature, so that the stacking fault
density was reduced. On the other hand, the ratio between the
emission intensity at the Deep Level and the emission intensity
near the band edge scarcely varied so long as the in-plane mask
tilt angle .theta. was not more than 10.degree., and the ratio
sharply increased when the angle exceeded 10.degree., so that the
emission at the Deep Level became dominant.
[0422] As described above, as seen from the measurement results of
the XRC full width at half maximum, the effect of improving the
crystal quality which is achieved by dislocation density reduction
can be realized at least in the range of the in-plane mask tilt
angle .theta. from 0.degree. to 35.degree.. To obtain the effect of
reducing the stacking fault density in addition to the crystal
quality improving effect, it is necessary to narrow the range of
the in-plane mask tilt angle. It was proved from the experimental
results that the angle .theta. is in the range of 0.degree. to
10.degree..
[0423] It was proved from the results of FIG. 35 that, even when
the in-plane mask tilt angle is not 0.degree., the effect of
reducing the stacking fault density can be obtained. As previously
described in the section of Inventive Example 1, in order to
improve the surface flatness, it is desired that the in-plane mask
tilt angle is greater than 0.degree.. When .theta. is greater than
0.degree., the migration effect is enhanced, and the surface
flatness can be improved. Also, the recessed portions 850 can be
designed so as to have a larger area while the flatness is
maintained.
[0424] FIG. 36 shows the variation of the surface morphology which
occurred when the in-plane mask tilt angle was varied from
0.degree. to 10.degree.. It can be seen that only the variation
from 0.degree. to 5.degree. resulted in that pits in the surface
vanished away. Thus, it is appreciated that the surface flatness
was improved. Further, when .theta.=10.degree., the rms roughness
further decreased to 20 nm.
[0425] It was found from the above results that a range of the
in-plane mask tilt angle in which both the dislocation density and
the stacking fault density are concurrently reduced while migration
is enhanced and the surface flatness can also be improved
concurrently is, for example, from 0.degree. to 10.degree..
Inventive Example 5
[0426] Hereinafter, the results of examination of the effect of
stacking faults and the dependence of the L width/S width are
described.
[0427] Inventive Example 4 demonstrated that, so long as the
in-plane mask tilt angle is in the range of 0.degree. to
10.degree., not only the dislocation density but also the stacking
fault density can be effectively reduced, so that excellent optical
characteristics can be obtained. However, it is the result obtained
when the L width and the S width were constant at 5 .mu.m and 10
.mu.m, respectively. The L width refers to the width of the
ridge-shaped nitride semiconductor layers 830 that are the seed
crystal (in a plan view, the length along a direction which is
perpendicular to the extending direction of the ridge-shaped
nitride semiconductor layers 830). In growth from this region in
which the ridge-shaped nitride semiconductor layers 830 are
provided, the crystal quality realized in the seed crystal is
maintained as it is, so that the effect of reducing the dislocation
density and the stacking fault density which is achieved by
selective growth is hardly obtained. Therefore, in a semiconductor
layer grown from this region, the dislocation density and the
stacking fault density are high as compared with a semiconductor
layer grown from the recessed portions 850, so that its
crystallinity deteriorates. Therefore, it is inferred that the
optical characteristics of an m-plane nitride semiconductor film
obtained by the Pendeo regrowth are also affected by the ratio
between the L width and the S width.
[0428] In view of such, in this Inventive Example 5, the variation
of the optical characteristics with varying ratios between the L
width and the S width was mainly examined by a PL evaluation
method.
[0429] In this example, samples were prepared with the in-plane
mask tilt angle .theta. being constant at 5.degree.. The other
growth conditions were the same as those of Inventive Examples 1
and 2. The reason why the in-plane mask tilt angle .theta. was
constant at 5.degree. is to clarify the relationship between the L
width and the S width, without being affected by the in-plane mask
tilt angle .theta., in the range of the in-plane mask tilt angle
from 0.degree. to 10.degree. in which the effect of the stacking
fault density can be sufficiently prevented as previously described
in the section of Inventive Example 4. Note that, although the
in-plane mask tilt angle .theta. of the samples was 5.degree., it
was confirmed that there is no large variation in the relationship
between the ratio of L width and S width and the optical
characteristics which will be described later so long as the
in-plane mask tilt angle .theta. is in the range of 0.degree. to
10.degree.. In this example, the samples were prepared by Pendeo
growth without a mask such as shown in FIGS. 4(a) to 4(d) rather
than Pendeo growth with the use of the mask 820 such as shown in
FIGS. 3(a) to 3(d).
[0430] In this example, the L width was constant at 5 .mu.m while
the S width was varying, and the ratio of the stripe width was
defined as S width/(L width+S width). That ratio of the stripe
width and the optical characteristics were compared. In this
example, the L width was constant at 5 .mu.m, but the L width is
not limited to this example. The L width can be selected from a
wide range, and the range of the L width may be not less than 0.1
.mu.m and not more than 10 .mu.m. Note that, as shown in FIG. 8(a),
when the lateral surfaces of the recessed portions 850 are sloped,
the L width refers to the width of the base of the ridge-shaped
nitride semiconductor layers.
[0431] The unevenly-processed substrates 910 were prepared with
varying S widths as described above, and a heterogeneous nitride
semiconductor substrate 920 which was obtained by regrowth of a
nitride semiconductor film was evaluated.
[0432] FIG. 37 shows the PL spectra of m-plane GaN samples obtained
by Pendeo regrowth with varying stripe width ratios, S width/(L
width+S width), which were measured at the low temperature (10K).
When the stripe width ratio, S width/(L width+S width), is in the
range of 0.29 to 0.58, the ratio between the peak intensity of the
emission that is attributed to the donor bound exciton (D0, X),
which is seen near 3.48 eV, and the peak intensity of the emission
that is attributed to the stacking faults (near 3.42 eV) is
generally at the same level, or rather, the peak that is attributed
to the faults has a higher intensity. However, it was found that,
when this stripe width ratio, S width/(L width+S width), is not
less than 0.6, the ratio of the peak intensity of the emission that
is attributed to the donor bound exciton (D0, X) to the peak
intensity that is attributed to the stacking faults greatly
increases.
[0433] FIG. 38 shows the relationship of the intensity ratio
between the emission that is attributed to the donor bound exciton
(D0, X) near 3.48 eV and the emission that is attributed to the
stacking faults near 3.42 eV, which were measured by PL measurement
at the low temperature 10K, with respect to the stripe width ratio,
S width/(L width+S width). The vertical axis represents the value
of the intensity of the emission that is attributed to stacking
faults which is divided by the intensity of the emission that is
attributed to the donor bound exciton. As this value decreases, the
effect of the stacking faults also decreases. It can be seen that,
when the stripe width ratio, S width/(L width+S width), is near
0.6, the emission intensity ratio greatly varies. When the stripe
width ratio, S width/(L width+S width), was not more than 0.6, the
intensity of the emission that is attributed to the stacking faults
is higher than or generally equal to the intensity of the emission
that is attributed to the donor bound exciton, i.e., the value was
not less than 1. In a stripe width structure of not less than 0.6,
the relative intensity of the donor bound exciton emission, which
was the emission near the band edge, abruptly improved to be not
more than 0.5. That is, it was found that, in an m-plane nitride
semiconductor structure that was obtained by Pendeo regrowth in
which the stripe width ratio, S width/(L width+S width), was not
less than 0.6, deterioration of the optical characteristics due to
the stacking fault density can be prevented.
[0434] It is inferred that the same effect can be obtained in a
Pendeo regrown film in which an m-plane nitride semiconductor
structure grown on a heterogeneous substrate is the basic body,
irrespective of the sapphire substrate, and also can be obtained in
the case of the previously-described semi-polar plane (11-22)
nitride semiconductor.
[0435] Next, the importance for device applications of setting the
stripe width ratio, S width/(L width+S width), to a value not less
than 0.6 such that the effect of the stacking fault density is
reduced as much as possible was confirmed by a method which will be
described below.
[0436] A blue light-emitting InGaN quantum well structure was
formed on a Pendeo regrown m-plane GaN film of a sample in which
the stripe width ratio, S width/(L width+S width), was 0.67 (L
width/S width=5 .mu.m/10 .mu.m), and the optical characteristics of
the structure were evaluated by PL measurement at the low
temperature (10K). For the sake of comparison, the optical
characteristics of InGaN quantum well structures formed on an
m-plane GaN bulk substrate of which stacking fault density was
approximately 0 and on a sample in which the stripe width ratio, S
width/(L width+S width), was 0.29 were evaluated on the other
hand.
[0437] The quantum well structure of the emission layer was formed
by 15 cycles of 3 nm thick InGaN well layers in which the In mole
fraction was 0.13 and 12.5 nm thick InGaN barrier layers in which
the In mole fraction was 0.03. Note that, between the ridge-shaped
nitride semiconductor layers and the emission layer, there was a
GaN layer which had a thickness of about 1 .mu.m.
[0438] FIG. 39 shows the PL spectra at the low temperature (10K) of
the aforementioned three quantum well structures. FIG. 39(a) shows
the result for the case where a quantum well structure was grown on
an m-plane GaN bulk substrate. FIG. 39(b) shows the result for the
case where a quantum well structure was grown on a Pendeo epitaxial
lateral overgrowth m-plane GaN in which S width/(L width+S width)
was 0.67. FIG. 39(c) shows the result for the case where a quantum
well structure was grown on a seed crystal m-plane GaN which did
not undergo the epitaxial lateral overgrowth.
[0439] From the quantum well structure grown on the m-plane GaN
bulk substrate of FIG. 39(a), a unimodal blue emission peak was
observed.
[0440] On the other hand, in the quantum well structure formed on
the seed crystal m-plane GaN of FIG. 39(c), not only a blue
emission peak which was attributed to the quantum well but also
another emission in a long wavelength region which was derived from
a different origin were observed in comparison to the quantum well
structure formed on the GaN bulk substrate. Further, the emission
intensity itself was weak as compared with the result of the
structure grown on the bulk substrate, and the half-value width of
the emission spectrum was broad. It is considered that the origin
of the emission which was thus seen in the long wavelength region
other than the quantum well was attributed to the stacking faults.
Thus, in the structure which underwent the measurement of FIG.
39(c), the crystal defect density was large.
[0441] This is because stacking faults are present in the seed
crystal m-plane GaN with high density as compared with the GaN bulk
substrate and Pendeo epitaxial lateral overgrowth film.
[0442] Applied Physics Express 2, 041002 (2009) suggests that
segregation of 1n occurs near stacking faults. Thus, it is inferred
that the origin of the emission of which wavelength was longer than
the original emission of the quantum well as seen in FIG. 39(c) was
attributed to segregation of the In mole fraction which occurred
due to the stacking faults.
[0443] In comparison to the above results, the emission spectrum
shown in FIG. 39(b) of the quantum well structure on a
heterogeneous m-plane GaN formed by Pendeo epitaxial lateral
overgrowth in which the stripe width ratio, S width/(L width+S
width), was 0.67 was similar to the result of the structure grown
on the bulk substrate. It can be seen that the original blue
emission of the quantum well structure was dominant, and the
emission in the long wavelength region which was attributed to the
stacking faults was greatly reduced.
[0444] That is, it was found that, when the ratio of S width/(L
width+S width) is not less than 0.6, the stacking fault density can
be effectively reduced. By reducing the stacking fault density, the
emission efficiency can be improved, and control of the emission
wavelength can be readily achieved.
[0445] The experimental fact that setting the ratio of S width/(L
width+S width) to a value not less than 0.6 as described above
enables to greatly reduce deterioration of the optical
characteristics due to the stacking faults suggests that, in the
surface area of a heterogeneous m-plane nitride semiconductor film
920 which is obtained by Pendeo epitaxial lateral overgrowth (shown
in FIG. 4), how much the proportion of the regions including the
stacking faults (i.e., the ridge-shaped nitride semiconductor
layers (seed crystal regions) 830) can be reduced relative to the S
width that is the low defect density region is important.
Therefore, the range in which the stacking fault density can be
reduced can be defined by relative values of the L width and the S
width.
[0446] Considering that a range of the L width which is desirable
from the viewpoint of realizing the epitaxial lateral overgrowth
and achieving sufficient dislocation density reducing effect is not
less than 0.1 .mu.m and not more than 10 .mu.m, or not less than 1
.mu.m and not more than 5 .mu.m, an optimum range of the S width in
which the effect of the stacking fault density can be sufficiently
reduced may be as follows because the ratio of S width/(L width+S
width) is greater than 0.6. The stripe structure may be formed such
that, for example, the S width is not less than 0.15 .mu.m when the
L width is 0.1 .mu.m; the S width is not less than 1.5 .mu.m when
the L width is 1 .mu.m; the S width is not less than 7.5 .mu.m when
the L width is 5 .mu.m; and the S width is not less than 15 .mu.m
when the L width is 10 .mu.m.
[0447] Next, the optimum region of S width/(L width+S width) was
examined. What was clarified in this example is that the effect of
the stacking faults on the optical characteristics can be reduced
by securing a relatively large S width region relative to the L
width. In this example, the L width was constant at 5 .mu.m. In
this case, as shown in FIG. 38, it was confirmed that the emission
of (D0,X) that was an emission near the band edge was dominant when
the ratio of S width/(L width+S width) was in the range of not less
than 0.6 and not more than 0.99, and that the emission which was
attributed to the stacking faults can be sufficiently
prevented.
[0448] In the case where the epitaxial lateral overgrowth in
heterogeneous m-plane nitride semiconductor growth of the present
embodiment is carried out, allowing nitride semiconductors which
are formed by the epitaxial lateral overgrowth in the regions of
the recessed portions 850 to be sufficiently combined and forming
the connecting portion 890 so as to obtain a flat nitride
semiconductor layer (for example, the heterogeneous m-plane nitride
semiconductor substrate 920 shown in FIG. 4(d)) are sometimes
important with some device structure fabrication and process steps.
In this case, if the width of the recessed portions 850, i.e., the
S width region, is excessively large, it is necessary to continue
the growth for a long period of time in order to form a sufficient
cover over the lateral growth film in the regrowth process, causing
the problem of cost increase of the process steps, for example.
[0449] According to the examinations carried out by the present
inventors, when the growth method employed is a MOVPE growth
method, the upper limit of the S width which is suitable for
realizing a nitride semiconductor layer with no gaps in the lateral
regrowth step is about 30 .mu.m. If the S width is greater than
that, the growth time becomes long, leading to an increase in the
process cost.
[0450] That is, to sufficiently reduce the effect of the stacking
faults on the optical characteristics and obtain a flat nitride
semiconductor film 860 with no gaps in regrowth, the S width is
desirably limited to a value not more than 30 .mu.m, and
furthermore, and the ratio of S width/(L width+S width) is
desirably designed to be not less than 0.6. In this case, the upper
limit of S width/(L width+S width) is, for example, 0.996 when the
L width is 0.1 .mu.m. When the L width is 1.0 .mu.m, the upper
limit of S width/(L width+S width) is 0.968. When the L width is
5.0 .mu.m, the upper limit of S width/(L width+S width) is
0.857.
[0451] That is, so long as the aforementioned ranges of the L width
and the S width, and the upper limit of the S width is 30 .mu.m,
and so long as the ratio of S width/(L width+S width) is in the
range of not less than 0.6 and not more than 0.996, a flat regrown
film can be obtained while the effect of the stacking faults is
reduced.
[0452] On the other hand, in the sample of Pendeo lateral regrowth
film, in view of device applications, nitride semiconductor films
regrown from the ridge-shaped nitride semiconductor layers 830 that
are the seed crystals do not necessarily need to connect to each
other so as to be flat as a whole. With some process methods or
final device structure designs, films regrown from the respective
ridge-shaped nitride semiconductor layers 830 can be deliberately
made independent of one another at the completion of a final device
structure. In this case, the connecting portion 890 is not formed,
and therefore, there is a merit that deterioration of the device
characteristics in defect regions formed by connection can be
prevented.
[0453] The results of epitaxial lateral overgrowth which was
carried out with the S width being deliberately set within a wide
range exceeding 30 .mu.m as described above are shown in FIG.
40.
[0454] FIG. 40 shows laser microscope images obtained from the
front surface side (i.e., m-axis side) of a heterogeneous nitride
semiconductor substrate 920 with the L width being constant at 5
.mu.m and varying S widths, (a) 10 .mu.m, (b) 50 .mu.m, (c) 100
.mu.m, (d) 200 .mu.m, and (e) 300 .mu.m. There are two features
observed in the samples of which S width was not less than 50
.mu.m: a stripe structure of an m-plane GaN regrown from the
ridge-shaped nitride semiconductor layers 830 that served as the
starting points; and a region in which the sapphire substrate of
the recessed portions 850 was exposed. Further, in these samples,
the S width was not less than 50 .mu.m, i.e., extremely large, but
a nitride semiconductor would not grow on the sapphire substrate
surface of the recessed portions 850. It can be seen that the
supplied source materials underwent migration through the sapphire
substrate surface and were taken into the ridge-shaped nitride
semiconductor layers 830.
[0455] It is known from the examinations carried out by the present
inventors that prevention of regrowth of the nitride semiconductor
in such a region in which the sapphire substrate surface of the
recessed portions 850 is exposed can be sufficiently achieved by
optimizing the regrowth conditions for the nitride
semiconductor.
[0456] In a structure which is grown such that regrown nitride
semiconductor films formed in the recessed portions 850 are not
reconnected deliberately and the sapphire substrate surface of the
recessed portions 850 remains as it is as shown in FIG. 40, the
source materials formed in sapphire substrate regions of the
recessed portions 850 are not entirely taken into the sapphire
substrate but into the m-plane GaN of the ridge structure.
Therefore, a thickness of the GaN of the ridge structure along the
vertical direction (m-axis direction) or horizontal direction (in
this example, a direction inclined by 5.degree. from the c-axis in
the a-axis direction) sharply increases, and the crystallinity and
the optical characteristics greatly improve.
[0457] These vertical and horizontal thicknesses and the growth
speed can be appropriately controlled by means of the regrowth
conditions. For example, in the case of the experimental results
shown in FIG. 40, the lateral growth speed was mainly varied. As
for the growth speed in the horizontal direction, assuming that it
is 1 when the S width is 10 .mu.m, the growth speed is about two
times faster when the S width is 50 .mu.m, and the growth speed is
about three times faster when the S width is 100 .mu.m.
[0458] As described above, when the unevenly-processed substrate
900 is prepared using a stripe structure of which S width exceeds
30 .mu.m to fabricate a heterogeneous nitride semiconductor
substrate 920, a regrown film can be obtained which is not a flat
film in the overall sample but which has a large thickness,
although partially, by the same process time.
[0459] Thanks to the effect of increasing the thickness of the
film, the dislocation density and the stacking fault density in the
regrown GaN film of the ridge portion are further reduced, and as a
result, remarkable improvement in the crystal quality is
achieved.
[0460] In a Pendeo regrown film structure in which connecting
regions are not formed deliberately in the recessed portions 850,
it is desirable that the S width is not less than 30 .mu.m. The
upper limit of the S width is, for example, 300 .mu.m.
[0461] Thus, in the unevenly-processed substrate 900 which has
large recessed portions 850 of which S width is not less than 30
.mu.m, the ratio of S width/(L width+S width) with which the
above-described effect of the stacking faults can be sufficiently
reduced is not less than 0.75 and less than 1 when the preferred
range of the L width is not less than 0.1 .mu.m and not more than
10 .mu.m, for example. When the range of the L width is not less
than 1 .mu.m and not more than 5 .mu.m, it is not less than 0.857
and less than 1.
Embodiment 3
[0462] In exemplary Embodiment 3, a nitride-based semiconductor
device is described in which a high quality heterogeneous m-plane
nitride semiconductor substrate 920 that is fabricated by Pendeo
epitaxial lateral overgrowth in Embodiments 1 and 2 is used as the
substrate. The nitride-based semiconductor device of exemplary
Embodiment 3 is, for example, a nitride-based semiconductor
light-emitting device. The nitride-based semiconductor device of
the present embodiment may be a LED. FIG. 41 is a schematic diagram
showing the structure of a nitride-based semiconductor
light-emitting device (LED) of exemplary Embodiment 3. FIG. 41
schematically shows a cross-sectional configuration of the
nitride-based semiconductor light-emitting device 801. This
nitride-based semiconductor light-emitting device 801 is, for
example, a semiconductor device which has a nitride-based
semiconductor multilayer structure which is made of a GaN-based
semiconductor.
[0463] The nitride-based semiconductor light-emitting device 801 of
the present embodiment includes a heterogeneous nitride
semiconductor substrate 920 which is obtained by forming an m-plane
nitride semiconductor film by Pendeo growth on an m-plane sapphire
substrate 811 and of which growing plane is the m-plane, a
semiconductor multilayer structure 802 formed thereon, and
electrodes 807, 808 formed on the semiconductor multilayer
structure 802. The heterogeneous nitride semiconductor substrate
920 used is the unevenly-processed substrate 910 (a structure for
growth of a nitride semiconductor layer) of Embodiment 1 or 2, and
can be formed by regrowing a nitride semiconductor to form the
nitride semiconductor film 870. The heterogeneous nitride
semiconductor substrate 920 includes the previously-described
raised-portion nitride semiconductor layers 830 and recessed
portions 850, although in this diagram illustration of these
components is omitted for the sake of simple illustration of the
entire configuration. The semiconductor multilayer structure 802 is
an m-plane semiconductor multilayer structure which is formed by
m-plane regrowth, and its growing plane is the m-plane.
[0464] The m-plane nitride-based semiconductor light-emitting
device 801 may be, for example, a device from which the m-plane the
sapphire substrate 811 was omitted. Further, it may be a device
from which the m-plane sapphire substrate 811 and part of the
nitride semiconductor film 870 are omitted. Omission of these
components may be realized by, for example, polishing or the like
after growth of the device structure of the m-plane nitride-based
semiconductor light-emitting device 801.
[0465] So long as impurity doping for conductivity control is
carried out on the raised-portion nitride semiconductor layers 830
and nitride semiconductor film 870 grown on the m-plane sapphire
substrate as described above, an electrode can be directly formed
at the interface 810 between the sapphire and the m-plane nitride
semiconductor film after removal of the m-plane sapphire substrate
811. For example, when the raised-portion nitride semiconductor
layers 830 and nitride semiconductor film 870 are doped with Si,
the n-type conductivity can be obtained, and an n-electrode can be
formed at the interface 810. In this case, in comparison to the
structure of the present embodiment, a vertical structure is
obtained which has electrodes at the upper surface and lower face
of the device.
[0466] The semiconductor multilayer structure 802 of FIG. 41
includes an active layer 804 including an Al.sub.aIn.sub.bGa.sub.cN
layer (a+b+c=1, a.gtoreq.0, b.gtoreq.0, c.gtoreq.0), and an
Al.sub.dGa.sub.eN layer (d+e=1, d.gtoreq.0, e.gtoreq.0) 805. The
Al.sub.dGa.sub.eN layer 805 is on the opposite side to the
substrate with respect to the active layer 804. Here, the active
layer 804 is an electron injection region in the nitride-based
semiconductor light-emitting device 801.
[0467] The active layer 804 of the present embodiment has a
GaInN/GaN multi-quantum well (MQW) structure (e.g., 81 nm thick) in
which Ga.sub.0.9In.sub.0.1N well layers (e.g., 9 nm thick) and GaN
barrier layers (e.g., 9 nm thick) are alternately stacked.
[0468] On the active layer 804, a p-type Al.sub.dGa.sub.eN layer
805 is provided. The thickness of the p-type Al.sub.dGa.sub.eN
layer 805 is, for example, 0.2 to 2 .mu.m. A region of the
Al.sub.dGa.sub.eN layer 805 bordering on the active layer 804 may
be provided with an undoped GaN layer 806.
[0469] The semiconductor multilayer structure 802 of the present
embodiment includes other layers. There is an
Al.sub.uGa.sub.vIn.sub.wN layer (u+v+w=1, u.gtoreq.0, v.gtoreq.0,
w.gtoreq.0) 803 which is formed between the active layer 804 and
the heterogeneous m-plane nitride semiconductor substrate 920. The
Al.sub.uGa.sub.vIn.sub.wN layer 803 of the present embodiment is a
first conductivity type (n-type) Al.sub.uGa.sub.vIn.sub.wN layer
803.
[0470] In the Al.sub.dGa.sub.eN layer 805, the mole fraction of Al,
d, is not necessarily uniform along the thickness direction. In the
Al.sub.dGa.sub.eN layer 805, the mole fraction of Al, d, may vary
continuously or stepwise along the thickness direction. That is,
the Al.sub.dGa.sub.eN layer 805 may have a multilayer structure in
which a plurality of layers having different Al mole fractions d
are stacked, and the concentration of the dopant may vary along the
thickness direction.
[0471] An electrode 807 is provided on the semiconductor multilayer
structure 802. The electrode 807 of the present embodiment is in
contact with a p-type semiconductor region and serves as part of
the p-electrode (p-side electrode). The electrode 807 is realized
by, for example, an Ag layer or a structure partially containing
Ag. The thickness of the electrode 807 is, for example, 100 to 500
nm.
[0472] In the configuration of the present embodiment, an electrode
808 (n-electrode) is provided on part of an n-type
Al.sub.uGa.sub.vIn.sub.wN layer (e.g., 0.2 to 2 .mu.m thick) 803 on
the m-plane nitride semiconductor substrate 920. In the shown
example, in a region of the semiconductor multilayer structure 802
in which the electrode 808 is to be formed, recessed portions 809
are formed such that the n-type Al.sub.uGa.sub.vIn.sub.wN layer 803
is partially exposed. An electrode 808 is provided on the surface
of the n-type Al.sub.uGa.sub.vIn.sub.wN layer 803 which is exposed
at the recessed portions 809. The electrode 808 is realized by, for
example, a multilayer structure of a Ti layer, an Al layer, and a
Pt layer. The thickness of the electrode 808 is, for example, 100
to 200 nm.
Other Embodiments
[0473] The above-described light-emitting device of an embodiment
of the present disclosure may be used as it is as a light source.
However, if the light-emitting device of the embodiment is combined
with a resin including a phosphoric material that produces
wavelength conversion, for example, the device can be used
effectively as a light source with an expanded operating wavelength
range (such as a white light source).
[0474] FIG. 42 is a schematic representation illustrating an
example of such a white light source. The light source shown in
FIG. 42 includes a light-emitting device 930 with the structure
shown in FIG. 41 and a resin layer 940 in which particles of a
phosphor such as YAG (yttrium aluminum garnet) are dispersed to
change the wavelength of the light emitted from the light-emitting
device 930 into a longer one. The light-emitting device 930 is
mounted on a supporting member 950 on which a wiring pattern has
been formed. And on the supporting member 950, a reflective member
960 is arranged so as to surround the light-emitting device 930.
The resin layer 940 has been formed so as to cover the
light-emitting device 930.
[0475] The principal surface that is the growing plane of the
m-plane sapphire which is used as the substrate 811 of the
light-emitting device 930 does not need to be perfectly parallel to
the m-plane but may be inclined from the m-plane by a predetermined
angle. The angle of the inclination is defined by an angle formed
by the normal to the actual growing plane of the nitride
semiconductor layer and the normal to the m-plane (m-plane without
inclination). The actual growing plane can be inclined from the
m-plane (m-plane without inclination) in the direction of a vector
which is represented by a direction that is based on a certain
crystal orientation such as, for example, c-axis, a-axis, and
<11-22> directions. For example, the absolute value of the
tilt angle .theta. may be not more than 5.degree., or not more than
1.degree., in the c-axis direction. In the a-axis direction, the
absolute value of the tilt angle .theta. may be not more than
5.degree., or not more than 1.degree.. Specifically, in the present
disclosure, the "m-plane" includes a plane which is inclined from
the m-plane (m-plane without inclination) in a predetermined
direction by an angle in the range of .+-.5.degree.. Within such an
tilt angle range, the growing plane of the nitride semiconductor
layer, as a whole, is inclined from the m-plane. However, it is
inferred that, upon closer observation, a large number of m-plane
regions are exposed. Thus, it is expected that a plane which is
inclined from the m-plane by an angle of not more than 5.degree.
(absolute value) has the same characteristics as those of the
m-plane. When the absolute value of the tilt angle .theta. is not
more than 5.degree., decrease of the internal quantum efficiency
due to a piezoelectric field can be prevented. Also, the "m-plane"
of the present disclosure is globally inclined from the m-plane and
includes a plane which has a plurality of m-plane steps.
[0476] The heterogeneous m-plane nitride semiconductor substrate
920 obtained by the m-plane nitride semiconductor Pendeo growth on
the m-plane sapphire substrate according to the embodiment can
naturally be used as an m-plane nitride semiconductor regrowth
substrate for a non-LED light-emitting device (such as a
semiconductor laser diode) or a device other than a light-emitting
device (such as a transistor or a photodetector), and can realize
cost reduction of these devices.
[0477] As described above, according to an embodiment, in epitaxial
lateral overgrowth of an m-plane nitride-based semiconductor film
by the Pendeo method, excellent surface flatness and higher quality
can be realized using an m-plane sapphire substrate which is
inexpensive and which can have a large diameter, and therefore, a
high-efficiency non-polar m-plane light-emitting device can be
provided at a low cost.
[0478] That is, according to an embodiment of the present
disclosure, heterogeneous m-plane nitride semiconductor epitaxial
lateral overgrowth is realized, and a high quality m-plane nitride
semiconductor film which has a reduced density of dislocations and
defects and a nitride-based semiconductor device which employs that
m-plane nitride semiconductor film as the basic body can be
provided.
[0479] As described above, in the heterogeneous nitride
semiconductor growth, an m-plane nitride semiconductor film which
has high quality and excellent surface flatness can be obtained by
a epitaxial lateral overgrowth method, and abnormal growth of a
semi-polar plane nitride semiconductor, such as the (11-22) plane,
can be prevented. Therefore, light-emitting devices, such as LEDs
and semiconductor lasers, and nitride-based semiconductor devices,
such as electronic devices, which employ this heterogeneous m-plane
nitride semiconductor film as the substrate, can be realized.
[0480] Next, the differences between the embodiments of the present
disclosure and the prior art are described.
[0481] There are some reports on the Pendeo growth method of an
m-plane nitride semiconductor grown on a heterogeneous substrate.
In Applied Physics Letters 93, 142108 (2008), a SiO.sub.2 mask is
formed over an m-plane nitride semiconductor grown on a SiC
substrate of which principal surface is the m-plane. Thereafter,
regions in which the SiC substrate is exposed, which are to be
raised-portion nitride semiconductor region and recessed portions,
are formed by etching, and epitaxial lateral overgrowth is carried
out, such that reduction of the dislocation density and stacking
fault density is realized.
[0482] However, in the previously-described method of Applied
Physics Letters 93, 142108 (2008), an expensive SiC substrate is
used as the substrate, resulting in a cost increase. In nitride
semiconductor growth with the principal surface being the c-plane,
the lattice constant of the a-axis of the SiC substrate is closer
to the lattice constant of the a-axis of the GaN, so that the
lattice mismatch degree is small, and growth of a film which has
relatively high quality is possible. However, in heterogeneous
growth with the principal surface being the m-plane or the a-plane
which is non-polar plane growth, the atomic arrangement in the
c-axis direction of the nitride semiconductor is 2H. (The numeral
before "H" represents the number of unit layers of Group III atoms
and N atoms included in one period. The wurtzite crystal structure
that is the crystal structure of the nitride semiconductor has two
unit layers in one period.) On the other hand, the atomic
arrangement in the c-axis direction of the SiC is 6H or 4H
structure. Thus, there is a mismatch in the atomic arrangement in
the c-axis direction. This is the reason that high density stacking
faults are likely to occur in a nitride semiconductor film grown on
a non-polar SiC substrate. Thus, in the nitride semiconductor
growth with the principal surface being the m-plane, the SiC
substrate is not necessarily suitable in view of the crystal
quality.
[0483] When carrying out the Pendeo growth using a non-polar SiC
substrate, the problem of semi-polar abnormal growth, which can
occur in the Pendeo growth with the use of an m-plane sapphire
substrate, would not occur. This is because the semi-polar abnormal
growth is a problem which is inherent in the m-plane sapphire
substrate.
[0484] In PCT INTERNATIONAL APPLICATION PUBLICATION NO.
2008/047907, a sapphire substrate is unevenly processed, and
epitaxial lateral overgrowth is carried out, resulting in
successful regrowth of an m-plane nitride semiconductor. The
substrate used is a sapphire substrate of which principal surface
is the a-plane. In this surface, an uneven structure having a shape
of thin and elongated stripes, extending in the m-axis direction in
the plane of the a-plane sapphire, are periodically formed by
etching. In this case, the front surface (principal surface) of the
raised portions to be formed is the a-plane, while the lateral
surface is a facet of the c-plane (or a plane inclined from the
c-plane). PCT INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907
discloses that, when a nitride semiconductor film is grown on the
thus-processed a-plane sapphire substrate, the film can be grown
from only the c-plane facet that is the lateral surface of the
raised portions under some growth conditions.
[0485] On the c-plane sapphire, a nitride semiconductor of which
principal surface is the c-plane grows. In this case, crystal
growth occurs such that the a-axis of the sapphire and the m-axis
of the nitride semiconductor film are parallel to each other. That
is, the crystal orientation shifts by 30.degree. in the plane. In
PCT INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907, this
relationship is utilized to grow the c-plane nitride semiconductor
in a lateral direction from the c-plane sapphire facet plane of the
raised portions. Further, the relationship that the a-axis of the
sapphire and the m-axis of the nitride semiconductor film are
parallel to each other is utilized to successfully grow a nitride
semiconductor film of which principal surface is the m-plane.
[0486] In PCT INTERNATIONAL APPLICATION PUBLICATION NO.
2008/047907, as described above, the nitride semiconductor film is
grown in a lateral direction from the c-plane facet that has a
normal component which is parallel to a direction perpendicular to
the longitudinal direction of the thin and elongated raised portion
regions of the a-plane sapphire, and therefore, it is necessary to
carry out etching somewhat deeply such that sufficient lateral
surface regions are secured. In PCT INTERNATIONAL APPLICATION
PUBLICATION NO. 2008/047907, unevenly-processed sapphire is formed
so as to have a depth of not less than 700 nm. However, the
sapphire is very hard, and the etching selection ratio with the
mask material is small. Thus, in general, etching deep is
difficult.
[0487] In PCT INTERNATIONAL APPLICATION PUBLICATION NO.
2008/047907, a nitride semiconductor layer is directly grown from a
sapphire substrate. Therefore, nitride semiconductor films having
the same plane orientation (in PCT INTERNATIONAL APPLICATION
PUBLICATION NO. 2008/047907, -c plane GaN) grow from opposite
lateral surfaces of the processed a-plane sapphire substrate, and
the crystal plane orientation at connecting portions is
discontinuous (in PCT INTERNATIONAL APPLICATION PUBLICATION NO.
2008/047907, the -c planes connect to each other), and therefore,
high density defects occur at the connecting portions. PCT
INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907 explains that
these problems can be solved by appropriately selecting the growth
conditions.
[0488] According to an embodiment of the present disclosure, for
example, the Pendeo growth method such as illustrated in FIG. 4 is
used as the basic epitaxial lateral overgrowth method. That is, the
m-plane nitride semiconductor regions of the raised portions formed
by the etching processing serve as the starting points of the
epitaxial lateral overgrowth. Therefore, regrowth is started from
the nitride semiconductor film. On the other hand, in PCT
INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907, regrowth
occurs from the processed sapphire lateral surfaces. In the present
embodiment, as described herein, the nitride semiconductor regions
serve as the core of the regrowth, and therefore, for example, in
the case where the opposite lateral surfaces of the nitride
semiconductor regions of the raised portions are the c-plane
facets, these lateral surfaces are the +c plane and the -c plane,
and occurrence of defects at the connecting portion 890 of FIG.
4(d) can be reduced.
[0489] In the embodiment of the present disclosure, it is only
necessary to form regions from which nitride semiconductor layers
are removed by etching, such as the recessed portions 850. Since
growth lateral surfaces are formed as in PCT INTERNATIONAL
APPLICATION PUBLICATION NO. 2008/047907, it is not necessary to
deeply etch the sapphire substrate that is difficult to
process.
[0490] PCT INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907
also describes an example in which an m-plane sapphire substrate is
processed in the same way as the a-plane sapphire substrate. In
that description, a problem that, in the case where the m-plane
sapphire substrate is unevenly processed and a nitride
semiconductor is regrown, a semi-polar plane of the (11-22) plane
grows, is discussed. That is, it can also be seen from the results
of PCT INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907 that
Pendeo growth with the use of an m-plane nitride semiconductor film
on an m-plane sapphire substrate is difficult to realize.
[0491] In the embodiment of the present disclosure, an m-plane
nitride semiconductor film grown on an m-plane sapphire substrate
is unevenly processed, and the raised portion m-plane nitride
semiconductor regions are formed as the seed crystal, whereby a
method for realizing Pendeo growth in which the nitride
semiconductor regions rather than the sapphire substrate serve as
the starting points of the regrowth is provided. In Pendeo growth
in which the m-plane sapphire substrate is used as described above,
the m-plane sapphire surface is exposed at the recessed portions
850, a problem occurs such that a semi-polar plane nitride
semiconductor film grows concurrently with an m-plane nitride
semiconductor. According to the embodiment of the present
disclosure, the semi-polar plane abnormal growth can be prevented
by appropriately selecting the uneven processing conditions, such
as the in-plane mask tilt angle, the L&S pattern interval, the
etching depth, etc. A high-quality and low-cost heterogeneous
m-plane nitride semiconductor substrate can be realized in which
the dislocation density and the stacking fault density are reduced
by epitaxial lateral overgrowth. When such a wafer is used as the
substrate of a light-emitting device such as LED, an internal
electric field caused by spontaneous electrical polarization or
piezoelectric polarization would not occur in a layer stacking
direction of the active layer (the normal direction of the
principal surface of the substrate) because the m-plane nitride
semiconductor is a non-polar plane, and improvement in the emission
efficiency is expected.
[0492] The present disclosure is applicable to a GaN-based
semiconductor light-emitting device such as a light-emitting diode
or a laser diode that operates at wavelengths over the ultraviolet
range and the entire visible radiation range, which covers blue,
green, orange and white parts of the spectrum. Such a
light-emitting device is applicable to the fields of display,
lighting, and optical data processing, for example. Also, the
light-emitting device is applicable to electronic devices, for
example.
[0493] Next, the results of examinations of improvement in the
quality of a nitride-based semiconductor light-emitting device
which is achieved by reduction of the polarization degree of light
emitted from the nitride-based semiconductor light-emitting device
are described.
[0494] It is known that, in a light-emitting device which includes
a nitride semiconductor of which principal surface is a non-polar
plane or semi-polar plane, light emitted from that active layer
region has a polarization characteristic. As described in the
article of "APPLIED PHYSICS LETTERS 92, (2008) 091105", this
polarization characteristic is attributed to the optical anisotropy
that is due to a low degree of symmetry of the crystal structure of
non-polar and semi-polar plane nitride semiconductors. In the case
of a nitride-based semiconductor light-emitting device which is
fabricated on the c-plane that is a polar plane, the c-axis is
parallel to the normal line of the principal surface (growing
plane). For example, in the case of a nitride semiconductor with no
strain, polarized light of which electric field vector is oriented
in a direction perpendicular to the c-axis is mainly obtained.
Therefore, in the case of conventional c-plane growth, the degree
of symmetry of the crystal is high so that, when seen in the c-axis
direction, each of the a-axis and the m-axis is present with the
intervals of 60.degree. (this is referred to as "sixfold
symmetry"), and thus, light emitted in the c-axis direction is
non-polarized light.
[0495] However, in non-polar plane and semi-polar plane growth in
which the growing plane is a crystal plane which is different from
the c-plane, the symmetry degree is low, so that emitted light is
polarized light. For example, in a nitride-based semiconductor
light-emitting device of which principal surface is the m-plane,
polarized light of which electric field vector is parallel to the
a-axis is mainly obtained from that surface.
[0496] Further, such a polarization characteristic is due to the
structure of the valence band and therefore varies depending on the
mole fraction of the Group III atom of the nitride semiconductor or
the strain.
[0497] A semiconductor light-emitting device described in Japanese
Laid-Open Patent Publication No. 2009-117641 has an active layer
which is made of a Group III nitride semiconductor and of which
growth principal surface is a non-polar plane or semi-polar plane
in order to emit polarized light which has a high polarization
ratio. The semiconductor light-emitting device includes an emission
section which is configured to emit polarized light from the active
layer, and slits which are provided in a light extraction surface
from which the polarized light is to be extracted and which are
lines and spaces that are narrower than the wavelength of the
polarized light.
[0498] A light-emitting device described in Japanese Laid-Open
Patent Publication No. 2008-305971 includes, in order to prevent
decrease of the output efficiency of polarized light produced in an
active layer, an emission section which is made of a Group III
nitride semiconductor of which principal surface is a non-polar
plane or semi-polar plane, in which a first semiconductor layer of
the first conductivity type, the active layer, and a second
semiconductor layer of the second conductivity type are stacked in
this order, and which is configured to produce polarized light from
the active layer, and an output section in which a plurality of
stripe-shaped grooves extending in a direction perpendicular to the
polarization direction of the polarized light are arranged along
the polarization direction and has an output surface in the shape
of a sawtooth wave. The polarized light is transmitted from the
emission section through the output section and output from the
output surface.
[0499] In this specification, light of which electric field
intensity is deviated in a specific direction is referred to as
"polarized light". For example, light of which electric field
intensity is deviated in the X-axis direction is referred to as
"X-axis direction polarized light". The X-axis direction is
referred to as "polarization direction". Note that the "X-axis
direction polarized light" not only means linearly-polarized light
which is polarized in the X-axis direction but may include
linearly-polarized light which is polarized in a different axial
direction. More specifically, the "X-axis direction polarized
light" means light in which the intensity (electric field
intensity) of light transmitted through a "polarizer which has a
polarization transmission axis extending in the X-axis direction"
is higher than the electric field intensity of light transmitted
through "a polarizer which has a polarization transmission axis
extending in a different axial direction". Therefore, the "X-axis
direction polarized light" includes not only linearly-polarized
light and elliptically-polarized light which are polarized in the
X-axis direction but also non-coherent light in which
linearly-polarized light and elliptically-polarized light which are
polarized in various directions are mixed together.
[0500] While the polarization transmission axis of the polarizer is
rotated around the optical axis, the electric field intensity of
light transmitted through the polarizer exhibits the strongest
intensity, Imax, and the weakest intensity, Imin. The polarization
degree is defined by the following formula (1):
Polarization degree=|Imax-Imin|/|Imax+Imin| Formula (1)
[0501] In the case of the "X-axis direction polarized light", when
the polarization transmission axis of the polarizer is parallel to
the X-axis, the electric field intensity of the light transmitted
through the polarizer is Imax. When the polarization transmission
axis of the polarizer is parallel to the Y-axis, the electric field
intensity of the light transmitted through the polarizer is Imin.
In the case of perfectly linearly-polarized light, Imin=0, and
therefore, the polarization degree is equal to 1. On other hand, in
the case of perfectly unpolarized light, Imax-Imin=0, and
therefore, the polarization degree is equal to 0.
[0502] A nitride-based semiconductor light-emitting device that
includes an active layer of which growing plane is the m-plane
mainly emits the a-axis direction polarized light as described
above. Here, the device also emits the c-axis direction polarized
light and the m-axis direction polarized light. However, the c-axis
direction polarized light and the m-axis direction polarized light
have smaller intensities than the a-axis direction polarized
light.
[0503] In the present embodiment, an active layer of which growing
plane is the m-plane is discussed as an example, and the discussion
is focused on the a-axis direction polarized light. Note that, in a
nitride-based semiconductor light-emitting device grown on
semi-polar planes, such as -r plane, (20-21) plane, (20-2-1) plane,
(10-1-3) plane, and (11-22) plane, and other non-polar planes such
as the a-plane, the degree of symmetry of the crystal is low, and
therefore, light emitted from the active layer of that device has
polarization characteristics. Thus, the same also applies to
polarized light in a specific crystal direction.
[0504] In Japanese Laid-Open Patent Publication No. 2008-305971,
maintaining the polarization characteristics that the
light-emitting device has is intended. On the other hand, reducing
the polarization characteristics is sometimes necessary depending
on the use of light-emitting device.
[0505] In the case where a light-emitting device which has
polarization characteristics is used as a light source, the
reflectance varies depending on the angle between the orientation
of the polarization (the orientation of the electric field vector
of the polarization) and the incidence angle, and therefore, such a
problem arises that the light distribution characteristics vary as
compared with a light-emitting device which has no polarization.
For example, in the case of an m-plane nitride-based semiconductor
light-emitting device, the a-axis direction polarized light is
emitted. Therefore, light from the light-emitting device is
distributed larger in the c-axis direction than in the a-axis
direction. That is, such a problem arises that the light
distribution characteristics are largely distributed in the c-axis
direction, and the light distribution characteristics are asymmetry
between the a-axis direction and the c-axis direction and are
therefore not uniform. Note that such light distribution
characteristics are not seen in a conventional light-emitting
device on the c-plane in which light emitted from the active layer
has no polarization characteristics. That is, the light
distribution characteristics in the conventional light-emitting
device on the c-plane exhibit a uniform pattern with a high degree
of symmetry.
[0506] The above problem that, in the m-plane nitride-based
semiconductor light-emitting device of which principal surface is a
non-polar plane, the light distribution characteristics are
asymmetry is attributed to the fact that light emitted from the
active layer of that device is polarized. This is attributed to the
fact that the reflectance at the interface between the
semiconductor layer and the air or between the semiconductor layer
and the substrate varies depending on whether it is a p-wave in
which the electric field vector of the polarized light is present
in the incidence plane or an s-wave in which the electric field
vector of polarization is perpendicular to the incidence plane.
[0507] Therefore, in the case where an LED device is applied to a
commonly-employed lighting device or the like, if light emitted
from the LED device is polarized light, it is important to reduce
the polarization characteristics as much as possible, thereby
improving the light distribution characteristics.
[0508] The present inventors examined the relationship, in a
nitride-based semiconductor light-emitting device of which
principal surface is the m-plane, between the polarization
characteristics and light distribution characteristics of light
emitted from the active layer and a plurality of stripe-shaped
structures (stripe structures) that are provided on the side from
which the light is to be emitted. As a result, the polarization
characteristics of emitted light depends on the direction of a
major electric field vector of polarized light produced in the
active layer of the nitride-based semiconductor light-emitting
device and the shape of the stripe structure which is formed so as
to be traversed by the polarized light. Hereinafter, an embodiment
of a light-emitting device according to the present disclosure is
described with reference to the drawings.
[0509] In the drawings mentioned below, for the sake of simple
description, elements which perform substantially the same
functions are sometimes denoted by the same reference numerals. The
present disclosure is not limited to the embodiments which will be
described below.
Fourth Embodiment
[0510] FIG. 43 schematically shows a cross-sectional configuration
of a light-emitting device 10 according to the fourth embodiment.
The light-emitting device 10 is a nitride-based semiconductor
light-emitting device and has, for example, a nitride-based
semiconductor multilayer structure which is made of an
Al.sub.xIn.sub.yGa.sub.zN (where x+y+z=1, x.gtoreq.0, y.gtoreq.0,
z.gtoreq.0) semiconductor. The light-emitting device 10 is, for
example, an LED device.
[0511] The light-emitting device 10 of the present embodiment
includes a heterogeneous nitride semiconductor substrate 600, a
semiconductor multilayer structure 20 provided on the heterogeneous
nitride semiconductor substrate 600, and a p-electrode (p-side
electrode) 30 and a n-electrode (n-side electrode) 40 which are
provided on the semiconductor multilayer structure 20. The
heterogeneous nitride semiconductor substrate 600 is obtained by
growing a nitride semiconductor film 320 on a substrate for growth
100 of which principal surface is the m-plane. The growth substrate
100 is, for example, a sapphire substrate. Between the growth
substrate 100 and the nitride semiconductor film 320, a plurality
of stripe-shaped gaps 60 are formed by a selective growth method.
The semiconductor multilayer structure 20 is a nitride-based
semiconductor multilayer structure which is formed by regrowth of a
nitride semiconductor, and its growing plane is the m-plane.
[0512] In the present embodiment, the growth substrate 100 and the
growing plane of an active layer 24 may be inclined by an angle of
not more than 5.degree. with respect to the m-plane. This tilt
angle is defined by an angle between the normal line of the surface
of the sapphire substrate and the normal line of the m-plane. The
direction of the inclination may be inclined in a certain direction
in the m-plane which is crystallographically defined. For example,
it may be the c-axis, a-axis, or <11-22> axis direction.
[0513] A plane which is inclined by an angle of not more than
5.degree. with respect to the m-plane has the same characteristics
as those of the m-plane. Therefore, the "m-plane" of the present
disclosure includes a plane which is inclined by an angle of not
more than 5.degree. with respect to the m-plane. The -r plane,
(20-21) plane, (20-2-1) plane, (10-1-3) plane, and (11-22) plane,
and the a-plane of the present disclosure may include planes which
are inclined by an angle of not more than 5.degree. with respect to
these planes.
[0514] The semiconductor multilayer structure 20 includes the
active layer 24 that is realized by an Al.sub.aIn.sub.bGa.sub.cN
layer (where a+b+c=1, a.gtoreq.0, b.gtoreq.0, c.gtoreq.0). The
active layer 24 is an electron injection region in the
light-emitting device 10 and is configured to mainly emit polarized
light which is polarized in the a-axis direction. Since the growing
plane of the active layer 24 is the m-plane, decrease of the
emission efficiency due to piezoelectric polarization or the like
is prevented, so that high efficiency emission light can be
obtained.
[0515] The active layer 24 has a multi-quantum well (MQW) structure
in which, for example, well layers having a thickness of not less
than 1 nm and not more than 20 nm, which are made of
Al.sub.uGa.sub.vIn.sub.wN (where u+v+w=1, u.gtoreq.0, v.gtoreq.0,
w.gtoreq.0), and barrier layers having a thickness of not less than
3 nm and not more than 50 nm, which are made of
Al.sub.uGa.sub.vIn.sub.wN (where u+v+w=1, u.gtoreq.0, v.gtoreq.0,
w.gtoreq.0), are alternately stacked. The number of cycles in the
quantum well structure may be not less than 2 cycles and not more
than 30 cycles, for example. Also, the thickness of the barrier
layer in the quantum well structure may be 9 nm, for example.
[0516] By respectively adjusting the mole fractions of Al and In in
the multi-quantum well structure and the thicknesses of the well
layer and the barrier layer, the state of the polarized light can
be controlled.
[0517] For example, it is known that in a nitride-based
semiconductor light-emitting device of which principal surface is
the (11-22) plane that is a semi-polar plane, the direction of
polarization varies depending on the mole fraction of In and the
thickness of the well layer. However, in the case of a
nitride-based semiconductor light-emitting device of which
principal surface is the m-plane, the direction of polarization
would not vary. The polarization state in the a-axis direction is
maintained, while only the degree of polarization varies depending
on the mole fractions and the quantum well structure. In the case
of a nitride-based semiconductor light-emitting device of which
principal surface is the (11-22) plane, variation of the
polarization direction occurs in a region in which the mole
fraction of 1n is as high as not less than 30%. In an active layer
in which blue light emission with the mole fraction of In being
greater than 20% is mainly obtained, the polarization direction
maintains the polarization state in the m-axis direction that is
perpendicular to the c-axis direction.
[0518] The multi-quantum well structure that forms the active layer
24 may be realized by well layers and barrier layers in both of
which the mole fraction of Al is 0. In this case, it changes from
the quaternary compound semiconductor to the ternary compound
semiconductor, so that control of the composition is easy.
[0519] On the active layer 24, a p-type layer 25 is provided. The
p-type layer is, for example, an Al.sub.dGa.sub.eN layer (where
d+e=1, d.gtoreq.0, e.gtoreq.0). The thickness of the p-type layer
25 is, for example, not less than 0.2 .mu.m and not more than 2
.mu.m. A region of the p-type layer 25 bordering on the active
layer 24 may be provided with an undoped layer 26. The undoped
layer 26 may be a GaN layer.
[0520] The mole fraction of Al in the p-type layer 25, d, does not
need to be uniform along the thickness direction. In the p-type
layer 25, the mole fraction of Al, d, may vary continuously or
stepwise along the thickness direction. That is, the p-type layer
25 may have a multilayer structure in which a plurality of layers
having different Al mole fractions d are stacked. The concentration
of the dopant may vary along the thickness direction.
[0521] Between the active layer 24 and the heterogeneous nitride
semiconductor substrate 600, an n-type layer 22 is provided. The
n-type layer 22 is, for example, an Al.sub.uGa.sub.vIn.sub.wN layer
(where u+v+w=1, u.gtoreq.0, v.gtoreq.0, w.gtoreq.0). The thickness
of the n-type layer 22 is, for example, not less than 0.2 .mu.m and
not more than 2 .mu.m.
[0522] The p-electrode 30 is in contact with a p-type semiconductor
region (p-type layer 25). The p-electrode 30 has, for example, Ag
or a structure which contains Ag. The thickness of the p-electrode
30 is, for example, from 100 nm to 500 nm.
[0523] In the p-type layer 25, the mole fraction of Al near the
interface with the p-electrode 30 may be 0 (Al mole fraction d=0).
This enables improvement of the activation rate of the p-type
impurity which serves as the dopant. The p-type layer 25 may be
replaced by an InGaN layer which contains In. A region of the
p-type layer 25 near the interface with the p-electrode 30 may have
a higher p-type impurity concentration than in the other regions of
the p-type layer 25 so as to function as a contact layer. In other
words, a contact layer in which the p-type impurity concentration
is higher than in the p-type layer 25 may be provided between the
p-type layer 25 and the p-electrode 30.
[0524] The n-type layer 22 is partially exposed. On that exposed
region, an n-electrode (n-side electrode) 40 is provided. The
n-electrode 40 does not necessarily need to be provided on the
surface of the n-type layer 22 but may be provided on the nitride
semiconductor film 320. In this case, it is desirable that the
nitride semiconductor film 320 also has n-type conductivity.
[0525] In the example shown in the drawing, a region of the
semiconductor multilayer structure 20 in which the n-electrode 40
is to be formed has a recessed portion 42 such that the n-type
layer 22 is partially exposed. The n-electrode 40 is provided on
the surface of the n-type layer 22 which is exposed at this
recessed portion 42. The n-electrode 40 has a structure which
contains Al, for example. The thickness of the n-electrode 40 is,
for example, not less than 100 nm and not more than 500 nm.
[0526] In the present embodiment, a plurality of gaps 60 extending
in a stripe arrangement are provided near the interface 50 between
the growth substrate 100 and the nitride semiconductor film 320.
The gaps 60 may be provided, for example, in a region on the growth
substrate 100 side or may be provided in a region on the nitride
semiconductor film 320 side. In the nitride semiconductor film 320,
a portion lying between adjacent two of the gaps 60 has a stripe
structure. The gap 60 itself has a stripe structure which has a
different refractive index from that of the nitride semiconductor.
For example, the gaps 60 may be vacuum. The gaps 60 may contain a
gas, e.g., the atmosphere employed in the crystal growth.
Alternatively, the gaps 60 may contain a solid, e.g., a resin for
sealing. Still alternatively, the gaps 60 may contain these gas and
solid.
[0527] Such stripe structures may not necessarily be provided on
the interface 50 but may be provided at a position which is
traversed by the polarized light. That is, the stripe structures
may be provided in any region between the active layer 24 and the
growth substrate 100, in the major emission direction of light from
the active layer 24. For example, the stripe structures may be
provided in any region of the n-type layer 22, the nitride
semiconductor film 320, and the growth substrate 100.
[0528] For example, the stripe structures may be provided on a
nitride semiconductor layer which has a thickness of not less than
0 .mu.m and not more than 10 .mu.m from the interface 50.
Alternatively, the stripe structures may be provided in a region
inside the growth substrate 100 which is separated from the
interface 50 by a distance of not less than 0 .mu.m and not more
than 10 .mu.m.
[0529] In the light-emitting device 10, part of light emitted from
the active layer 24 which is output upward in the drawing, i.e.,
light traveling from the active layer 24 toward the p-type layer
25, is reflected by the p-electrode which contains Ag and which has
a high reflectance. Therefore, finally, the light from the
light-emitting device is not emitted toward the p-electrode 30. The
light emitted from the active layer 24 is mainly extracted from the
growth substrate 100 side. Thus, the polarization characteristics
can be reduced by providing the stripe-shaped gaps 60 on the light
emission side. Hereinafter, the structure of the gaps 60 is
described in detail.
[0530] The structure of the gaps 60 is described with reference to
FIG. 44(a) and FIG. 44(b). FIG. 44(a) schematically shows a
cross-sectional configuration near the interface between the growth
substrate 100 and the nitride semiconductor film 320 in which the
gaps 60 are provided. FIG. 44(b) shows a planar configuration where
the structure shown in FIG. 44(a) is seen from the side of the
m-plane that is the growing plane, i.e., seen in the m-axis
direction.
[0531] As shown in FIG. 44(b), where the angle between the
extending direction of the stripe-shaped raised portions (ridge
portions) 51, i.e., the extending direction of the stripe-shaped
gaps 60, and the a-axis direction in the active layer 24 is defined
as the in-plane tilt angle .beta., the in-plane tilt angle .beta.
is set to a value which is not less than 3.degree. and not more
than 45.degree.. Note that, as will be described later, all of the
plurality of gaps 60 do not need to have equal angles. The
respective gaps 60 may be formed to have different in-plane tilt
angles .beta. within the range of not less than 3.degree. and not
more than 45.degree., for example. The gaps 60 may be periodically
formed. By forming the gaps 60 within this range of angle .beta.,
the polarization characteristics of light emitted from the active
layer 24 can be reduced. As a result, the light distribution
characteristics of emitted light is improved, so that the light
extraction efficiency can be improved. The in-plane tilt angle
.beta. may be not less than 3.degree. and not more than 35.degree..
More specifically, the in-plane tilt angle .beta. may be not less
than 3.degree. and not more than 10.degree..
[0532] In the example shown in FIG. 44(a), the gaps 60 are provided
in the nitride semiconductor film 320. Provision of the gaps 60
leads to that a portion of the nitride semiconductor film 320 near
the interface has an uneven shape, and as a result, a plurality of
raised portions 51 are provided. Each of the raised portions 51 is
formed such that its crest portion is oriented downward, i.e.,
toward the growth substrate 100 side, and has a bottom surface 52
which has a plane parallel to the interface 50 and at least one
slope surface 53 which is not parallel to the interface 50. Note
that, however, the raised portions 51 do not necessarily need to
have the bottom surface 52.
[0533] FIG. 45(a) and FIG. 45(b) and FIGS. 46(a) to 46(c) show some
examples of the cross-sectional configuration of the stripe-shaped
gaps 60. These cross-sectional diagrams show cross-sectional shapes
which are taken along a direction perpendicular to the extending
direction of the stripe-shaped gaps 60.
[0534] For example, as shown in FIG. 45(a) and FIG. 45(b), the
cross-sectional shape of the raised portions 51 may be an inverted
trapezoidal shape or may be a trapezoidal shape. Alternatively, it
may be a quadratic shape, although not shown. As shown in FIG.
46(a), it may be an inverted triangular shape which does not have
the bottom surface 52. Alternatively, as shown in FIG. 46(b), the
slope surface has a curved semicircular shape.
[0535] The cross-sectional structure of the gaps 60 may have a
triangular shape or an inverted triangular shape as shown in FIG.
45(a) and FIG. 45(b). Alternatively, it may have a trapezoidal
shape or a curved lateral surface, although not shown.
[0536] Alternatively, as shown in FIG. 46(c), the slope surface 53
of the gaps 60 may be formed by a plurality of slope surface
portions. For example, as shown in FIG. 46(c), where the angle of
the slope surface 53 is defined as .alpha..sub.i, the raised
portions structure 51 may have a plurality of slope surfaces
defined by .alpha..sub.i+1, .alpha..sub.i+2, . . . , .alpha..sub.k,
for example. When the number of angles .alpha. of this slope
surface, k, is extremely large and the angles .alpha. are different
from one another, the slope surface 53 is approximate to a curved
surface, and the cross-sectional shape is semicircular or
semielliptical as shown in FIG. 46(b). Further, the gaps 60 between
these stripe-shaped structures do not necessarily need to be
regularly arranged with equal intervals. Even when the periods are
partially irregular, the effects of the present embodiment can be
obtained.
[0537] In the present embodiment, providing stripe-shaped uneven
structures on the side from which light from the active layer 24 is
to be emitted and designing those uneven structures so as to
partially have the slope surface 53 (angle
.alpha..noteq.90.degree.) are important. Therefore, for example, in
the range where the gaps 60 are formed, the range of the angle
.alpha..sub.i of the slope surface 53 can be selected from a wide
range. The range of a plurality of angles .alpha..sub.i may be from
0.degree. to 180.degree.. More specifically, the range of the
angles .alpha..sub.i may be not less than 0.degree. and not more
than 150.degree..
[0538] The height of each of the gaps 60 may be not less than
.lamda./(4.times.n) where .lamda. is the wavelength, and n is the
refractive index of the nitride semiconductor film 320. More
specifically, the height of each of the gaps 60 may be not less
than .lamda./(4.times.n) and not more than 10 .mu.m. For example,
when the wavelength of light emitted from the active layer 24 is
450 nm, the refractive index of the GaN layer in this wavelength
range is about 2.5. Therefore, the height of the gaps 60 may be at
least not less than 45 nm. As is the case with the above-described
periods of the stripe-shaped gaps 60, the height of the gaps 60
does not need to be equal among all of the structures.
[0539] As shown in FIG. 44(b), the stripe-shaped gap 60 forms a
line 54 extending in the extending direction of the gaps 60. This
line 54 is formed by, for example, the gap 60 and the wall surface
of the nitride semiconductor film 320. This line 54 does not
necessarily need to be a single line. As shown in FIG. 47, it may
be formed by a stripe-shaped gap 60 which has an in-plane
inclination that is defined by a plurality of angles .beta..
Alternatively, the gaps 60 may be formed in a zigzag shape.
Alternatively, the gaps 60 may be discontinuous as shown in FIG.
48(a). The range of the angle .beta. may be
3.degree..ltoreq..beta..ltoreq.45.degree.. As shown in FIG. 48(b),
the gaps 60 do not necessarily need to be inclined in the same
direction in the plane. For example, the gaps 60 may be inclined in
the +c axis direction of the nitride semiconductor from the a-axis
by an angle of 3.degree..ltoreq..beta..ltoreq.45.degree.. On the
contrary, the gaps 60 may be inclined in the -c axis direction.
When the proportion of the gaps 60 which have such an tilt angle
range is not less than 50% of all of the gaps 60, the remarkable
effects of the present embodiment can be obtained.
[0540] As shown in FIG. 49(a), stripe structures which have
different width dimensions may be periodically provided as the
plurality of gaps 60. Alternatively, as shown in FIG. 49(b),
structures which are only partially discontinuous may be provided
as the plurality of gaps 60.
[0541] Next, the relationship between the gaps 60 and the
polarization characteristics and light distribution characteristics
of the light emitted from the light-emitting device 10 of which
principal surface is the m-plane is described.
[0542] The light emitted from the nitride-based semiconductor
light-emitting device of which principal surface is the m-plane is
mainly polarized in the a-axis direction. As shown in FIG. 50(a),
the propagation vector of the a-axis polarized light is in a
direction perpendicular to the a-axis direction. Therefore, where
the propagation vectors of the emitted light are, for example, k1,
k2, . . . , these propagation vectors are present in a flat plane
that is formed by the m-axis and the c-axis (which is referred to
as "mc-plane" in this specification) and are parallel to this
plane. Specifically, as shown in FIG. 50(b), light emitted from the
m-plane nitride-based semiconductor light-emitting device is mainly
emitted in a direction perpendicular to the a-axis so that, in the
a-axis direction, the proportion of the emitted light relatively
decreases. That is, the light has such light distribution
characteristics that the radiation angle is wide in a direction
perpendicular to the a-axis.
[0543] FIG. 51 schematically shows an example where the
stripe-shaped gaps 60 are formed so as to be parallel to the a-axis
of the light-emitting device 10 of which principal surface is the
m-plane (angle .beta.=0.degree.).
[0544] The stripe-shaped gaps 60 are provided at the interface
between the growth substrate 100 on the light emission surface side
and the nitride semiconductor film 320. In this case, the a-axis
polarized light of emitted light is mainly incident as a s-wave
onto the slope surface 53 of the gaps 60 and the bottom surface 52
of the raised portions 51. As previously described, in this case,
c-axis polarized light is rarely emitted, and the propagation
vector of the a-axis polarized light is parallel to the mc-plane.
Therefore, the p-wave component is smaller than the s-wave
component and is approximately 0. Therefore, in the case where the
stripe-shaped gaps 60 are formed such that the in-plane tilt angle
.beta.=0.degree., the a-axis polarization characteristics of the
light extracted from the light-emitting device 10 of which
principal surface is the m-plane to the outside are maintained.
[0545] In the example of FIG. 51, when the stripe-shaped gaps 60
(raised portions 51) have the in-plane tilt angle .beta.=0.degree.,
the a-axis polarized light emitted from the active layer 24 is
incident on the stripe-shaped gaps 60 such that almost all of the
incident light is the s-wave. The light extracted to the outside is
also maintained as the s-wave, and therefore, the a-axis polarized
state is likely to be maintained.
[0546] When the angle .beta. is greater than 0.degree., the
incident light includes not only the s-wave component but also the
p-wave component, and therefore, the polarization degree is
reduced. The effect of reducing this polarization degree can be
obtained likewise even in the case of inclination from the a-axis
direction of the nitride semiconductor to the +c axis direction or
to the -c axis direction.
[0547] FIG. 52 schematically shows an example where the
stripe-shaped gaps 60 are provided perpendicular to the a-axis of
the light-emitting device 10 of which principal surface is the
m-plane (in-plane tilt angle .beta.=90.degree.). In this case also,
the propagation vector of light is parallel to the mc-plane. That
is, it has such a light distribution characteristic that the
radiation angle is wide in a direction perpendicular to the a-axis
direction. For example, as in the case of the propagation vector k1
emitted from point Q, when the propagation direction is parallel to
the m-axis direction, light is incident mainly as the p-wave on the
slope surface 53 of the gap 60 and the bottom surface of the raised
portion 51, while the s-wave is substantially 0. Thus, for the
a-axis polarized light that has a propagation vector which is
parallel to the m-axis, the polarization characteristic in the
a-axis direction is maintained.
[0548] However, in this case, only a small portion of the light has
a propagation vector which is parallel to the m-axis, while almost
all of the light is parallel to the mc-plane, but its propagation
direction is inclined in the c-axis direction as in the case of the
propagation vector k2 shown in FIG. 52. When polarized light such
as the propagation vector k2 is incident on the slope surface 53 of
the gap 60, the incident light is an associated wave which contains
not only the p-wave component but also the s-wave component, and
therefore, the polarization is not maintained.
[0549] As described hereinabove, it is understood that the
polarization degree of the light emitted from the light-emitting
device structure depends on the in-plane tilt angle .beta. of the
gaps 60, and that the polarization degree can be maintained only
when .beta. is around 0.degree..
[0550] In this specification, the variation of the polarization
degree with respect to the angle .beta. is evaluated using a value
which is referred to as "specific polarization degree", and
discussed. The specific polarization degree refers to a value of
the polarization degree that is obtained when the gaps 60 are
inclined in the plane by an arbitrary angle, which is normalized
with the polarization degree that is obtained when the in-plane
tilt angle .beta.=0.degree.. This is defined by Formula (2) as
follows:
Specific polarization degree=(the polarization degree of a
light-emitting device which has stripe structures with arbitrary
in-plane tilt angle .beta.)/(the polarization degree of a
light-emitting device which has stripe structures with in-plane
tilt angle .beta.=0.degree.) Formula (2)
[0551] The specific polarization degree represented by Formula (2)
has a value which is approximately 1 when the angle .beta. is
smaller than about 3.degree.. When the angle .beta. is not less
than 3.degree., it is understood that, the specific polarization
degree abruptly decreases so that the polarization degree can be
reduced.
[0552] As illustrated in FIG. 50(b), light emitted from a
nitride-based semiconductor light-emitting device of which
principal surface is the m-plane is a-axis polarized. Therefore,
its propagation vector is parallel to the mc-plane, and it is
rarely present in the ma-plane. Expressing this as the distribution
of light, it can be said that a wider light distribution is
detected in the c-axis direction than in the a-axis direction.
[0553] Such a characteristic can be a cause of the asymmetry of the
light distribution characteristics. That is, an m-plane
nitride-based semiconductor light-emitting device which has a flat
light emission surface, and which does not have stripe structures
on the light emission surface side, has different light
distribution characteristics in the a-axis direction and the c-axis
direction, leading to an asymmetrical result. The light
distribution characteristic is widely distributed in the c-axis
direction than in the a-axis direction. The light distribution
characteristic in the c-axis direction is 0.degree., i.e., there is
a tendency that the light intensity in a direction which is
inclined in the c-axis direction is stronger than in the m-axis
direction.
[0554] This asymmetry of the light distribution characteristics can
also be reduced by forming the gaps 60 of the present embodiment on
the light emission surface side. FIG. 53(a) schematically shows an
example of a light-emitting device which has an interface between
GaN and sapphire and which has a flat emission surface. FIG. 53(b)
schematically shows an example of a light-emitting device which has
gaps 60 with the angle .beta.=0.degree. in the emission surface. As
shown in FIG. 53(a), in the case where the emission surface is
flat, when light emitted from the GaN layer side is transmitted
with a certain incidence angle .theta.1 to the sapphire substrate
side, .theta.2 of the transmitted light is greater than .theta.1,
and it is likely to be emitted as light which is more inclined to
the c-axis direction.
[0555] This is attributed to the fact that .theta.2>.theta.1
always holds true because the refractive index of the nitride
semiconductor layer is higher than that of the sapphire or air
according to the Snell's law which will be described later. This is
a cause of an increase in the asymmetry of the light distribution
characteristics which have been previously described.
[0556] On the contrary, in the case where the gaps 60 shown in the
present embodiment are formed on the light emission surface side,
for example, when the stripe-shaped gaps 60 are provided in the
a-axis direction of the angle .beta.=0.degree. as shown in FIG.
53(b), the incidence light which has a propagation vector in the
mc-plane is likely to be incident on the interface 50 at a smaller
angle than in the case of FIG. 53(a). As a result, in the mc-plane,
the incident light is likely to be inclined in the m-axis direction
than in the c-axis direction. That is, the light distribution
characteristics of a conventional m-plane nitride-based
semiconductor light-emitting device which has a flat light emission
surface are widely distributed in the c-axis direction. On the
other hand, in the case where the gaps 60 of the present embodiment
are provided, the light intensity distribution on the m-axis side,
i.e., in the major axis direction of the light-emitting device
(vertical direction), is stronger. Thus, the light distribution
characteristic in the c-axis direction is improved, and the
asymmetry with the a-axis direction can be improved. Further, as
described herein, the gaps 60 shown in the present embodiment also
contribute to improvement of the light extraction efficiency.
[0557] FIG. 54 shows incidence, reflection, and refraction of the
p-wave and the s-wave at the interface of a material. The energy
reflectances at such an interface ("Rp" for the p-wave and "Rs" for
the s-wave) can be obtained according to the Snell's law (Formula
(3)) and the Fresnel's formula (Formula (4), Formula (5)) as
follows:
n.sub.1 sin .theta..sub.1=n.sub.2 sin .theta..sub.2 Formula (3)
R.sub.p=tan.sup.2(.theta..sub.1-.theta..sub.2)/tan.sup.2(.theta..sub.1+.-
theta..sub.2) Formula (4)
R.sub.s=sin.sup.2(.theta..sub.1-.theta..sub.2)/sin.sup.2(.theta..sub.1+.-
theta..sub.2) Formula (5)
[0558] In the light-emitting device according to the present
embodiment, light emitted from the active layer 24 propagates from
the nitride semiconductor layer of a high refractive index to the
air layer of a low refractive index.
[0559] FIG. 55(a) shows the calculation results of the incidence
angle .theta..sub.1 dependence of the energy reflectances R.sub.p,
R.sub.s of the p-wave and s-wave in the case where, in FIG. 54, the
layer of the refractive index n.sub.1 is the GaN layer, and the
layer of the refractive index n.sub.2 is the air.
[0560] In a region where the incidence angle is smaller than
23.degree., the reflectances for both of the p-wave and the s-wave
are relatively low, and the light emitted from the nitride
semiconductor layer can be readily extracted to the outside. In
this angular range, the reflectance of the p-wave is lower than the
reflectance of the s-wave. Near 22.degree., the reflectance of the
p-wave is 0. This is called a Brewster's angle.
[0561] On the other hand, as in the present embodiment, it is known
that when light travels from a material of a high refractive index
to a material of a low refractive index, total reflection occurs.
An incidence angle at which total reflection occurs is referred to
as "critical angle .theta..sub.c". The critical angle .theta..sub.c
refers to a value of .theta..sub.1 which is obtained when
.theta..sub.2=90.degree. in the Snell's law represented by Formula
(3). That is, at the interface between the nitride semiconductor
and the air, the critical angle .theta..sub.c is near 23.degree.,
and therefore, light which is incident on the interface at an angle
greater than this angle is totally reflected, so that the light
cannot be efficiently extracted to the outside of the
light-emitting device. When such a phenomenon occurs, the light is
not extracted to the outside, and it can be a cause of decrease of
the external quantum efficiency of the light-emitting device.
[0562] FIG. 55(b) and FIG. 55(c) show the calculation results at
the interface between the GaN layer and the sapphire and the
interface between the GaN layer and the SiO.sub.2 for the sake of
comparison.
[0563] The gap 60 of the present embodiment has a slope surface 53
in part of its structure. With such a slope surface formed, the
incidence angle is smaller than the critical angle as compared with
a flat light emission surface in some cases (see FIG. 53(b), for
example). In this case, the reflectance is low so that large part
of the light is extracted to the outside, and thus, the light
extraction efficiency and the external quantum efficiency can be
improved.
[0564] The above-described effect of improving the light
distribution characteristics and the light extraction efficiency is
attributed to such features that light emission obtained from an
m-plane nitride semiconductor is a-axis polarized and that the
propagation vector of that light includes large components which
are parallel to the mc-plane. Accordingly, the light intensity
distribution is widely distributed in the c-axis direction than in
the a-axis direction. Therefore, by forming the stripe-shaped
uneven structure so as to be perpendicular to the c-axis, the light
distribution characteristics and the light extraction efficiency
can be improved. From the viewpoint of improving the light
distribution characteristics and the light extraction efficiency,
it is designed such that the polarization direction of the light
emitted from the active layer 24 and the extending direction of the
stripe structures are parallel to each other.
[0565] Therefore, when the a-axis polarization degree in the
m-plane nitride-based semiconductor light-emitting device is
maintained, it is desirable that the angle .beta. is near
0.degree., i.e., the stripe structures are formed along the a-axis
direction. Even under this condition, the light distribution
characteristics and the light extraction efficiency can be
improved.
[0566] On the other hand, by forming the stripe-shaped gaps 60 so
as to be traversed by the emitted light, the light distribution
characteristics and the light extraction efficiency can be
improved. In addition to these effects, by determining the angle
.beta. of the stripe-shaped gaps 60 with respect to the a-axis
direction so as to be greater than 0.degree., the polarization
degree of the emitted light can be reduced.
[0567] That is, the range of the in-plane tilt angle .beta. of the
stripe-shaped gaps 60 in which reduction of the a-axis polarization
degree and the effect of improving the light distribution
characteristics and the light extraction efficiency, which are the
major objects of the present embodiment, can be achieved is not
less than 3.degree. and not more than 45.degree.. The value of the
angle .beta. may be not less than 3.degree. and not more than
35.degree.. More specifically, the value of the angle .beta. may be
not less than 3.degree. and not more than 10.degree..
[0568] <Configuration of Light-Emitting Device 10 with Ridge
Portions 51 and Gaps 60 Lying Therebetween>
[0569] Hereinafter, the configuration and manufacturing method of a
nitride-based semiconductor light-emitting device which has gaps 60
near the interface are described in detail.
[0570] In the present embodiment, the nitride semiconductor layer
is premised on a structure of which principal surface is a
non-polar or semi-polar plane. Light emitted from such a nitride
semiconductor structure of which principal surface is a non-polar
plane or semi-polar plane has polarization, and the stripe-shaped
gaps 60 provided on the light emission surface side enable
reduction of the polarization degree and improvement of the light
distribution characteristics and the light extraction
efficiency.
[0571] In the present embodiment, a method for forming the stripe
structures near the interface between the growth substrate 100 and
the grown nitride semiconductor film 320 is described in detail,
rather than a case where the stripe structures are formed in the
rear surface of the growth substrate 100, i.e., a surface of the
growth substrate 100 which is opposite to the principal surface on
which epitaxial growth is to be carried out.
[0572] For example, in the case of a non-polar plane or semi-polar
plane nitride-based semiconductor light-emitting device which is
grown on the growth substrate 100 using sapphire, the effects of
the present embodiment can be obtained by forming the stripe
structures on the rear surface of the sapphire substrate. However,
the sapphire substrate that is commonly employed in crystal growth
of a nitride semiconductor has high hardness and is difficult to
process.
[0573] In view of such, in the present embodiment, the rear surface
of the growth substrate 100 is not processed. The stripe-shaped
gaps 60 are formed in the front surface of the growth substrate 100
before growth of the nitride semiconductor film 320, or in the
nitride semiconductor film 320 having a thickness of about several
nanometers to several micrometers which is grown on the substrate
100.
[0574] By using such a method, the stripe-shaped gaps 60 can be
readily formed on the principal surface of the growth substrate 100
without directly processing the substrate, even when the hardness
of the growth substrate 100 is high. Here, the stripe structures
are provided on the light emission surface side when seen from the
active layer 24, and therefore, the polarization degree of emitted
light can be reduced, and the light distribution characteristics
and the light extraction efficiency can be improved.
[0575] As the growth substrate 100 of the present embodiment, a
substrate on which a nitride semiconductor that has a non-polar
plane or semi-polar plane can be grown is used. For example, the
growth substrate 100 used may be a GaN bulk substrate which is a
nitride semiconductor substrate. That is, the gaps 60 may be
provided near the interface between the nitride semiconductor film
320 and the GaN bulk substrate. The GaN bulk substrate may be a
substrate on which a non-polar plane or semi-polar plane can be
grown. It may also be a GaN bulk substrate of which principal
surface is the a-plane or m-plane, or a GaN bulk substrate of which
principal surface is a semi-polar plane, such as (11-22) plane,
(2-201) plane, or (2-20-1) plane.
[0576] However, in view of the existing technology, a GaN bulk
substrate which is used in crystal growth of a nitride-based
semiconductor device of which principal surface is a non-polar
plane or semi-polar plane is expensive, and furthermore, increasing
the diameter of the substrate is difficult.
[0577] For example, the price of an existing GaN bulk substrate of
which principal surface is the m-plane is higher than that of a
sapphire substrate of the same size by two or more orders of
magnitude. The size of the m-plane GaN substrate is a square of
about several centimeters on each side. Even in the case of a
c-plane GaN bulk substrate, increasing the diameter so as to exceed
about 5.1 cm (=2 inches) is difficult according to the existing
technology. On the other hand, a sapphire substrate, which is one
of the heterogeneous substrates, is presently inexpensive, e.g.,
about several thousands of Japanese yens for the two-inch size, and
increase of the diameter to about 10.2 cm (=4 inches) or about 15.2
cm (=6 inches) or greater has already been realized.
[0578] Thus, even when a GaN bulk substrate is used as the growth
substrate 100, a similar polarization degree reducing effect can be
obtained. However, from the viewpoint of cost and increase of the
diameter, using a substrate which is made of a material different
from the nitride semiconductor, i.e., a hetero-substrate, is
desired.
[0579] As the hetero-substrate, for example, sapphire, silicon
carbide, (SiC), silicon (Si), gallium oxide (Ga.sub.2O.sub.3),
lithium aluminum oxide (LiAlO.sub.2) zinc oxide (ZnO), or the like,
can be suitably used. For example, as the substrate for m-plane
nitride semiconductor growth, m-plane sapphire, m-plane SiC, and
(100) LiAlO.sub.2 substrates have been reported.
[0580] In the case of a nitride-based semiconductor light-emitting
device of which principal surface is the m-plane, the surface of
the active layer 24 may be at least parallel to the m-plane or
controlled to have an angle of .+-.5.degree. with respect to the
m-plane. Within a range where that condition is met, a
hetero-substrate can be appropriately selected.
[0581] The effect of the present embodiment is valid even when
silicon (Si) is used for the hetero-substrate. The Si substrate is
inexpensive, and increase of the diameter of the Si substrate can
be readily achieved. Further, it is known that growth of a nitride
semiconductor of a semi-polar plane or non-polar plane is enabled
by a growth method in which a facet plane of Si is used. In the
present embodiment, the stripe-shaped gaps 60 are provided near the
interface between Si and the nitride semiconductor layer, whereby
the above-described effect can be obtained. However, since the Si
substrate absorbs visible light, the light extraction efficiency
decreases. Thus, when using a substrate which absorbs emitted light
from the active layer 24, such as the Si substrate, it is desired
that the Si substrate is removed after formation of the gaps
60.
[0582] In the case of, for example, sapphire which absorbs only
small part of the visible light in the growth substrate 100, the
growth substrate 100 does not necessarily need to be removed.
However, even when the substrate is removed, the effect of the
present embodiment can be obtained so long as the gaps 60 are
provided in the nitride semiconductor layer which has an interface
with the substrate before removal.
[0583] In the present embodiment, a manufacturing method of the
heterogeneous nitride semiconductor substrate 600 in the
light-emitting device 10 in which an m-plane nitride semiconductor
on an m-plane sapphire substrate is the basic body is described
with reference to the drawings. In the drawings mentioned below,
for the sake of simple description, elements which perform
substantially the same functions are denoted by the same reference
numerals. Note that the present disclosure is not limited to the
embodiments which will be described below.
[0584] In the present embodiment, as the seed crystal film or
regrown film which is to be grown on a hetero-substrate, a gallium
nitride layer (GaN layer) is mainly described. However, these
layers may be layers which contain at least one of Al, In, and B.
Further, the seed crystal or regrown film does not need to be
formed by only a GaN layer but may contain, for example, only a
single Al.sub.xGa.sub.yIn.sub.zN layer (where 0.ltoreq.x, y,
z.ltoreq.1, x+y+z=1) or may contain a plurality of
Al.sub.xGa.sub.yIn.sub.zN layers (0.ltoreq.x, y, z.ltoreq.1,
x+y+z=1) having different compositions which are alternately
stacked. Alternatively, the film may have a configuration in which
boron (B) is further contained in these layers.
[0585] FIG. 56(a) to FIG. 56(d) illustrate a method for forming
stripe-shaped gaps in a region near the interface between a growth
substrate and a nitride semiconductor film grown thereon.
[0586] First, as shown in FIG. 56(a), a nitride semiconductor layer
110 is grown on a growth substrate 100 of which growing plane is
the m-plane. Then, a mask 120 which is made of a dielectric or
oxide is selectively formed. The mask material used may be, for
example, silicon oxide (SiO.sub.2), silicon nitride (SiN), silicon
oxynitride (SiON), zirconium oxide (ZrO), zinc oxide (ZnO), gallium
oxide (Ga.sub.2O.sub.3), or aluminum oxide (Al.sub.2O.sub.3).
[0587] Then, as shown in FIG. 56(b), etching is carried out on the
nitride semiconductor layer 110 and upper part of the growth
substrate 100 using the mask 120, whereby recessed portions 210 are
formed at which the growth substrate 100 is exposed through the
mask 120.
[0588] As described above, an unevenly-processed substrate 500 is
formed that has stripe-shaped nitride semiconductor layers 110a,
which remain without being removed by the etching and which are
raised portions that serve as starting points of regrowth, and
recessed portions 210 at which the growth substrate 100 is exposed
by etching. Note that etching which is carried out on the upper
part of the growth substrate 100 is over-etching which is carried
out such that the nitride semiconductor layer 110 does not remain.
Details of this over-etching will be described later.
[0589] Then, as shown in FIG. 56(c), nitride semiconductor portions
310 are regrown on the unevenly-processed substrate 500, so that
regrowth occurs preferentially from the stripe-shaped nitride
semiconductor layers 110a. Further, by appropriately selecting the
growth conditions, regrown nitride semiconductor portions 310
undergo lateral growth from the opposite lateral surfaces of the
stripe-shaped nitride semiconductor layers 110a, and the growth
advances so as to cover the recessed portions 210 in which the
growth substrate 100 is exposed.
[0590] Then, the growth is continued so that the laterally-grown
nitride semiconductor portions 310 connect to one another, thereby
forming a connecting portion 410 as shown in FIG. 56(d). As a
result, the bottom surfaces 210a of the recessed portions 210
formed in the exposed growth substrate 100 are covered with the
regrown film. The growth is further continued so that, in this
turn, the nitride semiconductor portions 310 grow in a direction
perpendicular to the principal surface of the growth substrate 100,
i.e., in the m-axis direction, so as to entirely cover the mask
120, so that connecting portions 400 are also formed above the mask
120. Finally, a flat nitride semiconductor film can be
obtained.
[0591] In this case, between the recessed portions 210 and the
laterally-grown nitride semiconductor portions 310, the gaps 60 are
produced in which the epitaxial film is not present. These gaps 60
have a structure extending in the shape of a stripe and include the
slope surfaces 53 and the bottom surface 52.
[0592] As described herein, uneven processing is carried out on the
growth substrate 100 and the nitride semiconductor layer 110
regrown on the growth substrate 100 such that the
unevenly-processed substrate 500 is prepared that has the
stripe-shaped nitride semiconductor layers 110a which form the
raised portions and the recessed portions 210 in which the growth
substrate 100 is exposed. When regrowth of the nitride
semiconductor portions 310 is carried out on this prepared
unevenly-processed substrate 500, the stripe-shaped gaps 60 can be
formed parallel to the extending direction of the stripe-shaped
nitride semiconductor layers 110a that form the raised
portions.
[0593] In the step of obtaining the stripe-shaped gaps 60 by the
above-described method, regrowth of the nitride semiconductor
portions 310 starts from the stripe-shaped nitride semiconductor
layers 110a and advances in lateral directions. Therefore, some of
dislocations which occur at that interface in heterogeneous growth
and which are included in the nitride semiconductor layer 110 bend
in lateral directions rather than in the m-axis direction that is
the vertical direction. As a result, thanks to the recessed
portions 210, the density of dislocations and defects can be
greatly reduced. Accordingly, the quality of the surface region of
the regrown nitride semiconductor portions 310 can be improved.
[0594] The method described hereinabove is a growth method which is
commonly referred to as "epitaxial lateral overgrowth". The method
of forming the gaps 60 according to the present embodiment employs
the epitaxial lateral overgrowth method so that the effect of
reducing the polarization degree can be achieved while the
dislocation density and the stacking fault density can be reduced.
In such epitaxial lateral overgrowth, it is generally considered
that the depth of the recessed portions 210 formed by etching is
desirably as large as possible, as compared with the stripe-shaped
nitride semiconductor layers 110a that form the raised portions.
This is because there is a probability that the nitride
semiconductor portions 310 grow from the surface of the growth
substrate 100 in the recessed portions 210 which is exposed by
etching in the regrowth.
[0595] In this growth method, regrowth occurs only from the
stripe-shaped nitride semiconductor layers 110a so that the density
of dislocations and defects can be reduced. Thus, it is important
to prevent regrowth from the bottom surfaces 210a of the recessed
portions 210 formed by etching. If the regrowth occurs, it is
important to prevent the regrowth from affecting the lateral
regrowth.
[0596] As the etching depth becomes larger, it is more difficult
for the source materials used in regrowth to reach the bottom
surfaces 210a of the recessed portions 210. The growth occurs
preferentially only from the nitride semiconductor layers 110 that
form the raised portions so that epitaxial lateral overgrowth is
enhanced. Even if growth occurs in the recessed portions 210, the
larger etching depth contributes to reduction of the effect on and
the interference with the regrown film.
[0597] The method of forming the gaps 60 according to the epitaxial
lateral overgrowth which has previously been described with
reference to FIG. 56 can be realized without using the mask 120
that is made of a dielectric or the like. Removing the mask 120
that is made of a dielectric has such an advantage that, for
example, contamination with impurities from the material itself
that forms the mask 120 can be prevented.
[0598] (A Variation of the Manufacturing Method)
[0599] FIG. 57(a) to FIG. 57(d) illustrate a growth method in the
case where a mask for regrowth is not used.
[0600] First, in the step shown in FIG. 57(a), a nitride
semiconductor layer 110 and a mask 121 are sequentially formed on a
growth substrate 100 as in FIG. 56(a).
[0601] Then, as shown in FIG. 57(b), an unevenly-processed
substrate 510 which has unevenness in the surface is formed by
etching with the use of the mask. Thereafter, in this variation,
the mask 121 is removed.
[0602] Then, as shown in FIG. 57(c), the unevenly-processed
substrate 510 from which the mask 121 has been removed is used as a
substrate for regrowth, and a nitride semiconductor film 320 is
regrown from the stripe-shaped nitride semiconductor layers 110a
that form the raised portions. Here, the regrowth occurs such that
lateral growth occurs so as to cover the recessed portions 210
while growth also occurs at the upper surfaces of the stripe-shaped
nitride semiconductor layers 110a. Finally, as a result of
continuation of the regrowth, the flat nitride semiconductor film
320 can be obtained as shown in FIG. 57(d).
[0603] In this variation, the mask 121 is not present in the
nitride semiconductor film 320, and therefore, contamination with
impurities from the mask that is made of a dielectric, such as
SiO.sub.2, would not occur. Therefore, there is such an advantage
that a high quality nitride semiconductor film 320 can be
obtained.
[0604] Furthermore, a commonly-employed resist film can be used as
the mask 121, and therefore, the step of forming the dielectric
mask can be omitted. Thus, there is another advantage that the
manufacture cost can be reduced.
[0605] Hereinafter, a method for forming the gaps 60 which have the
in-plane tilt angle .beta. with the use of the structure of the
variation of FIG. 57 from which the mask 121 is to be removed is
described. The difference from the formation method of FIG. 56 is
only the presence/absence of the mask. Therefore, the polarization
degree reducing effect, for example, can also be obtained likewise
in the case of FIG. 56.
[0606] FIG. 58(a) and FIG. 58(b) show the cross-sectional
configuration and planar configuration of the unevenly-processed
substrate 510 for growth of a nitride semiconductor layer according
to the fourth embodiment.
[0607] As shown in FIG. 58(a), the unevenly-processed substrate 510
includes the growth substrate 100 which is made of sapphire of
which growing plane is the m-plane and nitride semiconductor layers
110a which have a plurality of stripe-shaped raised portions
provided on the growing plane of the growth substrate 100. Between
respective ones of the plurality of stripe-shaped nitride
semiconductor layers 110a, the recessed portions 210 are provided.
In the example shown in FIG. 58(b), the stripe-shaped nitride
semiconductor layers 110a extend in a direction which is inclined
by the in-plane tilt angle .beta. with respect to the a-axis
direction of the m-plane nitride semiconductor.
[0608] As described above, the crystal axes of the m-plane nitride
semiconductor layer and the m-plane sapphire in the plane deviate
by 90.degree.. That is, it is appreciated that epitaxial growth
occurs such that, although the m-axis that is the principal surface
is parallel, the a-axis (c-axis) in the plane of the nitride
semiconductor is parallel to the c-axis (a-axis) of the
sapphire.
[0609] Thus, defining the angle .beta. relative to the growth
substrate 100, the angle .beta. is 0.degree. when it is parallel to
the c-axis direction of the growth substrate 100. When it is
parallel to the a-axis direction of the growth substrate 100, the
angle .beta. is 90.degree..
[0610] That is, the unevenly-processed substrate 510 is designed
such that the angle .beta. between the extending direction of the
stripe-shaped nitride semiconductor layers 110a and the a-axis of
the nitride semiconductor layers 110a is not less than 3.degree.
and not more than 45.degree.. Then, the nitride semiconductor film
320 is regrown on the formed unevenly-processed substrate 510,
whereby the heterogeneous nitride semiconductor substrate 600 can
be obtained that has the stripe-shaped gaps 60 which have equal
in-plane tilt angles .beta.. In the light-emitting device 10 with
the m-plane principal surface which is manufactured on the
heterogeneous nitride semiconductor substrate 600, the polarization
degree of light emitted from the active layer 24 is reduced, while
the light distribution characteristics and the light extraction
efficiency are improved.
[0611] At the bottom surfaces 210a of the recessed portions 210,
the m-plane of the growth substrate 100 is exposed. The source
material particles of the nitride semiconductor are unlikely to
adhere to the m-plane of the sapphire. Thus, in regrowth of the
nitride semiconductor film 320 with the use of the stripe-shaped
nitride semiconductor layers 110a as seed crystals, the source
material particles are likely to adhere to the stripe-shaped
nitride semiconductor layers 110a rather than to the bottom
surfaces 210a of the recessed portions 210. Thus, growth of the
nitride semiconductor film 320 of low crystallinity from the bottom
surfaces 210a can be prevented.
[0612] FIG. 58(a) and FIG. 58(b) show an example where a mask is
not provided on the upper surface of the stripe-shaped nitride
semiconductor layers 110a. In the present embodiment, the mask 120
such as shown in FIG. 56(b) may be provided.
[0613] The unevenly-processed substrate 510 of the present
embodiment may be formed using the growth substrate 100 which is in
a wafer form, for example. FIG. 58 shows part of the wafer.
Respective elements are shown in consideration of visibility, and
the actual scale of the respective elements is not limited to the
scale of the elements shown in the drawings.
[0614] In the present embodiment, as described above, the growing
plane of the growth substrate 100 may be inclined by an angle of
not more than 5.degree. with respect to the m-plane. Also, a
surface exposed at the bottom surfaces 210a of the recessed
portions 210 may be inclined by an angle of not more than 5.degree.
with respect to the m-plane. Also, the growing plane of the
stripe-shaped nitride semiconductor layers 110a may be inclined by
an angle of not more than 5.degree. with respect to the
m-plane.
[0615] The range of the tilt angle .beta. in the plane of the gaps
60 may be not less than 3.degree. and not more than 35.degree.. In
the range of the in-plane tilt angle .beta.>35.degree., abnormal
growth of the nitride semiconductor film 320 that has a semi-polar
plane is likely to occur from the wall surfaces 220 of the recessed
portions 210 formed by etching. Therefore, it was proved from the
examinations carried out by the present inventors that, in the
nitride semiconductor film 320 obtained by lateral regrowth, the
m-plane and the semi-polar plane coexist so that the crystal
quality and the surface flatness significantly deteriorate.
Specifically, when the plurality of gaps 60 are formed with the
tilt angle being within the range of
3.degree..ltoreq..beta..ltoreq.35.degree., the polarization degree
can be reduced, and the light distribution characteristics and the
light extraction efficiency can be improved. Furthermore, abnormal
growth which could occur at the semi-polar plane from the growth
substrate 100 exposed in the recessed portions 210 is prevented,
and the m-plane nitride regrown film 320 and the heterogeneous
nitride semiconductor substrate 600, which have excellent surface
flatness and crystal quality, can be obtained.
[0616] The absolute value of the in-plane tilt angle .beta. of the
gaps 60 may be not less than 3.degree. and not more than
10.degree.. In this case, the stacking fault density can be
particularly reduced. The details will be described with reference
to the measurement results of examples which will be described
later.
[0617] The stripe-shaped nitride semiconductor layers 110a which
are formed by a plurality of raised portions according to the
present embodiment are not limited to a simple arrangement of lines
and spaces such as shown in FIG. 58(b). As previously described, it
may be the structure shown in FIG. 47, FIG. 48, or FIG. 49.
[0618] The shape of a cross section which is perpendicular to the
extending direction of the stripe-shaped gaps 60 may be
appropriately selected. For example, it may be any of the
configurations shown in FIG. 45 and FIG. 46.
[0619] As shown in FIG. 59(a), in the present embodiment, lateral
surfaces 110A of the stripe-shaped nitride semiconductor layers
110a may be inclined with respect to the normal line of the growing
plane of the growth substrate 100. In this case, the angle .gamma.
inside the ridge between the lateral surface 110A which is parallel
to the extending direction of the stripe-shaped nitride
semiconductor layers 110a and the m-plane may be greater than
0.degree. and smaller than 150.degree..
[0620] The cross section of the stripe-shaped nitride semiconductor
layers 110a is not limited to a quadrangular or trapezoidal shape
but may be a triangular shape or another polygonal shape, or may
include a curve.
[0621] In the present embodiment, in part of some or one of the
plurality of stripe-shaped nitride semiconductor layers 110a, the
condition that the extending direction of the stripe-shaped nitride
semiconductor layers 110a has an inclination of not less than
3.degree. and not more than 45.degree. with respect to the a-axis
direction of the nitride semiconductor may not be satisfied. In
this case, at least 50% of the plurality of stripe-shaped nitride
semiconductor layers 110a may satisfy the condition that the angle
between the extending direction of the stripe-shaped nitride
semiconductor layers 110a and the a-axis of the nitride
semiconductor is not less than 3.degree. and not more than
45.degree..
[0622] The thickness of the growth substrate 100 is, for example,
not less than 0.1 mm and not more than 1 mm. The diameter of the
growth substrate 100 (wafer) is, for example, not less than about
2.5 cm (1 inch) and not more than about 20.3 cm (8 inches).
[0623] The thickness of the stripe-shaped nitride semiconductor
layers 110a is, for example, not less than 10 nm and not more than
10 .mu.m. The width L along a direction perpendicular to the
extending direction of the stripe-shaped nitride semiconductor
layer 110a shown in FIG. 59(a) may be set to a value which is not
less than 0.1 .mu.m and not more than 10 .mu.m, for example. The
width S of the recessed portion 210 shown in FIG. 59(a) may be set
to a value which is not less than 1 .mu.m and not more than 30
.mu.m.
[0624] The width L and width S shown in FIG. 59(a) are generally
equal to the period of the uneven structure that is formed in a
direction perpendicular to the extending direction of the stripe
structures.
[0625] To obtain only the effect of reducing the polarization
degree and the effect of improving the light distribution
characteristics and the light extraction efficiency, which are the
major objects of the present embodiment, the period which is
determined from the above-described width L and width S may be on
the level of the wavelength of light, i.e., may be not more than 1
.mu.m.
[0626] On the other hand, in the case of obtaining the effect of
improving the crystallinity of the heterogeneous nitride
semiconductor film concurrently with the polarization control
effect, the period which is determined from the above-described
width L and width S can be controlled in a still wider range. As
described above, even if the period was 40 .mu.m, that effect was
confirmed.
[0627] In the step of manufacturing the unevenly-processed
substrate 510, the etching for formation of the recessed portions
210 may be carried out somewhat deeper in order to avoid the
stripe-shaped nitride semiconductor layers 110a remaining at the
bottom surfaces 210a of the recessed portions 210. In this case,
the upper part of the growth substrate 100 is also removed, so that
the wall surfaces 220 which are formed by the growth substrate 100
are exposed at the lower part of the recessed portions 210 as shown
in FIG. 58(a).
[0628] As shown in FIG. 59(b), in forming the recessed portions
210, the recessed portions 210 may not be necessarily formed such
that the upper part of the growth substrate 100 is removed. In this
case, the bottom surfaces 210a of the recessed portions 210 are at
the same height as the interface between the growth substrate 100
and the stripe-shaped nitride semiconductor layers 110a.
[0629] The depth of the bottom surfaces 210a of the recessed
portions 210 relative to the interface between the growth substrate
100 and the stripe-shaped nitride semiconductor layers 110a may be,
for example, not less than 0 nm and not more than 500 nm, or not
less than 0 nm and not more than 150 nm.
[0630] In the present embodiment, it has previously been explained
that, assuming that the absolute value of the angle between the
extending direction of the stripe-shaped nitride semiconductor
layers 110a and the a-axis of the nitride semiconductor film 320 of
which principal surface is the m-plane is 35.degree., abnormal
growth which is attributed to the semi-polar plane occurs. However,
this problem can be avoided by decreasing the area of the sapphire
which is exposed at the lateral surfaces of the stripe-shaped
nitride semiconductor layers 110a.
[0631] For example, when the depth of the bottom surfaces 210a of
the recessed portions 210 relative to the interface between the
growth substrate 100 and the stripe-shaped nitride semiconductor
layers 110a is not less than 0 nm and not more than 150 nm,
abnormal growth of the nitride semiconductor film 320 from the
lateral surfaces of the stripe-shaped nitride semiconductor layers
110a, which is attributed to the semi-polar plane, can be
avoided.
[0632] As shown in FIG. 59(b), in a configuration where the area of
the wall surfaces 220 inside the recessed portions 210 in the
growth substrate 100 is as small as possible, the amount of a
semi-polar nitride semiconductor which grows from the growth
substrate 100 that is present at the lower part of the lateral
surfaces of the stripe-shaped nitride semiconductor layers 110a is
small. Therefore, even if the angle between the extending direction
in the growing plane of the stripe-shaped nitride semiconductor
layers 110a and the a-axis of the nitride semiconductor film 320 is
not less than 35.degree., abnormal growth of the nitride
semiconductor film 320 which is attributed to the semi-polar plane
is prevented, so that the flatness of the surface can be
improved.
[0633] When a plane which is different from the m-plane is exposed
at the bottom surfaces 210a of the recessed portions 210 between
the stripe-shaped nitride semiconductor layers 110a, regrowth of
the nitride semiconductor film 320 from the bottom surfaces 210a of
the recessed portions 210 is a problem, and therefore, it is
necessary to form the recessed portions 210 deeper. In the present
embodiment, the bottom surfaces 210a of the recessed portions 210
are made of the m-plane sapphire, the nitride semiconductor film
320 is unlikely to grown from the bottom surfaces 210a. Thus, the
recessed portions 210 can be formed shallow, without consideration
of growth from the bottom surfaces 210a.
[0634] Further, the absolute value of the angle between the
in-plane direction in which the stripe-shaped nitride semiconductor
layers 110a extend and the a-axis of the nitride semiconductor film
320 of which principal surface is the m-plane may be not less than
3.degree. and not more than 35.degree., and furthermore, the depth
of the bottom surfaces 210a of the recessed portions 210 relative
to the interface between the growth substrate 100 and the
stripe-shaped nitride semiconductor layers 110a may be more than 0
nm and not more than 150 nm. Thanks to this arrangement, even when
the depth of the bottom surfaces 210a is more than 0 nm, the amount
of the semi-polar nitride semiconductor that grows from a portion
of the growth substrate 100 which is exposed at the lower part of
the lateral surfaces of the stripe-shaped nitride semiconductor
layers 110a can be further reduced.
[0635] Alternatively, the absolute value of the angle between the
in-plane direction in which the stripe-shaped nitride semiconductor
layers 110a extend and the a-axis of the nitride semiconductor film
320 of which principal surface is the m-plane may be not less than
0.degree. and not more than 3.degree., and furthermore, the depth
of the bottom surfaces 210a of the recessed portions 210 relative
to the interface between the growth substrate 100 and the
stripe-shaped nitride semiconductor layers 110a may be more than 0
nm and not more than 150 nm. Thanks to this arrangement, the amount
of the semi-polar nitride semiconductor that grows from a portion
of the growth substrate 100 which is exposed at the lower part of
the lateral surfaces of the stripe-shaped nitride semiconductor
layers 110a can be further reduced, and the polarized light can be
maintained.
[0636] As described hereinabove, the stripe-shaped nitride
semiconductor layers 110a are provided on the growth substrate 100
that is made of m-plane sapphire, and regrowth is carried out,
whereby the stripe-shaped gaps 60 that are parallel to the
stripe-shaped nitride semiconductor layers 110a can be formed.
Thanks to this arrangement, the light-emitting device 10 of which
principal surface is the m-plane can be obtained in which the
stripe-shaped gaps 60 are provided near the heterogeneous interface
in the heterogeneous nitride semiconductor substrate 600.
Fifth Embodiment
[0637] Hereinafter, the fifth embodiment is described with
reference to the drawings.
[0638] FIG. 60(a) and FIG. 60(b) show schematic cross-sectional
configuration and planar configuration of the heterogeneous nitride
semiconductor substrate 600 which has the stripe-shaped gaps 60 and
which is obtained by the manufacturing method described in the
fourth embodiment. Here, the configuration shown in FIG. 60(a) is
essentially identical with that of FIG. 57(d).
[0639] When the unevenly-processed substrate 510 is provided and
the heterogeneous nitride semiconductor substrate 600 is formed by
regrowth of the nitride semiconductor film 320, in general, the
density of dislocations and defects in part of the nitride
semiconductor film 320 formed above the recessed portions 210 is
lower than in part of the nitride semiconductor film 320 formed on
the stripe-shaped nitride semiconductor layers 110a, so that the
quality of crystal is likely to be high.
[0640] This is because, in the stripe-shaped nitride semiconductor
layers 110a, dislocations and defects produced at the heterogeneous
interface extend in the m-axis direction that is the growth
direction, while in the nitride semiconductor film 320 lying above
the recessed portions 210 which is obtained by lateral growth,
dislocations and defects bend, for example, so that they are
unlikely to extend in the m-axis direction.
[0641] As a result, as shown in FIG. 60(a) and FIG. 60(b), a
gradation of the density of dislocations and defects is formed in
the plane of the heterogeneous nitride semiconductor substrate 600.
The regions in which the density of dislocations and defects is
high are formed on the stripe-shaped nitride semiconductor layers
110a that form the raised portions, and therefore, as shown in FIG.
60(b), when seen from the m-axis side, they are formed parallel to
the stripe-shaped gaps 60 and have equal in-plane tilt angles
.beta..
[0642] FIG. 61 schematically shows a cross-sectional configuration
of a semiconductor light-emitting device according to the fifth
embodiment. As shown in FIG. 61, the light-emitting device 11 of
the present embodiment includes a heterogeneous nitride
semiconductor substrate 600, and a semiconductor multilayer
structure 20 which is provided on the heterogeneous nitride
semiconductor substrate 600 and which includes an active layer 24.
The heterogeneous nitride semiconductor substrate 600 is formed by
a growth substrate 100 and a nitride semiconductor film 320 which
is formed on the growth substrate 100 by selective growth and which
includes a plurality of gaps 60 that have the in-plane tilt angle
.beta.. The semiconductor multilayer structure 20 is provided with
a n-electrode 40 and a p-electrode 30. Light emitted from the
active layer 24 is mainly reflected by the p-electrode 30 and
outgoes from the growth substrate 100 side.
[0643] As previously described with reference to FIG. 60, in the
present embodiment, above a region in which the stripe-shaped
nitride semiconductor layers 110a that form the raised portions are
to be formed, dislocations and defects which have a higher density
than in the recessed portions 210 are formed parallel to the
stripe-shaped gaps 60. Commonly, this defect density sometimes
varies by one or more orders of magnitude. Therefore, in the
light-emitting device 11 which employs the structure of the present
embodiment as the basic body, the presence of the stripe-shaped
nitride semiconductor layers 110a that are the raised portions
which produce high dislocation density/high defect density regions
in the nitride semiconductor film 320 can be a cause of decrease in
the emission efficiency.
[0644] That is, in the fourth embodiment shown in FIG. 43, an
electric current is injected into the entire active layer 24, and
therefore, emission of light can also be obtained from the active
layer 24 provided on the high dislocation density/high defect
density regions. However, there is a probability that the emission
efficiency decreases in those regions.
[0645] In the present embodiment, to avoid this problem of decrease
of the emission efficiency, a plurality of insulating films 140 are
selectively formed at the interface between the p-electrode 30 and
a p-type nitride semiconductor layer 25. The respective insulating
films 140 are formed on the stripe-shaped nitride semiconductor
layers 110a that form the raised portions so as to form stripe
structures which have the same angle .beta. as the gaps 60. With
such insulating films 140 provided, portions of the active layer 24
lying above the stripe-shaped nitride semiconductor layers 110a
would not contribute to emission of light, while only portions of
the active layer 24 lying above the recessed portions 210 which
contain less dislocations and defects contribute to emission of
light, so that the emission efficiency can be improved.
[0646] The insulating films 140 may be made of a material which is
capable of transmitting polarized light emitted from the active
layer 24. For example, SiO.sub.2, SiN, ZrO, Ga.sub.2O.sub.3,
Al.sub.2O.sub.3, ZnO, or the like, may be used for the insulating
films 140.
[0647] In the light-emitting device 11 of the present embodiment,
an electric current is not allowed to flow through the high
dislocation density/high defect density regions, and the operation
of a low emission efficiency region in the active layer 24 is
prevented, so that the emission efficiency can be improved.
[0648] Note that the stripe-shaped insulating films 140 may not be
formed. For example, the p-electrode 30 itself may have a stripe
shape, and the p-electrode 30 may be provided only in portions
lying above the respective recessed portions 210. However, with
such a configuration, polarized light emitted from the active layer
24 is not reflected by the regions lying above the stripe-shaped
nitride semiconductor layers 110a in which the p-electrode 30 is
not provided. As a result, the light extraction efficiency in the
light-emitting device 11 decreases.
[0649] Therefore, the insulating films 140 may be provided, between
the p-type layer 25 and the p-electrode 30, in regions which are
parallel to the gaps 60 and which are lying above the stripe-shaped
nitride semiconductor layers 110a. Here, the p-electrode 30 may be
provided so as to cover the plurality of insulating films 140. The
thickness of the insulating films 140 may be small so long as the
insulative property is ensured. For example, the thickness of the
insulating films 140 is not less than 20 nm and not more than 200
nm.
[0650] As described above, by forming a light-emitting device
structure 11 that has the gaps 60 which have the tilt angle .beta.
with respect to the a-axis direction in the plane of the principal
surface of the heterogeneous nitride semiconductor substrate 600,
the a-axis polarization degree of polarized light emitted from the
active layer 24 can be prevented, while the light distribution
characteristics and the light extraction efficiency can be
improved. The range of the angle .beta. of the gaps 60 may be not
less than 3.degree. and not more than 45.degree.. More
specifically, the angle .beta. may be not less than 3.degree. and
not more than 35.degree.. More specifically, the angle .beta. may
be not less than 3.degree. and not more than 10.degree..
[0651] As shown in FIG. 57(d) and FIG. 60(a), in the step of
preparing the heterogeneous nitride semiconductor substrate 600
which has the stripe-shaped gaps 60, connecting portions 410 that
are formed by nitride semiconductor film portions 320 which are
selectively grown in lateral directions so as to connect to one
another are parallel to the stripe-shaped gaps 60.
A Variation of the Fifth Embodiment
[0652] As described above, at each of the connecting portions 410,
nitride semiconductor film portions 320 regrown from different
stripe-shaped nitride semiconductor layers 110a connect to each
other. Therefore, small deviations occur in the crystal plane and
crystal orientation and can be causes of additional defects and
dislocations. Thus, if the active layer 24 is formed over the
connecting portions 410, they serve as non-emission regions and can
be a cause of decrease in the emission efficiency.
[0653] FIG. 62 and FIG. 63 show a light-emitting device 12 of this
variation which is unlikely to be affected by the connecting
portions 410. Here, FIG. 62 and FIG. 63(b) show cross-sectional
configurations which are seen in the a-axis direction. FIG. 63(a)
shows a cross-sectional configuration which is seen in the c-axis
direction.
[0654] In this variation, to prevent the connecting portions 410
from being included in the active layer 24, in regrowing the
nitride semiconductor film 320 on the unevenly-processed substrate
510, portions of the semiconductor multilayer structure 20 up to
the active layer 24 are grown, and then, in regrowth of the p-type
layer 25, the nitride semiconductor film portions 320 regrown from
the respective stripe-shaped nitride semiconductor layers 110a are
allowed to connect to each other, whereby the connecting portions
410 are obtained.
[0655] When the light-emitting device 12 is manufactured in this
way, the connecting portions 410 are not included in the active
layer 24, so that decrease of the emission efficiency can be
prevented. Furthermore, also in this variation, by forming the
stripe-shaped gaps 60 that have the slope surface 53 of which tilt
angle .beta. is controlled within the range of not less than
3.degree. and not more than 45.degree. with respect to the a-axis
direction in the plane of the principal surface of the
heterogeneous nitride semiconductor substrate 600, the polarization
degree of emitted light can be reduced, while the light
distribution characteristics and the light extraction efficiency
can be improved.
[0656] In this variation, the n-electrode 40 of the light-emitting
device 12 may be formed as shown in FIG. 63(b). Specifically, in
the light-emitting device 12, the n-type layer 22 may have a
discontinuous configuration along the c-axis direction of the
nitride semiconductor film 320 due to the plurality of gaps 60.
Therefore, to achieve passage of an electric current in all of the
n-type layer portions 22 that are isolated from one another along
the c-axis direction, the n-electrode 40 may be formed so as to
continuously cover not only the top and lateral surfaces of the
respective n-type layer portions 22 but also the wall and bottom
surfaces of the gaps 60 in the heterogeneous nitride semiconductor
substrate 600.
[0657] By employing such a configuration, an electric current is
allowed to flow through the active layer 24 formed on the
heterogeneous nitride semiconductor substrate 600, and emission of
light can be obtained from the entire device.
[0658] The configuration of the n-electrode 40 according to this
variation shown in FIG. 63(b) is suitably used in a configuration
where the plurality of active layer portions 24 and the plurality
of n-type layer portions 22 are not connected to one another.
[0659] Thus, in the previously-described light-emitting device 11,
the position of formation of the n-electrode 40 is not particularly
limited so long as the surface of the n-type layer 22 can be formed
by etching.
Sixth Embodiment
[0660] Hereinafter, the sixth embodiment is described with
reference to the drawings.
[0661] FIG. 64(a) and FIG. 64(b) show details of a fabrication
method of a heterogeneous nitride semiconductor substrate 601
according to the sixth embodiment.
[0662] First, as shown in FIG. 64(a), a nitride semiconductor layer
110 of which principal surface is a non-polar plane is formed on
the growth substrate 100. In the present embodiment, for example,
the growth substrate 100 is an m-plane sapphire substrate, and the
nitride semiconductor layer 110 is a nitride semiconductor layer
which is grown on the m-plane sapphire substrate and of which
principal surface is the m-plane. Then, on the nitride
semiconductor layer 110, a stripe-shaped mask 120 is selectively
formed. The mask 120 is inclined in the principal surface of the
nitride semiconductor layer 110. For example, the range of the tilt
angle .beta. with respect to the a-axis direction is not less than
3.degree. and not more than 45.degree..
[0663] The above steps are the same as those of the fourth
embodiment and the fifth embodiment. In the present embodiment, as
shown in FIG. 64(b), etching is not carried out on the nitride
semiconductor layer 110, but regrowth is carried out on the surface
of the nitride semiconductor layer 110 via the mask 120, whereby
the nitride semiconductor film 320 is obtained.
[0664] When the regrowth is carried out with such a configuration,
regrowth of the nitride semiconductor film 320 would not occur on
the mask 120, and regrowth preferentially occurs from the surfaces
of exposed regions 200 of the nitride semiconductor 110 which are
not covered with the mask 120. Under some growth conditions, growth
of the nitride semiconductor film 320 is enhanced also in lateral
directions. As a result, the connecting portions 410 at which
regrown film portions connect to each other are formed above the
mask 120, while the stripe-shaped gaps 60 are formed above the mask
120.
[0665] For the material of the mask 120, for example, SiO.sub.2,
SiN, ZrO, ZnO, Ga.sub.2O.sub.3 or Al.sub.2O.sub.3, or an oxide
containing part of these compounds, or a dielectric material may be
used.
[0666] Alternatively, a metal may be used for the material of the
mask 120. For example, it may be aluminum (Al), titanium (Ti),
nickel (Ni), tungsten (W) or tantalum (Ta), or an alloy material
containing part of these elements.
[0667] In the present embodiment, regrowth of the nitride
semiconductor film 320 is carried out on the surface of the nitride
semiconductor layer 110 with the mask 120 formed thereon. Usually,
growth of the nitride semiconductor is carried out at a high
temperature, e.g., not less than 600.degree. C. and not more than
1300.degree. C. Therefore, even under such growth conditions, the
material may be appropriately selected such that the shape of the
mask is maintained, and thermal decomposition or reaction is
unlikely to occur. For example, an oxide, a dielectric, or a metal
material may be appropriately used. Note that, however, the metal
material generally has a high optical absorption coefficient, and
therefore, in consideration of application to a light-emitting
device, it is preferred that the material of the mask 120 is
capable of transmitting or reflecting visible-range polarized light
emitted from the active layer 24. Therefore, the material of the
mask 120 may be an oxide film or dielectric film which has high
transmittance.
[0668] As described hereinabove, in the step of FIG. 64(a), the
mask 120 is selectively formed on the nitride semiconductor layer
110, and furthermore, the nitride semiconductor film 320 is
regrown. Thus, in the step of FIG. 64(b), the heterogeneous nitride
semiconductor substrate 601 that has the gaps 60 which have the
tilt angle .beta. with respect to the a-axis can be fabricated.
[0669] FIG. 65 schematically shows the cross-sectional
configuration of a light-emitting device 13 manufactured on a
heterogeneous nitride semiconductor substrate 601 according to the
sixth embodiment. As shown in FIG. 65, the light-emitting device 13
of the present embodiment includes the heterogeneous nitride
semiconductor substrate 601 and a semiconductor multilayer
structure 20 which is provided on the heterogeneous nitride
semiconductor substrate 600 and which includes an active layer 24.
The heterogeneous nitride semiconductor substrate 601 is formed by
a growth substrate 100, a nitride semiconductor layer 110 provided
on the growth substrate 100, and a nitride semiconductor film 320
which is selectively grown on the nitride semiconductor layer 110
via a mask 120 and which includes a plurality of gaps 60 that have
the in-plane tilt angle .beta.. The semiconductor multilayer
structure 20 is provided with a n-electrode 40 and a p-electrode
30. Light emitted from the active layer 24 is mainly reflected by
the p-electrode 30 and outgoes from the growth substrate 100
side.
First Variation of Sixth Embodiment
[0670] In the present embodiment also, as in the fourth embodiment
1 and the fifth embodiment, portions of the nitride semiconductor
film 320 overlying the mask 120 which are obtained by lateral
growth have lower dislocation density and lower defect density than
portions of the nitride semiconductor film 320 which are regrown on
exposed regions 200 that are not covered with the mask 120.
[0671] That is, portions of the nitride semiconductor film 320
overlying the exposed regions 200 that are not covered with the
mask 120 have a higher density of dislocations and defects than
portions overlying the mask 120. Usually, these densities are
different by one or more orders of magnitude in some cases. Thus,
in the light-emitting device 13 which employs the structure of the
present embodiment as the basic body, regions of the active layer
24 lying above the exposed regions 200 that are not covered with
the mask 120, which are high dislocation density/high defect
density regions, can be a cause of decrease in the emission
efficiency.
[0672] This cause can be avoided by forming the insulating films
140 between the p-electrode 30 and the p-type nitride semiconductor
layer 25 as in the fifth embodiment.
[0673] FIG. 66 shows a light-emitting device 14 which is unlikely
to be affected by the exposed regions 200. The insulating films 140
are provided above the exposed regions 200 and therefore have
stripe structures which have equal angle .beta. to that of the gaps
60. When the insulating films 140 are formed in this way, portions
of the active layer 24 lying above the exposed regions 200 that are
not covered with the mask 120 do not contribute to emission of
light. Instead, only portions of the active layer 24 lying above
the mask 120, which contain smaller dislocations and defects,
contribute to emission of light, so that the emission efficiency
can be improved.
[0674] Here, the insulating films 140 may be made of a material
which is capable of transmitting polarized light emitted from the
active layer 24. For example, SiO.sub.2, SiN, ZrO, or the like, may
be used for the insulating films 140. The thickness of the
insulating films 140 may be thin so long as the insulation is
ensured. For example, the thickness of the insulating films 140 may
be not less than 20 nm and not more than 200 nm.
[0675] In the light-emitting device 14 of the present embodiment,
an electric current is not allowed to flow through the high
dislocation density/high defect density regions, and the operation
of a low emission efficiency region in the active layer 24 is
prevented, so that the emission efficiency can be improved.
[0676] Note that the stripe-shaped insulating films 140 may not be
formed. For example, the p-electrode 30 itself may have a stripe
shape, and the p-electrode 30 may be provided only in portions
lying above the exposed regions 200 that are not covered with the
mask 120. However, with such a configuration, polarized light
emitted from the active layer 24 is not reflected by the portions
lying above the exposed regions 200 that are not covered with the
mask 120, in which the p-electrode 30 is not provided, so that the
emission efficiency can decrease.
[0677] Therefore, the insulating films 140 may be provided, between
the p-type layer 25 and the p-electrode 30, in regions which are
parallel to the gaps 60 and which are lying above the exposed
regions 200 that are not covered with the mask 120. Here, the
p-electrode 30 may be provided so as to cover the plurality of
insulating films 140.
[0678] As described above, by forming a light-emitting device
structure 13, 14 that has the gaps 60 which have the tilt angle
.beta. with respect to the a-axis direction in the plane of the
principal surface of the heterogeneous nitride semiconductor
substrate 601, the a-axis polarization degree of polarized light
emitted from the active layer 24 can be prevented, while the light
distribution characteristics and the light extraction efficiency
can be improved. In the case of a light-emitting device
manufactured on the heterogeneous nitride semiconductor substrate
601, the range of the angle .beta. of the gaps 60 may be not less
than 3.degree. and not more than 45.degree.. Specifically, the
angle .beta. may be not less than 3.degree. and not more than
10.degree..
Second Variation of Sixth Embodiment
[0679] As previously described with reference to FIG. 60 and FIG.
61 according to the fifth embodiment, in the present embodiment
also, in the step of preparing a heterogeneous nitride
semiconductor substrate 601 which has a plurality of stripe-shaped
gaps 60, the connecting portions 410 are formed in the nitride
semiconductor film 320 so as to be parallel to the stripe-shaped
mask 120. These connecting portions 410 are formed by the nitride
semiconductor film portions 320, which are regrown from the exposed
regions 200 that are not covered with the mask 120, connecting to
each other.
[0680] At each of the connecting portions 410, nitride
semiconductor film portions 320 regrown from different exposed
regions 200 of the nitride semiconductor layer 110 connect to each
other. Therefore, small deviations occur in the crystal plane and
crystal orientation and can be causes of additional defects and
dislocations. Thus, if the active layer 24 is formed over the
connecting portions 410, they serve as non-emission regions and can
be a cause of decrease in the emission efficiency.
[0681] FIG. 67 shows a light-emitting device 15 of this variation
which is unlikely to be affected by the connecting portions 410. As
shown in FIG. 67, as in a variation of the fifth embodiment, in
manufacturing the light-emitting device 15, the connecting portions
410 are not included in the active layer 24, and mutual connection
of regrown film portions is achieved during the growth of the
p-type layer 25. Thanks to this arrangement, in the light-emitting
device 15 of this variation, decrease of the emission efficiency
can be prevented.
[0682] Further, also in this variation, by forming the
stripe-shaped gaps 60 that have the slope surface 53 of which tilt
angle .beta. is controlled within the range of not less than
3.degree. and not more than 45.degree. with respect to the a-axis
direction in the plane of the principal surface of the
heterogeneous nitride semiconductor substrate 601, the polarization
degree of emitted light can be reduced, while the light
distribution characteristics and the light extraction efficiency
can be improved.
[0683] In the light-emitting device 15 of this variation, the
n-electrode 40 may be formed on the surface of the nitride
semiconductor layer 110. For example, the surface of the nitride
semiconductor layer 110 is exposed by etching, and thereafter, the
n-electrode 40 may be formed on the exposed region of the nitride
semiconductor layer 110. Here, the nitride semiconductor layer 110
needs to have the n-type conductivity.
Seventh Embodiment
[0684] Hereinafter, the seventh embodiment is described with
reference to the drawings.
[0685] FIG. 68(a) schematically shows a cross-sectional
configuration of a light-emitting device according to the seventh
embodiment. FIG. 68(b) schematically shows a cross-sectional
configuration of a light-emitting device according to a variation
of the seventh embodiment. FIG. 68(a) shows a configuration which
is obtained by removing the growth substrate 100 from the
light-emitting device 10 of the fourth embodiment. FIG. 65(b) shows
a configuration which is obtained by removing the growth substrate
100 from the light-emitting device 13 of the sixth embodiment.
[0686] In the present disclosure, the polarization degree of
polarized light emitted from the active layer 24 is reduced by
stripe-shaped ridge portions which are provided at or near the
interface between the growth substrate 100 and the nitride
semiconductor film 320 and gaps 60 which are present between the
ridge portions, so that the effect of improving the light
distribution characteristics and the light extraction efficiency
can be obtained. Thus, the growth substrate 100 is not necessarily
required in the light-emitting device and may be removed
therefrom.
[0687] Particularly when at least a small portion of emitted light
from the active layer 24 is absorbed in the growth substrate 100,
it is desired that the growth substrate 100 is removed. Removal of
the growth substrate 100 improves the light extraction efficiency
and hence improves the characteristics of the light-emitting
device.
[0688] Note that, however, in the case where a material by which
absorption of visible range light is substantially negligible, such
as sapphire, is used for the growth substrate 100, the substrate
for growth does not necessarily need to be removed.
[0689] In order to remove the growth substrate 100, a preferred
method may be appropriately used depending on the material of the
growth substrate 100. For example, a laser lift-off method, an
abrasion method, a wet etching method, a dry etching method, or the
like, may be used.
[0690] In the present embodiment, a plurality of gaps 60 are
provided near the interface between the growth substrate 100 and
the nitride semiconductor film 320. Therefore, for example, the
width along a direction which is perpendicular to the extending
direction of the stripes in the nitride semiconductor layers 110a
that form stripe-shaped raised portions (width L in FIG. 59) is
made as small as possible to increase the formation region of each
of the gaps 60, so that the growth substrate 100 can be readily
removed.
[0691] For example, the growth substrate 100 can be removed by
using a commonly-employed wafer bonding method.
[0692] FIG. 68(a) shows an example of the light-emitting device 10
from which the growth substrate 100 has been removed using a wafer
bonding method. As shown in FIG. 68(a), a supporting substrate 27
which is used for removing the growth substrate 100 is bonded onto
the p-type layer 25. In the present embodiment, the material of the
supporting substrate 27 may be a material which has p-type
conductivity. For example, it may be a p-type Si substrate or
p-type GaAs substrate. However, these materials absorb emitted
light from the active layer 24 so that the emission efficiency
deteriorates. Therefore, as the substrate material, a p-type SiC
substrate or an oxide substrate or diamond substrate which has
p-type conductivity may be used. Furthermore, a joining layer may
be provided between the p-type layer 25 and the supporting
substrate 27 for joining substrates together.
[0693] When this joining layer is made of a material of high
reflectance, e.g., Ag, Al, Rh, or the like, or a material which
contains these elements in part thereof, the previously-described
material which causes absorption of light, such as Si, GaAs, or the
like, may be used as the supporting substrate.
[0694] In the light-emitting device 10 of the present embodiment,
for the purpose of removing the growth substrate 100, when the
conductivity type of the nitride semiconductor film 320 is n-type,
for example, the p-electrode 30 is provided on the supporting
substrate 27, and the n-electrode is provided on the rear surface
of the nitride semiconductor film 320, whereby the p-electrode 30
and the n-electrode 41 are at opposite positions to each other.
With such a configuration employed, an injected electric current
flows in a top-to-bottom direction of the light-emitting device 10,
i.e., in a vertical direction. Thus, since concentration of the
injected electric current would not occur, it is suitable to a
large-current operation.
[0695] In the case of the light-emitting device 13 according to a
variation shown in FIG. 68(b), not only the conductivity type of
the nitride semiconductor film 320 but also the conductivity type
of the nitride semiconductor layer 110 needs to be n-type.
[0696] The n-electrodes 41 shown in FIG. 68(a) and FIG. 68(b) are
both provided on the light emission surface side. Since it is
provided on the light emission surface side, the n-electrode 41 may
be made of a material which has high transmittance. For example, it
may be a transparent electrode which is made of a material
containing In.sub.2O.sub.3 and SnO.sub.2, called Indium Tin Oxide
(ITO), a transparent electrode containing ZnO, or a metal electrode
which has a small thickness of not more than 100 nm.
[0697] The n-electrode 41 may be provided over the entire light
emission surface as shown in FIG. 68(a). When the conductivity of
the nitride semiconductor film 320 that has n-type conductivity is
high, the n-electrode 41 may be provided over part of the light
emission surface.
Eighth Embodiment
[0698] Hereinafter, the eighth embodiment is described with
reference to the drawings.
[0699] FIG. 69 schematically shows a cross-sectional configuration
of a light-emitting device according to the eighth embodiment.
[0700] The light-emitting device 16 of the present embodiment has
the same light-emitting device configuration as that of the
light-emitting device 10 of the fourth embodiment. However, the
difference resides in that the rear surface of the growth substrate
100 also has a stripe-shaped uneven structure 70.
[0701] Since the stripe-shaped uneven structure 70 is thus provided
in the rear surface of the growth substrate 100, the polarization
degree of emitted light which cannot be sufficiently reduced by the
stripe-shaped gaps 60 provided in the nitride semiconductor film
320 can be further reduced. Accordingly, the light distribution
characteristics and the light extraction effect in the
light-emitting device 16 can be further improved.
[0702] The cross-sectional configuration and planar configuration
of the stripe-shaped uneven structure 70 provided in the rear
surface of the growth substrate 100 may be configurations shown in
FIG. 45, FIG. 46, FIG. 47, FIG. 48, and FIG. 49.
[0703] The range of the in-plane tilt angle in the stripe-shaped
uneven structure 70 may be equal to the above-described range of
the angle .beta. of the stripe-shaped gaps 60. For example,
assuming that the in-plane tilt angle of the stripe-shaped uneven
structure 70 is .beta.' and, in the light-emitting device 16 of
which principal surface is the m-plane, the a-axis direction in the
plane is defined as .beta.'=0.degree., the range of .beta.' may be
not less than 3.degree. and not more than 45.degree..
[0704] The stripe-shaped uneven structure 70 may be formed by
carrying out patterning and etching processes directly on the rear
surface of the growth substrate 100. Note that, however, in the
case where the growth substrate 100 is made of a material which is
difficult to process, e.g., sapphire, a processible thin film for
formation of unevenness is formed, or bonded, in addition to the
growth substrate 100, and the unevenness processing may be carried
out on that thin film. The stripe-shaped uneven structure 70 which
has the in-plane tilt angle .beta.' may be formed by, for example,
depositing SiO.sub.2 or the like on the rear surface of the growth
substrate 100 and carrying out etching or the like on this
deposited film.
[0705] The in-plane tilt angle .beta. in the gaps 60 and the
in-plane tilt angle .beta.' in the stripe-shaped uneven structure
70 formed in the rear surface of the growth substrate 100 do not
necessarily need to be identical with each other.
[0706] Also, the gaps 60 and the stripe-shaped uneven structure 70
do not need to have exactly identical structures. The structures
illustrated in the fourth embodiment may be independently employed
at respective sides so long as the polarization degree can be
reduced.
[0707] The configuration of the present embodiment in which the
stripe-shaped uneven structure 70 is provided in the rear surface
of the growth substrate 100 is also applicable to the fifth to
seventh embodiments.
First Variation of Each of Embodiments
[0708] In each of the embodiments, the gaps 60 which are configured
to be inclined by a predetermined angle .beta. with respect to the
polarization direction, in the in-plane direction of the growing
plane of the semiconductor multilayer structure 20 that is a
constituent of the light-emitting device, may be filled with a
material which has a different refractive index from that of the
nitride semiconductor and which is different from air, such as a
dielectric, for example.
[0709] For example, in each of the fourth to eighth embodiments,
the gaps 60 may be partially, or entirely, filled with a
dielectric, such as SiO.sub.2, SiN, or the like.
[0710] In each of the embodiments, stripe-shaped ridge portions
sandwiched by the gaps 60 are provided on the emission side of
light from the active layer 24, the angle of the extending
direction of the stripe-shaped ridge portions is within the range
of not less than 3.degree. and not more than 45.degree. with
respect to the a-axis direction of the nitride semiconductor.
Further, it is important that the ridge portions sandwiched by the
gaps 60 have the slope surface 53.
[0711] Therefore, in the configuration shown in each embodiment,
the effect of this variation can be obtained even when each of the
gaps 60 is filled with a dielectric, for example.
[0712] Further, in this variation, for example, the interface that
forms the slope surface 53 is the nitride semiconductor and a
dielectric film (e.g., SiO.sub.2 film). Therefore, as shown in FIG.
55(c), the critical angle .theta..sub.c is large as compared with
the case of the gaps 60 (FIG. 53(a)), and therefore, it is an
advantageous configuration in respect of light extraction.
[0713] As described hereinabove, when the stripe-shaped gaps 60 are
filled with a dielectric, for example, and the in-plane tilt angle
.beta. of the stripe-shaped dielectric is not less than 3.degree.
and not more than 45.degree., the polarization degree of light
emitted from the active layer 24 can be reduced, while the light
distribution characteristics can be improved and the light
extraction efficiency can also be improved.
Second Variation of Each of Embodiments
[0714] In each of the embodiments, the gaps 60 may be formed by
carrying out patterning and etching directly on the principal
surface that is the growing plane of the growth substrate 100
instead of forming the gaps 60 in the nitride semiconductor film
320 formed on the growth substrate 100. In this way, the
stripe-shaped ridge portions may be formed by directly performing
unevenness processing on the growth substrate 100. In this case,
the effect of this variation can be obtained so long as the
stripe-shaped gaps 60 which have previously been described in the
fourth embodiment are formed in the surface of the growth substrate
100, and the nitride semiconductor film 320 can be regrown on a
growth substrate 60 which has the gaps 60.
[0715] For example, in the case where the growth substrate 100 is
made of sapphire, as shown in FIG. 55(b), the critical angle
.theta..sub.c is large as compared with the gaps 60 shown in FIG.
55(a) and SiO.sub.2 shown in FIG. 55(c), so that it is advantageous
in respect of the light extraction efficiency.
[0716] As described hereinabove, in a configuration where the
nitride semiconductor film 320 is regrown on the growth substrate
100 which has the gaps 60 formed in the principal surface, the
polarization degree of light emitted from the active layer 24 can
be reduced, while the light distribution characteristics can be
improved and the light extraction efficiency can be improved, so
long as the in-plane tilt angle .beta. of the stripe-shaped ridge
structure is not less than 3.degree. and not more than
45.degree..
Third Variation of Each of Embodiments
[0717] Each of the embodiments is likewise applicable to a device
of which principal surface is a semi-polar plane.
[0718] For example, in the configurations from the fourth
embodiment to the above-described second variation, the same
effects can be obtained even when the principal surface (growing
plane) of the nitride semiconductor layer 110 regrown on the growth
substrate 100, the nitride semiconductor film 320, and the
semiconductor multilayer structure 20 is the (11-22) plane that is
a semi-polar plane.
[0719] It is known that a nitride-based semiconductor
light-emitting device of which principal surface is the (11-22)
plane mainly emits light which is polarized in the m-axis.
[0720] Thus, in the configurations from the fourth embodiment to
the above-described second variation, it is assumed that the
in-plane tilt angle of the stripe structures is .eta., and the
.eta. is defined as the angle between the m-axis in the (11-22)
plane nitride semiconductor and the extending direction of the
stripe structures. When the angle n is in the range of not less
than 3.degree. and not more than 45.degree., the polarization
degree reducing effect, the light distribution characteristic
improving effect and the light extraction efficiency improving
effect can be obtained as in the above-described light-emitting
devices in which the m-plane nitride semiconductor is employed.
Example A
Relationship Between the Angle .beta. and the Polarization Degree
and Light Distribution Characteristics in a Light-Emitting Device
which has Stripe Structures in a Light Emission Surface
Manufacture of Inventive Example 6, Reference Example 1, and
Comparative Example 1
[0721] In this example, to examine the polarization degree reducing
effect and the effects on the light distribution characteristics
and light extraction efficiency with varying in-plane tilt angles
.beta. of the stripe structures, the experimental results of
light-emitting devices in which a stripe-shaped ridge structure is
formed in the rear surface of a growth substrate, which is a simple
structure, are first described.
[0722] FIG. 70 schematically shows a light-emitting device which
was examined in EXAMPLE A. In the light-emitting device of EXAMPLE
A, the growth substrate 100 used was a GaN bulk substrate of which
principal surface was the m-plane. In EXAMPLE A, the stripe
structures were not formed near the interface 50 but in the rear
surface of the growth substrate 100, i.e., the light emission
surface.
[0723] First, on the principal surface of the growth substrate 100,
a nitride semiconductor film 320 which was made of GaN and a
semiconductor multilayer structure 20 on the nitride semiconductor
film 320 were formed by epitaxial growth. For the epitaxial growth,
a metal organic chemical vapor deposition (MOCVD) method was
used.
[0724] Specifically, the nitride semiconductor film 320 was grown
on the principal surface of the growth substrate 100, and then, an
n-type layer 22 which was made of GaN was grown. The dopant used
for the n-type layer 22 was silicon (Si). Here, the nitride
semiconductor film 320 which was the first one that was grown on
the growth substrate 100 was also doped with Si. Silane (SiH.sub.4)
gas was used as the source material of Si. From the viewpoint of
decreasing the electric resistance of the light-emitting device,
the nitride semiconductor film 320 may be doped. Note that,
however, the nitride semiconductor film 320 does not necessarily
need to be doped but may be an undoped layer. The total thickness
of the nitride semiconductor film 320 and the n-type layer 22 was
from about 3 .mu.m to 8 .mu.m. The growth temperature for the
respective layers was 1050.degree. C.
[0725] Then, an active layer 24 was grown on the n-type layer 22.
The active layer 24 has a multi-quantum well structure in which
In.sub.0.13Ga.sub.0.87N well layers and GaN barrier layers were
alternately stacked. The thicknesses of the well layer and the
barrier layer were 3 nm and 12.5 nm, respectively. The number of
periods of the quantum well structure was from 9 to 16. The growth
temperature was from 700.degree. C. to 800.degree. C.
[0726] On the active layer 24, firstly, an undoped GaN layer 26 was
grown. Then, two layers, a p-type Al.sub.0.14Ga.sub.0.86N layer and
a p-type GaN layer, were sequentially grown to form a p-type layer
25. The p-type dopant used was Cp.sub.2Mg (bis cyclopentadienyl
magnesium) which was the source material of magnesium (Mg). The
thickness of the p-type layer 25 was 250 nm. The growth temperature
was 875.degree. C.
[0727] Then, on the p-type layer 25, a p-type GaN contact layer was
grown to such a thickness that the contact resistance of the
p-electrode 30 can be decreased (not shown). The p-type contact
layer had a higher Mg concentration than the Mg concentration of
the p-type layer 25.
[0728] In this way, the semiconductor multilayer structure 20 was
formed on the growth substrate 100 that was an m-plane GaN bulk
substrate by a metal organic chemical vapor deposition method.
[0729] Then, a recessed portion 42 was formed by lithography and
dry etching such that the n-type layer 22 was partially exposed
from the semiconductor multilayer structure 20. Then, on the
surface of the n-type layer 22 which was exposed at the recessed
portion 42, a n-electrode 40 was formed of 100 nm thick aluminum
(Al). Then, on the p-type GaN contact layer, a p-electrode 30 was
formed of 400 nm thick silver (Ag). Note that the order of
formation of the n-electrode 40 and the p-electrode 30 is not
particularly limited. Thereafter, the growth substrate 100 which
was made of bulk GaN was abraded such that the thickness of the
growth substrate 100 was about 100 .mu.m. In this way, the
light-emitting device was manufactured.
[0730] Then, a method for forming the stripe-shaped uneven
structure 70 is described.
[0731] Firstly, a SiO.sub.2 film was formed as the hard mask
material on the rear surface of the growth substrate 100. The
SiO.sub.2 film was formed by, for example, a plasma CVD (plasma
chemical vapor deposition) method. Then, a resist film was applied
over the SiO.sub.2 film, and the resist film was patterned using an
electron beam lithography apparatus so as to form stripe structures
with periods of 300 nm. Thereafter, the patterned resist film was
used as the mask in order to etch the SiO.sub.2 film by dry etching
with the use of a CF.sub.4 gas and a O.sub.2 gas, whereby a hard
mask was formed from the SiO.sub.2 film.
[0732] Then, dry etching with a chlorine (Cl.sub.2) gas was carried
out on the rear surface of the growth substrate 100 with the hard
mask formed thereon such that a stripe-shaped uneven structure 70
was formed in the rear surface of the growth substrate 100. Here,
the shape of a cross section of the uneven structure 70 taken along
a direction perpendicular to the extending direction of the stripes
of the uneven structure 70 is trapezoidal. Then, the SiO.sub.2 film
which was used as the hard mask was removed by wet etching.
[0733] By such a fabrication method, the stripe-shaped uneven
structure 70, in which the period was 300 nm and the height
difference of the recessed/raised portions was 300 nm, was formed
in the rear surface of the growth substrate 100. The angle between
the slope surface 53 and the surface of the growth substrate 100,
i.e., the m-plane, was about 60.degree..
[0734] In this example, light-emitting devices which had the
stripe-shaped uneven structures 70 with varying in-plane tilt
angles .beta. were manufactured in the same way. In this example,
three light-emitting devices were manufactured, where the
configuration of .beta.=0.degree. is referred to as Reference
Example 1, the configuration of .beta.=45.degree. is referred to as
Inventive Example 6, and the configuration of .beta.=90.degree. is
referred to as Comparative Example 1.
Manufacture of Inventive Example 7, Reference Example 2, and
Comparative Example 2
[0735] The stripe-shaped uneven structure 70 was formed in the rear
surface of the growth substrate 100 that is an m-plane GaN bulk
substrate by the same fabrication method as that employed for
Inventive Example 6, Reference Example 1, and Comparative Example
1. The same constituent material of the hard mask, SiO.sub.2, was
used.
[0736] The period of the stripe structures was 8 .mu.m. The height
difference of the uneven structure was 4 .mu.m. The cross-sectional
shape was a near trapezoidal shape. The angle between the slope
surface 53 and the substrate surface, i.e., the m-plane, was about
60.degree..
[0737] By the same manufacturing method as that employed for
Inventive Example 6, the stripe-shaped uneven structure 70 was
formed in the rear surface of the growth substrate 100, and
light-emitting devices were manufactured with angles .beta. being
0.degree. (Reference Example 2), 45.degree. (Inventive Example 7),
and 90.degree. (Comparative Example 2).
Manufacture of Inventive Example 8, Reference Example 3, and
Comparative Example 3
[0738] The semiconductor multilayer structure 20 was fabricated on
the growth substrate 100 that is an m-plane GaN bulk substrate by
the same fabrication method as that employed for Inventive Example
6, Reference Example 1, and Comparative Example 1. Thereafter,
light-emitting devices which had the stripe-shaped uneven structure
70 in the rear surface of the growth substrate 100 with varying
angles .beta., angle .beta.=0.degree., 5.degree., 30.degree.,
45.degree., and 90.degree., were manufactured.
[0739] In Inventive Example 8 and other examples, the stripe-shaped
uneven structure 70 was fabricated by a different method from that
employed for Inventive Example 6, Reference Example 1, and
Comparative Example 1. In this example, the resist film was used as
the mask, instead of using the hard mask of SiO.sub.2. That is, dry
etching was carried out using the patterned resist film as the mask
so as to form the stripe-shaped uneven structure 70 in the rear
surface of the growth substrate 100. A residue of the resist film
after the etching was removed using an etchant that was a mixture
solution of a sulfuric acid and a hydrogen peroxide solution.
[0740] In Inventive Example 8, Reference Example 3, and Comparative
Example 3, the period of the stripe-shaped uneven structure 70 was
8 .mu.m, and the height difference of the recessed/raised portions
was 2.5 .mu.m. The dry etching was carried out such that the
cross-sectional shape of the stripe-shaped uneven structure 70 is
triangular or elliptical. That is, as shown in FIG. 46(a) and FIG.
46(b), etching was carried out on the growth substrate 100 such
that the area of the bottom surface of the stripe-shaped uneven
structure 70 was decreased, whereby a stripe-shaped uneven region
70 was formed.
[0741] Here, the configuration where the extending direction of the
stripe-shaped uneven structure 70 was identical with the a-axis,
i.e., .beta. was 0.degree., is referred to as Reference Example 3.
The configuration where the angles .beta. formed with respect to
the a-axis were 5.degree., 30.degree., and 45.degree. is referred
to as Inventive Example 8. The configuration where the angle .beta.
formed with respect to the a-axis was 90.degree. is referred to as
Comparative Example 3.
Manufacture of Comparative Example 4
[0742] On the growth substrate 100 of a light-emitting device which
was manufactured by the same manufacturing method as that employed
for Inventive Example 6, a light-emitting device was manufactured
which was different only in that the stripe-shaped uneven structure
70 was not provided. This device is referred to as Comparative
Example 4. The configuration of Comparative Example 4 was such
that, in the configuration of FIG. 70, the uneven structure 70 is
not provided in the rear surface of the growth substrate 100. Thus,
it has a flat light emission surface.
[0743] (Method for Measuring the Polarization Degree)
[0744] FIG. 71 schematically shows a measurement system for the
polarization degree. A light-emitting device 1 of a nitride-based
semiconductor, which is a measurement object, is powered by a power
supply 6 to emit light. The emission of the light-emitting device 1
is checked using a stereoscopic microscope 3. The stereoscopic
microscope 3 has two ports. A silicon photodetector 4 is attached
to one of the ports, and a CCD camera 5 is attached to the other
port. A polarizing plate 2 is provided between the light-emitting
device 1 and the stereoscopic microscope 3. While the polarizing
plate 2 is rotated, the maximum and the minimum of the emission
intensity are measured using the silicon photodetector 4.
[0745] (Method for Measuring the Light Distribution
Characteristics)
[0746] For the manufactured light-emitting device, OL700-30 LED
GONIOMETER manufactured by Optronic Laboratories, Inc. was used.
The light distribution characteristic in the a-axis direction and
the light distribution characteristic in the c-axis direction were
measured based on condition A (the distance between the tip of the
light-emitting device and the light receiving section 7 is 316 mm),
which is described in CIE127 published by the International
Commission on Illumination (CIE).
[0747] FIG. 72(a) and FIG. 72(b) schematically show the measurement
system for the light distribution characteristics.
[0748] The light distribution characteristic in the a-axis
direction shown in FIG. 72(a) is a value obtained by measuring the
luminous intensity while a semiconductor light-emitting chip 700 is
rotated around the c-axis of the semiconductor light-emitting chip
700, with the angle formed between the m-axis direction [1-100],
which is the normal direction of the m-plane of the active layer of
the semiconductor light-emitting chip 700, and the measurement line
8 extending between the semiconductor light-emitting chip 700 and
the measuring device 7 being selected as the measurement angle.
[0749] The light distribution characteristic in the c-axis
direction shown in FIG. 72(b) is a value obtained by measuring the
luminous intensity while the semiconductor light-emitting chip 700
is rotated around the a-axis of the semiconductor light-emitting
chip 700, with the angle formed between the m-axis direction
[1-100], which is the normal direction of the m-plane of the active
layer of the semiconductor light-emitting chip 700, and the
measurement line 8 extending between the semiconductor
light-emitting chip 700 and the measuring device 7 being selected
as the measurement angle.
[0750] Here, the evaluation of the light distribution
characteristics was made with values obtained by normalizing the
luminous intensities in the a-axis direction and the c-axis
direction at the same angle with respect to the normal direction
using the luminous intensity in the normal direction [1-100] of the
m-plane that was the principal surface, i.e., the luminous
intensity at 0.degree., on the assumption that the luminous
intensity in the m-axis direction [1-100] is 1.
[0751] The measured angle range was from -90.degree. to +90.degree.
with the m-axis direction being at the median, i.e., 0.degree.. The
measurement was carried out in each of the a-axis direction and the
c-axis direction.
[0752] The measurement method described hereinabove was used to
evaluate the polarization degree and the light distribution
characteristics of the light-emitting device.
[0753] Firstly, the results of the light distribution
characteristics of a light-emitting device of Comparative Example 4
which has a flat emission surface, and which does not have stripe
structures, such as the uneven structure 70 and the gaps 60, on the
light emission side, are shown in FIG. 73.
[0754] It is understood form FIG. 73 that the light distribution
characteristics are apparently asymmetry with respect to the a-axis
direction (open circles) and the c-axis direction (solid circles).
The distribution of light emitted in the c-axis direction was such
that the intensity was distributed over a wider angle range than
the distribution of light in the a-axis direction. As previously
described, this is attributed to a fact that the emission from a
light-emitting device grown on an m-plane nitride semiconductor is
the a-axis polarized light. As illustrated in FIG. 50(b), the
propagation vector of the polarized light is mainly present in the
mc-plane, and the intensity of light which is present in the
ma-plane relatively decreases.
[0755] Thus, it was confirmed that, in a normal nitride-based
semiconductor light-emitting device of which principal surface is
the m-plane and in which no processing is performed on the light
emission surface, the propagation vector is mainly present in the
mc-plane, and the distribution of the emission intensity is wider
in the c-axis direction than in the a-axis direction, resulting in
an asymmetry distribution.
[0756] FIG. 74(a) and FIG. 74(b) show the evaluation results of the
light distribution characteristics of the light-emitting devices of
Inventive Example 6, Reference Example 1, and Comparative Example 1
in which the in-plane tilt angles .beta. were 0.degree.,
45.degree., and 90.degree., respectively.
[0757] FIG. 74(a) and FIG. 74(b) show the evaluation results of the
light distribution characteristics in the a-axis direction and the
c-axis direction, respectively. Here, as for the light distribution
characteristic in the a-axis direction, assuming that, relative to
the c-axis of the light-emitting device, the angle was 0.degree.
when the m-axis and the optical axis were parallel to each other as
illustrated in FIG. 72(a), the light intensity distribution was
evaluated with the angle increasing in a direction inclined from
the m-axis to the a-axis. As for the evaluation of the light
distribution characteristic in the c-axis direction, the
measurement was carried out as illustrated in FIG. 72(b).
[0758] As seen from the results shown in FIG. 74(a), the light
distribution characteristic in the a-axis direction does not depend
on the in-plane tilt angle .beta.. On the other hand, as seen from
the results shown in FIG. 74(b), the light distribution
characteristic in the c-axis direction which was obtained when the
in-plane tilt angle .beta. was not more than 45.degree. is
apparently different from the result of the light distribution
characteristic in the c-axis direction of Comparative Example 4
that did not have the stripe structures, which is shown in FIG. 73.
That is, it is appreciated that, when the in-plane tilt angle
.beta. of the stripe structures was not more than 45.degree., the
light distribution characteristics in the a-axis direction and the
c-axis direction were close to each other so that the asymmetry was
improved.
[0759] Next, the relationship between the angle .beta. and the
polarization degree reducing effect was evaluated using Inventive
Example 8, Reference Example 3, and Comparative Example 3. FIG. 75
shows the relationship between the in-plane tilt angle .beta. of
the stripe structures and the specific polarization degree. Here,
the specific polarization degree refers to a value normalized with
the polarization degree that is obtained when .beta.=0.degree. at
which the polarization state of light emitted from the active layer
24 can be best maintained (the polarization degree of Reference
Example 3) as previously shown in Formula (2).
[0760] As seen from FIG. 75, the in-plane tilt angle .beta. was
slightly greater than 0.degree., the polarization degree was
abruptly reduced. When .beta.=5.degree., the specific polarization
degree was not more than 0.4. When .beta.=30.degree., the specific
polarization degree further decreased to 0.25 or smaller. The
specific polarization degree was the minimum near .beta.=45.degree.
and was somewhat greater at .beta.=90.degree..
[0761] Thus, the polarization degree of light emitted from the
active layer 24 strongly depends on the in-plane tilt angle .beta.
of the stripe-shaped uneven structure 70. The polarization degree
reducing effect greatly varies in the range of 0.degree. to
5.degree.. In the angle range of not less than 5.degree., the
polarization degree reducing effect varies relatively moderately.
Near .beta.=45.degree., the specific polarization degree reached
the minimum value.
[0762] In the structure of the angle .beta.=0.degree., it is
considered that, if it is less than about .+-.3.degree., the
polarization degree of light emitted from the active layer 24 can
be maintained. Therefore, it is considered that the in-plane tilt
angle .beta. at which the effect of reducing the polarization
degree of the light can be achieved may be not less than
3.degree..
[0763] As described hereinabove based on the experimental results,
it was found that, when the in-plane tilt angle .beta. of the
stripe-shaped uneven structure 70 is set to an angle greater than
0.degree., the effect of reducing the polarization degree of light
can be obtained.
[0764] However, the degree of reduction of the polarization degree
depends on the shape of the stripe-shaped uneven structure 70 to
some extent. For example, in the configuration of the uneven
structure 70 of Inventive Example 8, the raised portions 51 have
substantially no bottom surface, and their wall surfaces are formed
by the slope surface 53 or curved surfaces as shown in FIG. 46(a)
and FIG. 46(b).
[0765] On the other hand, when the cross-sectional shape of the
uneven structure 70 is trapezoidal, the uneven structure 70 has a
bottom surface, and the polarization degree reducing effect cannot
be obtained in that region. That is, the polarization degree
reducing effect varies to some extent depending on the shape of the
uneven structure 70.
[0766] However, it was confirmed from comparative examination of
Reference Example 2, Inventive Example 7, and Comparative Example 2
that, so long as the stripe-shaped uneven structure 70 or the gaps
60 are present on the light emission surface side of the
light-emitting device and they include the slope surface 53 in part
thereof, the in-plane tilt angle .beta. dependence of the
polarization characteristics would not substantially vary. The same
effects can be obtained.
[0767] Next, the effect of light extraction was evaluated. FIG. 76
shows the relationship between the light extraction efficiency and
the in-plane tilt angle .beta. in light-emitting devices of
Inventive Example 6, Reference Example 1, and Comparative Example
1. Here, the light extraction efficiency was evaluated as the
specific light extraction efficiency. The specific light extraction
efficiency refers to a value of the external quantum efficiency of
each of Inventive Example 6, Reference Example 1, and Comparative
Example 1 which is normalized with the external quantum efficiency
of Comparative Example 4 that does not have the stripe structures
but a flat light emission surface. The external quantum efficiency
can be represented by Formula (6) shown below using the internal
quantum efficiency and the light extraction efficiency:
External Quantum Efficiency=Internal Quantum Efficiency.times.Light
Extraction Efficiency Formula (6)
[0768] The semiconductor multilayer structures 20 of Inventive
Example 6, Reference Example 1, Comparative Example 1, and
Comparative Example 4 have identical configurations, and therefore,
it can be assumed that they have identical internal quantum
efficiencies. Under this assumption, the light extraction
efficiency was evaluated by comparison of the external quantum
efficiencies.
[0769] First, as seen from FIG. 76, when the stripe-shaped uneven
structure 70 is provided on the light emission surface side, the
specific light extraction efficiency exhibits a large value which
is not less than 1.1. The most important point is that, in a
light-emitting device in which the in-plane tilt angle .beta. is
not less than 0.degree. and not more than 45.degree., the specific
light extraction efficiency exhibits a further increased value,
which is not less than 1.2. This is because, for example, as
illustrated in FIG. 50, the polarization direction of light emitted
from the active layer 24 that is made of an m-plane nitride
semiconductor is the a-axis direction, and its propagation vector
is mainly present in the mc-plane. That is, the light extraction
effect achieved by the stripe structures is attributed to the fact
that the extending direction of the stripe structures is preferably
closer to the a-axis direction (.beta.=0.degree.) than the c-axis
direction (.beta.=90.degree.).
[0770] Thus, it is because, when the stripe structures are formed
in such a way, light which is mainly propagating in the c-axis
direction is more susceptible to the unevenness of the stripe
structures.
[0771] Furthermore, it was confirmed from those experimental
results that, when the angle .beta. is in the range of not less
than 0.degree. and not more than 45.degree., the light extraction
effect at substantially the same level can be obtained.
[0772] As described hereinabove, it was found that, in a
nitride-based semiconductor light-emitting device of which
principal surface is the m-plane, when the stripe-shaped uneven
structure 70 is provided on the light emission surface side and its
in-plane tilt angle .beta. is not less than 3.degree. and not more
than 45.degree., the polarization degree of light emitted from the
active layer 24 can be reduced, while improvement of the light
distribution characteristics and the light extraction efficiency
improving effect can be obtained.
Example B
Characteristics of a Light-Emitting Device in which Gaps are
Provided at the Interface Between the Growth Substrate and the
Nitride Semiconductor Film
[0773] In EXAMPLE A, it was explained based on the experimental
results that an m-plane nitride-based semiconductor light-emitting
device which has a stripe-shaped uneven structure with a certain
in-plane tilt angle .beta. on the light emission surface side can
achieve reduction of the polarization degree, improvement of the
light distribution characteristics, and improvement of the light
extraction efficiency.
[0774] In EXAMPLE B, it was experimentally confirmed that, in a
semiconductor light-emitting device in which gaps 60 are provided
near the interface between the growth substrate and the nitride
semiconductor film, which constitute a major structure according to
the fourth embodiment, the same effects as those described in
EXAMPLE A are obtained. Hereinafter, details of this feature are
described.
[0775] (Fabrication of the Heterogeneous Nitride Semiconductor
Substrate 600)
[0776] First, a method for fabricating the heterogeneous nitride
semiconductor substrate 600 is described.
[0777] In this example, a heterogeneous nitride semiconductor
substrate 600 of which principal surface was the m-plane was
fabricated based on a fabrication method in which the nitride
semiconductor film 320 is regrown after removal of the mask 121
that is made of a resist which is used in formation of the nitride
semiconductor layer 110 in the shape of raised portions as
illustrated in FIG. 57.
[0778] 1. Growth of a Nitride Semiconductor Layer on a Growth
Substrate which was Made of m-Plane Sapphire
[0779] In EXAMPLE B, an m-plane sapphire substrate was used as the
growth substrate 100 shown in FIG. 57(a). The thickness of the
m-plane sapphire substrate was 430 .mu.m. The diameter of the
m-plane sapphire substrate was about 5.1 cm (=2 inches). The angle
between the normal line of the principal surface of the m-plane
sapphire substrate and the normal line of the m-plane was
0.degree..+-.0.1.degree..
[0780] Then, a nitride semiconductor layer 110 of which growing
plane was the m-plane was grown on the growth substrate 100 by a
metal organic chemical vapor deposition method. It is commonly
believed that growth of a low temperature buffer layer is required
for growth of an m-plane nitride semiconductor on a substrate that
is made of m-plane sapphire. In this example, an AlN layer was used
as the low temperature buffer layer.
[0781] After the growth of the low temperature buffer layer, the
substrate temperature was increased to a temperature in the range
of 900.degree. C. to 1100.degree. C. in order to grow the nitride
semiconductor layer 110 of which principal surface was the m-plane.
The thickness of the nitride semiconductor layer 110 was from about
1 .mu.m to 3 .mu.m.
[0782] 2. Fabrication of the Unevenly-Processed Substrate 510
[0783] Then, the mask 121 was formed of a resist on the nitride
semiconductor layer 110 by a known lithography method as shown in
FIG. 57(a). For the mask 121, a typical line & space (L&S)
pattern, i.e., a pattern of thin and elongated stripes, was used.
In the L&S pattern used in this example, the width of the line
portions of the mask 121, L, was 5 .mu.m, and the width of the
space portions, S, was 10 .mu.m. The thickness of the mask 121
after the photolithography step was finished was about 2 .mu.m to 3
.mu.m.
[0784] By pattern formation of the mask 121, the in-plane tilt
angle .beta., which is the extending direction of the stripe-shaped
gaps 60, can be appropriately determined.
[0785] Then, the nitride semiconductor layer 110 that serves as a
seed crystal was etched through the mask 121 using an inductively
coupled plasma etching (ICP etching) apparatus such that the growth
substrate 100 was selectively exposed through the nitride
semiconductor layer 110, whereby the stripe-shaped nitride
semiconductor layers 110a that form the raised portions and the
recessed portions 210 provided therebetween were formed. In forming
the recessed portions 210 by etching, the upper part of the growth
substrate 100 was also etched away such that part of the nitride
semiconductor layer 110 would not remain. That is, the etching was
carried out till the GaN layer exposed through the space portions
of the mask 121 was removed and the growth substrate 100 was
exposed, whereby the recessed portions 210 were formed. Thereafter,
the mask 121 remaining on the stripe-shaped nitride semiconductor
layers 110a was removed, whereby the unevenly-processed substrate
510 shown in FIG. 57(b) was obtained.
[0786] FIG. 77(a) and FIG. 77(b) shows an example of the
unevenly-processed substrate 510 of this example. FIG. 77(a) and
FIG. 77(b) are scanning electron microscopic images (SEM images)
that were obtained after the stripe-shaped nitride semiconductor
layers 110a and the recessed portions 210 in which the surface of
sapphire was exposed by etching were formed using a stripe-shaped
L&S pattern mask. Here, a cross-sectional image (left) which is
perpendicular to the extending direction of the stripe-shaped
nitride semiconductor layers 110a and a perspective image (right)
are shown. The cross-sectional shape of the stripe-shaped nitride
semiconductor layers 110a can be controlled by appropriately
selecting the formation conditions for the mask 121 and the etching
conditions.
[0787] As shown in FIG. 77, the cross-sectional shape of the
stripe-shaped nitride semiconductor layers 110a can be controlled
to be a trapezoidal cross-sectional shape such as shown in FIG.
77(a) or a triangular cross-sectional shape such as shown in FIG.
77(b). In this example, a GaN layer with a trapezoidal
cross-sectional shape was used as the stripe-shaped nitride
semiconductor layers 110a. Further, as illustrated in FIG. 58(a),
in this example, the etching was carried out such that the sapphire
was exposed at the wall surfaces in the lower part of the recessed
portions 210. In this case, the depth of the recessed portions 210,
i.e., the height of the wall surfaces 220 of the growth substrate
100 in the recessed portions 210, was about 250 nm.
[0788] In this example, the thickness of the stripe-shaped nitride
semiconductor layers 110a was from about 1 .mu.m to 3 .mu.m,
although the thickness of the nitride semiconductor layers 110a may
be appropriately adjusted. To realize epitaxial lateral overgrowth
of the nitride semiconductor film 320 of which principal surface
was the m-plane according to this example, it is only necessary to
form the stripe-shaped nitride semiconductor layers 110a that are
the seed crystal of which principal surface is the m-plane and from
which regrowth of the nitride semiconductor is to start, and the
recessed portions 210 in which the growth substrate 100 of the
m-plane sapphire is exposed at the lower part.
[0789] 3. Fabrication of the Heterogeneous Nitride Semiconductor
Substrate 600 (Regrowth of a Nitride Semiconductor Film on an
Unevenly-Processed Substrate)
[0790] Then, a nitride semiconductor film 320 was regrown on the
unevenly-processed substrate 510.
[0791] The unevenly-processed substrate 510 was again carried into
the metal organic chemical vapor deposition apparatus. Then, the
growth temperature was set to a temperature in the range of about
900.degree. C. to 1000.degree. C. for carrying out regrowth.
[0792] The important point for growth of the nitride semiconductor
film 320 is that regrowth of the nitride semiconductor film 320
starting from the stripe-shaped nitride semiconductor layers 110a
that form the raised portions is enhanced in lateral directions. In
this example, the growth was carried out under the conditions that
the value of the V/III ratio, which is the ratio of the source
materials for the Group V element and the Group III element, was
about 160, the growth pressure was about 13.3 kPa, and the growth
speed was about 4 .mu.m/h. Under these conditions, the nitride
semiconductor film 320 was formed by regrowth so as to have a
thickness of 4 .mu.m to 10 .mu.m, whereby a heterogeneous nitride
semiconductor substrate 600 of which principal surface was the
m-plane was obtained.
[0793] In that process, the growth conditions for enhancing the
growth speed of the lateral growth and the width L and width S of
the raised portions of the unevenly-processed substrate 510 are
appropriately selected such that the recessed portions 210 can be
covered with the nitride semiconductor film 320. Further, the
cross-sectional shape of the gaps 60 formed above the recessed
portions 210 can also be controlled.
[0794] FIG. 78(a) and FIG. 78(b) show some fabricated examples of
the heterogeneous nitride semiconductor substrate 600 which were
obtained as described above. It can be seen that a plurality of
gaps 60 were provided in an interface region between the growth
substrate 100 and the nitride semiconductor film 320 as explained
in the section of the fourth embodiment. It can be seen that, by
adjusting the fabrication conditions for the heterogeneous nitride
semiconductor substrate 600, the configuration and cross-sectional
shape of the stripe-shaped gaps 60 can be controlled. For example,
FIG. 78(a) and FIG. 78(b) are SEM images on the same scale. The
height of the gaps 60 shown in FIG. 78(b) is higher than the height
of the gaps 60 shown in FIG. 78(a).
[0795] The heterogeneous nitride semiconductor substrate 600
fabricated by the above-described fabrication method, which
includes the gaps 60 in the interface region, can obtain the effect
of greatly improving the crystallinity in addition to the effect of
reducing the polarization degree of light, which is one of the
major effects of this example.
[0796] Hereinafter, experimental results of that example are
described.
[0797] Table 2 shows the results of the half-value width of the
(1-100) plane X-ray .omega. rocking curve (XRC) in a regrown
m-plane GaN film of the heterogeneous nitride semiconductor
substrate 600 that was obtained in this example. Here, the X-ray
was incident so as to be parallel to the a-axis direction and
c-axis direction of the GaN. For the sake of comparison, the values
of the XRC full width at half maximum in an m-plane GaN layer
directly grown on an m-plane sapphire substrate are also shown in
Table 2.
[0798] The half-value width of the m-plane GaN layer directly grown
on the m-plane sapphire substrate exhibited a high value which was
not less than 1000 seconds. Further, when the X-ray was incident in
the a-axis direction and the c-axis direction of the GaN layer, the
XRC full width at half maximum for the case where the X-ray was
incident in the c-axis direction of the GaN layer was further
increased by a factor of about two. This is because information of
stacking faults was reflected in the XRC full width at half maximum
which was obtained when the X-ray was incident in the c-axis
direction.
[0799] That is, it is understood that, in measurement of the XRC
full width at half maximum in the m-plane GaN layer directly grown
on the m-plane sapphire substrate, when asymmetry is seen in the
measurement results of the X-ray incidence in the a-axis direction
and the c-axis direction, the crystal includes many stacking faults
in addition to usual dislocations.
TABLE-US-00002 TABLE 2 GaN: a-axis GaN: c-axis direction incidence
direction incidence Directly grown m-plane GaN 1326 seconds 2325
seconds Regrown m-plane GaN 548 seconds 746 seconds
[0800] On the other hand, when the m-plane GaN film was regrown
after the unevenly-processed substrate 510 was formed, the XRC full
width at half maximums in the a-axis direction and the c-axis
direction decreased to 548 seconds and 746 seconds,
respectively.
[0801] When the X-ray was incident in the a-axis direction in the
regrown m-plane GaN layer, it decreased to 1/2 or a smaller value.
This means that the dislocation density was greatly reduced by the
regrowth. Further, as compared with the results of the
directly-grown m-plane GaN layer, the values of the half-value
width which were obtained when the X-ray was incident in the a-axis
direction and the c-axis direction in the regrown m-plane GaN layer
were close to each other, so that the degree of symmetry was
improved. This demonstrates that, in the regrown m-plane GaN layer
of this example, not only the dislocation density but also the
stacking fault density decreased.
[0802] As described hereinabove, it is understood that, when the
stripe-shaped gaps 60 of this example are provided near the
interface between the growth substrate 100 and the nitride
semiconductor film 320, the effect of reducing the polarization
degree of light and the effect of reducing the density of
dislocations and defects in the nitride semiconductor film 320
itself, i.e., the crystallinity improving effect, can be
obtained.
[0803] Such a crystallinity improving effect was also confirmed by
a transmission electron microscope (TEM).
[0804] FIG. 79(a) and FIG. 79(b) show cross-sectional TEM images of
the heterogeneous nitride semiconductor substrate 600. FIG. 79(a)
corresponds to a region around one of the stripe-shaped nitride
semiconductor layers 110a of FIG. 57(d). FIG. 79(b) shows an
enlarged image of a portion around the gap 60 shown in FIG.
79(a).
[0805] First, as seen from FIG. 79(a), a portion of the nitride
semiconductor film 320 which was grown on the stripe-shaped nitride
semiconductor layer (raised portion) 110a contains dislocations
(black line-like portions in the drawing) at a high density as
compared with portions grown on the recessed portions 210.
[0806] Examining the dislocation density, the GaN layer on the
raised portion 110a had a dislocation density on the order of
10.sup.10 cm.sup.-2, while the GaN layer on the recessed portions
210 had a dislocation density of not more than 10.sup.9 cm.sup.-2.
It was found that the dislocation densities were different by one
or more orders of magnitude.
[0807] As in the case of the results of Table 2, it was also
confirmed from the TEM images that, by configuring the
heterogeneous nitride semiconductor substrate 600 so as to include
the stripe-shaped gaps 60, the density of dislocations and defects
is reduced and the crystallinity is improved.
[0808] The region of high dislocation/defect density, which is seen
in FIG. 79, is identical with the region which has previously been
illustrated in FIG. 60.
[0809] Thus, in this example, particularly when a light-emitting
device is manufactured on the heterogeneous nitride semiconductor
substrate 600 that has the gaps 60, high dislocation density/high
defect density regions are formed above the raised portions
110a.
[0810] FIG. 79(b) shows an enlarged image of the cross-sectional
shape of the gap 60. It can be confirmed that, by regrowing the
nitride semiconductor film 320, the connecting portion 410 and the
gap 60 having the slope surfaces 53 were formed.
[0811] By the above-described fabrication method, the heterogeneous
nitride semiconductor substrate 600 that has the stripe-shaped gaps
60 near the region of the interface 50 between the growth substrate
100 and the nitride semiconductor film 320 can be prepared.
[0812] 4. Manufacture of the Light-Emitting Device 10 on the
Heterogeneous Nitride Semiconductor Substrate 600
[0813] Next, a semiconductor multilayer structure 20 was formed on
the heterogeneous nitride semiconductor substrate 600 that had the
stripe-shaped gaps 60, which was fabricated by the above-described
method.
[0814] The in-plane tilt angle .beta. of the gaps 60 in the
heterogeneous nitride semiconductor substrate 600 of this example
was 5.degree..
[0815] As illustrated in FIG. 75 of EXAMPLE A, it was confirmed
that, in a light-emitting device in which the stripe-shaped uneven
structure 70 is formed in the rear surface of the growth substrate
100, the polarization degree reducing effect can be obtained so
long as the in-plane tilt angle .beta. is 5.degree..
Manufacture of Inventive Example 9
[0816] A light-emitting device 10 was formed on the heterogeneous
nitride semiconductor substrate 600 including the gaps 60 with the
in-plane tilt angle .beta. of 5.degree. at the interface 50 between
the growth substrate 100 and the nitride semiconductor film
320.
[0817] The configuration and manufacturing method of the
light-emitting device 10 are equivalent to those of Inventive
Example 6 of EXAMPLE A.
Manufacture of Comparative Example 5
[0818] A light-emitting device was manufactured by an equivalent
method to that of Inventive Example 6, which was however only
different from the light-emitting device of Inventive Example 6 in
that the stripe-shaped uneven structure 70 was not provided. This
light-emitting device is referred to as Comparative Example 5.
[0819] The light emission surface of the light-emitting device of
Comparative Example 5 did not have the stripe-shaped uneven
structure 70 illustrated in FIG. 70 but had a flat surface. In
terms of the configuration, it was basically the same as the
above-described Comparative Example 4. However, for the sake of
strict comparison with the characteristics of the light-emitting
device 10 manufactured on the heterogeneous nitride semiconductor
substrate 600 of Inventive Example 9, the multilayer structure of
the light-emitting device of Comparative Example 5 was grown
concurrently with the light-emitting device 10 of Inventive Example
9, by a metal organic chemical vapor deposition method under the
same growth conditions.
[0820] (Comparison of the Light Distribution Characteristics and
the Polarization Degree)
[0821] The characteristics of the light-emitting devices of
Inventive Example 9 and Comparative Example 5 were compared and
evaluated. FIG. 80(a) and FIG. 80(b) show the evaluation results of
the light distribution characteristics in the a-axis direction and
the c-axis direction of the light-emitting devices. FIG. 80(a)
shows the results of Comparative Example 5. FIG. 80(b) shows the
results of Inventive Example 9 which had the gaps 60.
[0822] Comparative Example 5 had basically the same configuration
as that of Comparative Example 4 described in EXAMPLE A. Therefore,
the light distribution characteristic in the c-axis direction
exhibited a wider distribution than in the a-axis direction, so
that it was asymmetry, such as illustrate in FIG. 73.
[0823] On the other hand, it can be seen that, in the case of
Inventive Example 9 where the heterogeneous nitride semiconductor
substrate 600 had the gaps 60, the degree of asymmetry was
apparently reduced as compared with Comparative Example 5, and the
difference between the light distribution characteristic in the
a-axis direction and the light distribution characteristic in the
c-axis direction was small.
[0824] Next, comparison of the polarization degree between
Comparative Example 5 and Inventive Example 9 was made. The
polarization degree is a value which is determined by the
definition of Formula (1) shown above. The measurement method used
was the method which has previously been described in EXAMPLE A.
The results of the comparison of the polarization degree are shown
in Table 3.
TABLE-US-00003 TABLE 3 Sample 1 Sample 2 Average Comparative
Example 5 0.642 0.665 0.654 Inventive Example 9 0.294 0.313
0.304
[0825] In this example, a plurality of light-emitting device chips
were manufactured, and the polarization degree under a
current-injected condition was evaluated. Table 3 shows only the
data of two samples for each example.
[0826] The polarization degree of the light-emitting device of
Inventive Example 9 which had the gaps 60 near the interface 50
exhibited apparently small values as compared with the results of
the light-emitting device of Comparative Example 5 which had a flat
light emission surface. The value of Inventive Example 9 normalized
with the value of Comparative Example 5 was 0.46.
[0827] This value is compared with FIG. 75 of EXAMPLE A. Note that,
however, the results of FIG. 75 of EXAMPLE A are normalized with
the polarization degree which was obtained when the in-plane tilt
angle .beta. of the stripe structures was 0.degree. Therefore,
strictly speaking, it is different from the value of this EXAMPLE
B. On the other hand, Comparative Example 5 is a light-emitting
device which did not have the stripe structures but a flat light
emission surface.
[0828] However, the difference in polarization degree between a
case where the in-plane tilt angle .beta. of the stripe structures
was zero (.beta.=0.degree.) and a case where no stripe structures
were provided is very small. Therefore, although both sides had
some errors, the comparison was made with consideration for this
point.
[0829] Comparing the results of Table 3 with the results of FIG.
75, the value of the specific polarization degree which was
obtained in this example, 0.46, is generally close to the value of
the specific polarization degree at the in-plane tilt angle
.beta.=5.degree. of EXAMPLE A, 0.37. It was found from this
comparison that the same effects as those obtained in EXAMPLE A are
obtained even in the light-emitting device 10 that has the gaps 60
near the interface 50.
[0830] The small difference in specific polarization degree between
the results of EXAMPLE B and the results of EXAMPLE A may be
attributed to the above-described errors, while on the other hand,
the difference in shape of the stripe structures may be one of the
causes.
[0831] As described hereinabove, it was substantiated by the
results of EXAMPLE A and EXAMPLE B that, by forming a plurality of
gaps 60 near the interface 50 between the growth substrate 100 and
the nitride semiconductor film 320, the polarization degree of
light can be reduced, and improvement of the light distribution
characteristics and improvement of the light extraction efficiency
can be achieved.
[0832] A shape of the stripe structures (e.g., gaps) which can
achieve the effects of the embodiment may be such that the range of
the angle .beta. is not less than 3.degree. and not more than
45.degree., where .beta. is the angle between the extending
direction of the stripe structures and the a-axis direction of the
nitride semiconductor film 320, i.e., the electric field vector
direction of polarized light, and a direction parallel to the
a-axis direction is .beta.=0.degree..
Example C
Fabrication Method of the Heterogeneous Nitride Semiconductor
Substrate 601 According to the Sixth Embodiment and the
Characteristics of a Light-Emitting Device
[0833] In this EXAMPLE C, the experimental results of the
light-emitting device 13 that had the gaps 60 described in the
section of the sixth embodiment are shown.
[0834] (Fabrication of the Heterogeneous Nitride Semiconductor
Substrate 601)
[0835] First, a method for fabricating the heterogeneous nitride
semiconductor substrate 601 is described.
[0836] In this EXAMPLE C, a heterogeneous nitride semiconductor
substrate 601 of which principal surface was the m-plane was
fabricated based on the fabrication method of the heterogeneous
nitride semiconductor substrate 601 which is illustrated in FIG.
64.
[0837] 1. Growth of a Nitride Semiconductor Layer on a Growth
Substrate which was Made of m-Plane Sapphire
[0838] In this EXAMPLE C, an m-plane sapphire substrate was used as
the growth substrate 100 of FIG. 64(a). The thickness of the
m-plane sapphire substrate was 430 .mu.m. The diameter of the
m-plane sapphire substrate was about 5.1 cm (=2 inches). The angle
between the normal line of the principal surface of the m-plane
sapphire substrate and the normal line of the m-plane was
0.degree..+-.0.1.degree..
[0839] Then, a nitride semiconductor layer 110 of which growing
plane was the m-plane was grown on the growth substrate 100 by a
metal organic chemical vapor deposition method. It is commonly
believed that growth of a low temperature buffer layer is required
for growth of an m-plane nitride semiconductor on a substrate that
is made of m-plane sapphire. In this example, an AlN layer was used
as the low temperature buffer layer.
[0840] After the growth of the low temperature buffer layer, the
substrate temperature was increased to a temperature in the range
of 900.degree. C. to 1100.degree. C. in order to grow the nitride
semiconductor layer 110 of which principal surface was the m-plane.
The thickness of the nitride semiconductor layer 110 was from about
1 .mu.m to 3 .mu.m.
[0841] 2. Fabrication of the Heterogeneous Nitride Semiconductor
Substrate 601
[0842] Next, as shown in FIG. 61(a), a SiO.sub.2 film having a
thickness of 200 nm, for example, was formed on the nitride
semiconductor layer 110 by a plasma CVD method. Then, a resist
pattern with lines & spaces (L&S) of 5 .mu.m & 5 .mu.m
was formed on the SiO.sub.2 film by a known lithography method.
Thereafter, the SiO.sub.2 film was dry-etched using the formed
resist pattern as a mask such that a mask 120 is formed by the
SiO.sub.2 film. By pattern formation of the mask 120, the in-plane
tilt angle .beta., which is the extending direction of the
stripe-shaped gaps 60, can be appropriately determined.
[0843] Then, the nitride semiconductor film 320 which was made of
m-plane GaN was regrown on the nitride semiconductor layer 110
which had the patterned mask 120. Specifically, the nitride
semiconductor layer 110 which had the mask 120 on its surface was
again carried into the metal organic chemical vapor deposition
apparatus. Then, the growth temperature was increased to a
temperature in the range of about 900.degree. C. to 1000.degree. C.
for carrying out regrowth.
[0844] The important point for growth of the nitride semiconductor
film 320 is that regrowth of the nitride semiconductor film 320
starting from the stripe-shaped nitride semiconductor layers 110a
that were exposed through the mask 120 is enhanced in lateral
directions. In this example, the growth was carried out under the
conditions that the value of the V/III ratio was about 160, the
growth pressure was about 13.3 kPa, and the growth speed was about
4 .mu.m/h. Under these conditions, the nitride semiconductor film
320 was formed by regrowth so as to have a thickness of 4 .mu.m to
10 .mu.m, whereby a heterogeneous nitride semiconductor substrate
601 of which principal surface was the m-plane was obtained.
[0845] Thus, in the above-described fabrication method of the
heterogeneous nitride semiconductor substrate 601, by appropriately
selecting the dimensions of the mask 120 and the growth conditions
of the nitride semiconductor film 320, the upper surface of the
mask 120 can be covered with the nitride semiconductor film 320.
Further, the cross-sectional shape of the gaps 60 formed above the
mask 120 can also be controlled.
[0846] FIG. 81(a) and FIG. 81(b) show some fabricated examples of
the heterogeneous nitride semiconductor substrate 601 which were
obtained as described above. It can be seen that a plurality of
gaps 60 were provided at the interface between the nitride
semiconductor layer 110 and the nitride semiconductor film 320 as
explained in the section of the sixth embodiment. The regrowth
conditions for the nitride semiconductor film 320 were different
between FIG. 81(a) and FIG. 81(b). It can be seen that, by
appropriately selecting the regrowth conditions, the configuration
and cross-sectional shape of the stripe-shaped gaps 60 can be
controlled.
[0847] The heterogeneous nitride semiconductor substrate 601
fabricated by the above-described fabrication method, which
includes the gaps 60 in the interface region, can obtain the effect
of greatly improving the crystallinity as in EXAMPLE B, in addition
to the effect of reducing the polarization degree of light.
[0848] As demonstrated by the results of EXAMPLE C, in the sixth
embodiment also, the plurality of gaps 60 can be formed in a region
near the interface 50 between the growth substrate 100 and the
nitride semiconductor film 320. A heterogeneous nitride
semiconductor substrate 601 having such a configuration is
prepared, and a light-emitting device is manufactured on the
prepared heterogeneous nitride semiconductor substrate 601. As a
result, as previously described in EXAMPLE A and EXAMPLE B, the
polarization degree can be reduced, while improvement of the light
distribution characteristics and improvement of the light
extraction efficiency can be realized.
[0849] A shape of the stripe structures (e.g., gaps) which can
achieve the effects of this example may be such that the range of
the angle .beta. is not less than 3.degree. and not more than
45.degree., where .beta. is the angle between the extending
direction of the stripe structures and the a-axis direction of the
nitride semiconductor film 320, i.e., the electric field vector
direction of polarized light, and a direction parallel to the
a-axis direction is .beta.=0.degree..
Example D
Experimental Grounds for the Range of the in-Plane Tilt Angle
.beta. (not Less than 3.degree. and not More than 35.degree.) in
the Heterogeneous Nitride Semiconductor Substrate 600 that has the
Gaps 60
[0850] In the above-described EXAMPLES A, B, and C, it has been
explained that, by providing the stripe structures on the light
emission surface side, the polarization degree of polarized light
emitted from the active layer 24 can be reduced, while improvement
of the light distribution characteristics and improvement of the
light extraction efficiency are possible. It has also been
demonstrated based on the experimental results that, when the
in-plane tilt angle .beta. of the stripe structures is in the range
of not less than 3.degree. and not more than 45.degree., the
effects of EXAMPLES A to C are obtained.
[0851] On the other hand, in the configuration illustrated in
EXAMPLE B, when a light-emitting device which has the gaps 60 is
manufactured using an m-plane sapphire substrate as the growth
substrate 100 and the heterogeneous nitride semiconductor substrate
600 as the basic body, the polarization degree reducing effect and
the light distribution characteristic improving effect are
achieved, and meanwhile, a problem occurs in regrowth of the
nitride semiconductor film 320. Therefore, the angle .beta. between
the extending direction of the stripe-shaped gaps 60 and the a-axis
may be designed to be not less than 3.degree. and not more than
35.degree..
[0852] In this EXAMPLE D, the reason why the angle .beta. is not
less than 3.degree. and not more than 35.degree. is described.
[0853] In this example, the structure of the heterogeneous nitride
semiconductor substrate 600, which is an underlayer, was mainly
fabricated and evaluated, and a light-emitting device 20 was not
manufactured on the heterogeneous nitride semiconductor substrate
600.
[0854] The condition of the in-plane tilt angle .beta. of not less
than 3.degree. and not more than 35.degree., which is described in
this example, is limited to only the following sequence: the
m-plane sapphire substrate is used as the growth substrate 100; the
nitride semiconductor layer 110 is etched to form the recessed
portions 210 in which the surface of the growth substrate 100 is
exposed as illustrated in FIG. 56 and FIG. 57; and thereafter, the
heterogeneous nitride semiconductor substrate 600 including the
regrowth step is included in the light-emitting device.
[0855] Thus, in EXAMPLE A in which the stripe-shaped uneven
structure 70 is not formed in the rear surface of the growth
substrate 100 and EXAMPLE C in which the recessed portions 210 are
not formed in the nitride semiconductor layer 110, the in-plane
tilt angle .beta. is not limited to the range of not less than
3.degree. and not more than 35.degree.. .beta. may be in the range
of not less than 3.degree. and not more than 45.degree..
[0856] The present inventors examined the in-plane tilt angle
.beta. dependence of the nitride semiconductor film 320 based on
the configuration of the heterogeneous nitride semiconductor
substrate 600 for which the growth substrate 100 of which growing
plane (principal surface) was the m-plane was used, and found that
the surface flatness and the crystallinity largely depend on the
angle .beta..
[0857] (Angle .beta. and Surface Flatness)
[0858] FIG. 82(a) to FIG. 82(h) show surface microscopic images of
the heterogeneous nitride semiconductor substrates 600 which had
the stripe-shaped gaps 60 with varying angles .beta.. In this
experiment, the angle .beta. was varied from 0.degree. to
90.degree.. Part of the results of the experiment is shown in FIG.
82.
[0859] In this experiment, a plurality of samples of the
heterogeneous nitride semiconductor substrate 600 were fabricated
on the growth substrate 100 of which principal surface was the
m-plane under the same conditions as those described in EXAMPLE B,
with only the angle .beta. being varied, and the fabricated samples
were evaluated. It can be seen from FIG. 82 that the flatness of
the surface of the heterogeneous nitride semiconductor substrate
600 greatly varied depending on the angle .beta..
[0860] In these samples which were different only in angle .beta.,
the nitride semiconductor film 320, which was regrown GaN, was
grown under the same growth conditions. However, the surface of
each of the obtained samples apparently depended on the angle
.beta., and the lateral growth was sufficient in some samples but
was insufficient in the other samples. For example, when the angle
.beta.=0.degree., the nitride semiconductor film portions 320
regrown from the stripe-shaped nitride semiconductor layers 110a
that form the raised portions did not at all connect to each other.
However, when the angle .beta. was increased, e.g., when the angle
.beta. was not less than 17.degree., the nitride semiconductor film
portions 320 sufficiently connected to each other to form a regrown
film having a flat surface.
[0861] The cause of such a phenomenon was analyzed as described
below.
[0862] When .beta.=0.degree., a surface perpendicular to the
extending direction of the stripe-shaped nitride semiconductor
layers 110a, i.e., a lateral surface of the ridge structure, is
mainly the c-plane. On the other hand, when .beta.=90.degree., a
lateral surface of the ridge structure is the a-plane. Therefore,
as the angle .beta. increases, the lateral surface of the ridge
structure transitions from the c-plane facet to the a-plane facet
of the GaN.
[0863] In general, comparing the crystal growth in the nitride
semiconductor between the c-plane facet and the a-plane facet, the
a-plane facet is more thermally stable, and further, the growth
speed at the a-plane facet is faster. Therefore, from the viewpoint
of enhancing lateral growth, the a-plane facet growth may be
employed rather than the c-plane facet growth.
[0864] FIG. 83 shows the relationship between the surface roughness
(rms roughness) which was obtained by a laser microscope and the
angle R. As previously described, as the angle .beta. increases,
the surface flatness dramatically improves. According to the
various examinations carried out by the present inventors, this
surface flatness improving effect is sufficiently achieved only by
slightly increasing the angle .beta. from 0.degree.. For example,
it can be seen from FIG. 83 that, when the angle .beta. is not less
than about 3.degree., the surface rms roughness is not more than
100 nm, and it was proved that the flatness improving effect can be
obtained.
[0865] On the other hand, when based on the above-described
assumption, the surface roughness only requires that the angle
.beta. should be not less than 3.degree.. The surface must be the
flattest when the angle .beta. is around 90.degree.. This is
because the lateral surface of the ridge structure is wholly formed
by only the a-plane facet. However, it was found from the results
of FIG. 82 and FIG. 83 that the surface roughness deteriorated
again when the angle .beta. exceeded 35.degree..
[0866] This deterioration of the surface roughness is obviously
different from the cause of deterioration of the surface roughness
which occurred when the angle .beta. was small. That is, as seen
from the result at .beta.=47.degree. in FIG. 82(g) and the result
at .beta.=80.degree. in FIG. 82(h), when .beta.>35.degree.,
three-dimensional protrusions were seen in the surface obtained
after the regrowth, and the density and number of the protrusions
increased as the angle .beta. increased.
[0867] (Cause of Protrusions Produced when Angle
.beta.>35.degree.)
[0868] To clarify what the protrusions seen in FIG. 82(g) and other
drawings are, X-ray diffraction measurement was carried out on the
heterogeneous nitride semiconductor substrate 600 of this
example.
[0869] FIG. 84 shows, as an example, the X-ray 2.theta.-.omega.
measurement results for the cases where the in-plane tilt angles
.beta. were 0.degree., 43.degree., and 90.degree.. When the angle
.beta. was 0.degree., only the diffraction peaks of m-plane
sapphire (3-300) and m-plane GaN (2-200) were observed. This
demonstrates that the nitride semiconductor film 320 formed on the
m-plane sapphire substrate by regrowth was oriented only in the
m-axis direction.
[0870] On the other hand, when .beta.=43.degree., another
diffraction peak was observed on the higher angle side in addition
to the above-described diffraction peaks of m-plane GaN and m-plane
sapphire. This diffraction peak is at the diffraction peak position
of the (11-22) plane that is a semi-polar plane. This means that,
in the heterogeneous nitride semiconductor substrate 600, in
forming the stripe-shaped nitride semiconductor 110a at
.beta.=43.degree. and regrowing the nitride semiconductor film 320,
the nitride semiconductor film 320 includes not only a
semiconductor of which principal surface is the m-plane but also a
semiconductor of which principal surface is the (11-22) that is a
semi-polar plane.
[0871] Further, it can be seen that, when .beta.=90.degree., the
diffraction intensity from this (11-22) plane is greater.
[0872] FIG. 85 shows the angle .beta. dependence of the value of
the integrated intensity ratio of the (11-22) plane and the m-plane
(2-200) plane, which was estimated from the X-ray 2.theta.-.omega.
measurement results. It can be seen from FIG. 85 that the
integrated intensity of the (11-22) plane that is a semi-polar
plane increased after the angle exceeded around 35.degree. at which
the surface roughness started to increase. Such a variation of the
XRD measurement results is identical with the variation of the
surface morphology shown in FIG. 83.
[0873] From the above results, it is inferred that the cause of the
deterioration of the surface flatness and protrusions which occur
when the in-plane tilt angle .beta. exceeds 35.degree. is
attributed to abnormal growth which occurs in the semi-polar plane.
It was found that the semi-polar plane is growth with the (11-22)
plane being the principal surface.
[0874] Next, the cause of the abnormal growth at the semi-polar
plane of which principal surface is the (11-22) plane, which occurs
when the angle .beta. of the nitride semiconductor layer 110 of the
ridge structure exceeds 35.degree., is described.
[0875] It is commonly known that the (11-22) plane that is a
semi-polar plane can also be grown on the m-plane sapphire
substrate. In that case, the epitaxy relationship with the sapphire
is the growth starting from the r-plane facet of the m-plane
sapphire. It is also commonly known that the a-plane nitride
semiconductor can be grown on the r-plane sapphire. That is, it may
be considered that the mechanism of the (11-22) plane growth on the
m-plane sapphire is obtained as a result of growth of the a-plane
nitride semiconductor on the inclined r-plane facet in the m-plane
sapphire surface.
[0876] Therefore, simply, when a nitride semiconductor is grown on
the m-plane sapphire that has a surface in which many r-plane
facets are to be formed, the nitride semiconductor film 320 of
which principal surface is the (11-22) plane is likely to be
obtained.
[0877] FIG. 86(a) schematically shows the facet structure of the
(11-22) plane nitride semiconductor. FIG. 86(b) schematically shows
the facet structure of the m-plane sapphire. The r-plane facet of
the sapphire is oriented in the c-axis direction. Therefore, it is
inferred that, when both an a-plane facet and a c-plane facet are
formed in the surface of the m-plane sapphire by dry etching, for
example, the r-plane facet is more likely to be formed in the
c-plane facet than in the a-plane facet.
[0878] FIG. 87(a) and FIG. 87(b) show the epitaxy relationship
between the m-plane sapphire (growth substrate 100) and the m-plane
GaN (nitride semiconductor layer 110) in the unevenly-processed
substrate 510 where the in-plane tilt angles are .beta.=0.degree.
and .beta.=90.degree..
[0879] The relationship of the crystal orientation between the
m-plane GaN and the m-plane sapphire is such that the growing
planes (principal surfaces) are the same m-axis, but the in-plane
crystal axes are deviated by 90.degree.. That is, it is such a
relationship that, for example, the c-axis of the GaN and the
a-axis of the sapphire are parallel to each other.
[0880] In forming the stripe-shaped nitride semiconductor layer
110a that forms a raised portion, it is necessary to remove the
nitride semiconductor layer 110 in the recessed portions 210, and
therefore, part of the sapphire substrate is removed due to
overetching in many cases. In such a case, if the in-plane tilt
angle .beta. of the stripe-shaped nitride semiconductor layers 110a
is 0.degree., the wall surfaces 220 of the sapphire substrate are
the a-plane as shown in FIG. 87(a), and the wall surfaces of the
stripe-shaped nitride semiconductor layer 110a that is made of GaN
are the c-plane.
[0881] In the case of .beta.=90.degree., on the contrary to the
above, as shown in FIG. 87(b), the wall surfaces 220 of the
sapphire substrate are the c-plane, and the wall surfaces of the
stripe-shaped nitride semiconductor layer 110a are the a-plane.
[0882] That is, when the angle .beta. is large, the wall surfaces
220 of the sapphire substrate transition from the a-plane facet to
the c-plane facet, so that the r-plane facet is more likely to be
formed. It is considered that, when the r-plane facet is formed, as
described above, the probability of growth of the (11-22) plane
increases.
[0883] Inferring from the experimental results, when the in-plane
tilt angle .beta. was not more than 35.degree., there were small
number of r-plane facets that served as the starting points of
abnormal growth occurring in the semi-polar plane, and regions of
the semi-polar plane were very few so that they could not detected
even by the XRD measurement. Therefore, it can be considered that
the effects of those regions are negligible as compared with the
regrown m-plane regions.
[0884] On the other hand, when the angle .beta. exceeds 35.degree.,
growth of the semi-polar plane is remarkable, and the source
materials supplied during the regrowth undergo not only growth on
the stripe-shaped nitride semiconductor layers 110a but also growth
starting from the r-plane facets in the wall surfaces 220 of the
growth substrate 100. As a result, it is inferred that, a regrown
film (nitride semiconductor film 320) was obtained in which the
m-plane and the (11-22) plane coexisted.
[0885] It was found from the above results that it is desirable to
control the range of the in-plane tilt angle .beta. of the gaps 60
within the range of not less than 3.degree. and not more than
35.degree. in order to realize high quality crystal growth of a
nitride semiconductor layer film 320 that is a heterogeneous
m-plane nitride semiconductor, while the polarization control
effect is maintained, in the heterogeneous nitride semiconductor
substrate 600 that is obtained by forming the nitride semiconductor
layer 110 on an m-plane sapphire which is used as the growth
substrate 100 and carrying out etching on the nitride semiconductor
layer 110 such that the surface of the growth substrate 100 is
selectively exposed to form the recessed portions 210.
[0886] (Abnormal Growth Occurring in the Semi-Polar Plane and the
Height Dependence of the Wall Surfaces 220 of the Sapphire
Substrate in the Unevenly-Processed Substrate 510)
[0887] Hereinabove, it has been described that, in the process of
fabricating the unevenly-processed substrate 510, the wall surfaces
220 of the sapphire substrate which are formed in the recessed
portions 210 serve as starting points of abnormal growth from the
semi-polar plane that is the (11-22) plane. In addition, it has
been described that this problem can be avoided by controlling the
in-plane tilt angle .beta. in the stripe structures so as to be not
less than 3.degree. and not more than 35.degree..
[0888] On the other hand, the present inventors carried out further
examinations of the above-described cause of occurrence of abnormal
growth in the semi-polar plane and, as a result, found that there
is another method for preventing the abnormal growth from the
semi-polar plane, other than optimizing the in-plane tilt angle
.beta.. The method is to decrease the area of the wall surfaces 220
of the growth substrate 100 shown in FIG. 87(a) and FIG. 87(b) at
which the semi-polar plane growth occurs. In principle, when the
depth of the wall surfaces 220 of the growth substrate 100 which is
made of the m-plane sapphire is close to 0, the abnormal growth at
the semi-polar plane is unlikely to occur, so that dependence on
the in-plane tilt angle .beta. decreases.
[0889] It is also inferred that, even if abnormal growth at the
semi-polar plane occurs, it must occur in an extremely small area
as compared with the nitride semiconductor film 320 that is the
original m-plane nitride semiconductor region, and the effect of
the nitride semiconductor film 320 that regrows on the entirety is
small.
[0890] Based on such an inference, the depth dependence of the wall
surfaces 220 of the growth substrate 100 in the heterogeneous
nitride semiconductor substrate 600 was examined.
[0891] FIG. 88(a) and FIG. 88(b) show the in-plane tilt angle
.beta. dependence of the value of the intensity ratio of the X-ray
diffraction peak integration between the (11-22) plane and the
m-plane (2-200) plane of the nitride semiconductor layer, with
different depths of the wall surfaces 220 which were exposed at the
recessed portions 210 of the growth substrate 100.
[0892] FIG. 88(a) is equivalent to the results shown in FIG. 85. In
this case, the depth of the wall surfaces 220 of the growth
substrate 100 was about 250 nm. On the other hand, in FIG. 88(b),
the etching duration in the unevenness processing was decreased
such that the etching depth was about 150 nm.
[0893] In FIG. 88(a), as previously described, it can be seen that,
when the in-plane tilt angle .beta. exceeds 35.degree., the
diffraction intensity of the (11-22) plane increases, and the
semi-polar plane is also present together in the nitride
semiconductor film 320 that is the regrown film. On the other hand,
in the case of the sample shown in FIG. 88(b) with the etching
depth of about 150 nm, such a tendency that the diffraction
intensity of the (11-22) plane abruptly increases cannot be seen
even if the value of the in-plane tilt angle .beta. increases.
[0894] This experimental result demonstrates that the semi-polar
plane abnormal growth can be prevented by reducing the depth of the
wall surfaces 220 of the growth substrate 100 and decreasing the
area of these wall surface regions.
[0895] This is probably because, as previously described, the
number of r-plane facets which serve as starting points of the
abnormal growth from the semi-polar plane was reduced by decreasing
the depth of the wall surfaces 220 of the recessed portions 210 in
the growth substrate 100.
[0896] It was found from the above results that, in a fabrication
method for fabricating the heterogeneous nitride semiconductor
substrate 600 based on the nitride semiconductor film 320 that is
regrown on the m-plane sapphire substrate, the in-plane tilt angle
.beta. of the stripe structures and the etching depth in the growth
substrate 100 that is the m-plane sapphire substrate are controlled
so as to be within predetermined ranges, so that the abnormal
growth which could occur in the semi-polar plane can be effectively
prevented.
[0897] The abnormal growth from the semi-polar plane can be
prevented by controlling the in-plane tilt angle .beta. so as to be
within the range of not less than 0.degree. and not more than
35.degree.. The abnormal growth from the semi-polar plane can also
be prevented by controlling the etching depth of the wall surfaces
220 which are exposed from the growth substrate 100 so as to be
within the range of 0 nm to 150 nm, irrespective of the in-plane
tilt angle .beta..
[0898] Both the in-plane tilt angle .beta. and the etching depth of
the wall surfaces 220 which are exposed from the sapphire substrate
may be concurrently controlled.
[0899] By such a design, the effect of the r-plane facet serving as
the starting point of the abnormal growth that occurs in the
semi-polar plane, which is present in the wall surfaces 220 exposed
from the growth substrate 100 that is made of sapphire, is
dramatically reduced so that a high quality heterogeneous nitride
semiconductor substrate 600 can be fabricated.
[0900] As described hereinabove, it was found that it is desirable
to control the range of the in-plane tilt angle .beta. of the gaps
60 within the range of not less than 3.degree. and not more than
35.degree. and to design such that the height of the wall surfaces
220 of the sapphire substrate which are exposed from the lower part
of the recessed portions 210 is from 0 nm to 150 nm in order to
realize high quality crystal growth of a nitride semiconductor
layer film 320 that is a heterogeneous m-plane nitride
semiconductor, while the polarization control effect is maintained,
in the fabrication method of the heterogeneous nitride
semiconductor substrate 600 which includes the step of exposing the
surface of the growth substrate 100 by etching to form the recessed
portions 210, wherein m-plane sapphire is used as the growth
substrate 100.
Example E
Experimental Grounds for the Range of the in-Plane Tilt Angle
.beta. (not Less than 3.degree. and not More than 10.degree.) in
the Heterogeneous Nitride Semiconductor Substrate 600 that has the
Gaps 60
[0901] It is known that, in crystal growth in a nitride
semiconductor of which principal surface is a non-polar plane or
semi-polar plane, not only dislocations but also stacking faults
are likely to occur, and particularly, the stacking faults greatly
affect the characteristics of a nitride-based semiconductor
light-emitting device of which principal surface is a non-polar or
semi-polar plane.
[0902] Therefore, in crystal growth of a nitride semiconductor of
which principal surface is a non-polar plane or semi-polar plane,
not only reduction of the dislocation density but also reduction of
the stacking fault density are important. This problem is very
important particularly in a nitride semiconductor structure which
is grown on a hetero-substrate which is different from the nitride
semiconductor, such as a sapphire substrate, and of which principal
surface is a non-polar of semi-polar plane.
[0903] As the values of the half-value width of the X-ray co
rocking curve (XRC) have been presented in EXAMPLE B (see Table 2),
it is usual that the values of the XRC full width at half maximum
of a nitride semiconductor layer directly grown on a sapphire
substrate of which growing plane is the m-plane are asymmetric when
measured with X-rays being incident in the a-axis direction and the
c-axis direction. Usually, in the case of a nitride semiconductor,
the XRC full width at half maximum is larger when the X-ray is
incident in the c-axis direction. This is because the XRC full
width at half maximum measured in the c-axis direction includes the
information of stacking faults in addition to the information of
dislocations.
[0904] On the other hand, as seen from Table 2 of EXAMPLE B, in the
heterogeneous nitride semiconductor substrates 600 and 601 which
have the gaps 60 according to the fourth embodiment and the sixth
embodiment, not only the dislocation density but also the stacking
fault density are greatly reduced, so that the crystallinity and
the optical characteristics can be greatly improved. Further, it
was found that the stacking fault reducing effect illustrated in
EXAMPLE B varies depending on the in-plane tilt angle .beta. of the
gaps 60.
[0905] As illustrated in EXAMPLE A, the polarization degree
reducing effect and the effects of improving the light distribution
characteristics and improving the light extraction efficiency can
be obtained over a wide range of the in-plane tilt angle .beta. of
the stripe structures, which is not less than 3.degree. and not
more than 45.degree..
[0906] However, it was found that, in the heterogeneous nitride
semiconductor substrate 600, 601 in which a hetero-substrate which
is different from the GaN bulk substrate is used as the growth
substrate 100, reduction of the stacking fault density cannot be
sufficiently realized in some range of the in-plane tilt angle
.beta.. Therefore, in the heterogeneous nitride semiconductor
substrate 600, 601 which is fabricated on the growth substrate 100
that is a hetero-substrate which is likely to include many
dislocations and defects, controlling the in-plane tilt angle
.beta. under not only the above-described condition which can
achieve the polarization degree reducing effect but also the
condition which can sufficiently reduce the stacking fault density
is indispensable. That is, without appropriate control of the value
of the in-plane tilt angle .beta., stacking faults are included in
the active layer 24, so that the efficiency of the light-emitting
device greatly deteriorates.
[0907] As a result of the examinations carried out by the present
inventors, it was found that the range of the in-plane tilt angle
.beta. in the stripe-shaped gaps 60 in the heterogeneous nitride
semiconductor substrate 600, 601 may be not less than 3.degree. and
not more than 10.degree..
[0908] Hereinafter, details of the experimental results are
described.
[0909] In this example, the structure of the heterogeneous nitride
semiconductor substrate 600 that is formed by a nitride
semiconductor film grown on an m-plane sapphire substrate according
to the fourth embodiment was used as the basic body, and the
in-plane tilt angle .beta. dependence of the gaps 60 in the
dislocation density and the stacking fault density of these samples
was examined.
[0910] The dislocation density reducing effect was evaluated based
on the XRC full width at half maximum. In this case, the incidence
direction of the X-ray was the a-axis direction in the GaN. The
stacking fault density reducing effect was evaluated by measurement
of the photoluminescence (PL). This is because the evaluation with
the use of PL enables to more precisely examine the effect of the
stacking fault density.
[0911] In EXAMPLE E, the heterogeneous nitride semiconductor
substrate 600 was fabricated using an m-plane sapphire substrate
under basically the same conditions as those of EXAMPLE B, and the
in-plane tilt angle .beta. of these samples were varied from
0.degree. to 90.degree..
[0912] The effect of this example can also be obtained likewise in
the embodiments and examples described in the sections of the sixth
embodiment and EXAMPLE C in which regrowth is carried out on the
mask 120 that is made of SiO.sub.2. That is, in a configuration in
which a nitride semiconductor of which principal surface is the
m-plane on the hetero-substrate, the in-plane tilt angle .beta. in
the stripe structures may be controlled to be in the range of not
less than 3.degree. and not more than 10.degree. in order to reduce
the polarization degree of light and reduce the stacking fault
density.
[0913] Note that, however, when the configuration which includes
the gaps 60 at the interface between the nitride semiconductor
layer 110 on the growth substrate 100 and the nitride semiconductor
film 320 is a homoepitaxy-based configuration in which a GaN bulk
substrate, or the like, is used as the growth substrate 100, the
effect of the stacking fault density is not so large, and
therefore, a suitable range of the in-plane tilt angle .beta.
becomes wider. For example, it may be not less than 3.degree. and
not more than 45.degree..
[0914] FIG. 89 shows the XRC full width at half maximum of the
heterogeneous nitride semiconductor substrate 600 with the in-plane
tilt angle .beta. being varied from 0.degree. to 35.degree.. The
incidence direction of the X-ray was the a-axis direction of the
GaN.
[0915] In this experiment, m-plane GaN films on two types of
m-plane sapphire substrates were used as the nitride semiconductor
layer 110 for seed crystal. That is, it was different between the
samples with the in-plane tilt angles .beta. from 0.degree. to
15.degree. and the samples with .beta. from 17.degree. to
35.degree..
[0916] The broken line shown in FIG. 89 represents the value of the
XRC full width at half maximum of a typical directly-grown GaN film
which is shown in Table 2 (1326 seconds, 0.37 degree). The two
types of seed crystal GaNs of this example exhibited generally
equal XRC full width at half maximums. It can be seen that the XRC
full width at half maximum was approximately a half of the value of
the directly-grown GaN film over a wide range of the in-plane tilt
angle .beta. from 0.degree. to 35.degree..
[0917] The above results demonstrate that, by using the
heterogeneous nitride semiconductor substrate 600 which has the
gaps 60 such as illustrated in the section of the fourth
embodiment, not only control of the polarization degree of light
but also the crystallinity improving effect can be obtained.
[0918] The XRC full width at half maximum shown in FIG. 89 appears
to gradually deteriorate as the in-plane tilt angle .beta.
increases. However, this is attributed to the difference in
crystallinity between the m-plane GaN layers 110 of the two types
of seed crystals used in this example and, probably, is not the
evidence of the dependence of the in-plane tilt angle .beta..
[0919] FIG. 90(a) and FIG. 90(b) show the PL spectrum at room
temperature. In the PL evaluation, a He--Cd laser (continuous wave,
intensity: up to 30 mW) was used as the excitation source. As an
example, FIG. 90(a) shows the result for the case of the in-plane
tilt angle .beta.=5.degree., and FIG. 90(b) shows the result for
the case of the in-plane tilt angle .beta.=14.degree.. The emission
peak near the band edge of the GaN was seen at around 3.4 eV, while
the other emissions were resulted from Deep Level.
[0920] Both samples did not have a large difference in the value of
the XRC full width at half maximum as seen from FIG. 89. However,
the emission intensity at the band edge was high in the sample
shown in FIG. 90(a) with a small in-plane tilt angle,
.beta.=5.degree., while in the sample shown in FIG. 90(b) with
.beta.=14.degree., the Deep Level emission was dominant.
[0921] Considering that there is no difference in the XRC full
width at half maximum shown in FIG. 89, there is a small
probability that the cause of the difference in emission spectrum
between the two samples is the dislocation density. That is, the
cause of this difference is attributed to the stacking fault
density.
[0922] FIG. 91(a) to FIG. 91(c) show the results of the PL
measurement at the low temperature (10K). FIG. 91(a) to FIG. 91(c)
are the results for the cases where the in-plane tilt angles .beta.
were 0.degree., 5.degree., and 21.degree., respectively. For the
sake of comparison, FIG. 91(d) shows the spectrum in the nitride
semiconductor layer 110 that is the seed crystal GaN. The vertical
axis in FIG. 91 is normalized with the intensity of the peak which
is attributed to stacking faults (near 3.42 eV).
[0923] In the case shown in FIG. 91(c) with a large in-plane tilt
angle, .beta.=21.degree., and in the sample of the seed crystal
shown in FIG. 91(d), three peaks were mainly observed.
[0924] The values of the respective emission peaks have some
deviations depending on, for example, the strain amount of a grown
film in some cases. However, it is inferred from the systematic
analysis of the experimental results obtained herein and the
comparison with other document results that the emission near 3.42
eV was attributed to the stacking faults. That is, the peak near
3.48 eV was the emission that was attributed to the donor bound
exciton (D0, X). Further, the peak on the low energy side was the
emission that was attributed to dislocations and defects.
[0925] Firstly, comparing the result of the seed crystal GaN shown
in FIG. 91(d) and the result of .beta.=21.degree. shown in FIG.
91(c), in the sample of .beta.=21.degree., it can be seen that the
relative intensity with respect to the peak that was attributed to
stacking faults, which was the emission that is attributed to the
donor bound exciton (D0, X), increased. This is probably because,
as compared with the seed crystal that was obtained by direct
growth on an m-plane sapphire substrate, the stacking fault density
in the crystal of the heterogeneous nitride semiconductor substrate
600 that had the stripe-shaped gaps 60 decreased. However, in the
sample of .beta.=21.degree., the intensity of the emission that was
attributed to the donor bound exciton (D0, X) was generally
equivalent to that of the stacking faults, and the overall emission
intensity was small.
[0926] In comparison to these results, in the case of the sample
shown in FIG. 91(a) with a small in-plane tilt angle .beta.,
.beta.=0.degree., and the sample shown in FIG. 91(b) with
.beta.=5.degree., the emission which was attributed to the emission
which was attributed to the donor bound exciton (D0, X) near 3.48
eV was dominant, rather than the peak intensity which was
attributed to the stacking faults. The relative intensity with
respect to the peak that was attributed to the stacking faults
increased by one or more orders of magnitude as compared with
.beta.=21.degree..
[0927] Thus, in a regrown GaN film of which emission intensity near
the band edge was strong in the PL measurement at room temperature,
great reduction of the emission intensity which was attributed to
the stacking faults and increase of the (D0, X) intensity were
confirmed even in the PL measurement at the low temperature.
Therefore, it was found that, by appropriately selecting the range
of the in-plane tilt angle .beta., not only the dislocation
reducing effect but also the stacking fault density reducing effect
can be obtained.
[0928] FIG. 92 shows the in-plane tilt angle .beta. dependence of
the value of the ratio between the Deep Level emission intensity
obtained by the PL measurement at room temperature and the emission
intensity near the band edge.
[0929] When the in-plane tilt angle .beta. is in the range of
0.degree. to 10.degree., the value of the intensity ratio is small.
Even at room temperature, the emission near the band edge is
intense. It can be seen from this that the stacking fault density
was sufficiently reduced.
[0930] On the other hand, when the in-plane tilt angle .beta. was
not less than 10.degree., the value of the ratio between the Deep
Level emission intensity and the emission intensity near the band
edge abruptly increased, so that the Deep level emission was
dominant. As seen from the measurement result of the XRC full width
at half maximum shown in FIG. 89, the crystal quality improving
effect achieved by reduction of the dislocation density can be
realized even when at least the in-plane tilt angle .beta. is in
the range of 0.degree. to 35.degree..
[0931] It was proved by the experimental results of FIG. 89 to FIG.
92 that, to obtain the stacking fault density reducing effect in
addition to the above effect, it is necessary to narrow the range
of the in-plane tilt angle .beta. and that the range of the angle
.beta. may be not less than 0.degree. and not more than
10.degree..
[0932] It was proved by the results of FIG. 92 that the stacking
fault density reducing effect can be obtained even when the
in-plane tilt angle .beta. is not 0.degree..
[0933] Note that, as described in EXAMPLE D, when the in-plane tilt
angle .beta. is greater than 0.degree., reconnection of films
regrown from the ridge-shaped nitride semiconductor layer 110 is
enhanced, and the surface flatness of the heterogeneous nitride
semiconductor substrate 600, 601 is improved.
[0934] By controlling the in-plane tilt angle .beta. in the stripe
structures provided in the nitride semiconductor film 320 of which
principal surface is a non-polar plane, which is grown on a
hetero-substrate, and in which particularly the stacking fault
density is likely to be high, so as to be not less than 3.degree.
and not more than 45.degree. as in the fourth embodiment, the
polarization degree of emitted light can be reduced, while
improvement of the light distribution characteristics and
improvement of the light extraction efficiency can be achieved.
[0935] Further, as demonstrated by the results of EXAMPLE E, by
controlling the range of the angle .beta. so as to be not less than
3.degree. and not more than 10.degree., the effect of reducing the
density of stacking faults in a regrown nitride semiconductor can
be obtained while the above-described effects are maintained.
[0936] When the range of the angle .beta. is not less than
0.degree. and less than 3.degree., and particularly when .beta. is
0.degree. as seen from FIG. 75, the polarization degree of emitted
light is maintained. In this case, it can be seen from FIG. 89 and
FIG. 92 that the crystallinity of the nitride semiconductor is
excellent. Further, abnormal growth of a nitride semiconductor
which has a semi-polar plane can be prevented.
Other Embodiments
[0937] The light-emitting device according to each of the
above-described fourth to eighth embodiments may be used as a light
source device, without making any modification. However, when
combined with, for example, a resin material containing a
fluorescent material for wavelength conversion, the light-emitting
device according to each of the embodiments may be suitably used as
a light source device with an expanded wavelength range (e.g., a
white light source device). The configuration of the white light
source device is the same as that of the device shown in FIG. 42,
and therefore, description thereof is herein omitted.
[0938] According to the present embodiment, a light source device
according to the present embodiment is capable of reducing the
polarization degree of light and improving the light distribution
characteristics and the light extraction efficiency as the
light-emitting devices of the fourth to eighth embodiments are, and
furthermore, capable of controlling the wavelength band of emitted
light with the use of a wavelength conversion section which is
realized by a resin layer containing a phosphor dispersed
therein.
[0939] As described hereinabove, according to each of the
above-described embodiments, variations thereof, and examples, in a
nitride-based semiconductor of which principal surface (growing
plane) is a non-polar plane or semi-polar plane, a plurality of
stripe structures are provided on the light emission surface side.
The extending direction of the stripe structures is inclined, in
the m-plane, by not less than 3.degree. and not more than
45.degree. with respect to the a-axis direction of the nitride
semiconductor of which growing plane is the m-plane, so that the
polarization degree of emitted light from an active layer which is
made of a nitride semiconductor of which growing plane is the
m-plane can be greatly reduced, and the light distribution
characteristics can be improved.
[0940] The stripe structures may be provided on the light emission
surface side. For example, the stripe structures may be provided in
the rear surface of the growth substrate, and may be structures
which have an uneven shape along a direction perpendicular to the
extending direction of the stripe structures. For example, the
stripe structures are realized by a plurality of raised portions 51
provided in the lower part of the nitride semiconductor film 320
with gaps 60 interposed therebetween as shown in FIG. 44.
Alternatively, the gaps 60 themselves may form the stripe
structures. Still alternatively, as shown in FIG. 56(d), it can be
said that part of the nitride semiconductor portion 310 sandwiched
by two gaps 60 on adjacent mask portions 120 forms a stripe
structure. Still alternatively, as shown in FIG. 56(d) and FIG.
58(d), it can be said that the stripe-shaped nitride semiconductor
layers 110a provided on the principal surface of the growth
substrate 100 form the stripe structures. Still alternatively, when
the nitride semiconductor 320 and the nitride semiconductor layer
110 are integrated to surround the perimeter of the gaps 60 as
shown in FIG. 64(b), it can be said that part of the nitride
semiconductor 320 which is sandwiched by two adjacent gaps 60 also
forms a stripe structure. In this case, the stripe structures are
provided inside the nitride-based semiconductor multilayer
structure. Still alternatively, as shown in FIG. 69, it can be said
that the uneven structure 70 provided in the substrate 100 forms
the stripe structures.
[0941] The stripe structures may be made of two types of materials
which have different refractive indices. For example, when the
stripe structures are provided in the rear surface of the
above-described growth substrate, the interface, which has an
uneven shape of the stripe structures, is formed by two types of
materials, the air and the material of the growth substrate.
Alternatively, for example, other combinations, such as the
interface between the sapphire substrate and GaN layer, the
interface between the air and the GaN layer, etc., are
possible.
[0942] To enhance improvement of the light extraction efficiency,
it is important that the stripe structures themselves are made of a
material which causes only a small light absorption loss. For
example, as the stripe structures, deliberately forming gaps, using
a dielectric layer, such as SiO.sub.2, or using a metal layer which
has high reflectance is desirable. By periodically forming such
stripe structures with angle .beta., the effect of reducing the
polarization degree of light and the effect of improving the light
distribution characteristics can be obtained.
[0943] The stripe structures may be provided at and near the
interface between a nitride semiconductor layer of which growing
plane is a non-polar plane and a substrate for growth of the
nitride semiconductor layer. In general, a sapphire substrate which
is used as a substrate for growth of a nitride semiconductor layer
is very hard and difficult to process. Therefore, such a problem
can be avoided when the stripe structures are provided near the
interface of the nitride semiconductor layer bordering on the
sapphire substrate, so that control of the polarization degree and
the light distribution characteristics can be readily achieved.
[0944] The present disclosure is applicable to, for example,
general-purpose lighting devices.
[0945] While the present disclosure has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed disclosure may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the disclosure that
fall within the true spirit and scope of the disclosure.
* * * * *