U.S. patent application number 11/636496 was filed with the patent office on 2007-04-19 for semiconductor light emitting element, semiconductor light emitting device, and method for fabricating semiconductor light emitting element.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Kenji Orita.
Application Number | 20070085102 11/636496 |
Document ID | / |
Family ID | 35504673 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070085102 |
Kind Code |
A1 |
Orita; Kenji |
April 19, 2007 |
Semiconductor light emitting element, semiconductor light emitting
device, and method for fabricating semiconductor light emitting
element
Abstract
Projections/depressions forming a two-dimensional periodic
structure are formed in a surface of a semiconductor multilayer
film opposing the principal surface thereof, while a metal
electrode with a high reflectivity is formed on the other surface.
By using the diffracting effect of the two-dimensional periodic
structure, the efficiency of light extraction from the surface
formed with the projections/depressions can be improved. By
reflecting light emitted toward the metal electrode to the surface
formed with the projections/depressions by using the metal
electrode with the high reflectivity, the foregoing effect achieved
by the two-dimensional periodic structure can be multiplied.
Inventors: |
Orita; Kenji; (Osaka,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
Osaka
JP
|
Family ID: |
35504673 |
Appl. No.: |
11/636496 |
Filed: |
December 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11117406 |
Apr 29, 2005 |
7161188 |
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11636496 |
Dec 11, 2006 |
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Current U.S.
Class: |
257/98 ;
257/E33.068; 257/E33.073; 257/E33.074 |
Current CPC
Class: |
G02B 6/1225 20130101;
H01L 33/22 20130101; H01L 33/42 20130101; H01L 2933/0083 20130101;
H01L 33/20 20130101; H01L 33/54 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
257/098 ;
257/E33.074 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2004 |
JP |
2004-189892 |
Claims
1-20. (canceled)
21. A method for fabricating a semiconductor light emitting
element, the method comprising the steps of: (a) forming a first
two-dimensional periodic structure in a principal surface of a
substrate or in a principal surface of a semiconductor layer
provided on the substrate; and (b) successively forming a first
semiconductor layer, an active layer, and a second semiconductor
layer over the first two-dimensional periodic structure.
22. The method of claim 21, wherein the step (b) includes the step
of forming, in a back surface of the first semiconductor layer, a
second two-dimensional periodic structure which is complementary to
the first two-dimensional periodic structure by forming the first
semiconductor layer in such a manner that the first two-dimensional
periodic structure is filled therewith.
23. The method of claim 22, further comprising the step of: (c)
after the step (b), removing the substrate or the substrate and the
semiconductor layer to expose the second two-dimensional periodic
structure.
24. The method of claim 23, wherein the step (c) is performed by
polishing the substrate and by wet etching.
25. The method of claim 23, wherein the step (c) is performed by a
laser lift-off process.
26. The method of claim 23, wherein the substrate removed in the
step (c) is reusable.
27. The method of claim 21, further comprising the step of: after
the step (b), forming a high-reflection film composed of a metal
film containing at least one of an Au film, a Pt film, a Cu film,
an Ag film, an Al film, and a Rh film on a principal surface of the
second semiconductor layer.
28. The method of claim 23, further comprising the step of: after
the step (c), performing wet etching under a condition where an
etching speed differs depending on a crystal plane to form the
second two-dimensional periodic structure composed of projections
or depressions each configured as a polygonal pyramid.
29. The method of claim 21, wherein the step (b) includes the step
of forming the first semiconductor layer in such a manner as to
form a cavity in the first two-dimensional periodic structure.
30. The method of claim 21, wherein the first two-dimensional
periodic structure is composed of a structure made of at least any
one of an oxide, a nitride, and a metal.
31. The method of claim 23, wherein the second two-dimensional
periodic structure is composed of depressions, the method further
comprising the step of: after the steps (b) and (c), increasing a
depth of each of the depressions or changing a cross-sectional
configuration of each of the depressions by allowing electricity to
flow in the semiconductor light-emitting element in an electrolytic
solution.
32. The method of claim 21, wherein the step (a) includes the steps
of: (a1) coating a first resist on the principal surface of the
substrate or of the semiconductor layer; (a2) pressing, against the
first resist, a mold formed with a two-dimensional periodic
structure which is complementary to the first two-dimensional
periodic structure to transfer a pattern which is homologous to the
first two-dimensional periodic structure thereto; and (a3) forming
the first two-dimensional periodic structure by using, as a mask,
the first resist to which the pattern homologous to the first
two-dimensional periodic structure has been transferred.
33. The method of claim 21, wherein the first two-dimensional
periodic structure is composed of a resin provided on the
substrate, the method further comprising the steps of: (d) after
the step (b), removing the substrate; (e) coating a second resist
on a back surface of the first semiconductor layer; (f) placing the
substrate formed with the first two-dimensional periodic structure
on the second resist to transfer a pattern which is complementary
to the first two-dimensional periodic structure to the second
resist; and (g) forming a second two-dimensional periodic structure
which is complementary to the first two-dimensional periodic
structure in a back surface of the first semiconductor layer by
using, as a mask, the second resist to which the pattern
complementary to the first two-dimensional periodic structure has
been transferred.
34. The method of claim 21, further comprising the steps of:
mounting the semiconductor light emitting element on a mounting
substrate; and molding the semiconductor light emitting element
with a hemispherical resin by using a mold die formed with a
hemispherical cavity.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority under 35 U.S.C.
.sctn. 119(a) to Japanese Patent Application JP 2004-189892, filed
Jun. 28, 2004, the entire content of which is incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field to Which the Invention Pertains
[0003] The present invention relates to a light emitting element
using a semiconductor and to a method for fabricating the same.
[0004] 2. Prior Art
[0005] The use of a nitride-based compound semiconductor
represented by AlInGaN has enabled the commercialization of light
emitting elements which output light at the ultraviolet, blue, and
green wavelengths from which it has heretofore been difficult to
obtain a sufficient emission intensity, such as a light emitting
diode (LED) and a semiconductor laser, so that research and
development thereof has been conducted vigorously. Among
light-emitting elements, an LED is easier to fabricate and control
than a semiconductor laser and longer in lifespan than a
fluorescent lamp so that the LED, particularly using a
nitride-based compound semiconductor, is considered to be promising
as a light source for illumination.
[0006] FIG. 34 is a perspective view showing a conventional
nitride-based compound semiconductor LED. The conventional LED has
a structure in which an n-type GaN layer 1002, an InGaN active
layer 1003, and a p-type GaN layer 1004 are formed successively
through crystal growth on a sapphire substrate 1001. Each of the
InGaN active layer 1003 and the p-type GaN layer 1004 has been
removed partly by etching so that the n-type GaN layer 1002 is
exposed. An n-side electrode 1006 is formed on the exposed portion
of the n-type GaN layer 1002. A p-side bonding electrode 1007 is
provided on the p-type GaN layer 1004.
[0007] The following is the operation of the LED.
[0008] First, holes injected from the p-side bonding electrode 1007
are diffused laterally in a p-type transparent electrode 1005 to be
injected from the p-type GaN layer 1004 into the InGaN active layer
1003.
[0009] On the other hand, electrons injected from the n-side
electrode 1006 are further injected into the InGaN active layer
1003 through the n-type GaN layer 1002. The recombination of the
holes and the electrons in the InGaN active layer 1003 causes light
emission. The light is emitted to the outside of the LED through
the p-side transparent electrode 1005.
[0010] However, it cannot be said that such a conventional
structure has sufficiently high light extraction efficiency. The
light extraction efficiency is the ratio of light generated in the
active layer and emitted from the LED into an air to all the light
generated in the active layer. The cause of the low light
extraction efficiency of the conventional LED is the refractive
index of a semiconductor which is higher than that of the air so
that the light from the active layer is totally reflected by the
interface between the semiconductor and the air and confined to the
inside of the LED. For example, the refractive index of GaN is
about 2.45 at a wavelength of 480 nm so that a critical angle at
which total reflection occurs is as small as about 23 degrees. That
is, the light emitted from the active layer at an angle larger than
the critical refraction angle in terms of a normal to the interface
between the semiconductor and the air is totally reflected by the
interface between the semiconductor and the air so that the light
emitted from the active layer and extractable to the outside of the
LED accounts for only about 4% of all the light emitted from the
active layer. Accordingly, the problem is encountered that external
quantum efficiency (the ratio of light that can be extracted from
the LED to currents supplied to the LED) is low and power
conversion efficiency (the ratio of a light output that can be
produced to all the supplied power) is lower than that of a
fluorescent lamp.
[0011] As a solution to the problem, a technology which forms a
photonic crystal at the surface of the LED has been proposed, as
disclosed in Japanese Laid-Open Patent Publication No.
2000-196152.
[0012] FIG. 35 is a perspective view showing a conventional LED
having an upper surface formed with a photonic crystal. As shown in
the drawing, two-dimensional periodic projections/depressions are
formed in the p-type GaN layer 1004 according to the conventional
embodiment. In the structure, even light emitted from the active
layer at an angle larger than the critical refraction angle in
terms of the normal to the interface between the semiconductor and
the air can have the direction of emission at an angle smaller than
the critical refraction angle due to diffraction by the periodic
projection/depressions. This increases the ratio of light emitted
to the outside of the LED without being totally reflected and
improves the light extraction efficiency. In the present
specification, the wording "two-dimensional periodic" indicates
that a structure is formed to have given spacings (a given period)
along a first direction in a plane, while it is also formed to have
given spacings (a given period) along a second direction crossing
the first direction.
SUMMARY OF THE INVENTION
[0013] However, there are cases where the following problems occur
when projections/depressions are formed in the surface of the LED
close to the active layer, such as in the p-type GaN layer.
[0014] Since the p-type GaN layer 1004 has a high resistivity, the
film thickness thereof is preferably as thin as about 0.2 .mu.m in
terms of reducing the series resistance of the LED and achieving
high-efficiency light emission. To form the projections/depressions
in the upper surface of the p-type GaN layer 1004, however, it is
necessary to increase the film thickness of the p-type GaN layer
1004. As a result, there are cases where the series resistance of a
conventional LED as shown in FIG. 35 increases and the power
conversion efficiency thereof lowers. In addition, dry etching for
forming the projections/depressions in the p-type GaN layer 1004
causes a crystal defect in the surface of the p-type GaN layer
1004. Since such a crystal defect functions as an electron donor,
an electron density in the surface of the n-type GaN layer 1002
increases and the contact resistance thereof lowers. In the p-type
GaN layer 1004, however, the crystal defect resulting from an
etching damage compensates for the holes so that the formation of
an ohmic electrode becomes difficult. This leads to the problem
that the contact resistivity increases and the power conversion
efficiency lowers. Since the projections/depressions are close to
the active layer, etching-induced damage occurs in the active layer
during the formation of the projections/depressions. Consequently,
the problems of a reduction in internal quantum efficiency (the
ratio of electron-hole pairs that are recombined in the active
layer and converted to photons to all the electron-hole pairs
recombined in the active layer) in the active layer and a reduction
in the light emission efficiency of the LED are also likely to
occur.
[0015] The surface of the LED in which a two-dimensional photonic
crystal can conceivably be formed is the main or back surface
thereof. In this case, the back surface of the substrate and the
interface between the substrate and the semiconductor can be
considered as two locations at either of which the photonic crystal
is to be formed. In either case, however, the following problem is
encountered when the photonic crystal is formed by using the
conventional technology. In the case of forming the photonic
crystal at the back surface of the substrate, total reflection
occurs at the interface between the semiconductor and the substrate
so that the effect of improving the light extraction efficiency
achieved by the photonic crystal formed in the back surface of the
substrate is lower than when the photonic crystal is formed in the
semiconductor. In the case of forming the photonic crystal at the
interface between the semiconductor and the substrate, on the other
hand, the efficiency of diffraction by the periodic
projections/depressions lowers due to a small refractive index
difference between the semiconductor and the substrate so that the
effect of improving the light extraction efficiency is lower than
when the photonic crystal is formed at the uppermost surface of the
LED.
[0016] It is therefore an object of the present invention to
provide a semiconductor light emitting element with a light
extraction efficiency higher than achieved conventionally.
[0017] A first semiconductor light emitting element according to
the present invention comprises: an active layer made of a
semiconductor and generating light; a first semiconductor layer of
a first conductivity type formed through crystal growth on a
principal surface of the active layer; and a second semiconductor
layer of a second conductivity type provided on a back surface of
the active layer and having a back surface formed with a
two-dimensional periodic structure, the semiconductor light
emitting element outputting light generated in the active layer
from the back surface of the second semiconductor layer.
[0018] In the arrangement, the two-dimensional periodic structure
formed in the back surface of the second semiconductor layer
reduces damage received by the element during the fabrication
thereof and thereby improves power conversion efficiency to a level
higher than achieved conventionally.
[0019] The first semiconductor light emitting element further
comprises: a first electrode provided on a principal surface of the
first semiconductor layer and having a reflectivity of 80% or more
with respect to light at a peak wavelength of all the light
generated in the active layer; and a second electrode provided on a
region of the back surface of the second semiconductor layer in
which the two-dimensional periodic structure is not formed. The
arrangement allows the light emitted from the active layer toward
the first electrode to be reflected efficiently to the
two-dimensional periodic structure in the surface of the
semiconductor multilayer film and thereby allows the light
extraction efficiency achieved by the two-dimensional periodic
structure to be approximately doubled.
[0020] In particular, the first electrode is preferably a metal
film containing at least one of an Au film, a Pt film, a Cu film,
an Ag film, an Al film, and a Rh film in terms of practical
use.
[0021] A metal layer having a thickness of 10 .mu.m or more is
further provided in contact relation with the first electrode. The
arrangement allows effective conduction of heat generated in the
active layer so that the occurrence of a faulty operation is
suppressed.
[0022] In particular, the metal layer is composed of one metal
selected from the group consisting of Au, Cu, and Ag. The
arrangement is preferred since it allows efficient transfer of heat
to the outside and it can be formed easily by using a plating
technology.
[0023] When a period of the two-dimensional periodic structure is
.LAMBDA., a peak wavelength of the light generated in the active
layer is .lamda., and a refractive index of the second
semiconductor layer is N, 0.5.lamda./N<.LAMBDA.<20.lamda./N
is preferably satisfied. In the case where 0.5.lamda./N>.LAMBDA.
is satisfied, an angle change caused by diffraction is large and
the angle of the diffracted light exceeds the total reflection
critical angle so that it is not emitted to the outside of the
semiconductor light emitting element. In this case, diffraction
with a two-dimensional period cannot improve the light extraction
efficiency. In the case where .LAMBDA.>20.lamda./N is satisfied,
the period becomes extremely larger than the wavelength of the
light emitted from the active layer so that the effect of
diffraction is hardly expected.
[0024] When a height of the two-dimensional periodic structure is
h, a peak wavelength of the light generated in the active layer is
.lamda., a refractive index of a portion surrounding the
semiconductor light emitting element is n.sub.1, and a refractive
index of the second semiconductor layer is n.sub.2, h is an
integral multiple of .lamda./{2(n.sub.2-n.sub.1)}. This results in
a phase difference which causes a light component passing through
the upper portions of the projections/depressions of the
two-dimensional periodic structure and a light component passing
through the lower portions of the projections/depressions to
conductivity interfere wiht each other. Consequently, the
efficiency of diffraction caused by the two-dimensional periodic
structure can be maximized and the light extraction efficiency can
be improved.
[0025] Depending on a material composing the two-dimensional
periodic structure, a vertical cross section of the two-dimensional
periodic structure may be configured as either a quadrilateral or a
triangular wave.
[0026] A second semiconductor light emitting element according to
the present invention comprises: a substrate which transmits light;
a first semiconductor layer formed through crystal growth on a
principal surface of the substrate and having a principal surface
formed with a two-dimensional periodic structure; a second
semiconductor layer of a first conductivity type provided on the
principal surface of the first semiconductor layer; an active layer
provided on a principal surface of the second semiconductor layer,
made of a semiconductor, and generating light; and a third
semiconductor layer of a second conductivity type provided on a
principal surface of the active layer, the semiconductor light
emitting element outputting the light generated in the active layer
from a back surface of the substrate.
[0027] The arrangement allows the improvement of the diffraction
efficiency without removing the substrate and thereby allows easier
fabrication than in the case where the substrate is removed.
[0028] A material composing the second semiconductor layer is not
buried in the two-dimensional periodic structure. Since the
arrangement allows an increase in the refractive index difference
between the two-dimensional periodic structure and a portion
surrounding the two-dimensional periodic structure, it becomes
possible to increase the diffraction efficiency and improve the
light extraction efficiency.
[0029] Preferably, a material of the substrate is one selected from
the group consisting of GaAs, InP, Si, SiC, AlN, and sapphire. If
the substrate is made of GaAs, InP, or Si, it can be removed easily
by wet etching. Even if the substrate is made of AlN, sapphire, or
SiC, it can be removed by dry etching. If the substrate is made of
a transparent material such as AlN or sapphire, it can be removed
by a laser lift-off process.
[0030] The back surface of the substrate is rough and, when a peak
wavelength of the light generated in the active layer is .lamda.,
an autocorrelation distance T in the back surface of the substrate
satisfies 0.5.lamda./N<T<20.lamda./N and a height
distribution D in a perpendicular direction satisfies
0.5.lamda./N<D<20.lamda./N. This scatters the light and
allows the light generated in the active layer to be emitted
efficiently to the outside of the substrate.
[0031] A semiconductor light emitting device according to the
present invention comprises: a semiconductor light emitting element
having a first semiconductor layer of a first conductivity type, an
active layer provided on a back surface of the first semiconductor
layer, made of a semiconductor, and generating light, and a second
semiconductor layer of a second conductivity type provided on a
back surface of the active layer and having a back surface formed
with a two-dimensional periodic structure, the semiconductor light
emitting element outputting the light generated in the active layer
from the back surface of the second semiconductor layer; a mounting
substrate on which the semiconductor light emitting element is
mounted; and a molding resin formed into a hemispherical
configuration to mold the semiconductor light emitting element.
[0032] The arrangement allows significant improvement of the light
extraction efficiency. This is because the light extracted from the
semiconductor light emitting element into the resin by the
two-dimensional periodic structure is incident perpendicularly to
the interface between the resin and the air due to the
hemispherical form of the resin and emitted into the air with
approximately 100% efficiency.
[0033] A method for fabricating a semiconductor light emitting
element according to the present invention comprises the steps of:
(a) forming a first two-dimensional periodic structure in a
principal surface of a substrate or in a principal surface of a
semiconductor layer provided on the substrate; and (b) successively
forming a first semiconductor layer, an active layer, and a second
semiconductor layer over the first two-dimensional periodic
structure.
[0034] The method makes it possible to form the two-dimensional
periodic structure without damaging the first semiconductor.
[0035] The step (b) may also include the step of forming, in a back
surface of the first semiconductor layer, a second two-dimensional
periodic structure which is complementary to the first
two-dimensional periodic structure by forming the first
semiconductor layer in such a manner that the first two-dimensional
periodic structure is filled therewith.
[0036] The method further comprises the step of: (c) after the step
(b), removing the substrate or the substrate and the semiconductor
layer to expose the second two-dimensional periodic structure. The
arrangement allows an increase in the refractive index difference
between the material composing the second two-dimensional periodic
structure and a portion surrounding the second two-dimensional
periodic structure and thereby allows the improvement of the
diffraction efficiency of the semiconductor light emitting
element.
[0037] The step (c) may also be performed by polishing the
substrate and by wet etching.
[0038] The step (c) is performed by a laser lift-off process. This
allows the substrate to be removed in a relatively short period of
time.
[0039] The substrate removed in the step (c) is reusable. This
achieves a reduction in fabrication cost.
[0040] The method further comprises the step of: after the step
(b), forming a high-reflection film composed of a metal film
containing at least one of an Au film, a Pt film, a Cu film, an Ag
film, an Al film, and a Rh film on a principal surface of the
second semiconductor layer. The arrangement allows the fabrication
of the semiconductor light emitting element with a higher light
extraction efficiency.
[0041] The method further comprises the step of: after the step
(c), performing wet etching under a condition where an etching
speed differs depending on a crystal plane to form the second
two-dimensional periodic structure composed of projections or
depressions each configured as a polygonal pyramid. The arrangement
allows the fabrication of the semiconductor light emitting element
with a higher light extraction efficiency. By thus properly
adjusting the configuration of each of the projections or
depressions through wet etching, design flexibility can be
increased.
[0042] The step (b) may also include the step of forming the first
semiconductor layer in such a manner as to form a cavity in the
first two-dimensional periodic structure.
[0043] The first two-dimensional periodic structure may also be
composed of a structure made of at least any one of an oxide, a
nitride, and a metal.
[0044] The second two-dimensional periodic structure is composed of
depressions and the method further comprises the step of: after the
steps (b) and (c), increasing a depth of each of the depressions or
changing a cross-sectional configuration of each of the depressions
by allowing electricity to flow in the semiconductor light emitting
element in an electrolytic solution. The arrangement allows the
angle between the inner side surface of each of the depressions and
the surface of the substrate to consequently approach 90 degrees
and thereby allows the improvement of the light extraction
efficiency by using the second two-dimensional periodic
structure.
[0045] The step (a) includes the steps of: (a1) coating a first
resist on the principal surface of the substrate or of the
semiconductor layer; (a2) pressing, against the first resist, a
mold formed with a two-dimensional periodic structure which is
complementary to the first two-dimensional periodic structure to
transfer a pattern which is homologous to the first two-dimensional
periodic structure thereto; and (a3) forming the first
two-dimensional periodic structure by using, as a mask, the first
resist to which the pattern homologous to the first two-dimensional
periodic structure has been transferred. Since the arrangement
allows the transfer of an extremely fine pattern formed
preliminarily in the mold, it facilitates micro-processing
performed with respect to the thin film.
[0046] The first two-dimensional periodic structure is composed of
a resin provided on the substrate and the method further comprises
the steps of: (d) after the step (b), removing the substrate; (e)
coating a second resist on a back surface of the first
semiconductor layer; (f) placing the substrate formed with the
first two-dimensional periodic structure on the second resist to
transfer a pattern which is complementary to the first
two-dimensional periodic structure to the second resist; and (g)
forming a second two-dimensional periodic structure which is
complementary to the first two-dimensional periodic structure in a
back surface of the first semiconductor layer by using, as a mask,
the second resist to which the pattern complementary to the first
two-dimensional periodic structure has been transferred. Since the
arrangement allows the transfer of an extremely fine pattern formed
preliminarily in the mold, it facilitates micro-processing
performed with respect to the thin film.
[0047] The method further comprises the steps of: mounting the
semiconductor light emitting element on a mounting substrate; and
molding the semiconductor light emitting element with a
hemispherical resin by using a mold die formed with a hemispherical
cavity. The arrangement allows the fabrication of the semiconductor
light emitting element with higher light extraction efficiency. By
molding the resin by using the mold die, it becomes possible to
stably manufacture semiconductor light emitting elements having
uniform configurations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a perspective view showing a semiconductor light
emitting element-according to a first embodiment of the present
invention;
[0049] FIG. 2 is a view showing the result of theoretically
calculating the incident-angle dependence of an amount of light
emitted to the outside of the semiconductor light emitting
element;
[0050] FIG. 3A is a view showing a structure of an LED in a real
space and FIGS. 3B and 3C are views each showing a structure of the
light emitting element in a wave-number space;
[0051] FIGS. 4A and 4B are views each showing a structure of the
wave-number space when a two-dimensional periodic structure with a
0.1-.mu.m period has been formed in the surface of an n-type GaN
layer;
[0052] FIG. 5 is a view showing a refractive index at each portion
of a semiconductor layer having a surface formed with a periodic
structure;
[0053] FIGS. 6A to 6C are views each showing a structure of the
wave-number space when a two-dimensional periodic structure with a
0.2-.mu.m period has been formed on the surface of the n-type GaN
layer;
[0054] FIG. 7 is a view showing a structure of the wave-number
space when a two-dimensional periodic structure with a 0.4-.mu.m
period has been formed in the surface of the r-type GaN layer;
[0055] FIG. 8 is a view for illustrating a solid angle used to
calculate light extraction efficiency;
[0056] FIG. 9 is a view showing a value obtained by normalizing
light extraction efficiency obtained by using numerical calculation
to light extraction efficiency when the surface of the n-type GaN
layer is flat;
[0057] FIGS. 10A and 10B are plan views each showing the
arrangement of a two-dimensional periodic structure formed in the
surface of the n-type GaN layer;
[0058] FIGS. 11A to 11F are perspective views illustrating a method
for fabricating the semiconductor light emitting element according
to the first embodiment;
[0059] FIG. 12A is a view showing the respective current-voltage
characteristics of a conventional semiconductor light emitting
element and the semiconductor light emitting element according to
the first embodiment and FIG. 12B is a view showing the respective
current-light output characteristics of the conventional
semiconductor light emitting element and the semiconductor light
emitting element according to the first embodiment;
[0060] FIGS. 13A and 13B are perspective views each showing a
variation of the semiconductor light emitting element according to
the first embodiment;
[0061] FIG. 14 is a perspective view showing a variation of the
semiconductor light emitting element according to the first
embodiment;
[0062] FIGS. 15A and 15B are perspective views each showing a
variation of the semiconductor light emitting element according to
the first embodiment;
[0063] FIGS. 16A and 16B are perspective views each showing a
variation of the semiconductor light emitting element according to
the first embodiment;
[0064] FIG. 17 is a view showing the result of theoretically
calculating the dependence of light extraction efficiency on the
tilt angle of a depression;
[0065] FIG. 18A is a perspective view showing a semiconductor light
emitting element according to a second embodiment of the present
invention and FIG. 18B is a plan view when the semiconductor light
emitting element according to the second embodiment is viewed from
above;
[0066] FIG. 19A is a view showing the result of theoretically
calculating the transmittance T of light incident on the surface of
an n-type GaN layer when the surface (back surface) of the n-type
GaN layer is formed with projections configured as hexagonal
pyramids and FIG. 19B is a view showing the relationship between
the period of a two-dimensional periodic structure and light
extraction efficiency;
[0067] FIGS. 20A to 20F are perspective views illustrating a method
for fabricating the semiconductor light emitting element according
to the second embodiment;
[0068] FIGS. 21A to 21C are views illustrating a variation of the
method for fabricating the semiconductor light emitting element
according to the second embodiment;
[0069] FIGS. 22A and 22B are views illustrating a variation of the
method for fabricating the semiconductor light emitting element
according to the second embodiment;
[0070] FIGS. 23A and 23B are views illustrating the variation of
the method for fabricating the semiconductor light emitting element
according to the second embodiment;
[0071] FIGS. 24A and 24B are views illustrating a variation of the
method for fabricating the semiconductor light emitting element
according to the second embodiment;
[0072] FIGS. 25A to 25C are views illustrating a variation of the
method for fabricating the semiconductor light emitting element
according to the second embodiment;
[0073] FIG. 26 is a perspective view showing a semiconductor light
emitting device according to a third embodiment of the present
invention;
[0074] FIG. 27A is a view showing the result of theoretically
calculating the transmittance of light when a semiconductor light
emitting element is molded with a resin and FIG. 27B is a view
showing the result of theoretically calculating the dependence of
light extraction efficiency on the period of a two-dimensional
periodic structure in the semiconductor light emitting element
according to the third embodiment;
[0075] FIGS. 28A to 28D are perspective views illustrating a method
for fabricating the semiconductor light emitting element according
to the third embodiment;
[0076] FIG. 29 is a cross-sectional view showing a part of a
semiconductor light emitting element according to a fourth
embodiment of the present invention;
[0077] FIGS. 30A to 30E are cross-sectional views illustrating a
method for fabricating the semiconductor light emitting element
according to the fourth embodiment;
[0078] FIGS. 31A to 31E are perspective views illustrating a method
for fabricating a semiconductor light emitting element according to
a fifth embodiment of the present invention;
[0079] FIGS. 32A to 32G are perspective views illustrating a method
for fabricating a semiconductor light emitting element according to
a sixth embodiment of the present invention;
[0080] FIG. 33 is a perspective view showing a semiconductor light
emitting element according to a seventh embodiment of the present
invention;
[0081] FIG. 34 is a perspective view showing a conventional
semiconductor light emitting element; and
[0082] FIG. 35 is a perspective view showing a conventional
semiconductor light emitting element having an upper surface formed
with a photonic crystal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0083] The present invention will be described more specifically in
accordance with the individual embodiments thereof. In the present
specification, the surface of a semiconductor layer formed by
epitaxial growth which is in the direction of crystal growth will
be termed a principal surface and the surface thereof opposite to
the principal surface will be termed a back surface.
Embodiment 1
--Structure of Light Emitting Element--
[0084] FIG. 1 is a perspective view showing a semiconductor light
emitting element according to a first embodiment of the present
invention. As shown in the drawing, the semiconductor light
emitting element according to the present embodiment comprises: a
p-type GaN layer (first semiconductor layer) 3 formed by epitaxial
growth and having a thickness of 200 nm; a high-reflection p
electrode (first electrode) 2 formed on the crystal growing surface
(principal surface) of the p-type GaN layer 3, made of platinum
(Pt) and gold (Au) which are stacked in layers, and having a
thickness of 1 .mu.m; an Au plating layer 1 formed on the lower
surface of the high-reflection p electrode 2 and having a thickness
of 10 .mu.m; a non-doped InGaN active layer 4 formed on the back
surface of the p-type GaN layer 3 and having a thickness of 3 nm;
an n-type GaN layer (second semiconductor layer) 5 formed on the
back surface of the non-doped InGaN active layer 4, having a back
surface formed with a projecting two-dimensional periodic structure
6, and having a thickness of 4 .mu.m; and an n electrode (second
electrode) 7 formed on the back surface of the n-type. GaN layer 5,
made of titanium (Ti) and Al which are stacked in layers, and
having a thickness of 1 .mu.m. The lower surface used herein
indicates the surface positioned in the lower part of FIG. 1. In
the example shown in FIG. 1, the high-reflection p electrode 2 is
provided on the entire principal surface of the p-type GaN layer 3
and the n electrode 7 is provided on a part of the back surface of
the n-type GaN layer 5. The wording "non-doped" indicates that
intentional doping has not been performed with respect to a layer
of concern.
[0085] The semiconductor light emitting element according to the
present embodiment functions as an LED from which light is
extracted through the back surface of the n-type GaN layer 5. The
PL peak wavelength of the non-doped InGaN active layer 4 is 405 nm.
As will be described layer, MOCVD (Metal-Organic Chemical Vapor
Deposition), MBE (Molecular Beam Epitaxy), or the like will be used
as a method for the crystal growth of a nitride-based compound
semiconductor composing the semiconductor light-emitting
element.
[0086] The period of the two-dimensional periodic structure 6
formed in the back surface of the n-type GaN layer 5, i.e., the
spacing between the respective centers of adjacent projections in a
two-dimensional plane is 0.4 .mu.m and the height of each of the
projections is 150 nm.
--Description of Diffraction at Surface of n-Type GaN Layer--
[0087] A description will be given next to diffraction at the
surface (back surface) of the n-type GaN layer of the semiconductor
light emitting element according to the present embodiment based on
the result of simulation.
[0088] FIG. 2 is a view showing the result of theoretically
calculating the incident-angle dependence of the transmittance of
light emitted from the non-doped InGaN active layer and incident on
the back surface (the upper surface in the drawing) of the n-type
GaN layer, i.e., an amount of light emitted to the outside of the
LED. For the theoretical calculation, numerical analysis in
accordance with a FDTD (Finite Difference Time Domain) method was
used. It is assumed that the incident angle when the light is
incident perpendicularly to the back surface of the n-type GaN
layer is zero degree.
[0089] As shown in FIG. 2, when the surface of the n-type GaN layer
is smooth and flat, the transmittance is constant when the incident
angle is in the range of zero degree to a total reflection critical
angle .theta.c (since the refractive index of GaN is about 2.5,
.theta.c=sIn-1(1/2.5)=about 23 degrees is satisfied) and the value
thereof is about 90%. The cause of the reflection of 10% of
generated light and the returning of the reflected light to the
inside of the LED is Fresnel reflection resulting from the
refractive index difference between GaN and the air. The reason for
the transmittance which becomes approximately zero when the
incident angle exceeds the total reflection critical angle is that
total reflection occurs at the interface between GaN and the air.
The cause of the occurrence of the total reflection will be
described with reference to FIG. 3.
[0090] FIG. 3A is a view showing a structure of the LED in an
actual space (1) and FIGS. 3B and 3C are views each showing a
structure of the light emitting element in a wave-number space.
FIG. 3B shows the case where the incident angle of light is small
and FIG. 3C shows the case where the incident angle of light is
large. The semi-circle in each of FIGS. 3B and 3C is an
equi-frequency surface, which indicates the magnitude (Wave Number
k=2.pi.m/.lamda. where n is a refractive index and .lamda. is a
wavelength in vacuum) of a wave vector to be satisfied by an
incident wave, a reflected wave, and a transmitted wave in the LED
and in the air. The semi-circle indicates the conservation law of
photon energy (h.omega./2.pi., h is the Planck's constant). This is
because the relationship given by k=.omega.n/c is established (c is
a velocity of light in vacuum). When the structure in the actual
space has translational symmetry relative to a horizontal direction
as in the case (1) shown in FIG. 3A, there is the conservation law
of a wave number component in the horizontal direction which should
be observed by each of the incident wave, the reflected wave, and
the transmitted wave (which is related to the phase continuity of
an electromagnetic wave). The incident angle and an emission angle
are determined to satisfy the foregoing two laws.
[0091] When the incident angle of light is small as shown in FIG.
3B, the light can be emitted into the air because the emission
angle which satisfies the foregoing two laws exists. When the
incident angle of light is large as shown in FIG. 3C, however,
there is no emission angle which satisfies the wave number
component in the horizontal direction so that the light cannot be
emitted into the air. In this case, the transmittance T satisfies
T=0 so that, when there is no absorption at the interface between
the LED and the air, the reflectivity R satisfies R=1 since the
conservation law of optical energy, which is T+R=1, should be
satisfied. Consequently, light is totally reflected by the
interface between the LED and the air. Even if a structure which
reduces Fresnel reflection, such as a no-reflection film, is
introduced into the interface between the LED and the air, R=1 is
inevitably satisfied under a condition which satisfies the
transmittance T=0 so that total reflection is inevitable.
[0092] FIGS. 4A and 4B are views showing a structure of the
wave-number space when a two-dimensional periodic structure with a
0.1 -.mu.m period is formed in the surface of the n-type GaN. The
periodic structure formed in the surface causes diffraction so that
it is necessary for a wave number k.sub.1// in the horizontal
direction of the incident wave and a wave number k.sub.2// in the
horizontal direction of the transmitted wave to satisfy the
following condition based on Diffraction Vector G=2.pi./.LAMBDA.
(.LAMBDA. is a period): k.sub.2//=k.sub.1//.+-.mG (m is an order of
diffraction and m=0, .+-.1, .+-.2, . . . ). The transmitted wave
occurs when the wave number k.sub.2// which satisfies the foregoing
expression and the condition for the equi-frequency surface
mentioned above exists.
[0093] As shown in FIG. 4A, when the period of the structure is 0.1
.mu.m and the incident angle is zero degree, the magnitude of the
diffraction vector is excessively large so that, if it is assumed
that the transmitted wave is diffracted, total reflection occurs
since k.sub.2// is larger than the equi-frequency surface in the
air. In this case, therefore, no diffraction occurs. When the
incident angle is 70 degrees at which a 0-order transmitted wave is
totally reflected, as shown in FIG. 4B, the condition for total
reflection is satisfied even if the light is diffracted. In the
case with the period, therefore, total reflection occurs at an
incident angle not less than the total reflection critical angle in
the same manner as when the surface is flat.
[0094] FIG. 5 is a view showing a refractive index at each portion
of a semiconductor layer having a surface formed with the periodic
structure. As shown in FIG. 5, when the period of the periodic
structure is smaller than the wavelength of light, the effective
refractive index of the two-dimensional periodic structure in the
surface of the semiconductor layer lowers due to the
projections/depressions thereof so that the two-dimensional
periodic structure functions as a layer having a refractive index
in the middle of the refractivities of the air and the LED. In this
case, the refractive index difference between the air and the LED
is reduced so that it becomes possible to suppress Fresnel
reflection which occurs when the incident angle is smaller than the
total reflection critical angle and improve the transmittance of
light when the incident angle is smaller than the total reflection
critical angle, as shown in FIG. 2.
[0095] A description will be given next to the case where the
surface (back surface) of the n-type GaN layer has a
two-dimensional periodic structure with a 0.2-.mu.m period. FIGS.
6A to 6C are views showing a structure of the wave-number space
when the two-dimensional periodic structure with a 0.2-.mu.m period
has been formed in the surface of the n-type GaN layer.
[0096] Under this condition, total reflection occurs since
k.sub.2// when light is diffracted is larger than the
equi-frequency surface in the air provided that the incident angle
of the light is zero degree so that no diffraction occurs. In this
case, a 0-order transmitted wave passes through the interface
between the n-type GaN layer and the air.
[0097] By contrast, when the surface is flat and the incident angle
at which total reflection occurs is 30 degrees or 70 degrees,
k.sub.2// is smaller than the equi-frequency surface in the air so
that the diffracted transmitted wave (the order of diffraction: -1)
is allowed to pass through the interface, as shown in FIGS. 6B and
6C. As a result, the transmittance does not become zero even at an
angle not less than the total reflection critical angle, as shown
in FIG. 2. Since the diffraction efficiency also contributes to an
actual transmittance, the transmittance presents a complicated
curve. In this case, the diffraction vector is relatively large so
that diffraction of an order not lower than second does not
contribute to transmission.
[0098] FIG. 7 is a view showing a structure of the wave-number
space when a two-dimensional periodic structure with a 0.4-.mu.m
period has been formed in the surface of the n-type GaN layer.
Since the diffraction vector is relatively small with this period,
first-order diffraction is related to transmission even when the
incident angle of light is zero degree as shown in FIG. 7A. When
the incident angle is 35 degrees as shown in FIG. 7B, first- and
second-order diffraction contributes to transmission, while second-
and third-order diffraction contributes to transmission when the
incident angle is 70 degrees as shown in FIG. 7C. As a result, the
transmittance becomes relatively high even at an angle not less
than the total reflection critical angle, as shown in FIG. 2.
[0099] Thus, as the period of the structure formed in the surface
of the n-type GaN layer becomes larger, higher-order diffraction is
involved and the behavior of light becomes more complicated.
[0100] Based on the result of the foregoing analysis, it is assumed
that, in the semiconductor light emitting element according to the
present embodiment, 0.5.lamda./N<.LAMBDA.<20.lamda./N is
satisfied when the refractive index of the semiconductor layer
formed with the two-dimensional periodic structure is N and the
period of the two-dimensional periodic structure is .LAMBDA.. If
0.5.lamda./N>.LAMBDA. is satisfied, an angle change caused by
diffraction is large and the diffracted light is at an angle over
the total reflection critical angle so that the diffracted light is
not emitted to the outside of the semiconductor light emitting
element. In this case, diffraction with a two-dimensional period
cannot improve the light extraction efficiency. If
.LAMBDA.>20.lamda./N is satisfied, the period becomes extremely
larger than the wavelength of light emitted from the active layer
so that the effect of diffraction is hardly expected. In view of
the foregoing, the condition defined by
0.5.lamda./N<.LAMBDA.<20.lamda./N is preferably satisfied to
validate the effect of the two-dimensional periodic structure.
[0101] FIG. 8 is a view for illustrating a solid angle used to
calculate light extraction efficiency. As shown in the drawing, it
is necessary to obtain actual light extraction efficiency .eta. by
integrating a theoretical reflectivity with an incident angle in
consideration of the effect of a solid angle on a transmittance T
(.theta.) at each incident angle. Specifically, .eta. can be
derived from the following numerical expression:
.eta.=.intg.2.pi.T(.theta.).theta.d.theta..
[0102] FIG. 9 is a view showing a value obtained by normalizing
light extraction efficiency obtained from the foregoing numerical
expression to light extraction efficiency when the surface of the
n-type GaN layer is flat. As the parameters of calculation, the
period A and the height h of each of the projections/depressions
are considered. According to the result, the light extraction
efficiency is maximum when the height of each of the projections in
the surface of the GaN layer is 150 nm. This is because, when the
height h of each of the projections/depressions is
.lamda./{2(n.sub.2-n.sub.1)} (where .lamda. is a light emission
wavelength in an air or in vacuum, n.sub.1 is the refractive index
of the air, and n.sub.2 is the refractive index of a
semiconductor), the phase of the light component of light passing
through the projections/depressions which passes through each of
the projections and the phase of the light component thereof which
passes through each of the depressions intensify each other through
interference so that the diffraction effect achieved by the
projections/depression becomes maximum. In this case, h=about 130
nm is satisfied, which nearly coincides with the result of
numerical calculation in accordance with the FDTD method. Thus, in
the semiconductor light emitting element according to the present
embodiment, h is most preferably in the vicinity of an integral
multiple of .lamda./{2(n.sub.2-n.sub.1)}. It is assumed herein that
h approximates to .lamda.{2(n.sub.2-n.sub.1)} by considering
general performance variations resulting from the fabrication
process.
[0103] It can be seen from FIG. 9 that, when the height h of each
of the projections/depression is 150 nm, the light extraction
efficiency is improved by 2.6 times at the maximum compared with
the case where the surface of the n-type GaN layer is flat provided
that period .LAMBDA. is 0.4 to 0.5 .mu.m. To circumvent total
reflection, it is necessary to use higher-order diffraction as the
period .LAMBDA. is longer. However, since the diffraction
efficiency becomes lower as the order of diffraction is higher, the
light extraction efficiency becomes lower as the period is longer
when the period .LAMBDA. is in the range not less than 0.4 .mu.m.
For example, when the period of the structure shown in FIG. 2 is
2.0 .mu.m, the transmittance of light at an incident angle not
smaller than the total reflection critical angle is lower than that
with a 0.4-.mu.m period.
[0104] FIGS. 10A and 10B are plan views showing the arrangement of
the two-dimensional periodic structure formed in the surface of the
n-type GaN layer. As shown in the drawings, the two-dimensional
periodic structure formed in the semiconductor light emitting
element according to the present embodiment may be either a
tetragonal lattice or a triangular lattice.
--Method for Fabricating Light Emitting Element--
[0105] FIGS. 11A to 11F are perspective views illustrating a method
for fabricating the semiconductor light emitting element according
to the present embodiment which is shown in FIG. 1.
[0106] First, as shown in FIG. 11A, the sapphire substrate 8 is
prepared and the AlGaN layer 9 with a thickness of 1 .mu.m is
formed through crystal growth by MOCVD (Metal Organic Chemical
Vapor Deposition) on the principal surface of the sapphire
substrate 8. If the thickness of the AlGaN layer 9 is 1 .mu.m,
crystal defects occurring therein are reduced. The composition of
Al in the AlGaN layer 9 is assumed herein to be 100%, though the
AlGaN layer 9 may have any Al composition provided that it is
transparent to the wavelength of light used for a laser lift-off
process performed later.
[0107] Next, as shown in FIG 11B, the depressed-type
two-dimensional periodic structure 10 is formed in the principal
surface of the AlGaN layer through patterning. In the present step,
a resist for an etching mask is patterned by using electron beam
exposure, a stepper, and the like. Then, the etching of the AlGaN
layer can be performed by using a dry etching technology such as
RIE (Reactive Ion Etching) or ion milling, by photoelectrochemical
etching under the irradiation of ultraviolet light, or by using a
wet etching technology such as etching using a heated acid/alkali
solution for the etching of the nitride-based compound
semiconductor. In this example, the two-dimensional periodic
structure 10 is formed by electron beam exposure and RIE. It is
assumed that the period of the two-dimensional periodic structure
10 is 0.4 .mu.m and the depth of each of the depressions is 150 nm.
Although the configuration of the two-dimensional periodic
structure 10 is not particularly limited, the depression has a
cylindrical hole in the example shown in FIG. 11B.
[0108] Next, as shown in FIG 11C, an n-type GaN layer 11
(corresponding to the n-type GaN layer 5 in FIG. 1), a non-doped
InGaN active layer 12 (corresponding to the non-doped InGaN active
layer 4), a p-type GaN layer 13 (corresponding to the p-type GaN
layer 3) are formed in this order by MOCVD on the principal surface
of the AlGaN layer 9 formed with the two-dimensional periodic
structure 10. It is assumed herein that the respective thicknesses
of the n-type GaN layer 11, the non-doped InGaN active layer 12,
and the p-type GaN layer 13 are 4 .mu.m, 3 nm, and 200 nm. In the
present step, the crystal growth of the n-type GaN layer 11 is
performed by setting conditions for the growth such that the
two-dimensional periodic structure 10 is filled therewith.
[0109] Thereafter, the Pt/Au high-reflection p electrode 2
(composed of a multilayer film of Pt and Au) is formed on the
principal surface of the p-type GaN layer 13 by, e.g., electron
beam vapor deposition. Further, the Au plating layer 15 with a
thickness of about 50 .mu.m is formed by using the Au layer of the
high-reflection p electrode 2 as an underlying electrode.
[0110] Subsequently, as shown in FIG. 11D, a KrF excimer laser (at
a wavelength of 248 nm) is applied to the back surface of the
sapphire substrate 8 for its irradiation in such a manner as to
scan the surface of a wafer. The laser beam used for irradiation is
not absorbed by the sapphire substrate 8 and the AlGaN layer 9 but
is absorbed only by the n-type GaN layer 11 so that GaN is unbonded
by local heat generation in the vicinity of the interface with the
AlGaN layer 9. As a result, it becomes possible to separate the
AlGaN layer 9 and the sapphire substrate 8 from the n-type GaN
layer 11 and a device structure made of a GaN-based semiconductor
is obtainable. The light source used herein may be any light source
provided that it supplies light at a wavelength absorbed by the GaN
layer and transparent to the AlGaN layer and the sapphire layer. It
is also possible to use a third harmonic (at a wavelength of 355
nm) of a YAG laser or an emission line of a mercury lamp (at a
wavelength of 365 nm).
[0111] Next, as shown in FIG. 11E, the sapphire substrate 8 and the
AlGaN layer 9 are removed from the state shown in FIG. 11D. As a
result, the projecting-type two-dimensional periodic structure 6 is
formed by self alignment in the back surface of the n-type GaN
layer. Since such a semiconductor multilayer film (i.e., a
multilayer film composed of the n-type GaN layer 11, the non-doped
InGaN active layer 12, and the p-type GaN layer 13) resulting from
the removal of the substrate is an extremely thin film with a
thickness of about 5 .mu.m, it has been difficult to form an
extremely fine structure such as a photonic crystal by using a
conventional photolithographic technology. However, the method
according to the present invention allows easy transfer of the
extremely fine structure to the surface (back surface) of the
semiconductor multilayer film by merely depositing a semiconductor
multilayer film over the depressed two-dimensional periodic
structure 10 formed preliminarily in the substrate and removing the
substrate thereafter.
[0112] Then, as shown in FIG. 11F, the Ti/Al n electrode 7 having a
thickness of 1 .mu.m is formed on the region of the back surface of
the n-type GaN layer 11 in which the two-dimensional periodic
structure 6 is not formed by vapor deposition and lithography or
the like, whereby the semiconductor light emitting element
according to the present embodiment is fabricated.
--Effects of Semiconductor Light Emitting Element and Fabrication
Method Therefor--
[0113] FIGS. 12 show the characteristics of the semiconductor light
emitting element thus obtained, of which FIG. 12A is a view showing
the respective current-voltage characteristics of the semiconductor
light emitting elements according to the conventional and present
embodiments and FIG. 12B is a view showing the respective
current-light output characteristics of the semiconductor light
emitting elements according to the conventional and present
embodiments. In the drawings, the graphs of the dotted lines
represent the characteristics of the semiconductor element having
the conventional structure in which the surface of the LED is flat
and from which the sapphire substrate has not been removed and the
graphs of the solid lines represent the characteristics of the
semiconductor light emitting element according to the present
embodiment.
[0114] From the current-voltage characteristics shown in FIG. 12A,
it will be understood that the semiconductor light emitting
elements according to the present and conventional embodiments have
substantially equal current-voltage characteristics including
substantially the same rising voltages. From the comparison with
the conventional embodiment in which the projections/depressions
are not formed in the surface of the n-type GaN layer, it will be
understood that, in the semiconductor light emitting element
fabricated by the method according to the present embodiment, the
semiconductor multilayer film receives no processing-induced damage
resulting from the formation of the two-dimensional periodic
structure. Although the sapphire substrate 8 can be removed by
polishing and the AlGaN layer 9 can be removed by etching in the
substrate separating step shown in FIGS. 11D and 11E, a method
which uses a laser is more preferable because it is completed in a
shorter period of time.
[0115] From the current-light output characteristic shown in FIG.
12B, on the other hand, it will be understood that, under the same
current, the light output of the semiconductor light emitting
element according to the present embodiment has increased to a
value substantially five times the light output of the
semiconductor light emitting element according to the conventional
embodiment. The value is about double the theoretically calculated
value shown in FIG. 2. This is because, due to the two-dimensional
periodic structure in the surface of the LED, the efficiency of
light extraction from the surface has increased to about 2.5 times
the efficiency of light extraction from the flat surface of the
conventional LED and, in addition, the high-reflection p electrode
14 formed on the lower surface of the LED (the back surface of the
p-type GaN layer 13) allows the light emitted from the non-doped
InGaN active layer 12 toward the high-reflection p electrode 14 to
be reflected efficiently to the two-dimensional periodic structure
6.
[0116] From FIG. 12B, it will also be understood that the light
output of the conventional structure is saturated under a large
current, while the light output of the element according to the
present embodiment is not saturated even under a large current over
100 mA. This is because heat generated in the active layer of the
conventional structure is dissipated through the n-type
semiconductor layer as thick as several micrometers and through the
sapphire substrate with a poor heat conduction property. This is
also because the light emitting element according to the present
embodiment has an excellent heat dissipation property because heat
from the active layer can be dissipated from the p-type
semiconductor as thin as submicron meters through the Au plating
layer with a high thermal conductivity.
[0117] Thus, even under a large current, the Au plating layer 15
allows the retention of the improved light extraction efficiency
achieved by the two-dimensional periodic structure 6.
[0118] FIG. 13A and 13B, FIG. 14, and FIGS. 15A and 15B are
perspective views showing variations of the semiconductor light
emitting element according to the present embodiment. FIGS. 16A and
16B are perspective views each showing a variation of the method
for fabricating the semiconductor light emitting element according
to the present embodiment.
[0119] Although the projecting-type two-dimensional periodic
structure 6 has been formed by using the depressed-type
two-dimensional periodic structure 10 formed in the surface of the
AlGaN layer 9 on the sapphire substrate 8 as a "mold" in the
semiconductor light emitting element according to the present
embodiment, the same effects are also achievable by forming the
projecting-type two-dimensional periodic structure 16 in the
surface of the AlGaN layer 9 and thereby forming a depressed-type
two dimensional periodic structure 17 in the surface of the LED, as
shown in FIGS. 13A and 13B. That is, the incident light can be
diffracted provided that the two-dimensional periodic structure has
been formed whether the structure in the surface of the LED is of
the projecting type or the depressed type.
[0120] Besides the method which forms the projections/depressions
in the AlGaN layer 9, a method as shown in FIG. 14 which forms the
projecting or depressed two-dimensional periodic structure 16 in
the principal surface of the sapphire substrate 8 also allows the
implementation of a semiconductor light emitting element in which
the projecting-type or depressed-type two-dimensional periodic
structure is formed in the surface of the AlGaN layer 9 by using
the projecting or depressed two-dimensional periodic structure 16
as a mold.
[0121] Alternatively, as shown in FIG. 15A, it is also possible to
form the projecting-type two-dimensional periodic structure 16
structure by forming an oxide film such as a SiO.sub.2 film, a
nitride film such as a SiN film, or a metal film such as a tungsten
(W) film on the sapphire substrate 8 and then patterning the formed
film. As shown in FIG. 15B, a light emitting element having the
same characteristics as the semiconductor light emitting element
according to the present embodiment can also be fabricated by
forming the projecting two-dimensional periodic structure 16
composed of an oxide film, a nitride film, or a metal film in the
principal surface of the AlGaN layer 9.
[0122] If a SiC substrate is used in place of the sapphire
substrate, the substrate can be removed by selective dry etching
performed with respect to SiC and GaN. If a Si substrate is used,
the substrate can easily be removed by wet etching.
[0123] In the case where the depressed two-dimensional periodic
structure 17 formed in the back surface (upper surface) of the
n-type GaN layer 11 through the removal of the substrate has a
small depth or the inclination of the inner tilted surface of each
of the depressions is not perpendicular, the configuration of the
depression can be adjusted by performing processes as shown in
FIGS. 16A and 16B after the removal of the substrate.
[0124] Specifically, as shown in FIG. 16A, an LED structure and a
counter electrode made of Pt or the like are immersed in an
electrolytic solution such as an aqueous KOH solution and a voltage
is applied between the LED and the counter electrode by using the p
side of the LED as a positive electrode. Consequently, anodic
oxidation causes the etching of GaN, as shown in FIG. 16B, but only
the depression is etched due to the localization of an electric
field so that the depth of the depression is increased
successfully. The localization of the electric field to the
depression also causes the etching resulting from the anodic
oxidation to proceed perpendicularly. As a result, the depression
having a perpendicular tilted surface can be formed by the etching
resulting from the anodic oxidation even when the inner tilted
surface of the depression after the removal of the substrate is not
vertical.
[0125] FIG. 17 is a view showing the result of theoretically
calculating the dependence of the light extraction efficiency on
the tilt angle of the depression. It is assumed herein that the
tilt angle is obtained by subtracting, from 180 degrees, the angle
formed between the side surface of the depression and the upper
surface of the n-type GaN layer 11 in a vertical cross section, as
shown in the left part of the drawing. From the result shown in
FIG. 17, it will be understood that the light extraction efficiency
lowers abruptly when the tilt angle becomes 50 degrees or less.
That is, when the tilt angle of the two-dimensional periodic
structure that can be formed after the removal of the substrate is
small, the tilt angle can be increased by the anodic oxidation
etching described above and the high light extraction efficiency
can be implemented. Thus, in the semiconductor light emitting
element according to the present embodiment, the tilt angle of the
two-dimensional periodic structure is adjusted preferably to 50
degrees or more. Even when the two-dimensional periodic structure
has a projecting configuration, the tilt angle is preferably 50
degrees or more in terms of improving the light extraction
efficiency.
Embodiment 2
[0126] FIG. 18A is a perspective view showing a semiconductor light
emitting element according to a second embodiment of the present
invention. FIG. 18B is a plan view when the semiconductor light
emitting element according to the second embodiment is viewed from
above. The semiconductor light emitting element according to the
present embodiment is different from the semiconductor light
emitting element according to the first embodiment in that a
projecting two-dimensional periodic structure 18 formed in the
upper surface (back surface) of the n-type GaN layer 5 is
configured as polygonal pyramids.
[0127] As shown in FIGS. 18A and 18B, the semiconductor light
emitting element according to the present embodiment comprises: the
p-type GaN layer 3 formed by epitaxial growth and having a
thickness of 200 nm; the high-reflection p electrode 2 formed on
the crystal growing surface of the p-type GaN layer 3, made of
platinum (Pt) and gold (Au) which are stacked in layers, and having
a thickness of 1 .mu.m; the Au plating layer 1 formed on the lower
surface of the high-reflection p electrode 2 and having a thickness
of 10 .mu.m; the non-doped InGaN active layer 4 formed on the back
surface of the p-type GaN layer 3 and having a thickness of 3 nm;
the n-type GaN layer 5 formed on the back surface of the non-doped
InGaN active layer 4, having a back surface formed with a
two-dimensional periodic structure 18 composed of projections each
configured as a hexagonal pyramid, and having a thickness of 4
.mu.m; and the n electrode 7 formed on the back surface of the
n-type GaN layer 5, made of titanium (Ti) and Al which are stacked
in layers, and having a thickness of 1 .mu.m. In the same manner as
in the first embodiment, the PL peak wavelength of the non-doped
InGaN active layer 4 is 405 nm. The side surface of the projecting
structure in the back surface of the n-type GaN layer 5 is composed
of the {10-1-1} plane of GaN. The period of the two-dimensional
periodic structure 18, i.e., the spacing between the respective
centers of the adjacent projections in a two-dimensional plane is
1.0 .mu.m and the height of each of the projections is 950 nm.
[0128] FIG. 19A is a view showing the result of theoretically
calculating the transmittance T of light emitted from the active
layer and incident on the surface of the n-type GaN layer when the
projections each configured as the hexagonal pyramid are formed in
the surface (back surface) of the n-type GaN layer. FIG. 19B is a
view showing the relationship between the period of the
two-dimensional periodic structure and the light extraction
efficiency. FIG. 19B assumes 1 when the surface of the n-type GaN
layer is flat and shows, for comparison, the cases where the
two-dimensional periodic structure has a projecting configuration
and a projecting/depressed configuration (the same configuration as
shown in the first embodiment).
[0129] From the result shown in FIG. 19A, it will be understood
that, even when the period of the two-dimensional periodic
structure is as long as 1.0 .mu.m, the projecting structure shows a
high transmittance at an incident angle in the vicinity of 45
degrees. Thus, in the semiconductor light emitting element
according to the present embodiment in which the cross section of
the two-dimensional periodic structure is configured as a
triangular waveform, the angle between the tilted surface of the
two-dimensional periodic structure and the incident light
approaches 90 degrees when the angle of light emitted from the
active layer and incident upon the two-dimensional periodic
structure in the surface of the semiconductor light emitting
element is large so that the diffraction efficiency is increased.
Since the light at a large incident angle accounts for a large
proportion in the light emitted from the active layer, high light
extraction efficiency is achieved.
[0130] From the result shown in FIG. 19B, it will be understood
that the projecting structure shows the same high light emission
efficiency as achieved by the projecting/depressed structure and
particularly retains the effect of increasing the light extraction
efficiency even with a longer period. It is to be noted that, with
a period of 1.0 .mu.m, the light extraction efficiency obtained
from the surface formed with the two-dimensional periodic structure
increases to 2.7 times the original light extraction
efficiency.
[0131] A description will be given next to a method for fabricating
the semiconductor light emitting element according to the present
embodiment.
[0132] FIGS. 20A to 20F are perspective views illustrating the
method for fabricating the semiconductor light emitting element
according to the present embodiment. In the fabrication method
according to the present embodiment, the steps shown in FIGS. 20A
to 20E are substantially the same as those in the fabrication
method according to the first embodiment shown in FIG. 11 so that
the description thereof will be omitted. However, it is assumed
that the period of the depressed-type two-dimensional periodic
structure 10 formed in the principal surface of the AlGaN layer 9
is 1.0 .mu.m and the depth of the depression is 150 nm.
[0133] That is, in the fabrication method according to the present
embodiment, the sapphire substrate 8 is removed from the main body
of the light emitting element in the steps prior to and inclusive
of that shown in FIG. 20E so that the two-dimensional periodic
structure 6 composed of, e.g., cylindrical projections is formed by
self alignment on the back surface of the n-type GaN layer 11.
[0134] Next, in the step shown in FIG. 20F, wet etching using an
aqueous KOH solution is performed with respect to the n-type GaN
layer 11 formed with the projecting-type two-dimensional periodic
structure 6. It is well known that, in an etching process using
KOH, an etching speed has different conditions depending on crystal
planes. Under such conditions, the projecting-type two-dimensional
periodic structure 6 described above is changed by etching to the
two-dimensional periodic structure 18 of a hexagonal pyramid type
as shown in FIG. 20F. In the embodiment shown herein, etching is
performed by using an aqueous KOH solution at a concentration of
0.1 M to form the two-dimensional periodic structure 18 composed of
hexagonal pyramids each using the crystal plane {10-1-1} as a
tilted surface. The fabrication method is characterized in that,
since the specified crystal plane is used as the tilted surface,
the two-dimensional periodic structure having a triangular cross
section can be formed easily with high reproducibility.
[0135] In the semiconductor light emitting element according to the
present embodiment, the light extraction efficiency is improved to
about double the result of the theoretical calculation shown in
FIG. 19B (about 5.3 times the light extraction efficiency achieved
in the conventional embodiment) since, compared with the case where
the surface of the n-type GaN layer is flat, reflection from the
high-reflection p electrode 2 can also be used. In addition, heat
generated in the active layer can be dissipated through the p-type
GaN layer 13 which is as thin as submicron meters and through the
Au plating layer 15 having a high thermal conductivity.
Accordingly, the effect of improving the light extraction
efficiency achieved by the two-dimensional periodic structure is
also retained even when a large current of 100 mA flows in the
semiconductor light emitting element according to the present
embodiment. Although the high-reflection p electrode 2 may also be
composed of a material other than a multilayer film consisting of
Pt and Au films, it is preferable in terms of practical use for the
high-reflection p electrode 2 to have a reflectivity of 80% or more
with respect to the peak wavelength of light generated in the
active layer. Specifically, the high-reflection p electrode 2 is
preferably a metal film containing at least one of an Au film, a Pt
film, a Cu film, an Ag film, and a Rh film.
[0136] To retain the excellent heat dissipation property, the Au
plating layer 15 preferably has a thickness of 10 .mu.m or more. As
the material of the Au plating layer 15, Au is most preferable but
a metal such as Cu or Ag can also be used because of the relatively
high thermal conductivity thereof.
[0137] The fabrication method described above can reduce the damage
to the n-type GaN layer compared with the method which forms the
two-dimensional periodic structure directly by etching so that the
current-voltage characteristic is substantially the same as when
the two-dimensional periodic structure is not formed.
[0138] FIGS. 21A to 21C, FIGS. 22A and 22B, FIGS. 23A and 23B,
FIGS. 24A and 24B, and FIGS. 25A to 25C are views showing
variations of the method for fabricating the semiconductor light
emitting device according to the present embodiment.
[0139] Although the fabrication method according to the present
embodiment has formed the two-dimensional periodic structure having
a triangular vertical cross section in the surface of the
semiconductor (n-type GaN layer 11) by using the sapphire substrate
8 or the AlGaN layer 9 formed with the depressed two-dimensional
periodic structure, the depressed two-dimensional periodic
structure 17 may also be transferred to the semiconductor surface
by using the sapphire substrate 8 or the AlGaN layer 9 formed with
a projecting two-dimensional periodic structure 16 as shown in
FIGS. 21A to 21C. The method allows a depressed-type
two-dimensional periodic structure 19 having a triangular vertical
cross section to be formed in the semiconductor surface by using
the wet etching process described above. Even in the case where
each of the depressions of the two-dimensional periodic structure
19 has a configuration obtained by hollowing a hexagonal pyramid,
the same high light extraction efficiency as achieved by the
semiconductor light emitting element according to the present
embodiment is achievable.
[0140] If the two-dimensional periodic structure 20 is
preliminarily formed in the surface of the AlGaN layer 9 to have a
triangular vertical cross-sectional configuration as shown in FIGS.
22A and 22B and FIGS. 23A and 23B, the two-dimensional periodic
structures 18 and 19 composed of projections or depressions each
having a triangular vertical cross section can be formed by self
alignment in the surface of the semiconductor when the sapphire
substrate 8 and the AlGaN layer 9 are removed.
[0141] If the material of the layer in which the two-dimensional
periodic structure is formed is a hexagonal system semiconductor
such as AlGaN, hexagonal pyramids each having a tilted surface
composed of a specific crystal plane can be formed by the same
method as described above. For example, when a Ti film having an
opening corresponding to a portion to be processed into a depressed
configuration is formed as an etching mask 21 on the AlGaN surface,
as shown in FIG. 24A, and then etching is performed using an
aqueous KOH solution at 100.degree. C., the two-dimensional
periodic structure 20 is formed in the surface the AlGaN layer 9.
In this case also, the two-dimensional periodic structure can be
formed with high reproducibility since the tilted surface is
composed of a specific crystal plane such as {10-1-1}.
[0142] In the case where a substrate to be formed with the
two-dimensional periodic structure is made of a hexagonal system
semiconductor, such as a Si substrate using the (001) plane for the
principal surface, the etching mask 21 made of Ti is formed into a
tetragonal lattice configuration with a two-dimensional period, as
shown in FIG. 25A, and then etched in an aqueous KOH solution at
70.degree. C. This allows the two-dimensional periodic structure 20
configured as square pyramids to be formed easily in the substrate
with high reproducibility, as shown in FIG. 25B, and also allows
the two-dimensional periodic structure 18 composed of holes
configured as square pyramids to be transferred from the substrate
to the semiconductor surface, as shown in FIG. 25C.
Embodiment 3
[0143] FIG. 26 is a perspective view showing a semiconductor light
emitting device according to a third embodiment of the present
invention. The semiconductor light emitting device according to the
present embodiment is a resin-molded semiconductor light emitting
device obtained by mounting the semiconductor light emitting
element according to the first or second embodiment on a mounting
substrate 22 and then molding the periphery of the light emitting
element with a hemispherical dome-shaped resin 23. In FIG. 26,
those of the components of the semiconductor light emitting element
which are the same as shown in FIG. 1 are designated by the same
reference numerals.
[0144] By thus molding the light emitting element with the
dome-shaped resin, the light extraction efficiency of the
semiconductor light emitting element can be improved, as will be
described herein below.
[0145] FIG. 27A is a view showing the result of theoretically
calculating the transmittance of light when the semiconductor light
emitting element is molded with the resin. FIG. 27B is a view
showing the result of theoretically calculating the dependence of
light extraction efficiency on the period of a two-dimensional
periodic structure in the semiconductor light emitting device
according to the present embodiment. For comparison, FIG. 27A also
shows the case where the semiconductor light emitting element is
not molded with the resin and the case where the surface of the
semiconductor light emitting element is flat. In the calculation
the result of which is shown in the drawings, it is assumed that
the refractive index of the resin is 1.5. In the calculation the
result of which is shown in FIG. 27B, it is assumed that
projections/depressions each having a perpendicularly tilted
surface are arranged to form a two-dimensional periodic structure
and the height of each of the projections is 150 nm.
[0146] From the result shown in FIG. 27A, it will be understood
that, when the two-dimensional periodic structure with a 0.4-.mu.m
period is provided, the transmittance of incident light is higher
at substantially every angle in the semiconductor light emitting
element molded with the resin than in the semiconductor light
emitting element which is not molded with the resin. Since resin
molding can also increase the transmittance of light even in the
semiconductor light emitting element in which the two-dimensional
periodic structure is not provided, it will be understood that the
transmittance of light can significantly be improved by resin
molding irrespective of the period of the two-dimensional periodic
structure.
[0147] The reason for the improved transmittance of light is that,
even when the surface of the semiconductor light emitting element
is flat, the total reflection critical angle is enlarged by resin
molding and Fresnel reflection is reduced thereby even at an
incident angle not more than the total reflection critical angle.
In other words, the transmittance of light is improved due to a
reduction in the difference between the refractive index (which is
2.5) of the inside of the semiconductor light emitting element and
the refractive index (which is 1.5) of the outside thereof.
[0148] From the result shown in FIG. 27B, it will be understood
that resin molding can further enhance the effect of improving the
light extraction efficiency achieved by the two-dimensional
periodic structure and, at the maximum, the light extraction
efficiency has become 3.8 times the light extraction efficiency
achieved in the conventional embodiment. This is because the
molding resin is shaped like a hemispherical dome so that the light
extracted from the semiconductor light emitting element into the
resin is incident perpendicularly to the interface between the
resin and the air due to the two-dimensional periodic structure in
the surface of the semiconductor light emitting element and emitted
into the air with approximately 100% efficiency. By thus molding
the light emitting element formed with the two-dimensional periodic
structure with the dome-shaped resin, the light extraction
efficiency of the semiconductor light emitting device according to
the present embodiment has been improved greatly.
[0149] In the semiconductor light emitting device according to the
present embodiment, an actually measured value of the light
extraction efficiency has improved to about double the result of
the theoretical calculation shown in FIG. 27B (7.5 times the light
extraction efficiency achieved in the conventional embodiment)
since, compared with the case with the flat surface, reflection
from the back surface formed with the two-dimensional periodic
structure due to the high-reflection p electrode 2 can also be
used. In addition, due to excellent heat dissipation from the
p-type semiconductor, which is as thin as submicron meters, through
the Au plating layer with a high thermal conductivity, the effect
of enhancing the light extraction efficiency allows the retention
of the improved light extraction efficiency achieved by the
two-dimensional periodic structure even when a large current of 100
mA flows in the electrode.
[0150] A description will be given next to a method for fabricating
the semiconductor light emitting device according to the present
embodiment.
[0151] FIGS. 28A to 28D are perspective views illustrating the
method for fabricating the semiconductor light emitting device
according to the present embodiment.
[0152] First, as shown in FIG. 28A, the semiconductor light
emitting element according to the first or second embodiment is
fabricated by using the method for fabricating the semiconductor
light emitting element according to the first embodiment shown in
FIGS. 11 or the method for fabricating the semiconductor light
emitting element according to the second embodiment shown in FIGS.
12.
[0153] Next, as shown in FIG. 28B, the semiconductor light emitting
element is mounted on the mounting substrate 22. Thereafter, the
resin 23 is applied dropwise to the semiconductor light emitting
element.
[0154] Then, as shown in FIG. 28C, the resin 23 is pressed by using
a mold die 24 provided with a hemispherical cavity during the
period after the semiconductor light emitting element is covered
with the resin 23 and before the resin 23 is set. As a result, the
resin 23 is molded into a hemispherical dome-shaped configuration,
as shown in FIG. 28D. Thereafter, the resin is set under
ultraviolet light. By the method described above, the semiconductor
light emitting device according to the present embodiment is
fabricated.
[0155] Although it has been difficult to form the resin into a
hemispherical configuration with high reproducibility by using a
conventional technology which simply applies and molds a resin, the
fabrication method according to the present embodiment enables
stable molding of a resin into the same configuration.
[0156] It is to be noted that a method which molds a resin into a
hemispherical configuration using a mold die as described above is
also applicable to a semiconductor light emitting element according
to an embodiment other than the first and second embodiments of the
present invention.
Embodiment 4
[0157] FIG. 29 is a cross-sectional view showing a part of a
semiconductor light emitting element according to a fourth
embodiment of the present invention. The semiconductor light
emitting element according to the present embodiment is different
from the first and second semiconductor light emitting elements in
that the sapphire substrate 8 and the AlGaN layer 9 remain mounted
on the amounting substrate 22 without being removed and that the
high-reflection p electrode 2 and the n electrode 7 are formed on
the same side when viewed from the n-type GaN layer 5.
[0158] Specifically, the semiconductor light emitting element
according to the present embodiment shown in FIG. 29 comprises: the
p-type GaN layer 3 formed by epitaxial growth and having a
thickness of 200 nm; the high-reflection p electrode 2 formed on
the crystal growing surface (principal surface) of the p-type GaN
layer 3, made of platinum (Pt) and gold (Au) which are stacked in
layers, and having a thickness of 1 .mu.m; the non-doped InGaN
active layer 4 formed on the back surface of the p-type GaN layer 3
and having a thickness of 3 nm; an n-type GaN layer 5 formed on the
back surface of the non-doped InGaN active layer 4 and having a
thickness of 4 .mu.m; the n electrode 7 formed under the n-type GaN
layer 5, made of Ti and Al which are stacked in layers, and having
a thickness of 1 .mu.m; the AlGaN layer 9 provided on the back
surface of the n-type GaN layer 5 and having a principal surface
(surface facing the n-type GaN layer 5) formed with the
projecting-type two-dimensional periodic structure 16; and the
sapphire substrate 8 disposed on the back surface of the AlGaN
layer 9. In the example shown in FIG. 29, the semiconductor light
emitting device has been mounted on the mounting substrate 22 and
the high-reflection p electrode 2 and the n electrode 7 are
particularly connected to the mounting substrate 22 via bumps 25
made of Au. The period of the two-dimensional periodic structure
16, i.e., the spacing between the respective centers of the
adjacent projections in a two-dimensional plane is 0.4 .mu.m and
the height of each of the projections/depressions is 150 nm. In the
example shown in FIG. 29, the n-type GaN layer 5 is formed not to
be buried in the two-dimensional periodic structure 16. If the
n-type GaN layer 5 is formed to be buried in the two-dimensional
periodic structure 16, the light extraction efficiency lowers so
that it is formed preferably not to be buried therein.
[0159] By thus mounting the semiconductor light emitting element
with the substrate made of sapphire or the like being left, the
light emitted from the non-doped InGaN active layer 4 propagates in
the light emitting element without undergoing a loss caused by
total reflection or Fresnel reflection since there is substantially
no refractive index difference till the AlGaN layer 9 is reached.
In the conventional structure, however, the refractive index
difference between the sapphire substrate (with a refractive index
of 1.6) and the AlGaN layer (with a refractive index of 2.5) is
large so that light at a large incident angle is totally reflected
at the interface between the sapphire substrate and the AlGaN
layer, returns to the inside of the semiconductor multilayer film,
and is therefore unextractable to the outside of the LED. By
contrast, if a two-dimensional periodic structure is formed in the
back surface of an AlGaN layer as in the semiconductor light
emitting element according to the present embodiment, diffraction
by the two-dimensional periodic structure changes the direction of
propagation. As a result, if the back surface of the AlGaN layer is
flat, the light at a large incident angle that has been totally
reflected at the interface between the sapphire substrate and the
AlGaN layer and occupying a large proportion in the solid angle is
allowed to be incident on the sapphire substrate without being
totally reflected. Since the sapphire substrate is transparent and
the refractive index difference between itself and the air is
small, the majority of the light incident upon the sapphire
substrate is emitted into the air.
[0160] In the case where resin molding is performed, the refractive
index difference between the sapphire substrate and a resin (with a
refractive index of about 1.5) is further reduced and, if the resin
is configured as a hemispherical dome, the light extraction
efficiency can further be improved.
[0161] A description will be given next to a method for fabricating
the semiconductor light emitting element according to the present
embodiment. FIGS. 30A to 30E are cross-sectional views illustrating
the method for fabricating the semiconductor light emitting element
according to the present embodiment.
[0162] First, as shown in FIG. 30A, the AlGaN layer 9 is formed
through crystal growth by, e.g., MOCVD on the sapphire substrate 8.
The thickness of the AlGaN layer 9 is assumed herein to be 1 .mu.m
for a reduction in crystal defects. The composition of Al in the
AlGaN layer 9 is assumed herein to be 100%, though the AlGaN layer
9 may have any Al composition provided that it is transparent
relative to the wavelength of light used in a laser lift-off
process performed later. Subsequently, the AlGaN layer is pattered
into the depressed- or projecting-type two-dimensional periodic
structure 16 by exposure using a stepper and RIE. It is assumed
herein that the period of the two-dimensional periodic structure 16
is 0.4 .mu.m and the depth of each of the depressions (or the
height of each of the projections) is 150 nm.
[0163] Next, as shown in FIG. 30B, the n-type GaN layer 5, the
non-doped InGaN active layer 4, the p-type GaN layer 3 are formed
successively by MOCVD on the principal surface of the AlGaN layer 9
formed with the two-dimensional periodic structure 16. The crystal
growth of the n-type GaN layer 11 is performed by setting
conditions for the growth such that the two-dimensional periodic
structure is not filled therewith.
[0164] Thereafter, etching is performed with respect to a region to
partially expose the principal surface of the n-type GaN layer 5,
as shown in FIG. 30C. Then, the Pt/Au high-reflection p electrode 2
is formed on the principal surface of the p-type GaN layer 3, while
the Ti/Al n electrode 7 is formed on the exposed portion of the
principal surface of the n-type GaN layer 5, each by electron beam
vapor deposition.
[0165] Next, as shown in FIG. 30D, the semiconductor light emitting
element is mounted on the mounting substrate 22 formed with the
bumps 25 for the n electrode and for the high-reflection p
electrode, whereby the semiconductor light emitting element
according to the fourth embodiment shown in FIG. 30E is
obtained.
[0166] In the semiconductor light emitting element thus fabricated,
the light extraction efficiency is improved to about double the
result of the theoretical calculation shown in FIG. 27B (quadruple
the light extraction efficiency achieved in the conventional light
emitting element) since, compared with the case where the principal
surface of the AlGaN layer 9 is flat, light reflected from the
lower surface of the LED due to the high-reflection p electrode 2
can also be used.
[0167] In addition, the heat generated in the active layer can be
dissipated from the p-type GaN layer 3 as thin as submicron meters
through the bumps 25 each having a high thermal conductivity so
that an excessive temperature increase is prevented in the
semiconductor light emitting element according to the present
embodiment. Moreover, the increase rate of the light output from
the semiconductor light emitting element to an input current
thereto when the input current is small remains unchanged even when
a large current of 100 mA flows in the electrode.
[0168] Although the present invention has formed the
two-dimensional periodic structure in the principal surface of the
AlGaN layer 9 on the sapphire substrate 8, the two-dimensional
periodic structure may also be formed in the principal surface of
the sapphire substrate 8. The substrate may also be composed of any
material other than sapphire provided that it is transparent to the
light emitted from the active layer.
[0169] When the back surface (principal surface) of the sapphire
substrate 8 is rough, the light extraction efficiency is improved
to 4.5 times the light extraction efficiency achieved in the
conventional structure. This is because the presence of the rough
back surface reduces a loss resulting from total reflection at the
interface between the sapphire substrate and the air. When the back
surface of the sapphire substrate 8 is rough, the autocorrelation
distance T in the back surface of the sapphire substrate 8
preferably satisfies 0.5.lamda./N<T<20.lamda./N and a height
distribution D in a perpendicular direction preferably satisfies
0.5.lamda./N<D<20.lamda./N in terms of sufficiently reducing
the loss.
[0170] When the semiconductor light emitting element is molded with
a hemispherical resin, the light extraction efficiency is improved
to 6 times the light extraction efficiency achieved in the
conventional structure. This is because the small refractive index
difference between the resin and the sapphire reduces the loss
resulting from total reflection at the interface between the
sapphire substrate and the resin.
[0171] In the semiconductor light emitting element according to the
present embodiment, a substrate made of one selected from the group
consisting of GaAs, InP, Si, SiC, and AlN may also be used instead
of the sapphire substrate.
Embodiment 5
[0172] FIGS. 31A to 31E are perspective views illustrating a method
for fabricating a semiconductor light emitting element according to
a fifth embodiment of the present invention. The fabrication method
according to the present embodiment is a method for forming a
two-dimensional periodic structure in the principal surface of a
substrate by using a nano-printing method.
[0173] First, as shown in FIGS. 31A and 31B, a Si substrate or a
SiC substrate formed with a two-dimensional periodic structure 28
composed of projections each at a height of 400 nm and having a
period of 0.4 .mu.m is prepared. Then, the substrate is pressed as
a mold 26 against the principal surface of the sapphire substrate 8
coated with a resist 27 having a film thickness of 600 nm.
[0174] When the mold 26 is removed from the sapphire substrate 8
thereafter, a depressed two-dimensional periodic structure (the
depth of each of holes is 400 nm and the period is 0.4 .mu.m) is
transferred to the resist 27, as shown in FIG. 31C.
[0175] Next, as shown in-FIG. 31D, the resist remaining at the
bottom of each of the holes in the resist 27 is removed by O.sub.2
dry etching.
[0176] Next, as shown in FIG. 31E, dry etching is performed by
using the resist 27 as an etching mask and then the resist 27 is
removed, whereby the two-dimensional periodic structure composed of
depressions each at a depth of 150 nm and having a period of 0.4
.mu.m is formed in the principal surface of the sapphire substrate
8.
[0177] By thus using the nano-printing method, an extremely fine
structure on a submicron order can be formed through patterning
without using a high-cost manufacturing apparatus such as a stepper
or an EB exposure system. In addition, the fabrication method
according to the present embodiment can be implemented by merely
pressing the mold so that high-speed patterning is performed. If
the substrate produced by the foregoing process is used as a mold,
the semiconductor light emitting elements according to the first to
fourth embodiments can be fabricated at a low cost.
Embodiment 6
[0178] FIGS. 32A to 32G are perspective views illustrating a method
for fabricating a semiconductor light emitting element according to
a sixth embodiment of the present invention. The method for
fabricating the semiconductor light emitting element according to
the present embodiment is a method for forming a two-dimensional
periodic structure in the principal surface of a semiconductor thin
film by using a soft mold method.
[0179] First, as shown in FIG. 32A, a soft mold used for
micro-processing is produced. In the present step, a
two-dimensional periodic structure 31 composed of holes
(depressions) each at a depth of 400 nm and having a period of 0.4
.mu.m is formed in a resin 30 such as polysilane coated on a
substrate 29 such as a Si substrate or a SiC substrate by using a
photolithographic, EB lithographic, or nano-printing process. The
substrate with the resin thus produced is used as the soft mold for
a micro-processing step performed later.
[0180] Next, as shown in FIG. 32B, a thin-film semiconductor
multilayer film having the Au plating layer 15 is formed by the
method described in the first embodiment. In the method according
to the present embodiment, however, the surface of the substrate
used for the formation of the semiconductor multilayer film is flat
so that the surface of the semiconductor multilayer film is also
flat.
[0181] Next, as shown in FIG. 32C, the resist 27 is coated on the
principal surface of the semiconductor multilayer film. However,
evaporation of a solvent in the resist 27 by baking is not
performed herein. The soft mold described above is placed on the
resist 27. In this case, the soft mold is placed to exert minimum
pressure on the semiconductor multilayer film having a thickness of
several micro meters and thereby prevent the destruction
thereof.
[0182] As a result, capillarity occurs as a result of the
absorption of the solvent in the resist 27 by the resin 30 so that
the resist 27 penetrates in the resin of the soft mold in such a
mariner as to fill in the holes of the two-dimensional periodic
structure, as shown in FIG. 32D.
[0183] Thereafter, when the mold is removed from the semiconductor
multilayer film, a projecting two-dimensional periodic structure
(the height of each of the projections is 400 nm and the period is
0.4 .mu.m) is transferred to the resist 27, as shown in FIG.
32E.
[0184] Next, as shown in FIG. 32F, the resist 27 remaining at the
bottom of each of the holes in the resist is removed by O.sub.2 dry
etching.
[0185] Thereafter, as shown in FIG. 32G, dry etching is performed
with respect to the principal surface of the semiconductor
multilayer film by using the resist 27 as an etching mask and then
the resist is removed, whereby the two-dimensional periodic
structure (the height of each of projections is 150 nm and the
period is 0.4 .mu.m) is formed in the principal surface of the
semiconductor multilayer film.
[0186] By thus using the soft mold method, micro-processing on a
submicron order can be performed even with respect to a thin film
which is extremely difficult to process, such as a semiconductor
multilayer film having a thickness on the order of submicron
meters. In this case, since a flat substrate can be used
satisfactorily for the crystal growth of the semiconductor
multilayer film, the crystal growth becomes easier than in the case
of crystal growth on a substrate formed with
projections/depressions.
[0187] Although the embodiments described heretofore have
particularly disclosed the nitride-based compound semiconductor
which is difficult to process or the case where the period of the
projections/depressions becomes smaller in response to the
oscillation wavelength of a shorter wavelength of blue or purple
light so that micro-processing thereof becomes difficult, the
design of the present invention is also applicable to a
semiconductor light emitting element which emits infrared light or
red light using AlGaAs (with a refractive index of 3.6) or AlGaInP
(with a refractive index of 3.5) as a semiconductor.
Embodiment 7
[0188] FIG. 33 is a perspective view showing a semiconductor light
emitting element according to a seventh embodiment of the present
invention. As shown in the drawing, the semiconductor light
emitting element according to the present embodiment comprises: a
p-type AlGaN layer (first semiconductor layer) 43 formed by
epitaxial growth and having a thickness of 200 nm; a
high-reflection p electrode (first electrode) 42 formed on the
crystal growing surface (principal surface) of the p-type AlGaN
layer 43, made of Al, and having a thickness of 0.5 .mu.m; an Au
plating layer 41 formed on the lower surface of the high-reflection
p electrode 2 and having a thickness of 10 .mu.m; a non-doped
AlInGaN active layer 44 formed on the back surface of the p-type
GaN layer 43 and having a thickness of 3 .mu.m; an n-type AlGaN
layer (second semiconductor layer) 45 formed on the back surface of
the non-doped AlInGaN active layer 44, having a back surface formed
with a projecting-type two-dimensional periodic structure 46, and
having a thickness of 4 .mu.m; and an n electrode (second
electrode) 47 formed on the back surface of the n-type GaN layer 5,
made of titanium (Ti) and Al which are stacked in layers, and
having a thickness of 1 .mu.m. The lower surface used herein
indicates a surface of a certain layer located in the lower part of
FIG. 33.
[0189] The semiconductor light emitting element according to the
present embodiment functions as an ultraviolet LED from which light
is extracted through the back surface of the n-type AlGaN layer 45
and the PL peak wavelength of the non-doped AlInGaN active layer 44
is 350 nm.
[0190] The period of the two-dimensional periodic structure 46
formed in the back surface of the n-type AlGaN layer 45, i.e., the
spacing between the respective centers of adjacent projections in a
two-dimensional plane is 0.3 .mu.m and the height of each of the
projections is 130 nm.
[0191] The nitride-based compound semiconductor composing the
semiconductor light emitting element according to the present
embodiment can also be formed by MOCVD or MBE, similarly to the
nitride-based compound semiconductor composing the semiconductor
light emitting element according to the first embodiment.
[0192] The semiconductor light emitting element according to the
present embodiment also achieves the same high light extraction
efficiency and excellent heat dissipation property as achieved by
the semiconductor light emitting element according to the first
embodiment. Since the high-reflection p electrode 42 is made of Al,
the light generated in the-non-doped AlInGaN active layer 44 can be
reflected with a particularly high efficiency.
[0193] Thus, the structure of a semiconductor light emitting
element according to the present invention is also applied
effectively to a light emitting element which emits light at an
emission wavelength having a peak in the ultraviolet region.
[0194] In the semiconductor light emitting element functioning as
the ultraviolet LED according to the present embodiment, the
high-reflection p electrode (first electrode) 42 may also be made
of Al.
[0195] Thus, the semiconductor light emitting element according to
the present invention is useful as a light source having high
emission efficiency.
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