U.S. patent application number 15/154026 was filed with the patent office on 2016-09-01 for columnar crystal containing light emitting element and method of manufacturing the same.
This patent application is currently assigned to SOPHIA SCHOOL CORPORATION. The applicant listed for this patent is SOPHIA SCHOOL CORPORATION. Invention is credited to Akihiko KIKUCHI, Katsumi KISHINO.
Application Number | 20160254138 15/154026 |
Document ID | / |
Family ID | 36000057 |
Filed Date | 2016-09-01 |
United States Patent
Application |
20160254138 |
Kind Code |
A1 |
KIKUCHI; Akihiko ; et
al. |
September 1, 2016 |
COLUMNAR CRYSTAL CONTAINING LIGHT EMITTING ELEMENT AND METHOD OF
MANUFACTURING THE SAME
Abstract
A method of manufacturing a semiconductor element by forming, on
a substrate, columnar crystals of a nitride-base or an oxide-base
compound semiconductor, and by using the columnar crystals, wherein
on the surface of the substrate, the columnar crystals are grown
while ensuring anisotropy in the direction of c-axis, by
controlling ratio of supply of Group-III atoms and nitrogen, or
Group-II atoms and oxygen atoms, and temperature of crystal growth,
so as to suppress crystal growth in the lateral direction on the
surface of the substrate.
Inventors: |
KIKUCHI; Akihiko; (Tokyo,
JP) ; KISHINO; Katsumi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SOPHIA SCHOOL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
SOPHIA SCHOOL CORPORATION
Tokyo
JP
|
Family ID: |
36000057 |
Appl. No.: |
15/154026 |
Filed: |
May 13, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11574386 |
Feb 27, 2007 |
9362717 |
|
|
PCT/JP2005/015799 |
Aug 30, 2005 |
|
|
|
15154026 |
|
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Current U.S.
Class: |
438/507 |
Current CPC
Class: |
H01L 21/0254 20130101;
B82Y 20/00 20130101; H01S 5/041 20130101; C30B 29/403 20130101;
H01L 21/0237 20130101; H01S 5/34333 20130101; H01S 5/021 20130101;
H01L 21/02472 20130101; H01L 21/02483 20130101; H01L 21/02554
20130101; H01L 33/18 20130101; H01S 5/320225 20190801; H01L
21/02505 20130101; C30B 29/16 20130101; C30B 23/002 20130101; H01S
2304/04 20130101; H01L 21/02565 20130101; H01S 5/183 20130101; H01L
33/08 20130101; H01L 21/02381 20130101; H01S 5/3412 20130101; C30B
25/02 20130101; C30B 29/605 20130101; H01L 33/007 20130101; H01L
21/0242 20130101; H01L 21/0262 20130101; H01L 33/0093 20200501;
H01L 21/02603 20130101; H01L 21/02458 20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02; H01S 5/02 20060101 H01S005/02; H01S 5/343 20060101
H01S005/343; H01S 5/32 20060101 H01S005/32 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2004 |
JP |
2004-253267 |
Claims
1. A method of manufacturing a semiconductor element by forming, on
a substrate, columnar crystals of a nitride-base or an oxide-base
compound semiconductor, and by using thus-formed columnar crystals,
wherein on a surface of said substrate, said columnar crystals are
grown while ensuring anisotropy in a direction of c-axis, by
controlling ratio of supply of Group-III atoms and nitrogen, or
Group-II atoms and oxygen atoms, and temperature of crystal growth,
so as to suppress crystal growth in a lateral direction on the
surface of the substrate.
2. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein said ratio of supply of the source atoms and
nitrogen or oxygen atom, and the temperature of crystal growth are
adjusted with respect to a mode of allowing growth of the columnar
crystals while ensuring anisotropy, after the growing while
ensuring anisotropy in the direction of c-axis, so as to allow the
columnar crystals to grow not only in a longitudinal direction
thereof but also isotropically.
3. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein at a time of starting the growth of said
columnar crystals, dots which serve as nuclei for allowing said
columnar crystals to grow on the surface of the substrate are
formed according to a predetermined size and to a predetermined
distribution density.
4. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein said columnar crystals are separated from said
substrate, and bonded to another substrate.
5. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein gaps between adjacent columnar crystals are
filled with an insulating material.
6. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein a diameter of said columnar crystals is 50 nm
to 100 nm.
7. The method of manufacturing a semiconductor element as claimed
in claim 3, wherein the forming of said nucleis for allowing said
columnar crystals to grow comprises using a SiO.sub.2 film or a Ti
film as a mask and forming holes in the SiO.sub.2 film or in the Ti
film where a growth is desired to proceed, so as to make the
surface of said substrate exposed therein.
8. The method of manufacturing a semiconductor element as claimed
in claim 1, further comprising, in said columnar crystals, forming
a device structure having an optical or electronic function.
9. The method of manufacturing a semiconductor element as claimed
in claim 8, wherein said device structure having said optical or
electronic function is a region formed as an active region
expressing a functionality of one of controlling light emission and
direction of current flow.
10. The method of manufacturing a semiconductor element as claimed
in claim 1, wherein said columnar crystals comprises a light
emitting layer, when said columnar crystals is said nitride-base
compound semiconductor, as materials for said columnar crystals and
said light emitting layer, InGaN, GaN, AlGaInN, AlGaN, AlN or
hetero-structures composed of these materials is employed, and when
said columnar crystals is said oxide-base compound semiconductor,
as materials for said columnar crystals and said light emitting
layer, ZnO, CdZnO, MgZnO, MgZnCdO or hetero-structures composed of
these materials is employed.
11. The method of manufacturing a semiconductor element as claimed
in claim 1, comprising at the time of starting the growth of said
columnar crystals, forming dots having a diameter of 50 nm to 100
nm as nuclei for allowing said columnar crystals to grow on the
surface of the substrate are formed according to a predetermined
distribution density.
12. The method of manufacturing a semiconductor element as claimed
in claim 2, wherein the allowing the columnar crystals to grow not
only in the longitudinal direction thereof but also isotropically
comprises making top portions of the columnar crystals in a form of
a reverse pyramid, on which a crystal is grown as a continuous film
of the compound semiconductor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of application Ser. No. 11/574,386
filed Feb. 27, 2007, which is the National Stage of
PCT/JP2005-015799 filed Aug. 30, 2005, and which claims benefit of
Japanese Patent Application No. 2004-253267 filed Aug. 31, 2004;
the above noted prior applications are all hereby incorporated by
reference in their entirety.
[0002] The present invention relates to a semiconductor element of
semiconductor devices, such as diode, light emitting diode and
semiconductor laser, which is obtained by allowing a nitride-base
or oxide-base compound semiconductor layer to grow according to a
predetermined density in a form of uniform columnar crystals, and
by using thus-grown columnar crystals, and a method of
manufacturing the same.
BACKGROUND ART
[0003] Nitride-base compound semiconductors are known to show
direct transition over the entire compositional region thereof (for
example, AlN, GaN, InN and mixed crystals thereof), have wide band
gaps, and are known as materials for blue or violet light emitting
diode, in other words, materials for short-wavelength light
emitting elements.
[0004] The nitride-base compound semiconductors, however, have
crystal structures of the hexagonal system, for which there is no
lattice-matched substrate crystal unlike the conventional Group
III-V compound semiconductors. Therefore, they are generally grown
on the surface of sapphire substrate (SiC or Si is also adoptable)
having the hexagonal structure.
[0005] However, since crystal lattice of the (0001) surface of the
sapphire substrate and that of the (0001) surface of the
nitride-base compound semiconductor differ in the lattice
constants, there is lattice mismatch between them. Thus, growth of
the nitride-base compound semiconductor layer onto the sapphire
substrate results in an insufficient crystallinity as a continuous
thin film, and fails in obtaining an epitaxial film having a low
threading dislocation density.
[0006] Presence of a high density of such threading dislocation
results in degradation in light emitting characteristics, when the
nitride-base compound semiconductor is used as a material for
high-luminance light emitting diodes and semiconductor lasers.
[0007] For this reason, a GaN thin film having a predetermined
thickness is grown on the sapphire substrate, and thereafter a
stripe-patterned or a mesh-patterned mask composed of a thin film
of SiO.sub.2, SiN or metal is formed. Thereafter, re-growth of GaN
under specific conditions can allow GaN to grow only on the exposed
GaN portions, without causing crystal growth on the mask.
[0008] In this process, on the mask, GaN grows laterally and fuses
on the top surface of the mask, so that the entire surface can be
covered with GaN, and finally a flat continuous thin film of GaN
can be produced (see Non-Patent Documents 1 and 2).
[0009] The threading dislocation density can be largely reduced in
the GaN continuous thin film epitaxially grown laterally on the
mask, when compared to in GaN continuous thin films formed by the
general manufacturing methods.
[0010] As a technique similar to as described in the above, there
has been also proposed a technique of reducing the threading
dislocation, by forming steps on the sapphire substrate or the GaN
film so as to allow the lateral growth. [0011] [Non-Patent Document
1] A. Usui, H. Sunakawa, A. Sakai and A. Yamaguchi, "Thick GaN
epitaxial growth with low dislocation density by hydride vapor
phase epitaxy", Jpn. J. Appl. Phys., 36 (7B) 1997. [0012]
[Non-Patent Document 2]A. Sakai, H. Sunakawa and A. Usui, "Defect
structure in selectively grown GaN films with low threading
dislocation density", Appl. Phys. Lett., 71 (16) 1997
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0013] The general MOCVD (metal-organic chemical vapor deposition)
process would result in a threading dislocation density of
10.sup.9/cm.sup.2 in the GaN film grown on the sapphire substrate,
while the threading dislocation density in the above-described,
Non-Patent Documents 1 and 2 can be reduced to as low as
10.sup.6/cm.sup.2 or around, by employing the lateral epitaxial
growth.
[0014] The methods of growth shown in Non-Patent Documents 1 and 2,
however, suffer from complicated processes of manufacturing, need
larger numbers of process steps, and consequently need larger costs
as compared with the general method of growth.
[0015] As a consequence, commercially-available,
low-dislocation-density GaN wafer is a material that is extremely
expensive as one million yen or around per a single 2-inch
wafer.
[0016] There has been also reported GaN with an extremely low
dislocation density grown by the high-pressure synthetic process
using sodium or the like as a solvent in a research phase, but is
almost not commercially available because of difficulty in a real
expansion due to limitation in manufacturing processes.
[0017] In addition, for the case where the columnar crystals are
intended for use as light emitting elements, a trial of forming
electrodes on the upper portions of the columnar crystals by the
conventional method of forming a light emitting element undesirably
allows the electrode material to come around the side faces of the
columnar crystals, causing short-circuiting of semiconductor layers
disposed in the vertical direction of the columnar crystals, and
connection failure of the electrode between the adjacent columnar
crystals, and makes it difficult to form a large-area light
emitting element having a diameter of as large as several
micrometers or above containing the columnar crystals.
[0018] The present invention was conceived after considering the
above-described situations, and objects thereof reside in providing
a method of manufacturing semiconductor element, such as GaN
columnar crystals, having a low threading dislocation density at
low costs based on simple manufacturing processes, and in providing
a semiconductor element such as a high-luminance light emitting
element or a functional element, using the GaN columnar crystals
prepared by this method of manufacturing.
Means for Solving the Problems
[0019] A method of manufacturing a semiconductor element of the
present invention is a method of manufacturing a light emitting
element by forming, on a substrate (for example, on a substrate
having a predetermined crystal surface directed to the top surface
thereof), columnar crystals of a nitride-base or an oxide-base
compound semiconductor, by the molecular beam epitaxy (MBE)
process, or under different conditions by the MOCVD (metal-organic
vapor phase deposition) process, the HVPE (hydride vapor phase
epitaxy) process, sputtering or the like, and by using thus-formed
columnar crystals. In the method, on the surface of the substrate,
the columnar crystals are grown while ensuring anisotropy in the
direction of c-axis, by controlling ratio of supply of Group-III
atoms and nitrogen, or Group-II atoms and oxygen atoms (in other
words, Group-III source and Group-V source, or Group-II source and
Group-VI source), and by controlling temperature of crystal growth,
so as to suppress crystal growth in the lateral direction on the
surface of the substrate. That is, by adjusting the growth
temperature and the ratio of supply of Group-III atom and nitrogen
so as to adopt them to conditions expressed by region B in FIG. 2,
expressed by a range of crystal growth temperature from 750.degree.
C. to 950.degree. C., and by a range of ratio of supply of
Group-III atom and nitrogen from 1:2 to 1:100, and for example by
adjusting the crystal growth temperature within the range from
750.degree. C. to 950.degree. C., and by adjusting the ratio of
supply of Group-III atom and nitrogen atom to 1:2 or above, so as
to suppress the crystal growth in the lateral direction, the
columnar crystals are grown keeping anisotropy in the direction of
c-axis.
[0020] In other words, the crystal growth temperature is set higher
than 700.degree. C. which has generally been adopted, while keeping
a nitrogen-excessive condition, so as to suppress the crystal
growth in the lateral direction (direction normal to the c-axis
along which the side walls of the columnar crystals lie), and
thereby to allow the columnar crystals to grow having anisotropy in
the direction of c-axis.
[0021] In addition, the method of manufacturing a semiconductor
element of the present invention is such as forming, on a
substrate, columnar crystals of a nitride-base or an oxide-base
compound semiconductor, and using thus-formed columnar crystals,
wherein on the surface of the substrate, the columnar crystals are
grown while ensuring anisotropy in the direction of c-axis, by
controlling ratio of supply of a Group-III source and a Group-V
source, or a Group-II source and a Group-VI source, and temperature
of crystal growth, so as to suppress crystal growth in the lateral
direction on the surface of the substrate.
[0022] The sources used herein for forming the colunmar crystals
are not necessarily in a form of atom, but may be supplied in a
form of molecule, or in a gas form of, for example, organic Ga
compound (Ga) and ammonia (N).
[0023] The method of manufacturing a semiconductor element of the
present invention is such as adjusting the ratio of supply of the
Group-III source and the Group-V source, or the Group-II source and
the Group-VI source, and the temperature of crystal growth, with
respect to the mode of allowing growth of the columnar crystals
while ensuring anisotropy, at the point of time where the columnar
crystals reach a predetermined level of height, so as to allow the
columnar crystals to grow not only in the longitudinal direction
thereof but also isotropically.
[0024] In other words, the above-described method of manufacturing
is such as sequentially adjusting conditions for growth, allowing
not only anisotropic growth only in the longitudinal direction
(direction of the c-axis), but allowing also isotropic growth in
the crystal direction normal to the c-axis, so as to make the mode
of growth transit from the anisotropic growth into the isotropic
growth.
[0025] For example, the method of manufacturing a light emitting
element of the present invention is such as adjusting, at the point
of time where the columnar crystals reach a predetermined level of
height, the ratio of supply of the Group-III atom and nitrogen atom
and the crystal growth temperature, in the mode of allowing growth
of the columnar crystals while ensuring anisotropy, that is, by
adjusting the growth temperature and the ratio of supply of
Group-III atom and nitrogen so as to adopt them to conditions
expressed by region C in FIG. 2, expressed by a range of crystal
growth temperature from 500.degree. C. to 800.degree. C., and by a
range of ratio of supply of Group-III atom and nitrogen from 1:2 to
1:100, allowing the crystal to grow not only in the direction of
the c-axis, but also in the direction normal to the c-axis, so as
to make the top portions of the columnar crystals in a form of
reverse pyramid such as reverse cone or reverse polygonal pyramid,
on which a crystal is grown as a continuous film of a nitrogen-base
compound semiconductor.
[0026] The method of manufacturing a semiconductor element of the
present invention is such as forming, at the time of starting the
growth of the columnar crystals, dots which serve as nuclei for
allowing the columnar crystals to grow on the surface of the
substrate (which are growth nuclei, efficient when the columnar
crystals are grown on the Si substrate and on the sapphire
substrate by the MBE process), according to a predetermined size
and to a predetermined distribution density.
[0027] The method of manufacturing a semiconductor element of the
present invention is such as separating the columnar crystals from
the substrate, and bonding them to another substrate.
[0028] The method of manufacturing a semiconductor element of the
present invention is such as filling the gaps between adjacent
columnar crystals with an insulating material.
[0029] The insulating material herein may be an inorganic material
or an organic material, including dielectrics, and any of those
capable of forming a capacitor together with the semiconductor
layers placed thereon and thereunder.
[0030] A semiconductor element of the present invention is such as
having a substrate; columnar crystals arranged on the substrate
according to a predetermined density, and in which a device
structure having optical or electronic functions (a region formed
as an active region expressing functionality such as controlling
light emission or direction of current flow) is formed; and a
two-dimensionally continuous film layer formed over the columnar
crystals (a continuous thin film, effectively applicable as an
electrode by virtue of its continuous formation with respect to the
lower columnar crystals).
[0031] The semiconductor element of the present invention is such
that each of the columnar crystals has, as being provided therein
at a predetermined level of height, as the above-described device
structure, a functional portion (a region formed as an active
region expressing functionality such as controlling light emission
or direction of current flow) composed of a semiconductor layer
differed in the constituent material from the columnar crystals.
The functional portion includes, for example, light emitting region
or a region having a rectifying function and the like.
[0032] The semiconductor element of the present invention is such
as having, between the upper portions of the columnar crystals and
the film layer, a semiconductor layer composed of the same material
with the columnar crystals, gradually enlarging from the diameter
of the columnar crystals, in the direction of c-axis.
[0033] The semiconductor element of the present invention is such
as having a filling component composed of a dielectric material
between adjacent columnar crystals.
[0034] The present invention allows growth of a layer of reverse
truncated pyramids as being continued from the columnar crystals,
as visually observable in the scanning electron micrograph of FIG.
12, so as to finally form a continuous thin film by the top
portions of the reverse truncated pyramids. In the columnar crystal
growth mode (region B: substrate temperatures from 750.degree. C.
to 950.degree. C., ratios of supply of Group-V/III from 1:2 to
1:100) and in the reverse truncated pyramid growth mode (region C:
substrate temperatures from 500.degree. C. to 800.degree. C., and
ratios of supply of Group-V/III from 1:2 to 1:100), and within the
conditional ranges for the individual growth modes shown in FIG. 2,
the ratio of supply of nitrogen in the reverse truncated pyramid
growth mode is adjusted to be lower than that in the columnar
crystal growth mode for the case where the growth temperature was
set equal for both modes, whereas the growth temperature in the
reverse truncated pyramid growth mode is adjusted to be lower than
that in the columnar crystal growth mode for the case where the
ratio of supply of nitrogen was set equal for both modes, or
conditions for the reverse truncated pyramid growth mode is kept
within region C, through lowering the substrate temperature, and
elevation in the ratio of supply of nitrogen, as compared with
those in the columnar crystal growth mode.
[0035] The ratio of supply of Group-V (nitrogen) in Group-V/III, in
the process of growing the semiconductor layer composed of the
reverse truncated pyramids in region C of the present invention, is
elevated, aiming at preventing Ga from being excessively supplied
to the surface normal to the c-axis of the columnar crystals, in
order to make metal Ga less likely to deposit between the adjacent
columnar crystals.
Effect of the Invention
[0036] As has been explained in the above, according to the present
invention, the columnar crystals composed of nitride-base compound
semiconductor (GaN, for example) are formed, and the light emitting
portion is provided to each of the columnar crystals, thereby
making it possible to obtain semiconductor devices (semiconductor
elements) such as light emitting element, showing a high luminance
in short-wavelength emission regions, by virtue of characteristics
of a high-quality columnar crystals almost free from the threading
dislocation density.
[0037] According to the present invention, crystal conditions of
the columnar crystals are altered after a predetermined level of
height was reached, so as to switch the growth of the columnar
crystals from anisotropic growth to isotropic growth, thereby
allowing the upper portions thereof to grow into a geometry of
reverse truncated cone or reverse truncated pyramid, and allowing
the top portions of the reverse truncated cones or reverse
truncated pyramids (generally referred to as reverse truncated
pyramids) to come into contact, so as to finally give the upper
portions of the columnar crystals as a continuous thin film, to
thereby prevent the electrode materials from coming around the side
faces of the columnar crystals, making it easy to form the
electrodes in the manufacturing process of the light emitting
element.
[0038] According to the present invention, nuclei from which the
columnar crystals are grown are initially formed on the surface of
the substrate at predetermined intervals, and the columnar crystals
are grown from these nuclei under predetermined conditions, so that
the columnar crystals destined for formation of the light emitting
portions can readily be produced on the surface of the substrate at
predetermined intervals. According to the present invention, it is
possible to form semiconductor devices such as light emitting
elements having high-luminance characteristics at low costs.
BRIEF DESCRIPTION OF THE INVENTION
[0039] FIG. 1 is a conceptual drawing showing a structure of a
light emitting diode according to one embodiment of the present
invention;
[0040] FIG. 2 is a graph explaining growth conditions determined by
substrate temperature (growth temperature) and Group-V/III supply
ratio;
[0041] FIG. 3 is a conceptual drawing of an MBE apparatus;
[0042] FIG. 4 is a conceptual drawing explaining a method of
manufacturing a light emitting element;
[0043] FIG. 5 is a conceptual drawing explaining the method of
manufacturing the light emitting element;
[0044] FIG. 6 is a conceptual drawing explaining the method of
manufacturing the light emitting element;
[0045] FIG. 7 is a conceptual drawing explaining the method of
manufacturing the light emitting element;
[0046] FIG. 8 is a graph showing PL (photo-luminescence) spectra of
GaN columnar crystals, a GaN continuous thin film grown by MOCVD,
and a GaN continuous thin film grown by MBE;
[0047] FIG. 9 is a graph showing excitation light intensity
dependence of PL peak intensity;
[0048] FIG. 10 is a conceptual drawing showing a sectional
structure of a semiconductor laser using the columnar crystals of
the present invention;
[0049] FIG. 11 is a drawing of scanning electron microphotograph
showing sections of the columnar crystals formed by Steps S1 to S6
in an embodiment of the present invention; and
[0050] FIG. 12 is a graph showing a light emission characteristic
(correlation between current and light output) of a light emitting
diode composed of the columnar crystals formed by the present
invention.
EXPLANATION OF THE MARKS
[0051] 1 . . . substrate, 2 . . . columnar crystal, 2a . . .
reverse truncated cone portion (p-type cladding layer), 2b, 2d . .
. i-type blocking layer, 2c . . . light emitting layer, 2e . . .
cladding layer (n-type cladding layer), 3 . . . electrode layer
BEST MODES FOR CARRYING OUT THE INVENTION
<Structure of Light Emitting Element>
[0052] Paragraphs below will explain a structure of the light
emitting element (light emitting diode, for example) according to
one embodiment of the present invention referring to the attached
drawings. FIG. 1 is a block diagram showing a structure of the
light emitting element according to this embodiment.
[0053] As seen in this drawing, a light emitting element L has
columnar crystals 2 formed on the top surface of a wherein the
upper portion of the columnar crystals 2 are electrically connected
to an electrode layer 3.
[0054] Each of the columnar crystals 2 has, as a device structure,
as shown by the enlarged view on the right hand side in FIG. 1, a
reverse pyramid (reverse cone or reverse polygonal pyramid) portion
2a (p-type cladding layer), an i-type blocking layer 2b, a light
emitting layer 2c, an i-type blocking layer 2d, and an n-type
cladding layer 2e.
[0055] The columnar crystal 2 has such device structure as
described in the above, containing the light emitting layer 2c as a
region (portion) of the device structure showing a light emitting
function.
[0056] The reverse pyramid portion 2a herein is typically composed
of p-GaN:Mg (GaN containing Mg as an impurity, thereby showing
p-type conductivity) or p-AlGaN:Mq, the i-type blocking layers 2b
and 2d are composed of intrinsic GaN, the n-type cladding layer 2e
is composed of n-GaN:Si (GaN containing Si as an impurity, thereby
showing n-type conductivity) or n-AlGaN:Si, and the light emitting
layer 2c has a MQW (multiple quantum well) structure (or SQW:
single quantum well) composed of InGaN/GaN (or
In.sub.xGa.sub.1-xN/In.sub.yGa.sub.1-yN) or GaN/AlGaN,
Al.sub.xG.sub.1-xN/Al.sub.yG.sub.1-yN.
[0057] The i-type blocking layer 2b and the i-type blocking layer
2d are provided for the purpose of preventing diffusion of
impurities from the reverse pyramid portion 2a and the n-type
cladding layer 2e, respectively, to the light emitting layer 2c.
However, they are not essential, and structures having the
individual cladding layers directly bonded to the light emitting
layer 2c may be employed.
[0058] For example, the substrate 1 is composed of an
electro-conductive silicon (or silicon carbide substrate, metal
substrate, or sapphire substrate having an electro-conductive
finish), wherein on the (111) surface of such silicon substrate, or
on the (0001) surface of silicon carbide or sapphire substrate, a
hexagonal nitride-base semiconductor is grown in the direction of
c-axis (as shown in FIG. 2, in the direction normal to the
substrate plane, that is, in the axial direction of the columnar
crystals to be grown) as the above-described columnar crystals
2.
[0059] Epitaxial growth of a nitride-base compound semiconductor
having a hexagonal structure on the surface of a substrate having
different lattice constants generally results in a high level of
threading dislocation density due to difference in the lattice
constants.
[0060] This is because the atomic arrangements of the growth nuclei
slightly differ from each other, when the growth nuclei formed with
a high density in the early stage of growth of the columnar
crystals come into contact to form the continuous film.
[0061] However, by forming the columnar crystals as described in
the above, each of the columnar crystals can grow from a single
growth nucleus (seed), and contains almost no threading
dislocation. Since the columnar crystals contain no junction
portions between the adjacent nano-crystals causative of the
threading dislocation, it is possible to drastically lower the
threading dislocation density in the whole crystal.
[0062] Moreover, reduction in the sectional area of the growth
plane is also successful in suppressing distortion stress at the
interface to a low level, in suppressing occurrence of the
threading dislocation on the columnar crystal basis to a low level,
and in lowering the threading dislocation density in the
crystal.
[0063] Silicon is electro-conductive and inexpensive, but largely
differs from nitride-base compound semiconductor in the lattice
constants. Thus, any trial of growth of nitride-base compound
semiconductor on this substrate often results in crack generation
and a high level of treading density. This has promoted a general
use of sapphire substrate showing no electro-conductivity. The
present invention, however, made it possible to form the
nitride-base compound semiconductor on the silicon substrate as
described in the above, and thereby to improve efficiency of
fabrication on the device basis.
[0064] It is also allowable to use, as the substrate, a film
surface of GaN having a form of continuous film without causing
lattice mismatching, or other semiconductor elements, glass, metal
(Al, Ti, Fe, Ni, Cu, Mo, Pd, Ag, Ta, W, Pt, Au, or alloys partially
containing these elements). On these film surfaces, the growth
nuclei are formed in the early stage, and the columnar crystals
grow according to a predetermined density (the number of columnar
crystals per unit area).
[0065] The columnar crystals 2 in the present invention has a
structural feature in that they are formed according to a
predetermined density, while at least keeping a distance with other
columnar crystals, while not necessarily keeping a predetermined
distance, that is not composed of unified intervals. This
structural feature prevents the columnar crystals 2 from fusing in
the process of growth with other columnar crystals formed as being
buried, and from forming the continuous film.
[0066] Although some portions of the columnar crystals may
physically come into contact with each other, rather than keeping
complete uncontactness among them as described in the above, it is
essential to keep a state in which the adjacent columnar crystals
are not bound on the atomic basis. Formation of any bond on the
atomic basis may result in production of crystal defects at the
bonded surface.
[0067] There has been known, as a prior art, a method of forming
GaN columnar crystals on the sapphire substrate, and then forming a
continuous thin film on the columnar crystals (K. Kusakabe, Jpn. J.
Appl. Phys., 40, 2001, L192-L194). The method of fabrication
according to the prior art has, however, been suffering from a
problem in that Ga, which is a source material, deposits between
the adjacent columnar crystals, so that electric isolation between
the adjacent columnar crystals cannot be ensured.
[0068] Whereas the method of forming the columnar crystals of the
present invention can satisfy electric isolation between the
adjacent columnar crystals, without allowing a part of the material
to deposit between the adjacent columnar crystals as described in
the above.
[0069] The columnar crystals 2 of the present invention are
depicted by the structural features such as being grown up to a
predetermined level of height in the direction of c-axis while
keeping anisotropy under control of the growth conditions (detailed
later in the paragraphs for the method of manufacturing), then
being grown isotropically from the p-type cladding layer (including
growth not only in the direction of c-axis, but also in directions
normal to the c-axis), so as to form the p-type cladding layer as
the reverse pyramid portion 2a. Consequently, a continuous thin
film is grown as an electrode layer 3.
[0070] The geometry of the reverse pyramid portion herein includes
not only a geometry depicted by reverse cone or reverse pyramid
having side faces thereof continuously widened, but also structures
having diameters which change step-wisely, or such like steps, from
the columnar crystals as the growth proceeds in the direction of
c-axis.
[0071] It is still also allowable, without being limited to the
reverse pyramid geometry, to adopt a structure finally obtained as
a continuous thin film, after being grown so as to widen the
surface area of the top growth plane, as the growth proceeds in the
direction of c-axis.
[0072] As a consequence, the columnar crystals of the present
invention, more specifically the top portions of the individual
reverse pyramid portions 2a of the columnar crystals 2 formed on
the substrate 1, are electrically connected by the electrode layer
3, so that electrical connection with portions other than the
reverse pyramid portions 2 is more readily avoidable as compared
with the prior arts, and thereby the element characteristics can be
improved while simplifying process steps of fabricating
semiconductor elements.
<Method of Manufacturing Light Emitting Element; on Silicon
Substrate>
[0073] For the case where a 350-.mu.m-thick, Sb-doped, low
resistivity n-type silicon substrate (on the Si(111) surface) is
used as the substrate 1 for forming a light emitting diode having a
structure shown in FIG. 1, a manufacturing apparatus adopted herein
is an MBE (molecular beam epitaxy) apparatus shown in FIG. 4.
[0074] In this case, Ti for heat absorption is deposited on the
back surface (surface not destined for growth thereon of the
columnar crystals) typically by electron beam deposition, prior to
growth of the columnar crystals.
[0075] Degree of vacuum in the chamber is adjusted to 10.sup.-6 to
10.sup.-9 Pa (pascal) under a condition having no emission of
molecular beams of the individual material (for example, metals
such as In, Ga, Mg, Si and so forth, and activated nitrogen) from
the individual molecular beam irradiation cells, and to 10.sup.-2
to 10.sup.-6 Pa under a condition having emission of molecular
beams and nitrogen for crystal growth from the individual molecular
beam irradiation cells.
[0076] As the pretreatment, for the purpose of lowering resistivity
value of the bonding portion of n-type cladding layer 2e and the
substrate 1, and of making the resistivity values of the connection
portions of the individual columnar crystals uniform, the surface
of Si is washed by the RCA cleaning or by using hydrofluoric acid
so as to remove native oxide film on the surface of the Si
substrate, to thereby activate the surface.
[0077] Substrate temperatures described hereinafter are those of
the Ti film deposited on the back surface of the silicon substrate,
measured using an infrared radiation thermometer (on the basis of a
radiation coefficient of 0.37).
[0078] Molecular beam intensity was successively measured using a
nude ion gauge moved to the site of measurement of the substrate
every time the measurement takes place.
[0079] Dopant concentration (electron and hole concentrations) were
estimated by the CV method, or based on doping conditions of a
single-layered film.
[0080] Thickness of the individual electrodes was measured in situ
during the deposition process using a quartz oscillator thickness
gauge.
Step S1:
[0081] Ga is irradiated at a substrate temperature of 500.degree.
C. to 600.degree. C., and a degree of vacuum of 10.sup.-3 Pa to
10.sup.-6 Pa.
[0082] The Ga irradiation is stopped, and activated nitrogen is
then irradiated to form GaN dots, so as to allow growth of the
columnar crystals in the succeeding process steps using these GaN
dots as the nuclei for the growth.
[0083] It is to be noted that Step S1 is omissible if the condition
is any of those fall in range B described later.
[0084] FIG. 2 herein expresses ranges of growth conditions, in
correlation of the substrate temperature (ordinate: growth
temperature) and supply ratio of Group-V/III (abscissa).
[0085] In the correlation between the growth temperature and the
supply ratio of Group-V/III, a range expressed by region A is a
range not causative of the crystal growth due to decomposition of
GaN, a range expressed by region B is a range satisfying conditions
for a growth mode of the columnar crystals, a range expressed by
region C is a range satisfying conditions for a growth mode of the
columnar crystals so as to grow the upper portions thereof in a
form of reverse truncated pyramid, consequently allowing crystals
composing a continuous thin film to grow, and region D expresses a
growth mode promoting, similarly to as in region C, growth of the
reverse truncated pyramid up to the continuous film, while allowing
filling of metal Ga between adjacent columnar crystals.
[0086] Accordingly, the succeeding growth of the columnar crystals
from Step S2 to Step S5 adopts the conditions for region B, and the
growth of the reverse truncated pyramid in Step S6 adopts the
conditions for region C.
Step S2:
[0087] Next, the substrate temperature is adjusted to 860.degree.
C. to 880C (region B: 750.degree. C. to 950.degree. C.), under a
degree of vacuum of 10.sup.-1 Pa to 10.sup.-6 Pa, a ratio of supply
of Ga and N of 1:2 (nitrogen: 1, Ga: 0.5, region B: from 1:2 to
1:100), an intensity of Ga molecular beam of 6.times.10.sup.-4 Pa,
and under excessive supply of N, the n-type cladding layer 2e of
100 nm to 2,000 nm in height, for example, 750 nm in height
(thickness), is grown as the GaN:Si columnar crystals, using the
GaN dots as the growth nuclei. In this process, Si atoms as n-type
impurity atoms are supplied so as to attain n (electron
concentration)=10.sup.15/cm.sup.3 to 1.times.10.sup.21/cm.sup.3, at
room temperature.
[0088] In short, it is to be understood that the columnar crystals
can be formed by allowing the growth to proceed on the surface of
the sapphire substrate, at a temperature higher than the substrate
temperature generally adopted for growth of GaN (700.degree. C.),
under a condition expressed by the ratio of supply of Group-III
atom and nitrogen atom of 2 or above.
[0089] By allowing GaN to grow on the surface of sapphire substrate
while setting the substrate temperature not lower than 800.degree.
C., under a degree of vacuum of 10.sup.-3 Pa to 10.sup.-6 Pa, and a
ratio of supply of Group-III atom and nitrogen atom to 2 or above,
it is made possible to form the high-quality GaN columnar crystals
(nano-columns) while keeping anisotropy in the direction of c-axis
of the hexagonal system (while suppressing growth in the directions
of a-axis and b-axis which correspond to the side faces of the
columnar crystals) (see FIG. 5).
[0090] In this process, once the growth in a form of columnar
crystals starts, the mode of growth of the columnar crystals while
keeping anisotropy in the direction of c-axis is sustainable, even
in a low temperature region within the substrate temperature range
from 600.degree. C. to 950.degree. C.
[0091] A light-excitation-induced emission experiment using a
Nd:YAG pulse laser (wavelength 355 nm, maximum output 20 mJ, pulse
width 5 nm) carried out at this timing showed an induced emission
at an induction light intensity with an extremely low threshold
value, as shown in FIG. 9.
[0092] IL is also found, as shown in FIG. 9, that the GaN
nano-column of the present invention shows a threshold value of
intensity of excitation light for induced emission of 0.2
MW/cm.sup.2, which is decreased about a single order of magnitude
below 1.6 MW/cm.sup.2 shown by MOCVD-GaN, and 2.0 MW/cm.sup.2 shown
by MBE-GaN. Thus, the GaN nano-column of the present invention
achieves an excellent induced emission property.
Step S3:
[0093] Next, the substrate temperature is adjusted to 860.degree.
C. to 880.degree. C. (region B: 750.degree. C. to 950.degree. C.),
and under a degree of vacuum of 10.sup.-3 Pa to 10.sup.-6 Pa, Ga
and N atoms are supplied under a ratio of supply of 1:2 (region B:
1:2 to 1:100), so as to grow the i-type blocking layer 2d as an
i-GaN layer in a form of columnar crystals, as being continued from
the above-described GaN:Si cladding layer 2e, to a thickness of 10
nm.
Step S4:
[0094] Next, the substrate temperature is adjusted to 500.degree.
C. to 800.degree. C., and under a degree of vacuum of 10.sup.-3 Pa
to 10.sup.-6 Pa, the light emitting portion 2c having the MQW
structure is formed by alternately producing InGaN layers and GaN
layers, by repeating multiple number of times a process of forming
an InGaN layer to a thickness of 1 nm to 10 nm while adjusting the
compositional ratio of In, Ga and N as In.sub.z.times.Ga.sub.1-xN
(x=0 to 0.5), and a process of forming a GaN layer to a thickness
of 1 nm to 10 nm while supplying Ga and N atoms at a ratio of
(region B: 1:2 to 1:100).
[0095] In the above-described MQW structure, not only InGaN/GaN,
but also InGaN/InGaN, GaN/AlGaN and InAlGaN/AlGaN and AlGaN/AlGaN
are adoptable.
[0096] By this process, the light emitting portion 2c is grown as
the columnar crystal as being continued from the i-type blocking
layer 2d.
Step S5:
[0097] Next, the substrate temperature is adjusted to 680.degree.
C. to 700.degree. C. (region B: 500.degree. C. to 800.degree. C.),
and under a degree of vacuum of 10.sup.-3 Pa to 10.sup.-6 Pa, the
i-type blocking layer 2b which is an i-GaN layer is formed, while
adjusting the ratio of supply of Ga and N atoms to 1:2 (nitrogen:
1, Ga: 0.5, region B: 1:2 to 1:100), in a form of columnar crystals
as being continued from the above-described GaN:Si light emitting
portion 2c, to d thickness of 10 nm.
Step S6:
[0098] Next, the substrate temperature is adjusted to 680.degree.
C. to 700.degree. C. (region C: 500.degree. C. to 800.degree. C.),
under a degree of vacuum of 10.sup.-3 Pa to 10.sup.-6 Pa, and under
supply of Ga and N atoms at a ratio of 1:8 (nitrogen: 1, Ga: 0.125,
region C: 1:2 to 1:100), the crystal growth is switched from a mode
ensuring anisotropy in the direction of c-axis into a
near-isotropic mode of growth, allowing the crystal growth not only
in the direction of c-axis of the (0001) surface, but also in the
directions normal to the c-axis which correspond to the side faces
of the columnar crystal 2 (see FIG. 6).
[0099] By this process, the reverse pyramid portion 2a composing
the p-type cladding layer is formed to a height (thickness) of 100
nm to 1,000 nm, as the GaN:Mg columnar crystal 2 (see FIG. 7). In
this process, Mg atoms are supplied as p-type impurities at room
temperature, so as to attain p=1.times.10.sup.15/cm.sup.3 to
1.times.10.sup.18/cm.sup.3.
[0100] With progress of the lateral crystal growth of the reverse
pyramid portions 2a of the individual columnar crystals 2, the top
portions of the reverse pyramid portions 2a (that is, top portions
of the columnar crystal 2 including the reverse pyramid portions
2a) are brought into a state of growth while being fused with each
other, thereby forming the electrode layer in a form of continuous
thin film.
[0101] It is to be noted herein that AlGaN is adoptable for the
reverse pyramid portion 2a as the p-type cladding layer.
[0102] The substrate is finally taken out from the MBE apparatus. A
transparent electrode (semitransparent p-type electrode composed of
Ti/Al, for example) is formed on the surface of the substrate, and
more specifically on the top surface of the electrode layer 3,
thereby the light emitting diode is formed. The p-type electrode
composed of Ti/Al herein is formed as a stack of two metal films,
by depositing Ti of 2 nm thick and then depositing Al of 3 nm
thick, by the electron beam deposition process.
[0103] Characteristics of thus-formed light emitting diode are
shown in FIG. 12 (measured at room temperature: R.T.). In FIG. 12,
the abscissa corresponds to forward current, and the ordinate
corresponds to emission intensity of light induced by the forward
current.
[0104] It is also allowable herein to form the reverse pyramid
portion 2a as the p-type cladding layer (under growth conditions
similar to those for the n-type cladding layer 2e), while keeping
the growth mode of the columnar crystals (the state shown in FIG.
5) unchanged, and to fill with an insulating material such as the
above-described dielectric up to a level of height of the columnar
crystals, and then to form the transparent electrode as shown in
FIG. 10. Also this process can successfully prevent the electrode
material to come around.
[0105] Methods of filling with the above-described insulating
materials such as SiO.sub.2, TiO.sub.2, Al.sub.2O.sub.3, SiN and so
forth may be such as, for example, coating a liquid material
(particles of an insulating material are suspended in a solvent)
such as OCD-T7 (SiO.sub.2) from Tokyo Ohka Kogyo Co., Ltd., or an
oxide-containing polymer (TiO.sub.2, Al.sub.2O.sub.3) from Chemat,
by the spin coating process, which is followed by annealing so as
to allow a desired oxide to deposit between the adjacent columnar
crystals.
[0106] Besides this, the space between the adjacent columnar
crystals can also be filled with a desired oxide (SiO.sub.2,
TiO.sub.z, Al.sub.2O.sub.3 and so forth, for example) or a nitride
such as SiN, with vapor phase deposition process such as the plasma
CVD process.
[0107] In the above-described Steps S1 to S5, by raising the ratio
of supply of nitrogen as compared with the ratio of supply of the
Group-III atom, migration of the Group-III atom (Ga, In, Al and so
forth) over the crystal growth surface (or the c-surface) can be
suppressed, and the rate of growth in the direction of c-axis is
consequently increased as compared with the rate of lateral crystal
growth, so that the columnar crystals are prevented from being
fused with each other, that is, from forming a continuous film, by
being kept in the anisotropic growth mode up to a predetermined
level of height.
[0108] The growth at high temperatures also successfully removes
GaN with nitrogen-polarity, showing a low decomposition
temperature, on the sapphire substrate, and allows only growth
nuclei with Ga-polarity on AlN dots to grow in a selective
manner.
[0109] In addition, on the crystal surface normal to the c-axis,
which is the side face of the columnar crystal growing in the
direction of c-axis, Ga atoms adhered on this side face are
destined for re-elimination back into the space, or for rapid
migration over the (0001) surface which is the top surface of the
columnar crystals, under a condition characterized by high
temperature and a less number of atomic adsorption sites of surface
orientation.
[0110] It is consequently supposed that the GaN columnar crystals
grow while keeping anisotropy in the direction of c-axis, and more
specifically that the rate of growth in the direction normal to the
c-surface (0001) of this hexagonal structure (in the direction of
c-axis) is extremely larger than the rate of growth in parallel
with the c-surface, and thereby the crystals are grown as the
columnar crystals.
[0111] Another possibility besides the growth mechanism described
in the above is such that Ga metal aggregates on the top surface
(c-surface) of the columnar crystals, so as to proceed growth in
the vapor-liquid-solid phase (VLS) mode.
[0112] On the other hand, Step S6 allows growth of the p-type
cladding layer, that is, the reverse pyramid portion 2e
(reverse-pyramid-like GaN crystal).
[0113] In this process, the reverse pyramid portion 2e is grown by
lowering the substrate temperature to as low as 680.degree. C.
(500.degree. C. to 800.degree. C.) or around, in contrast to a
temperature of 7150.degree. C. to 950.degree. C. allowing the GaN
columnar crystals to grow in Steps S1 to S5, and by lowering the
amount of supply of Ga atom and N atom to 1:4 to 1:100 or around,
so as to proceed the growth of GaN crystal under an
extremely-nitrogen-excessive condition.
[0114] As a consequence, lowering of the substrate temperature as
compared with that in the growth of columnar crystals makes the
migration of Ga atoms slower, reduces difference between the rate
of growth of GaN crystal in parallel with the direction of c-axis
(normal to the c-surface) and the rate of growth normal to the
direction of c-axis (in parallel with the c-surface), and thereby
switches the growth mode of GaN crystal into that allowing the
growth also in the direction of the side faces of the columnar
crystals.
[0115] It is therefore supposed that the diameter of the columnar
crystals can gradually increase as the growth proceeds, so as to
form the reverse pyramid (reverse pyramid portion) structure.
[0116] Another possible cause is ascribable to Mg doping carried
out for the purpose of converting the crystal into p-type, wherein
the surface of the GaN columnar crystals initially having Ga
polarity might be converted to that of nitrogen polarity, resulting
in increase in the rate of growth of the columnar crystals in the
lateral direction.
<Method of Manufacturing Light Emitting Element; on Sapphire
Substrate>
[0117] An exemplary case of forming a light emitting diode using a
sapphire substrate (on the (0001) surface, with electro-conductive
finish) as the substrate 1, similarly to as in the production on
the Si substrate configured as shown in FIG. 1, will be explained.
A manufacturing apparatus adopted herein is, for example, an MBE
(molecular beam epitaxy) apparatus shown in FIG. 3. The MBE
apparatus has a chamber 21, a heater 22 for heating substrate, and
molecular beam irradiation cells 23a, 23b, 23c, 23d and 23e.
[0118] The sapphire substrate used herein has Ti deposited from
vapor phase to as thick as 350 nm or around, on the back surface
thereof (the surface opposite to that allowing thereon the columnar
crystal growth).
[0119] Prior to the columnar crystal growth, the sapphire substrate
is preliminarily subjected to surface treatment using activated N
(nitrogen) produced by raising an RF plasma at around 100 W to 450
W in N.sub.2 gas supplied at a flow rate of 0.1 to 10 cc/s.
Step S1:
[0120] Under conditions expressed by a substrate temperature of
700.degree. C. to 950.degree. C., a degree of vacuum of 10.sup.-3
Pa (in the presence of molecular beam supply) to 10.sup.-6 Pa (in
the absence of molecular beam supply), and a ratio of supply of Al
and N of 1:1, AlN is grown as a layer of several nanometers thick,
or of 1 nm to 20 nm thick. By this process, dots of growth nuclei
of AlN can be formed on the sapphire substrate according to a
predetermined density, and can thereby grow the columnar crystals
with excellent uniformity.
[0121] In this process, Al and N atoms migrate over the sapphire
substrate, AlN nuclei present within a predetermined range of
distance fuse with each other so as to gradually produce nodules.
AlN dots 4 of approximately 50 to 100 nm in diameter are formed
according to a density of 10.sup.10/cm.sup.2 or around, that is,
while keeping predetermined distance therebetween (see FIG. 4). The
GaN columnar crystals can be grown with an excellent
reproducibility, by allowing GaN to grow using the AlN dots 4 as
nuclei in Step S2 and thereafter.
[0122] Density of the AlN dots 4 can appropriately be altered, by
modifying the above-described conditions.
[0123] In Step S2 and thereafter, the columnar crystals are grown
similarly to as the growth on the silicon substrate.
[0124] FIG. 8 shows photo-luminescence (PL) spectra, measured at
room temperature, of the GaN columnar crystal grown on the (0001)
sapphire substrate by the method of fabrication of the present
invention based on the MBE process, a GaN continuous film (with a
threading dislocation density of 3 to 5.times.10.sup.9/cm.sup.2)
grown by the metal-organic chemical vapor deposition (MOCVD)
process, and a GaN continuous film (with a threading dislocation
density of approximately 8.times.10.sup.9/cm.sup.2) grown by the
MBE process.
[0125] An excitation light source used herein for the Pt.
spectrometry is a CW He-Cd laser having an emission wavelength of
325 nm and an intensity of 10 mW.
[0126] The diameter of the columnar crystals herein is 50 nm to 100
nm.
[0127] It was confirmed from FIG. 8 that, assuming the emission
peak intensity of GaN grown by MOCVD as 1, GaN formed by the MBE
process showed an intensity of 0.3, whereas the GaN columnar
crystals grown by the MBE process of the present invention showed
extremely strong emission expressed by intensities of 27
(nano-column A) to 286 (nano-column B).
[0128] One possible method of forming dots (AlN dots or GaN dots),
which can serve as the nuclei on the surface of the sapphire
substrate and the silicon substrate, periodically at predetermined
intervals (according to a predetermined density) is such as using a
SiO.sub.2 film or a Ti film as a mask, in other words, forming
holes in the SiO.sub.2 film or in the Ti film where the growth is
desired to proceed, so as to make the surface of the substrate
expose therein, thereby allowing the columnar crystals to grow on
the exposed portions.
[0129] Still another method is possibly such as modifying (for
example, scratching) the surface of the substrate by irradiating
electron beam or Ga beam at a predetermined energy, specifically in
portions where the columnar crystals are desired to grow, and
making use of the modified surface as the nuclei for growth of the
dots.
[0130] The portions where the columnar crystals are desired to grow
may be irradiated with electron beam at a predetermined energy.
[0131] By this process, carbon deposits where the electron beam is
irradiated, and thus deposited carbon can be used as a marking for
growth of the dots.
[0132] In addition, it is also allowable to design an atomic step
structure of the substrate on the wafer scale, aiming at aligning
micro-structures using such step structure as a template, and to
form the columnar crystals as being grown on the nuclei, making use
of selectivity of nuclei formation between the terrace and the step
band.
[0133] It is still also allowable to use a silicon super-precision
die or the like, wherein an irregular geometry is formed directly
on the surface of the substrate specifically in the portion
destined for growth of the columnar crystals, by the
nano/micro-imprinting technique, so as to form a periodic
structure.
[0134] By fabrication according to the above-described methods of
manufacturing, a high-quality GaN crystal almost free from
threading dislocation can readily be grown, and consequently hetero
junction or p-n junction can readily be formed in a continuous
manner in the process of growth of the columnar crystals.
[0135] In the process of forming the hetero junction, even growth
of the hetero structure largely differing in the lattice constants
and in the thermal expansion coefficients can largely reduce
distortion stress, as compared with the continuous film, by virtue
of the columnar crystal structure, and can thereby prevent cracks
from occurring.
[0136] According to the above-described method of manufacturing, it
is also expected to improve the efficiency of light emitting diodes
(LED) or photo-excitation elements in which the columnar crystals
are used, because the growth surface (that is, the top portion) of
the columnar crystal will have a nano-texture (surface
nano-structure, or fine irregular geometry on the surface) in a
self-organized manner, and this structure can contribute to large
efficiencies in extraction and introduction of light.
[0137] Besides the above-described substrate materials, also the
(0001) surface of SiC, metals (Al, Ti, Fe, Ni, Cu, Mo, Pd, Ag, Ta,
W, Pt, Au, or alloys partially containing these elements), and flat
substrate obtained by coating any of these metals on a
predetermined substrate (for example, the Si (111) surface of Si
substrate) can be used as the substrate allowing thereon growth of
the GaN crystal, that is, the columnar crystals of the Group-III
nitride semiconductor.
[0138] In the above-described method of manufacturing, Mg used as a
p-type dopant (impurity) in the process of forming the p-type
cladding layer may be replaced with Be.
[0139] It is still also allowable to simultaneously dope Be and Si,
or Be and O, in place of Mg, so as to form the p-type cladding
layer.
[0140] The present invention is aimed at providing light emitting
elements operating at a wavelength band from 200 nm (AlN) to 800 nm
(GaInN). In nitride-base semiconductors, as materials for the
columnar crystals and the light emitting layer, InGaN, GaN,
AlGaInN, AlGaN, AlN and hetero-structures composed of these
materials may be employed.
[0141] As other possible materials for the columnar crystals and
the light emitting layer in oxide-base semiconductors, ZnO, CdZnO,
MgZnO, MgZnCdO and hetero-structures composed of these materials
may be employed.
[0142] For the case where the substrate 1 has an insulating
property, it is also allowable to remove the substrate 1 by laser
lift-off or etching, and to form an electrode on the bottom portion
of the n-type cladding layer 2e of each columnar crystal 2, or to
transfer (transplant) the columnar crystals onto another
electro-conductive substrate.
[0143] The columnar crystals formed by the above-described method
of manufacturing are periodically arranged, while keeping voids
between the adjacent columnar crystals.
[0144] For the purpose of improving physical strength of the
element structure, an insulating material may therefore be filled
as a support material into the space between the adjacent columnar
crystals.
[0145] The filling component used herein may be SiO.sub.2,
Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, GdzO.sub.3, polyimide, epoxy
resin or the like.
[0146] For the case where the substrate 1 has an insulating
property, it is also allowable to remove the substrate 1 by laser
lift-off or etching, to carry out filling with the above-described
support material (insulating filling material), and to form an
electrode on the bottom portion of the n-type cladding layer 2e of
each columnar crystal 2, or to transfer (transplant) the columnar
crystals onto another electro-conductive substrate.
<Structure of Semiconductor Laser>
[0147] Next, a configuration of a semiconductor laser element
obtained by adopting the columnar crystals used in the
above-described light emitting diode and the method of
manufacturing will be explained, referring to FIG. 10 showing a
section of the structure.
[0148] The semiconductor laser element has, on the substrate 11
composed of an electro-conductive material (Si (111) surface, or
the SiC (0001) surface), a plurality of columnar crystals having a
device structure (light emitting function, or light- and
electron-confining function) individually based on a quantum
structure composed of an n-AlGaN DBR 12 (distributed Bragg
reflector layer), an active layer composed of AlGaN MQW 13
(multiple quantum well), and a p-AlGaN DBR 14 or the like, arranged
according to a predetermined periodicity, wherein the space between
every adjacent columnar crystals is filled with a transmissive
insulating material 15 (SiO.sub.2, for example).
[0149] It is also allowable, as in Step S6 described previously, to
grow the entire portions of the columnar crystals, while keeping
the growth mode of the columnar crystals (the state shown in FIG.
5) unchanged, to fill with the insulating material composed of a
dielectric, up to a level of height of the columnar crystals, and
then to form the electrode material on the surface thereof. Also
this process can successfully prevent the electrode material from
coming around.
[0150] Another possible structure is shown in FIG. 10, in which an
electrode 17 composed of a transmissive material is formed on a
continuous film 16, formed by bonding the top surfaces of the
reverse pyramid portions of the columnar crystals, while allowing
the crystal growth to proceed in the isotropic mode. In one
possible method of forming a p-side reflective mirror, the p-type
continuous film 16 may directly be formed, while omitting the
p-AlGaN DBR 14, but forming the semiconductor DBR in such
continuous film, or a reflective mirror composed of a dielectric
multi-layered film may be formed.
[0151] By virtue of arrangement of the columnar crystals
neighboring within a predetermined distance, supply of a
predetermined current between the upper and lower electrodes can
initiate light emission in the active layers of the individual
columnar crystals, wherein also a predetermined intensity of light
emitted from the active layers of the adjacent columnar crystals
comes therein to thereby raise induced emission, and finally
results in laser oscillation after phases of the emitted light from
the individual columnar crystals are synchronized.
[0152] Similarly to the light emitting diode described previously,
the above-described semiconductor laser element of the present
invention is improved in the light emission property as compared
with the conventional one, by virtue of the crystals almost free
from threading dislocation, and is simplified in formation of the
upper electrode 17, because the columnar crystals are sequentially
transformed from reverse cone or reverse polygonal pyramid geometry
of the reverse pyramid portions thereof, so as to finally give the
continuous film 16 on the upper portions thereof.
[0153] In the foregoing explanations, the columnar crystals are
grown by using a molecular beam epitaxy (MBE) apparatus, whereas
the light emitting element making use of the above-descried
columnar crystals can be formed also by MOCVD, HVPE or sputtering,
by appropriately controlling the substrate temperature and the
ratio of supply of Group-V/III.
[0154] The foregoing paragraphs have explained the present
invention referring to a light emitting diode and a semiconductor
laser, whereas the present invention is applicable, not only to
these light emitting devices, but also to semiconductor devices
having any other diode structures (device structures having
rectifying function).
INDUSTRIAL APPLICABILITY
[0155] According to the present invention, by forming the columnar
crystals of nitride-base compound semiconductor, and by providing
the light emitting portion to each of the columnar crystals, it is
possible to obtain semiconductor devices such as light emitting
elements having high luminance in short-wavelength emission region,
making an effective use of properties of the high-quality columnar
crystals almost free from threading dislocation density. In
addition, the electrode material is successfully prevented from
coming around the side faces of the columnar crystals, thereby
simplifying formation of the electrode in the process steps of
manufacturing light emitting elements.
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