U.S. patent application number 12/391531 was filed with the patent office on 2009-06-25 for semiconductor light emitting element and method for fabricating the same.
This patent application is currently assigned to Panasonic Corporation. Invention is credited to Hideto Adachi, Toshiya Fukuhisa, Akihiko Ishibashi, Satoshi Kamiyama, Isao Kidoguchi, Yasuhito Kumabuchi, Masaya Mannoh, Kiyoshi Ohnaka, Masakatsu Suzuki, Akira Takamori, Takeshi Uenoyama.
Application Number | 20090159924 12/391531 |
Document ID | / |
Family ID | 11637467 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090159924 |
Kind Code |
A1 |
Kamiyama; Satoshi ; et
al. |
June 25, 2009 |
SEMICONDUCTOR LIGHT EMITTING ELEMENT AND METHOD FOR FABRICATING THE
SAME
Abstract
The semiconductor laser of this invention includes an active
layer formed in a c-axis direction, wherein the active layer is
made of a hexagonal-system compound semiconductor, and anisotropic
strain is generated in a c plane of the active layer.
Inventors: |
Kamiyama; Satoshi;
(Hyogo-ken, JP) ; Suzuki; Masakatsu; (Osaka,
JP) ; Uenoyama; Takeshi; (Kyoto, JP) ; Ohnaka;
Kiyoshi; (Osaka, JP) ; Takamori; Akira;
(Osaka, JP) ; Mannoh; Masaya; (Osaka, JP) ;
Kidoguchi; Isao; (Osaka, JP) ; Adachi; Hideto;
(Osaka, JP) ; Ishibashi; Akihiko; (Osaka, JP)
; Fukuhisa; Toshiya; (Kyoto, JP) ; Kumabuchi;
Yasuhito; (Osaka, JP) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Panasonic Corporation
Osaka
JP
|
Family ID: |
11637467 |
Appl. No.: |
12/391531 |
Filed: |
February 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11759326 |
Jun 7, 2007 |
|
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12391531 |
|
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|
|
10891968 |
Jul 15, 2004 |
7368766 |
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11759326 |
|
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|
|
10011552 |
Nov 6, 2001 |
6861672 |
|
|
10891968 |
|
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|
09080121 |
May 15, 1998 |
6326638 |
|
|
10011552 |
|
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08588863 |
Jan 19, 1996 |
5787104 |
|
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09080121 |
|
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Current U.S.
Class: |
257/103 ; 257/13;
257/E33.003; 257/E33.026 |
Current CPC
Class: |
H01S 5/2068 20130101;
H01S 5/24 20130101; H01L 33/32 20130101; H01S 2304/12 20130101;
H01S 5/3201 20130101; H01S 5/2237 20130101; H01S 5/32308 20130101;
H01S 5/3403 20130101; Y10S 257/918 20130101; H01S 5/320225
20190801; H01S 2301/173 20130101; H01L 33/16 20130101; H01S 5/0237
20210101; H01S 5/2201 20130101; H01S 2304/04 20130101; B82Y 20/00
20130101; H01S 5/2232 20130101; Y10T 428/12528 20150115; H01S
5/34333 20130101; H01S 5/0213 20130101; H01S 5/3203 20130101; H01S
5/32341 20130101; H01S 5/0207 20130101 |
Class at
Publication: |
257/103 ; 257/13;
257/E33.026; 257/E33.003 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 19, 1995 |
JP |
07-006405 |
Claims
1. A semiconductor light emitting element, comprising: a
semiconductor substrate of a hexagonal crystalline system,
including a principal plane which has a plane direction tilted with
respect to a (0001) plane direction; and an active layer of
Al.sub.xGa.sub.yIn.sub.zN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1) arranged on the principal
plane of the semiconductor substrate and composed of wurtzite-type
crystal.
2. A semiconductor light emitting element according to claim 1,
wherein the active layer has a stress in an in-plane direction of
the principal plane of the semiconductor substrate.
3. A semiconductor light emitting element according to claim 1,
wherein the principal plane of the semiconductor substrate is a
(1100) plane, a (1120) plane or an R plane.
4. A semiconductor light emitting element according to claim 1,
wherein the active layer includes a well layer and a barrier layer.
Description
[0001] The present patent application is a continuation of U.S.
patent application Ser. No. 11/759,326, filed on Jun. 7, 2007,
which is a continuation of U.S. patent application Ser. No.
10/891,968, filed on Jul. 15, 2004 (now U.S. Pat. No. 7,368,766),
which is a divisional of U.S. patent application Ser. No.
10/011,552, filed on Nov. 6, 2001 (now U.S. Pat. No. 6,861,672),
which is a divisional of U.S. patent application Ser. No.
09/080,121, filed on May 15, 1998 (now U.S. Pat. No. 6,326,638),
which is a divisional of U.S. patent application Ser. No.
08/588,863, filed on Jan. 19, 1996 (now U.S. Pat. No. 5,787,104),
the contents of which are incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a short-wavelength
semiconductor light emitting element used in the fields of optical
communications, optical information processing, and the like, and a
method for fabricating the same.
[0004] 2. Description of the Related Art
[0005] In recent years, with increased demands for short-wavelength
semiconductor light emitting elements in various fields, studies
focusing mainly in ZnSe and GaN as the materials for such elements
have been vigorously conducted. As for ZnSe material, a
short-wavelength semiconductor laser with an oscillation wavelength
of about 500 nm has succeeded in oscillating consecutively at room
temperature. Now, study and development for practical use of this
material is under way. As for GaN material, a blue light emitting
diode with high luminance has recently been realized. The
reliability of this material as the light emitting diode is by no
means inferior to that of other materials for semiconductor light
emitting elements. GaN material is therefore expected to be
applicable to a semiconductor laser. However, the properties of GaN
material are not clearly known; moreover, GaN material has a
hexagonal-system crystalline structure. Therefore, it is uncertain
whether GaN material can provide characteristics durable enough for
practical use when it is used as an element having a structure
similar to that used for conventional cubic-system materials.
SUMMARY OF THE INVENTION
[0006] The semiconductor laser of this invention includes an active
layer formed in a c-axis direction, wherein the active layer is
made of a hexagonal-system compound semiconductor and anisotropic
strain is generated in a c plane of the active layer.
[0007] In another aspect of the present invention, a method for
fabricating a semiconductor laser is provided. The method includes
the step of forming an active layer made of a hexagonal-system
compound semiconductor in a c-axis direction, wherein the active
layer is formed so that anisotropic strain is generated in a c
plane.
[0008] Alternatively, the semiconductor light emitting element of
this example includes: a semiconductor substrate; a stripe groove
formed on a principal plane of the semiconductor substrate; and a
semiconductor light emitting layer formed on the other principal
plane of the semiconductor substrate.
[0009] Alternatively, the method for fabricating a semiconductor
light emitting, element of this example includes the steps of:
forming a stripe-shaped groove on a principal plane of a
semiconductor substrate; and forming a light emitting element
structure on the other principal plane of the semiconductor
substrate.
[0010] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
forming a stripe-shaped mask on a principal plane of a
semiconductor substrate; etching the semiconductor substrate
selectively using the mask; depositing material having a thermal
expansion coefficient different from that of the semiconductor
substrate on the semiconductor substrate selectively using the
mask; and forming a light emitting element structure on the other
principal plane of the semiconductor substrate.
[0011] Alternatively, the semiconductor light emitting element of
this example includes: a semiconductor substrate; a stripe-shaped
member formed on a principal plane of the semiconductor substrate,
the member being made of a material having a thermal expansion
coefficient different from that of the semiconductor substrate; and
a semiconductor light emitting layer formed on the other principal
plane of the semiconductor substrate.
[0012] Alternatively, the method for fabricating a semiconductor
light emitting element of this example includes the steps of:
forming a stripe-shaped member on a principal plane of a
semiconductor substrate, the member being made of a material having
a thermal expansion coefficient different from that of the
semiconductor substrate; and forming a light emitting element
structure on the other principal plane of the semiconductor
substrate.
[0013] Alternatively, the method for fabricating a semiconductor
light emitting element of this example includes the steps of:
forming a light emitting element structure on a surface of a
semiconductor substrate; and forming a stripe-shaped member on the
other surface of the semiconductor substrate at 300.degree. C. or
more, the member being made of a material having a thermal
expansion coefficient different from that of the semiconductor
substrate.
[0014] Alternatively, the method for fabricating a semiconductor
light emitting element of this example includes the steps of:
forming a light emitting element structure on a principal plane of
a semiconductor substrate; forming a stripe-shaped member on the
other surface of the semiconductor substrate, the member being made
of a material having a thermal expansion coefficient different from
that of the semiconductor substrate; and heat-treating the
semiconductor substrate at 500.degree. C. or more.
[0015] Alternatively, the semiconductor light emitting element of
this example includes: a semiconductor substrate; a first metal
formed on a principal plane of the semiconductor substrate; a
stripe-shaped second metal formed on the first metal; and a light
emitting element structure formed on the semiconductor
substrate.
[0016] Alternatively, the method for fabricating the semiconductor
light emitting element of this example includes the steps of:
forming a light emitting element structure on a principal plane of
a semiconductor substrate; depositing a first metal on the other
principal plane of the semiconductor substrate; and depositing a
stripe-shaped second metal on the first metal.
[0017] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
attaching a semiconductor substrate to a surface of a body which is
part of a curved surface of a cylinder; and forming a light
emitting element structure on the semiconductor substrate.
[0018] Alternatively, the semiconductor light emitting element of
this invention includes: a substrate having a principal plane; and
a wurtzite-type AlGaInN compound semiconductor formed on the
substrate, wherein the substrate is made of a material of which
thermal expansion coefficient is anisotropic in the principal
plane.
[0019] Alternatively, the semiconductor light emitting element of
this invention includes a substrate having a principal plane and a
wurtzite-type AlGaInN compound semiconductor formed on the
substrate, wherein the substrate is made of a material of which
thermal expansion coefficient is greater in a first direction in
the principal plane and smaller in a second direction vertical to
the first direction than the thermal expansion coefficient of the
wurtzite-type AlGaInN compound semiconductor.
[0020] Alternatively, the semiconductor light emitting element of
this invention includes a wurtzite-type AlGaInN compound
semiconductor where a total of a thermal strain in a first
direction in a substrate plane and a thermal strain in a second
direction vertical to the first direction generated when the
element is cooled from a growth temperature to room temperature is
zero.
[0021] Alternatively, the semiconductor light emitting element of
this invention includes: an active layer made of a wurtzite-type
compound semiconductor; a pair of carrier confinement layers
sandwiching the active layer; and a stripe-shaped strain generating
layer having a lattice constant different from that of the pair of
carrier confinement layers.
[0022] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
placing a semiconductor light emitting element having a
double-hetero structure on an anisotropic crystal; and securing the
semiconductor light emitting element to the anisotropic crystal at
100.degree. C. or more.
[0023] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
placing a semiconductor light emitting element having a
double-hetero structure on a bimetal; and securing the
semiconductor light emitting element to the bimetal at 100.degree.
C. or more.
[0024] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
placing a semiconductor light emitting element having a
double-hetero structure on a sub-mount; applying stress to the
semiconductor light emitting element from a top surface or a side
face thereof; and securing the semiconductor light emitting element
to the sub-mount.
[0025] Alternatively, the method for fabricating an AlGaInN
semiconductor light emitting element of this invention including a
substrate having a step and an AlGaInN double-hetero structure
formed on the substrate is provided. The method includes the steps
of: forming at least two strip grooves on an AlGaInN thin film to
obtain a mesa structure; and forming a multilayer structure
including the AlGaInN double-hetero structure on the entire top
surface of the substrate including the inside of the at least two
stripe grooves so that a crystal mixture ratio of AlGaInN on a flat
surface of the mesa structure is different from that on a slope
surface of the mesa structure.
[0026] Alternatively, the method for fabricating an AlGaInN
semiconductor light emitting element of this invention including a
substrate having a step and an AlGaInN double-hetero structure
formed on the substrate. The method comprising the steps of:
forming a stripe groove on an AlGaInN thin film to obtain a concave
groove structure; and forming a multilayer structure including the
AlGaInN double-hetero structure on the entire top surface of the
substrate including the inside of the stripe groove so that a
crystal mixture ratio of AlGaInN on a flat surface of the concave
groove structure is different from that on a slope surface of the
concave groove structure.
[0027] Alternatively, the method for fabricating a nitride compound
semiconductor of this invention includes the step of forming a
nitride compound semiconductor by vapor phase epitaxy while
selectively irradiating the nitride compound semiconductor, so as
to form an irradiated portion and a non-irradiated portion having
different lattice constants.
[0028] Alternatively, the method for fabricating a nitride compound
semiconductor of this invention includes the steps of: forming a
nitride compound semiconductor by vapor phase epitaxy while
selectively irradiating the nitride compound semiconductor, so as
to form an irradiated portion and a non-irradiated portion having
different lattice constants; and forming a nitride compound
semiconductor by vapor phase epitaxy at a temperature higher than a
temperature used for the former growth step.
[0029] Alternatively, the semiconductor light emitting element of
this invention includes: a substrate; a first cladding layer formed
on the substrate, an area of a plane parallel to the substrate
being smaller than an area of a surface of the substrate; a second
cladding layer formed on the first cladding layer, an area of a
plane parallel to the substrate being larger than the area of the
first cladding layer, the second cladding layer being made of
crystal having a lattice constant different from that of the first
cladding layer; an active layer formed on the second cladding
layer; and a third cladding layer formed on the active layer.
[0030] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
forming a first cladding layer on a substrate; forming a second
cladding layer on the first cladding layer; forming an active layer
on the second cladding layer; forming a third cladding layer on the
active layer; and etching so that the first cladding layer can be
etched faster than the substrate, the second cladding layer, the
active layer, and the third cladding layer.
[0031] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
forming a first cladding layer on a substrate; forming a second
cladding layer on the first cladding layer; forming an active layer
on the second cladding layer; forming a third cladding layer on the
active layer; forming an insulating film on faces of the substrate,
the first cladding layer, the second cladding layer, the active
layer, and the third cladding layer vertical to a depositing
direction; removing a portion of the insulating film so as to
expose the side face of the first cladding layer; and etching so
that the first cladding layer can be etched faster than the
insulating film.
[0032] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
forming a first conductive semiconductor on a substrate; forming an
insulating semiconductor on the first conductive semiconductor, the
insulating semiconductor having a lattice constant different from
that of the first conductive semiconductor; forming a semiconductor
layer of a double-hetero structure on the insulating semiconductor;
and etching the first conductive semiconductor by immersing the
substrate, the first conductive semiconductor, and the insulating
semiconductor in an electrolytic solution and attaching a positive
electrode and a negative electrode to the first conductive
semiconductor or the insulating semiconductor for applying a
voltage between the electrodes.
[0033] Alternatively, the semiconductor light emitting element of
this invention includes: a substrate; a semiconductor crystal
nucleus deposited on the substrate; a thin film spirally formed
around the crystal nucleus in parallel to the substrate; a first
cladding layer formed on the thin film; an active layer formed on
the first cladding layer; and a second cladding layer formed on the
active layer.
[0034] Alternatively, the method for fabricating a semiconductor
light emitting element of this invention includes the steps of:
forming a semiconductor crystal nucleus on a substrate under a
first pressure condition by vapor phase epitaxy; forming a thin
film around the crystal nucleus spirally in parallel to the
substrate under a second pressure condition; forming a first
cladding layer under a third pressure condition; forming an active
layer on the first cladding layer under the third pressure
condition; and forming a second cladding layer on the active layer
under the third pressure condition.
[0035] In still another aspect of the present invention, a
semiconductor light emitting device is provided. The device
includes a base having a concave portion and a semiconductor light
emitting element formed in the concave portion, wherein an active
layer of the semiconductor light emitting element is made of a
hexagonal-system compound semiconductor, and anisotropic strain is
generated in a c plane of the active layer due to stress from the
base.
[0036] Alternatively, the semiconductor light emitting device of
this invention includes a semiconductor light emitting element and
a stress applying portion for applying stress to an active layer of
the semiconductor light emitting element, wherein the active layer
of the semiconductor light emitting element is made of a
hexagonal-system compound semiconductor, and anisotropic strain is
applied to a c plane of the active layer from the stress applying
portion.
[0037] In still another aspect of the present invention, an
epitaxial method for epitaxially growing crystal on a substrate
causing lattice mismatching is provided. In the method, lattice
strain generated in an epitaxial layer due to the lattice
mismatching between crystals of the substrate and the epitaxial
layer is concentrated in a specific direction of the epitaxial
layer, so as to generate anisotropic strain in the epitaxial
layer.
[0038] Thus, the invention described herein makes possible the
advantages of (1) providing a semiconductor light emitting element
with high performance and a simple structure where the strain
characteristic of an electronic band structure unique to a
hexagonal-system compound semiconductor is utilized, i.e.,
providing a semiconductor light emitting element with a low
threshold current by applying anisotropic strain to the c plane of
a hexagonal-system compound semiconductor, and (2) providing a
method for fabricating such a semiconductor light emitting
element.
[0039] These and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 shows an electronic band structure of a valence band
of a GaN layer of an AlGaN/GaN quantum well structure.
[0041] FIG. 2 shows an electronic band structure of a valence band
of a GaN layer of an AlGaN/GaN quantum well structure when
anisotropic strain is applied to the c plane.
[0042] FIG. 3 shows the strain dependency of the threshold current
density when anisotropic strain is applied to the c plane.
[0043] FIG. 4 shows a semiconductor light emitting element of
Example 1 according to the present invention.
[0044] FIGS. 5A to 5E are sectional views showing a fabrication
process of the semiconductor light emitting element of Example 1
according to the present invention.
[0045] FIGS. 6A and 6B show another example of the semiconductor
light emitting element according to the present invention.
[0046] FIG. 7 shows still another example of the semiconductor
light emitting element according to the present invention.
[0047] FIG. 8 shows still another example of the semiconductor
light emitting element according to the present invention.
[0048] FIGS. 9A and 9B show a fabrication process of still another
example of the semiconductor light emitting element according to
the present invention.
[0049] FIG. 10 is a sectional view of a wurtzite-type InGaN/AlGaN
quantum well semiconductor laser of Example 2 according to the
present invention.
[0050] FIG. 11 shows two crystal plane directions perpendicular to
each other on a LiTaO.sub.3 substrate.
[0051] FIG. 12 shows the state where the LiTaO.sub.3 substrate is
inclined from the (1100) plane in the (0001) or (11 20)
direction.
[0052] FIG. 13 is a sectional view of a wurtzite-type InGaN/AlGaN
quantum well semiconductor laser of Example 3 according to the
present invention.
[0053] FIG. 14 shows strain locally generated in the vicinity of a
p-Al.sub.z.Ga.sub.1-z.N strain generating layer.
[0054] FIG. 15 shows the thermal expansion coefficients of
LiTaO.sub.3 and GaN.
[0055] FIGS. 16A and 16B are perspective views showing a
fabrication process of a semiconductor laser of Example 4 according
to the present invention.
[0056] FIG. 17 shows a sub-mount for the semiconductor laser of
Example 4.
[0057] FIGS. 18A and 18B show a fabrication process of a
semiconductor laser of Example 5 according to the present
invention.
[0058] FIG. 19 shows a sub-mount for the semiconductor laser of
Example 5.
[0059] FIGS. 20A and 20B show another sub-mount for the
semiconductor laser of Example 5.
[0060] FIG. 21 is a sectional view showing a fabrication process of
a semiconductor laser according to the present invention.
[0061] FIG. 22 is a sectional view showing another fabrication
process of a semiconductor laser according to the present
invention.
[0062] FIGS. 23A to 23C show a fabrication process of an AlGaInN
semiconductor light emitting element of Example 6 according to the
present invention.
[0063] FIGS. 24A to 24C show a fabrication process of an AlGaInN
semiconductor light emitting element of Example 7 according to the
present invention.
[0064] FIGS. 25A and 25B show a vapor phase epitaxy step including
selective laser irradiation and the relationship between the
lattice constant of GaN crystal grown by the vapor phase epitaxy
and the selective laser irradiation, respectively, of Example
8.
[0065] FIG. 26 shows the relationship between the laser irradiation
intensity and the GaN lattice constant.
[0066] FIG. 27 shows a vapor phase epitaxy step including selective
laser irradiation of Example 10.
[0067] FIGS. 28A to 28D are sectional views showing a fabrication
process of a semiconductor device of Example 11 according to the
present invention.
[0068] FIG. 29 is a schematic sectional view of the semiconductor
device of Example 11, together with a crystal structure of an
active layer of the semiconductor device.
[0069] FIGS. 30A to 30C schematically show a fabrication process of
a semiconductor device of Example 12 according to the present
invention.
[0070] FIG. 31 schematically shows a unit cell of a GaN spiral thin
film of Example 12.
[0071] FIGS. 32A and 32B are perspective views of a semiconductor
light emitting device of Example 14 according to the present
invention.
[0072] FIGS. 33A and 33H are sectional views of a semiconductor
light emitting device of Example 15 according to the present
invention.
[0073] FIG. 34 is a sectional view of a crystal growth apparatus
used in Example 13 according to the present invention.
[0074] FIGS. 35A to 35D are sectional views showing a crystal
growth process in Example 13.
[0075] FIG. 36 is a sectional view of epitaxial layers formed by
two-stage epitaxy as a comparative example.
[0076] FIG. 37 is a perspective view of epitaxial layers in Example
13.
[0077] FIG. 38 is a sectional view of epitaxial layers in the case
of using an SiO.sub.2 film in Example 13.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0078] The inventors have found that the effective mass of holes
near the top of a valence band is reduced when strain which is not
isotropic (anisotropic strain) is applied to the c plane of a
hexagonal-system compound semiconductor. Using this property, a
semiconductor light emitting element with a low threshold current
can be realized by applying anisotropic strain to the c plane of an
active layer composed of a hexagonal-system compound semiconductor
grown in a c-axis direction. The "isotropic" strain as used herein
refers to a strain applied to the c plane hydrostatically
(isotropically).
[0079] Hereinbelow, the strain characteristic of the electronic
band structure of a valence band of a hexagonal-system compound
semiconductor used in the present invention will be described with
reference to the accompanying drawings.
[0080] FIG. 1 shows an electronic band structure of a valence band
of a GaN quantum well layer of an AlGaN/GaN quantum well structure
when no strain is applied. The quantum well structure is composed
of an AlGaN barrier layer and the GaN quantum well layer. The
thickness of the GaN quantum well layer is 4 nm. In FIG. 1, a curve
a represents a first-level energy band of a heavy hole, a curve b a
second-level energy band of a heavy hole, a curve c a first-level
energy band of a light hole, and a curve d a second-level energy
band of a light hole.
[0081] It is observed from FIG. 1 that the effective mass of holes
near the top of the valence band (near wave number 0) is
significantly large compared with that of zincblende type compound
semiconductors. Also, when uniaxial strain in the c-axis direction
or an isotropic (biaxial) strain in the c plane is applied to the
GaN quantum well layer, the effective mass of holes near the top of
the valence band is almost the same as that obtained when no strain
is applied. The uniaxial strain in the c-axis direction refers to
the case where strain is applied only in the c-axis direction of
the hexagonal-system compound semiconductor. The biaxial strain in
the c plane refers to the case where strains of an equal magnitude
are applied along axes vertical to each other, which is also
referred to as the isotropic strain.
[0082] A deformation energy generated by the application of
anisotropic strain in the c plane of a hexagonal-system compound
semiconductor can be expressed by
D.sub.5(e.sub.xx-e.sub.yy+2ie.sub.xy) where D.sub.5 denotes the
deformation potential when anisotropic strain is applied in the c
plane, e.sub.xx and e.sub.yy denote the strains in two directions
perpendicular to each other in the c plane, and e.sub.xy denotes
the shearing strain in the c plane.
[0083] FIG. 2 shows an electronic band structure of the valence
band of the GaN quantum well layer of the AlGaN/GaN quantum well
structure when anisotropic strain having a deformation energy of 10
meV is applied in the c plane. As is observed from FIG. 2, the
"curvature of the valence band" in the region of small wave numbers
is reduced when anisotropic strain is applied. This indicates that
the effective mass of holes near the top of the valence band is
considerably small compared with the case where no strain is
applied. This means that the state density near the top of the
valence band decreases, and thus the injected current density
required for laser oscillation can be low. Accordingly, only a
small oscillation threshold current is required for a semiconductor
laser having an active layer where anisotropic strain is applied to
a quantum well layer thereof.
[0084] FIG. 3 shows the strain dependency of the threshold current
density observed when anisotropic strain is applied to the c plane.
The X axis represents the deformation energy and the Y axis
represents the threshold current density standardized by a value
obtained when no strain is applied. The results obtained when the
threshold gain is varied are shown in FIG. 3. As is observed from
FIG. 3, the threshold current density significantly decreases by
the application of anisotropic strain, irrespective of the
threshold gain values.
[0085] Though the shearing strain in the c plane is not taken into
consideration in FIG. 3, a similar effect to the above is observed
when shearing strain is applied to the c plane.
[0086] As described above with reference to FIG. 1 to 3, it has
been found that, as for the semiconductor light emitting element
using a hexagonal-system compound semiconductor as an active layer,
an element requiring only a small threshold current can be realized
by applying anisotropic strain to the active layer.
[0087] Now, the element where anisotropic strain is applied to a
hexagonal compound semiconductor and the method for fabricating the
same according to the present invention will be described by way of
examples.
Example 1
[0088] The semiconductor light emitting element of Example 1
according to the present invention will be described with reference
to FIG. 4. An AlGaInN material which is a III-V group compound
semiconductor is used as a hexagonal-system compound semiconductor.
Strain is applied in a direction parallel to the c plane. When a
wurtzite-type material is used for an AlGaInN light emitting layer
100, the band structure (valence band) can be changed by applying
uniaxial strain in a direction vertical to the (0001) axis
(parallel to the c plane). As a result, the characteristics of the
light emitting element improve as described above.
[0089] By forming stripe-shaped grooves 102 on a sapphire substrate
101 as shown in FIG. 4, the directivity of the thermal expansion
coefficient is exhibited on the substrate 101. By this formation of
the grooves 102, it is possible to apply uniaxial strain in the x
direction shown in FIG. 4 to the AlGaInN light emitting layer 100
grown on the surface of the substrate 101 opposite to the grooves
102. Using the light emitting layer 100 as an active layer, a
semiconductor light emitting element with a small threshold current
can be realized.
[0090] The method for fabricating the semiconductor light emitting
element of Example 1 will be described with reference to FIGS. 5A
to 5E.
[0091] First, a stripe-shaped mask 104 is formed on a principal
plane of a sapphire substrate 103. Then, the substrate 103 is
etched with an etchant such as hot sulfuric acid using the mask 104
so as to form stripe-shaped grooves 105. Then, a material such as
AlN is selectively grown on the substrate 103 using the mask 104 so
as to form AlN buried layers 106 only in the grooves 105. As a
result, the thermal expansion coefficient distribution is generated
in the thickness direction. This makes it possible to generate
uniaxial strain in the substrate when an AlGaInN light emitting
layer 107 is formed on the substrate by crystal growth at a high
temperature equal to or more than 1000.degree. C. in a later stage.
Metalorganic vapor phase epitaxy (MOVPE) is used for the crystal
growth. An appropriate temperature for the crystal growth of
AlGaInN is considered to be about 1100.degree. C. When the
temperature is lowered and resumes room temperature, the AlGaInN
light emitting layer 107 is in the state of having uniaxial
strain.
[0092] The strain applied to the AlGaInN light emitting layer 107
can be greater when the AlN buried layers 106 are formed, because
the formation of the buried layers increases the thermal expansion
and thus improves the heat transfer from a heater. The absolute
amount of the strain to be applied can be controlled by varying the
width and depth of the grooves 105, so as to obtain an optimal
structure for the light emitting element.
[0093] An alternative method for applying uniaxial strain to
crystal is to form stripe-shaped oxide films on a substrate. FIGS.
6A, 6B, and 7 show an alternative example of the semiconductor
light emitting element according to the present invention.
[0094] Before crystal growth, stripe-shaped oxide films 109 are
formed on a principal plane of a sapphire substrate 108.
[0095] When the temperature is raised to 1000.degree. C. or more
for the crystal growth, the substrate is curved in the z direction
shown in FIG. 7 due to the difference in the thermal expansion
coefficient between the sapphire substrate 108 and the oxide films
109. The crystal growth is conducted while the curved state being
maintained, and thereafter the temperature is lowered to room
temperature. As a result, crystal having strain in the z direction
is obtained. In this case, the strain amount can be controlled by
the width and pitch of the stripes. For example, when a
semiconductor laser is fabricated, the width and the pitch are
preferably 5 microns and 10 microns, respectively.
[0096] Alternatively, the AlGaInN light emitting layer can be first
formed by MOVPE. Then, the stripe-shaped oxide layers 109 are
formed at a high temperature of about 500.degree. C., so that a
curve similar to the above can be formed and thus uniaxial strain
can be generated in the crystal. Alternatively, the stripe-shaped
oxide films 109 can be heated to a high temperature after the
formation thereof.
[0097] A bimetal effect can be used to provide an effect similar to
the above. FIG. 8 shows still another example of the semiconductor
light emitting element according to the present invention. It is
effective to use SiC for a substrate 112. An AlGaInN light emitting
layer 116 is first formed on the SiC substrate 112 by MOVPE. An Ni
layer 113 is formed on the SiC substrate 112, and then
stripe-shaped first Au layers 114 are formed on the Ni layer 113.
The SiC substrate 112 is then curved in the z direction by the
bimetal effect, and thus uniaxial strain is applied to the AlGaInN
light emitting layer 116. In this case, the ohmic characteristic is
exhibited by the Ni layer 113 to the SiC substrate 112, which
provides an advantageous effect to the resultant semiconductor
light emitting element.
[0098] FIGS. 9A and 98 show yet another example of the
semiconductor light emitting device according to the present
invention. In this alternative example, stress is applied
externally before the crystal growth. A sapphire substrate 117 is
secured on a tray 119 having a curvature R with fixtures 118. A
light emitting layer is formed on the substrate 117, and then the
substrate 117 is removed from the tray 119. While the substrate 117
gradually resumes the original shape, strain is applied to the
light emitting layer. According to this method, the strain amount
to be applied can be controlled by mechanically changing the
curvature of the tray 119.
Example 2
[0099] FIG. 10 is a sectional view of a wurtzite-type
In.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN quantum well semiconductor
laser of Example 2 according to the present invention.
In.sub.xGa.sub.1-xN and Al.sub.yGa.sub.1-yN are used for a quantum
well layer and a barrier layer, respectively.
[0100] Referring to FIG. 10, an AlN buffer layer 202, an
n-Al.sub.zGa.sub.1-zN cladding layer 203, an Al.sub.yGa.sub.1-yN
first optical guide layer 204, an In.sub.xGa.sub.1-xN/GaN multiple
quantum well active layer 205 (a multilayer structure of
In.sub.xGa.sub.1-xN quantum well layers and GaN quantum well
layers), an Al.sub.yGa.sub.1-yN second optical guide layer 206, and
a p-Al.sub.zGa.sub.1-zN cladding layer 207 are consecutively formed
in this order on a (1100) LiTaO.sub.3 substrate 201 by MOVPE.
[0101] A ridge stripe 208 is formed by etching, and an SiO.sub.2
insulating film 209 is formed over the top surface of the resultant
structure. Openings 210 and 211 are formed at the SiO.sub.2
insulating film 209 for current injection. Finally, an anode
electrode 212 and a cathode electrode 213 are formed.
[0102] The layers constituting the wurtzite-type InGaN/AlGaN
quantum well semiconductor laser are formed at a temperature range
of 800 to 1100.degree. C., except for the AlN buffer layer 202,
when grown by MOVPE, for example, though the growth temperatures
for the layers are often different from one another depending on
the composition and material to be used. Accordingly, when room
temperature is resumed after the crystal growth process, strain is
generated in the crystal due to the difference in the thermal
expansion coefficient between the crystal and the substrate. The
crystal growth for all the layers after the polycrystalline AlN
buffer layer 202 is conducted using the AlN buffer layer 202 as a
seed crystal. Accordingly, the difference in the lattice constant
between the (1100) LiTaO.sub.3 substrate 201 and the other layers
hardly affect the strain. There may be the case where the lattice
constants are different among the hetero structure composed of the
n-Al.sub.zGa.sub.1-zN cladding layer 203 to the
p-Al.sub.zGa.sub.1-zN cladding layer 207, and this difference in
the lattice constant may affect the strain. In such a case,
however, suitable materials and thicknesses can be selected to
prevent an occurrence of misfit dislocation and the like. However,
the above-described strain due to the difference in the thermal
expansion coefficient cannot be prevented. This strain is therefore
positively utilized in this example.
[0103] FIG. 15 shows the thermal expansion coefficients of the
LiTaO.sub.3 substrate 201 and the wurtzite-type GaN crystal in the
plane. Since the (1100) LiTaO.sub.3 substrate is used in this
example, the thermal expansion coefficient is anisotropic in the
plane, which is expressed by the (0001) direction and the (11 20)
direction vertical to the (0001) direction, as shown in FIG. 11. As
for the wurtzite-type GaN material, crystal grows at the (0001)
orientation regardless of the crystal plane direction of the
substrate. Accordingly, each layer is formed vertically to the
(0001) direction. The thermal expansion coefficient of the
wurtzite-type GaN material is isotropic in the (0001) plane. The
thermal expansion coefficient of GaN is 5.6.times.10.sup.-6.
Materials of AlGaInN mixed crystal of any composition have the
thermal expansion coefficients near the above value. On the
contrary, the LiTaO.sub.3 substrate has a thermal expansion
coefficient of 1.2.times.10.sup.-6 which is smaller than that of
GaN in the (0001) direction. In the (11 20) direction, however, it
has a thermal expansion coefficient of 2.2.times.10.sup.-5 which is
extremely larger than that of GaN. Accordingly, in the
semiconductor laser shown in FIG. 10, when the thickness of the
(1100) LiTaO.sub.3 substrate 201 is sufficiently larger than the
total thickness of the n-Al.sub.zGa.sub.1-zN cladding layer 203 to
the p-Al.sub.xGa.sub.1-zN cladding layer 207 formed by crystal
growth and the growth temperature of each layer is as high as
1000.degree. C., a strain in the (0001) direction (e.sub.xx) of
-0.44% and a strain in the (11 20) direction (e.sub.yyn) of 1.6%
are generated in the crystals of the layers 203 to 207 when they
are cooled to room temperature. In this way, anisotropic strain can
be generated in the plane of the In.sub.xGa.sub.1-xN/GaN multiple
quantum well active layer 205. This greatly reduces the state
density of the valence band and thus reduces the threshold current
of the laser.
[0104] In the case where the total thickness of the crystal growth
layers 203 to 207 is large, the layers cannot bear the strain
generated by the difference in the thermal expansion coefficient
between the layers and the substrate, i.e., e.sub.xx and e.sub.yy.
This may cause dislocation defect and thus reduces the strain. In
such a case, as shown in FIG. 12, the LiTaO.sub.3 substrate may be
tilted by .theta. toward the (0001) direction from the (1100)
direction and by .phi.0 toward the (11 20) direction. By this
tilting, the strain generated in the respective directions in the
crystal growth layers can be reduced to:
e'.sub.xx=-0.4 cos .theta.
e'.sub.yy=1.6 cos .phi..
[0105] Accordingly, by appropriately selecting q and f, an
occurrence of the dislocation defect can be prevented. The effect
of preventing the dislocation defect is especially high by
selecting q and f so that e'.sub.xx+e'.sub.yy=0.
[0106] In this example, LiTaO.sub.3 was used for the substrate.
However, other nonlinear optical crystal materials such as
LiNbO.sub.3, KTiOPO.sub.4, KNbO.sub.3, and LiB.sub.6O.sub.13 can
also be used as long as they have large anisotropy in the thermal
expansion coefficient and are stable in the growth temperature.
Example 3
[0107] FIG. 13 is a sectional view of a wurtzite-type InGaN/AlGaN
quantum well, semiconductor laser of Example 3 according to the
present invention.
[0108] Referring to FIG. 13, an AlN buffer layer 302, an
n-Al.sub.zGa.sub.1-zN cladding layer 303, an Al.sub.yGa.sub.1-yN
first optical guide layer 304, an In.sub.xGa.sub.1-xN/GaN multiple
quantum well active layer 305, an Al.sub.yGa.sub.1-yN second
optical guide layer 306, a p-Al.sub.zGa.sub.1-zN first cladding
layer 307, and a p-Al.sub.z.Ga.sub.1-z.N strain generating layer
308 are consecutively formed in this order on a (0001) sapphire
substrate 301 by crystal growth. Then, the resultant structure is
taken out from a crystal growth apparatus, and the
p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308 is shaped into
a stripe with a width of 2 mm by etching. The resultant structure
is placed in the crystal growth apparatus again, and a
p-Al.sub.z.Ga.sub.1-z.N second cladding layer 309 is formed. An
SiO.sub.2 insulating film 310 is then formed over the top surface
of the resultant structure. Openings 311 and 312 are formed at the
SiO.sub.2 insulating film 310 for current injection. Finally, an
anode electrode 313 and a cathode electrode 314 are formed.
[0109] When the Al composition ratio z' of the
p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308 is made larger
than the Al composition ratio z of the p-Al.sub.zGa.sub.1-zN first
cladding layer 307, and the p-Al.sub.zGa.sub.1-zN second cladding
layer 309, the lattice constant of the former becomes smaller than
that of the latter. As a result, compression strain can be
generated in the surrounding crystals as shown in FIG. 14.
[0110] The above local strain can be generated because the width of
the p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308 is as small
as about 2 mm. If the width is larger, strain is only generated in
the p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308 itself, not
to the surrounding crystals. Since the p-Al.sub.z.Ga.sub.1-z.N
strain generating layer 308 is of a stripe shape, strain is
generated in the surrounding crystals in the plane vertical to the
stripe, while it is not in the plane parallel to the stripe. As a
result, strain is generated only in the plane of the
In.sub.xGa.sub.1-xN/GaN multiple quantum well active layer 305
vertical to the stripe, causing anisotropy in the strain and thus
reducing the hole state density. The strain in the
In.sub.xGa.sub.1-xN/GaN multiple quantum well active layer 305 is
greater as the multiple quantum well active layer 305 is nearer to
the p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308.
Accordingly, the strain can be adjusted by setting the thickness of
the p-Al.sub.zGa.sub.1-zN first cladding layer 307
appropriately.
[0111] In this example, the Al composition ratio z' of the
p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308 was made larger
than the Al composition ratio z of the p-Al.sub.zGa.sub.1-zN first
cladding layer 307 and the p-Al.sub.zGa.sub.1-zN second cladding
layer 309. Anisotropic strain can also be generated when the former
is made smaller than the latter. In this case, especially, an
optical waveguide structure can be realized by use of the
p-Al.sub.z.Ga.sub.1-z.N strain generating layer 308, because the
refractive index of the layer 308 is greater than that of the
adjacent p-Al.sub.zGa.sub.1-zN second cladding layer 309. Thus, a
refractive index waveguide structure can be easily realized.
Example 4
[0112] FIGS. 16A and 16B show a method for fabricating a
semiconductor laser according to the present invention.
[0113] As shown in FIG. 16A, a chip of a semiconductor laser 401
which was fabricated by forming an Al.sub.xGa.sub.yIn.sub.xN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1)
layer on the (0001) plane of a sapphire substrate by a crystal
growth method such as MOVPE is mounted on a sub-mount 402 at a high
temperature of 200.degree. C. As shown in FIG. 17, the sub-mount
402 includes an LiTaO.sub.3 dielectric 403 of anisotropic crystal
and a solder member 404. The semiconductor laser 401 is mounted on
a plane perpendicular to the (0001) plane, for example, on the
(1120) or (1100) plane.
[0114] The solder member 404 is composed of Pb--Sn and the like,
for example. The solder member melted at 200.degree. C. is
solidified when the temperature lowers to room temperature, so that
the semiconductor laser 401 is secured to the sub-mount 402. The
thermal expansion coefficient of the LiTaO.sub.3 dielectric 403 is
22.times.10.sup.-6/K in the a-axis direction and
1.2.times.10.sup.-6/K in the c-axis direction. That is, the thermal
expansions in the x-axis direction and the y-axis direction shown
in FIG. 16A are considerably different from each other. As a
result, non-uniform stress is applied to the semiconductor laser
401 when the semiconductor laser 401 is secured to the sub-mount
402. The amount of the uniaxial stress applied to the semiconductor
laser 401 can be controlled by adjusting the temperature to be
increased. In other words, larger stress can be applied as the
temperature is higher.
[0115] The semiconductor laser 401 of this example uses
wurtzite-type crystal, which can change the band structure of the
valence band by applying uniaxial stress in a direction vertical to
the (0001) axis. This reduces the effective mass and thus the state
density. As a result, a highly reliable semiconductor laser with
reduced threshold current and driving current can be obtained.
[0116] Thus, the characteristics of the semiconductor light
emitting element can be greatly improved by combining the
wurtzite-type semiconductor light emitting element with anisotropic
crystal of which thermal expansion coefficient varies depending on
the direction.
[0117] In this example, the semiconductor laser 401 was mounted on
a plane of the sub-mount vertical to the (0001) plane, for example,
on the (1120) or (1100) plane. However, the plane direction is not
limited to the above as long as the sub-mount can provide uniaxial
stress.
Example 5
[0118] FIGS. 18A to 18B show another method for fabricating a
semiconductor laser according to the present invention.
[0119] As shown in FIG. 18A, a chip of a semiconductor laser 501
which was fabricated by forming an Al.sub.xGa.sub.yIn.sub.zN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1)
layer on the (0001) plane of a sapphire substrate by a crystal
growth method such as MOVPE is mounted on a sub-mount 502 at a high
temperature of 180.degree. C. As shown in FIG. 19, the sub-mount
502 includes an Fe--Ni alloy 503, an Fe--Ni--Mn alloy 504, and a
Pb--Sn solder member 505. The Fe--Ni alloy which is called Invar
hardly changes its length as the temperature changes. On the other
hand, the Fe--Ni--Mn alloy exhibits significantly large thermal
expansion as the temperature rises. A sub-mount which curves as the
temperature changes can be obtained by laminating the Fe--Ni alloy
503 and the Fe--Ni--Mn alloy 504.
[0120] The solder member melted at 180.degree. C. is solidified
when the temperature lowers to room temperature, so that the
semiconductor laser 501 is secured to the sub-mount 502.
Non-uniform stress which is especially large in one direction is
applied to the semiconductor laser 501 when the semiconductor laser
501 is secured to the sub-mount 502. The amount of the uniaxial
stress applied to the semiconductor laser 501 can be controlled by
adjusting the temperature to be increased.
[0121] The semiconductor laser 501 of this example uses the
wurtzite-type crystal, which can change the structure of the
valence band by receiving uniaxial stress in a direction vertical
to the (0001) axis. This reduces the effective mass and thus the
state density. As a result, a highly reliable semiconductor laser
with reduced threshold current and driving current can be
obtained.
[0122] Thus, the characteristics of the semiconductor light
emitting element can be greatly improved by combining the
wurtzite-type semiconductor light emitting element with the
bimetal.
[0123] In this example, the sub-mount shown in FIG. 19 was used.
Instead, the effect of the present invention can be obtained by
using any sub-mount which can be curved in one direction. For
example, the structure shown in FIGS. 20A and 20B can also be used.
In this structure, a stripe-shaped Fe--Ni alloy 503 which does not
expand with a temperature change, is formed on the bottom surface
of an Fe--Ni--Mn alloy 504. The sub-mount of this structure can be
curved in a direction vertical to the stripe direction as shown in
FIG. 20B.
[0124] FIG. 21 shows another method for fabricating a semiconductor
laser according to the present invention.
[0125] As shown in FIG. 21, a chip of a semiconductor laser 551
which was fabricated by forming an Al.sub.xGa.sub.yIn.sub.zN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1)
layer on the (1120) plane of a sapphire substrate by a crystal
growth method such as MOVPE is mounted on a sub-mount 552.
Ultraviolet (UV)-curable resin 554 is formed on the sub-mount 552.
Then, the semiconductor laser 551 is applied with stress from above
by a collet 553, while the UV-curable resin 554 is irradiated with
UV light, so as to secure the semiconductor laser 551 to the
sub-mount 552. Vertical stress is applied to the semiconductor
laser 551 when the semiconductor laser 551 is secured to the
sub-mount 552.
[0126] The semiconductor laser 551 of this example uses the
wurtzite-type crystal, which can change the structure of the
valence band by receiving uniaxial stress in a direction vertical
to the (0001) axis. This reduces the effective mass and thus the
state density. As a result, a highly reliable semiconductor laser
with reduced threshold current and driving current can be
obtained.
[0127] In the above alternative example, the stress was applied to
the semiconductor laser from above as shown in FIG. 21. Instead,
the stress can also be applied from the sides of the semiconductor
laser as shown in FIG. 22. In this case, the semiconductor laser
551 is placed in a concave portion formed on a sub-mount 555. A
hexagonal-system compound semiconductor is used for the active
layer of the semiconductor laser, and the crystal is grown in the
c-axis direction.
[0128] As shown in FIG. 22, screws 557 for applying stress to the
sides of the semiconductor laser 551 via plate springs 556 are
disposed on the sides of the sub-mount 555. The semiconductor laser
551 is secured to the concave portion with the UV-curable resin
554. By screwing the screws 557, uniaxial strain is generated in
the c plane of the active layer of the semiconductor laser 551.
With the uniaxial strain, a semiconductor laser with a small
threshold current can be realized.
[0129] In the above alternative example, the semiconductor laser
having the (1120) substrate was used. The effect of the present
invention can be obtained for a structure where uniaxial stress is
applied in a direction vertical to the (0001) plane.
[0130] The UV-curable resin was used in the above examples. Any
other materials such as thermosetting resin can also be used as
long as they can secure the semiconductor laser to the
sub-mount.
Example 6
[0131] FIGS. 23A to 23C show a method for fabricating an AlGaInN
semiconductor light emitting element according to the present
invention.
[0132] The crystal growth in this method is conducted by low
pressure MOVPE. Two MOVPE processes are required to fabricate the
element. First, as shown in FIG. 23a, the first MOVPE process is
conducted after degreasing a 6H--SiC substrate 601. The process
will be described in detail.
[0133] Hydrogen gas is supplied in a reaction chamber of an MOVPE
apparatus, and the pressure in the reaction chamber is set at 1/10
atmospheric pressure. Then, the temperature of the substrate 601 is
raised up to 1100.degree. C. in the hydrogen gas atmosphere to
clean the surface of the 6H--SiC substrate 601.
[0134] After the temperature of the substrate 601 is lowered to
600.degree. C., ammonia gas as the V-group material and, after ten
seconds, trimethyl aluminum as the III-group material, are supplied
to a surface of the 6H--SiC substrate 601 so as to form a
non-monocrystalline AlN layer 602 with a thickness of 50 nm. The
supply of trimethyl aluminum is then stopped temporarily, in order
to raise the substrate temperature to 900.degree. C. Then,
trimethyl aluminum as the III-group material is supplied again, so
as to form a monocrystalline AlN layer 603 with a thickness of 5
.mu.m.
[0135] Then, as shown in FIG. 23B, using SiO.sub.2 as a mask for
etching, two stripe grooves with a width of 3 .mu.m are formed with
a distance of 2 .mu.m therebetween.
[0136] After removing the mask for etching, the second MOVPE
process is conducted in the following manner. Hydrogen gas is
supplied in the reaction chamber of the MOVPE apparatus, and the
pressure in the reaction chamber is set at 1/10 atmospheric
pressure. Then, the temperature of the 6H--SiC substrate 601 is
raised to 1100.degree. C. in an atmosphere which is a mixture of
hydrogen gas and ammonia gas, so as to clean the surface of the
substrate 601.
[0137] Then, as shown in FIG. 23C, after the temperature of the
substrate 601 is lowered to 1030.degree. C., trimethyl aluminum,
trimethyl indium, and trimethyl gallium as the III-group materials
are supplied, so as to form an Si-doped n-type AlGaInN cladding
layer 604 with a thickness of 3 .mu.m, an AlGaInN active layer 605
with a thickness of 20 nm, and an Mg-doped p-type AlGaInN cladding
layer 606 with a thickness of 2 .mu.m over the entire top surface
of the AlN layer 603 including the inside of the stripe grooves
consecutively in this order. Finally, a p-side electrode 607 and an
n-side electrode 608 are formed to complete the laser
structure.
[0138] At the formation of the double-hetero structure in the above
fabrication process, the efficiency by which each of the III-group
elements is introduced in the crystal in two stripe groove portions
609 is different from that in a flat portion 610 between the two
stripe grooves. As a result, the composition varies, which
corresponds to the variation in the lattice constant. Accordingly,
transverse stress is applied from the stripe groove portions 609 to
the flat portion 610 which is sandwiched by crystals of a different
lattice constant. This indicates that strain can be selectively
applied to the flat portion in a direction vertical to the stripe
direction. This substantially corresponds to uniaxial strain in the
plane of the active layer, which is effective in reducing the state
density of the valence band. Also, the active layer in the flat
portion 610 between the two stripe grooves is curved. Thus, a
semiconductor laser with a low threshold current and a stable
trans-verse mode can be realized by using the flat portion 610 as a
light emitting portion.
[0139] The amount of strain applied to the flat portion 610 as the
light emitting portion can be easily controlled by changing the
distance between the two stripe grooves, the depth of the stripe
grooves, and the thickness of the Si-doped n-type AlGaInN cladding
layer 604.
Example 7
[0140] FIGS. 24A to 24C show another method for fabricating an
AlGaInN semiconductor light emitting element according to the
present invention. The method of this example is different from the
method in Example 6 in that a concave portion is formed on a
substrate instead of the two stripe grooves. Strain can be applied
to an active layer of a flat portion on the bottom of the concave
portion by using slope portions of the concave portion.
[0141] The crystal growth in this method is conducted by Vacuum
MOVPE. Two MOVPE processes are required to fabricate the element.
As shown in FIG. 24A, the first MOVPE process is conducted after
degreasing a 6H--SiC substrate 651. The process will now be
described in detail.
[0142] Hydrogen gas is supplied in a reaction chamber of an MOVPE
apparatus, and the pressure in the reaction chamber is set at 1/10
atmospheric pressure. Then, the temperature of the substrate 651 is
raised up to 1100.degree. C. in the hydrogen gas atmosphere to
clean the surface of the substrate 651. After the temperature of
the substrate 651 is lowered to 600.degree. C., ammonia gas as the
V-group material and, after ten seconds, trimethyl aluminum as the
III-group material are supplied to a surface of the 6H--SiC
substrate 651, so as to form a non-monocrystalline AlN layer 652
with a thickness of 50 nm. The supply of trimethyl aluminum is then
stopped temporarily, to raise the substrate temperature to
900.degree. C. Then, trimethyl aluminum as the III-group material
is supplied again, so as to form a monocrystalline AlN layer 653
with a thickness of 5%.
[0143] Then, as shown in FIG. 24B, using SiO.sub.2 as a mask for
etching, a stripe groove with a width of 3 .mu.m is formed.
[0144] After removing the mask for etching, the second MOVPE
process is conducted in the following manner. Hydrogen gas is
supplied in the reaction chamber of the MOVPE apparatus, and the
pressure in the reaction chamber is set at 1/10 atmospheric
pressure. Then, the temperature of the 6H--SiC substrate 651 is
raised up to 1100.degree. C. in an atmosphere of mixture of
hydrogen gas and ammonia gas, so as to clean the surface of the
substrate 651.
[0145] Then, as shown in FIG. 24C, after the temperature of the
substrate 651 is lowered to 1030.degree. C., trimethyl aluminum,
trimethyl indium, and trimethyl gallium as the III-group material
are supplied, so as to form an Si-doped n-type AlGaInN cladding
layer 654 with a thickness of 3 .mu.m, an AlGaInN active layer 655
with a thickness of 20 nm, and an Mg-doped p-type AlGaInN cladding
layer 656 with a thickness of 2 .mu.m over the entire top surface
of the AlN layer 653 including the inside of the stripe groove
consecutively in this order. Finally, a p-side electrode 657 and an
n-side electrode 658 are formed to complete the laser
structure.
[0146] At the formation of the double-hetero structure in the above
fabrication process, the efficiency by which each of the III-group
elements is introduced in the crystal in stripe groove slope
portions 659 is different from that in a stripe groove flat portion
660. As a result, the composition varies, which corresponds to the
variation in the lattice constant. Accordingly, trans-verse stress
is applied from the stripe groove slope portions 659 to the stripe
groove flat portion 660 which is sandwiched by crystals of a
different lattice constant. This indicates that strain can be
selectively applied to the stripe groove flat portion 660 only in a
direction vertical to the stripe direction. That is, uniaxial
strain can be applied in the plane of the active layer of the
stripe groove flat portion 660. Thus, a semiconductor laser with a
low threshold current and a stable transverse mode can be realized
by using the stripe groove flat portion 660 as a light emitting
portion.
[0147] The amount of strain applied to the stripe groove flat
portion 660 as the light emitting portion can be easily controlled
by changing the width and depth of the stripe groove and the
thickness of the Si-doped n-type AlGaInN cladding layer 654.
Example 8
[0148] Referring to FIG. 25A, GaN crystal layers 802 and 803 are
formed on a (0001) sapphire substrate 801 by MOVPE using trimethyl
gallium (TMG) and ammonia (NH.sub.3). Hydrogen is used as a carrier
gas. The growth pressure is 100 Torr. At the vapor phase epitaxy, a
portion of the substrate is selectively irradiated with a light
beam emitted from an excimer laser and the like through a window
formed in a reaction chamber. The crystal growth is conducted at
500.degree. C., which is lower than the temperature at which
monocrystal is normally obtained. This low temperature is necessary
for forming the GaN crystal layers with different lattice constants
arranged two-dimensionally.
[0149] FIG. 26 shows data by which the inventors have found that
the lattice constant of GaN crystal varies depending on the
intensity of a laser beam radiated during vapor phase epitaxy. This
is considered to occur due to the following reason: When the growth
temperature is sufficiently low, resultant GaN crystal is
polycrystalline, which has a small apparent lattice constant. If a
laser beam with high intensity is radiated during the crystal
growth, the temperature of the irradiated portion selectively
increases, resulting in monocrystallizing the irradiated portion.
Referring to FIG. 25A, the (0001) sapphire substrate 801 is
selectively irradiated with an excimer laser beam with an intensity
of 10 kW during the MOVPE process. As a result, the GaN crystal
layer 802 having a lattice constant normally obtained for
monocrystal is formed in the laser beam irradiated region. On the
other hand, the polycrystalline GaN crystal layer 803 having a
larger lattice constant is formed in the region which is not
irradiated with a laser beam. As a result, as shown in FIG. 25B, a
two-dimensionally anisotropic strain state is realized in the
boundary area of the GaN crystal layers 802 and 803, where strain
is generated along the boundary and no strain is generated in a
direction perpendicular to the boundary. If this is applied to an
active layer of a gallium nitride semiconductor laser, for example,
significant improvement on characteristics thereof is expected. The
growth temperature is not limited to 500.degree. C., but a similar
effect can be obtained by 700.degree. C. or less at which
polycrystal is obtained.
Example 9
[0150] In this example, GaN is further deposited on the GaN crystal
layers 802 and 803 of Example 8 by MOVPE at 1000.degree. C. without
irradiation with a laser beam. As a result, a GaN crystal layer 805
in FIG. 27 formed on the polycrystalline GaN crystal layer 803 has
a large lattice constant, while a GaN crystal layer 804 formed on
the monocrystalline GaN crystal layer 802 has a small lattice
constant. Important is that the temperature for this crystal growth
should be higher than the temperature at which the GaN crystal
layers 802 and 803 were formed. Using the polycrystalline GaN
crystal layer 803 having a large lattice constant as a buffer
layer, the GaN crystal layer 805 which is more monocrystalline and
has, good crystallinity can be formed. As a result, a GaN
monocrystal layer with higher quality than that obtained by the
method only including laser beam irradiation as shown in FIG. 25A
can be realized.
Example 10
[0151] Information on lattice mismatching from the sapphire
substrate can be controlled by varying the thickness of the GaN
crystal layers 802 and 803 shown in FIG. 25A. Accordingly, by
varying the thickness of the GaN crystal layers 802 and 803, the
lattice constant of the GaN crystal layer 805 can be varied and
thus the amount of strain can be controlled. By using the GaN
monocrystal layer having two-dimensional strain fabricated by the
above method as an active layer of a gallium nitride semiconductor
laser, significant improvement on characteristics thereof is
expected.
[0152] In the above examples, the growth of GaN monocrystal was
described. A similar effect can also be obtained by using AlN, InN,
or a mixture thereof. Also, a similar effect can be obtained by
using a substrate made of SiC, ZnO, and the like, instead of the
sapphire substrate described above.
Example 11
[0153] FIGS. 28A to 28D show a fabrication process of Example 11
according to the present invention. A sapphire substrate 1101 is
placed in a reaction chamber of an MOVPE apparatus and heated to
1000.degree. C. Then, the following layers are formed by MOVPE by
supplying respective materials in the reaction chamber so as to
form a double-hetero structure: an AlGaN cladding layer 1102 with a
thickness of 5 .mu.m by supplying hydrogen, ammonia, trimethyl
aluminum, and trimethyl gallium; an n-type AlGaInN cladding layer
1103 with a thickness of 5 .mu.m by supplying hydrogen, ammonia,
trimethyl aluminum, trimethyl indium, and trimethyl gallium; an
InGaN active layer 1104 with a thickness of 0.01 .mu.m by supplying
hydrogen, ammonia, trimethyl indium, and trimethyl gallium; and a
p-type AlGaInN cladding layer 1105 with a thickness of 2 .mu.m by
supplying hydrogen, ammonia, trimethyl aluminum, trimethyl indium,
trimethyl gallium, and diethyl zinc.
[0154] The AlInGaN cladding layers 1103 and 1105 and the InGaN
active layer 1104 have lattice constants larger than that of the
AlGaN cladding layer 1102. Accordingly, compression strain is
generated in the resultant structure.
[0155] Then, as shown in FIG. 28A, an insulating film 1106 is
formed over the sides of the substrate 1101 and the layers 1102 to
1105 by thermal CVD.
[0156] The insulating film 1106 is selectively etched by
photolithography and reactive ion etching with carbon tetrafluoride
so that the side face of the AlGaN cladding layer 1102 is exposed
as shown in FIG. 28B.
[0157] The AlGaN cladding layer 1102 is then etched from the
exposed side face by 5 .mu.m by reactive ion beam etching with
chlorine, forming the structure as shown in FIG. 28C. The AlGaInN
cladding layer 1103, the InGaN active layer 1104, and the AlGaInN
cladding layer 1105 have lattice constants larger than the AlGaN
cladding layer 1102 because the former contain indium. Accordingly,
the lattice constants of these layers are reduced from their
original values and compression strain is generated in the portion
of these layers where the AlGaN cladding layer 1102 exists
underneath. On the contrary, in the other portion of these layers
where the AlGaN cladding layer 1102 has been removed by etching, no
stress is applied and thus their original lattice constants remain.
In FIG. 28C, therefore, these layers 1103 to 1105 are shown as
expanding in the longitudinal direction.
[0158] FIG. 29 shows a sectional view of the structure after
removing the insulating film, together with a plane view thereof as
is viewed from above. Regions 1107 and 1108 schematically show the
lattice constants of crystals of the InGaN active layer 1104 as is
viewed from above. The region 1107 of the InGaN active layer 1104
is located above the AlGaN cladding layer 1102 having a lattice
constant smaller than that of the active layer. Accordingly, the
region 1107 of the InGaN active layer 1104 receives stress from all
the directions vertical to the growth direction, generating
two-dimensional compression strain. The region 1107 expands in the
growth direction by receiving the compression strain.
[0159] The region 1108 of the InGaN active layer 1104 below which
the AlGaN cladding layer 1102 does not exist receives no stress in
the right direction as is seen from FIG. 29 among the directions
vertical to the growth direction. Accordingly, it is possible to
apply compression strain in a selective direction.
[0160] Thus, strain in directions shown by arrows in FIG. 29 is
generated in the active layer 1104 at the boundary of the region
1107 below which the AlGaN cladding layer 1102 exists and the
region 1108 below which the layer 1102 does not exist. This strain
corresponds to uniaxial strain in the plane, reducing the state
density of the valence band. In the region 1107, the active layer
1104 receives compression strain isotropically in the plane. This
strain is therefore not effective in reducing the threshold
current. However, anisotropic strain is generated at the boundary
of the regions 1107 and 1108, which is greatly effective in
reducing the threshold current.
[0161] Alternatively, as shown in FIG. 28D, an InGaN cladding layer
can be used instead of the AlGaN cladding layer 1102, and an
AlGaInN active layer can be used instead of the InGaN active layer
1104. In this alternative example, also, tensile strain can be
selectively applied to the AlGaInN active layer at the boundary of
the portion below which the InGaN cladding layer exists and the
portion below which the cladding layer does not exist in a manner
as described above.
[0162] The selective etching can be conducted by electrolysis,
instead of the patterning of the insulating film and the dry
etching such as reactive ion beam etching as described above. In
the etching by electrolysis, a layer to be etched is doped with
impurities with high concentration so as to be etched faster than
other layers, and a voltage is applied via electrodes in an
electrolytic solution. An insulating layer with a thickness of
about 1 .mu.m is required between the layer to be etched and other
layers constituting the device structure to prevent electrical
interference.
Example 12
[0163] FIGS. 30A to 30C schematically show a fabrication process of
Example 12 according to the present invention. A sapphire substrate
1201 is placed in a reaction chamber of an MOVPE apparatus and
heated to 1000.degree. C. Hydrogen, ammonia, and trimethyl gallium
are supplied in the reaction chamber so as to form a GaN crystal
nucleus 1202 on the substrate 1201. At this time, the pressure in
the reaction chamber is set as low as 10 Torr in order to obtain a
low density for the formation of the growth nucleus.
[0164] Then, while the pressure is further lowered to 5 Torr,
ammonia and trimethyl gallium are supplied to grow GaN crystal
1203. This extremely low pressure prevents a new crystal nucleus
from being formed on the substrate and the crystal from being
deposited vertically to the substrate. This is the condition where
monocrystal is most easily deposited on a crystal wall. Thus, the
GaN crystal 1203 is grown around the crystal nucleus 1202 spirally
in parallel to the substrate, forming a spiral thin film.
[0165] FIG. 31 schematically shows a unit cell 1204 of the GaN
crystal 1203. The unit cell 1204 does not receive stress in the
radial direction, but receives a tensile stress in the
circumferential direction which is stronger as the portion thereof
is closer to the outer circumference of the substrate. Accordingly,
as long as dislocation does not occur, the GaN crystal 1203 is a
crystal having asymmetric strain (anisotropic strain) where tensile
strain is generated only in the circumferential direction.
[0166] Thereafter, the pressure in the reaction chamber is raised
to 80 Torr, to allow crystal to be deposited vertically to the
substrate. Hydrogen, ammonia, trimethyl aluminum, and trimethyl
gallium are supplied onto the GaN spiral thin film 1203, so as to
form an n-type AlGaN cladding layer 1205 with a thickness of 5
.mu.m. Likewise, an InGaN active layer 1206 with a thickness of
0.01 .mu.m is formed by supplying hydrogen, ammonia, trimethyl
indium, and trimethyl gallium, and a p-type AlGaN cladding layer
1207 with a thickness of 2 .mu.m is formed by supplying hydrogen,
ammonia, trimethyl aluminum, trimethyl gallium, and diethyl zinc.
Thus, as shown in FIG. 30C, a double hetero structure is
formed.
[0167] According to the method of this example, crystal with
asymmetric strain can be easily formed only by varying the pressure
in the crystal growth process without the necessity of the steps
such as etching and selective re-growth. As a result, a
semiconductor laser with a small threshold current can be obtained
by this method.
Example 13
[0168] FIG. 34 is a schematic sectional view of a crystal growth
apparatus used in this example. Material gas is supplied from a gas
inlet 1012 into a reaction chamber 1011 made of quartz. A susceptor
1013 made of carbon is placed in the reaction chamber loll, on
which a sample substrate 1014 is mounted. The susceptor 1013 is
provided with a rotational mechanism for securing the in-plane
uniformity of the composition and pressure of an epitaxial layer.
The susceptor 1013 is heated by induction with a high frequency
coil 1015 disposed around the reaction chamber 1011. A thermocouple
1016 is disposed inside the susceptor 1013 for monitoring and
controlling the temperature of the substrate. A gas outlet 1017,
which is connected to a vacuum pump 1018, regulates the pressure in
the reaction chamber and exhausts gas outside.
[0169] A crystal growth method using the above apparatus will be
described with reference to FIGS. 34 and 35A to 35D. First, a
(0001) plane .alpha.-Al.sub.2O.sub.3 sapphire substrate 1031 of
which surface has been cleaned with chemical treatment with an
organic solvent and a hydrochloric acid group substance and by
washing with pure water is mounted on the susceptor 1013 and
secured thereto with a holder 1021. High-purity hydrogen gas
purified by a purifying apparatus is supplied from the gas inlet
1012 and replaces air in the reaction chamber 1011. After the
supply of the hydrogen gas for several minutes, the vacuum pump
1018 is driven to set the pressure in the reaction chamber at 10
Torr. Following the stabilization of the pressure, the susceptor
1013 is heated by induction with the high frequency coil 1015, so
as to raise the temperature of the sample substrate 1014 to
1200.degree. C. This temperature is kept for about 10 minutes, to
clean the surface of the substrate.
[0170] The temperature of the substrate is lowered to 400.degree.
C. Then, TMG (trimethyl gallium) and NH.sub.3 (ammonia) as the
material gas are supplied from the gas inlet 1012, so as to form an
amorphous GaN film 1035 with a thickness of 0.1 .mu.m as shown in
FIG. 35A. At this time, since the substrate temperature is low
compared with a normal growth condition, the decomposition
efficiency of NH.sub.3 is low. In consideration of this, the flow
rate ratio of NH.sub.3 to TMG is set at 10000:1. If the substrate
temperature at the crystal growth is higher than the above
temperature, crystal grows three-dimensionally, i.e., hexagon-pole
shaped crystals are grown like islands, failing in obtaining a
uniform amorphous GaN film.
[0171] The sample substrate is taken out from the reaction chamber
1011 after the temperature thereof is lowered. The amorphous GaN
film 1035 is then etched by photolithography to form stripes in a
direction crossing the R plane of the sapphire substrate 1031 as
shown in FIG. 35B. The R plane of the sapphire substrate 1031 is a
plane shown as the section in FIGS. 35A to 35D. The stripes extend
in a direction vertical to the plane of these figures, i.e.,
crossing the R plane. The width of the stripes and the distance
between the stripes are 5 .mu.m and 50 .mu.m, respectively.
[0172] The sample substrate 1014 is placed again in the reaction
chamber 1011 after being washed sufficiently with pure water.
NH.sub.3 gas is supplied in the reaction chamber this time, instead
of hydrogen gas, and the sample substrate 1014 is heated to
1100.degree. C. as in the manner described above, so as to clean
the surface of the sample substrate.
[0173] Then, GaN films are formed by a normal two-stage epitaxy by
supplying TMG and NH.sub.3 from the gas inlet 1012. More
specifically, first, the substrate temperature is lowered to
600.degree. C. to facilitate a GaN film 1033 with a thickness of
0.05 .mu.m to be formed three-dimensionally, i.e., hexagon-pole
shaped crystals to be grown like islands as shown in FIG. 35C.
Then, the substrate temperature is raised to 1050.degree. C. to
form a GaN film 1034 with a thickness of 5.0 .mu.m by epitaxy as
shown in FIG. 35D. The flow rate ratio of NH.sub.3 to TMG is 300:1.
The portions of the GaN film 1034 formed on the stripes of the
amorphous GaN film 1035 are amorphous because only amorphous
crystal is grown on an amorphous crystal. The amorphous portions of
the GaN film 1034 are specifically called amorphous GaN films
1036.
[0174] The other portions of the GaN film 1034 interposed between
the amorphous GaN films 1036 constitute element formation regions
1041. In these regions, the dislocation density is low and the
strain in the crystal is anisotropic. In other words, strain caused
by the difference in the lattice constant between the substrate
1031 and the GaN film 1034 is maintained in a direction parallel to
the stripes, while it is minimized in a direction perpendicular to
the stripes.
[0175] Thus, if a semiconductor laser, for example, is formed in
the element formation region 1041 having anisotropic strain, the
threshold current thereof can be reduced due to the anisotropic
strain in the region.
[0176] Now, the crystal quality of the epitaxial layers obtained by
this example will be described. For comparison, FIG. 36 shows a
sectional view of GaN epitaxial layers 1033 and 1034 with a
thickness of 5 .mu.m formed on a (0001) plane
.alpha.-Al.sub.2O.sub.3 sapphire substrate 1031 by a conventional
two-stage epitaxial method. This exhibits a distribution of
dislocation observed by a transmission electron microscope.
[0177] Dislocations 1032 generated uniformly due to strain caused
by lattice mismatching extend from an interface 1037 between the
substrate 1031 and the GaN epitaxial layer 1033 toward the surface
of the epitaxial layers while meandering. The dislocations which
disappear or come out midway show that they extend in the direction
vertical to the plane of the figure, not indicating that they are
distinguished. The dislocation density estimated from the image
obtained by the transmission electron microscope is
10.sup.9/cm.sup.2 or more, and the distribution is uniform. The
lattice strain applied to the epitaxial layers is isotropic in the
plane.
[0178] The epitaxially formed GaN films of this example will now be
described with reference to FIG. 37. The distribution of
dislocations observed by the transmission electron microscope is
also shown in FIG. 37 schematically. It is observed from FIG. 37
that a considerably large number of dislocations reach the portions
above the stripes of the amorphous GaN film 1035 where crystal
defects are concentrated. This occurs within the region of the GaN
films with a thickness of 3 .mu.m. In other words, while strain is
minimized in a direction 1039 vertical to the stripes, strain tends
to remain in the crystal in a direction 1040 parallel to the
stripes. As a result, epitaxial films where strain due to lattice
mismatching is mostly remained in the direction 1040 parallel to
the stripes are obtained.
[0179] Other effects are as follows. The dislocation density of the
element formation regions 1041 interposed between the stripes is
10.sup.5/cm.sup.2 or less. This indicates that the GaN films in
these regions have excellent crystallinity compared with the
comparative example shown in FIG. 36. Also, the strain in the
direction parallel to the stripes is differently generated from
that in the direction vertical to the stripes. That is, the element
formation regions 1041 have anisotropic strain.
[0180] In this example, the stripes of the amorphous GaN film 1035
were formed on the substrate. A similar effect can also be obtained
by using oxide films and nitride films such as SiO.sub.2 and
SiN.
[0181] FIG. 38 shows the case of using an SiO.sub.2 film. GaN films
1033 and 1034 are selectively formed on a substrate on which strips
of an SiO.sub.2 film 1038 with a thickness of 0.1 .mu.m have been
formed. In this case, as in the above case, epitaxial films where
strain due to lattice mismatching mostly remains in a direction
parallel to the stripes are obtained. Since the GaN films are not
formed on the stripes of the SiO.sub.2 film (selective growth), the
GaN films are formed like islands. In the island-like GaN film
1034, strain remains in the direction parallel to the stripes while
being minimized in the direction vertical to the stripes, realizing
anisotropic strain.
[0182] In this example, the GaN epitaxy on the (0001) plane
.alpha.-Al.sub.2O.sub.3 sapphire substrate was described. The
present invention is not limited to the above case, but can be
applied to any epitaxy causing lattice mismatching, providing
effects similar to the above.
[0183] In this example, a plurality of stripes of the GaN film 1035
were formed. However, with at least one stripe, anisotropic strain
can be generated in the element formation region of the GaN film.
This is because anisotropic strain always exists near the
stripe.
[0184] The two-stage epitaxial method was used to form the GaN
films on the substrate. However, the GaN film 1034 may be directly
formed on the substrate without forming the GaN film 1033.
[0185] Thus, it has been verified that, in the epitaxy causing
lattice mismatching, the method according to the present invention
is effective in obtaining an epitaxial film where lattice strain
generated due to the lattice mismatching can be concentrated in a
specific direction.
Example 14
[0186] FIGS. 32A and 32B show a semiconductor light emitting device
of Example 14 according to the present invention. In this example,
stress is physically applied to the sides of a semiconductor light
emitting element by use of a shape memory alloy so as to generate
strain in an active layer.
[0187] Referring to FIG. 32A, a concave portion 1403 is formed on a
Cu--Ni--Al shape memory alloy 1404. The width of the concave
portion 1403 is 480 .mu.m, which is slightly smaller than the width
of a semiconductor light emitting element 1402 to be described
later. The surfaces of the concave portion 1403 are lightly treated
for insulation to prevent the semiconductor light emitting element
1402 from being short-circuited when the element is placed in the
concave portion 1403.
[0188] First, the concave portion 1403 is mechanically enlarged,
and the semiconductor light emitting element 1402 is placed in the
concave portion 1403. The resultant structure is placed in a
heating chamber and heated to 80.degree. C. The shape memory alloy
is thus heated and resumes the original shape. As a result, stress
is applied to the semiconductor light emitting element 1402 in the
X direction vertical to the stripe. This makes it possible to
generate strain uniaxially (in the x direction) in the c plane of
an active layer, and thus reduce the threshold current of the light
emitting element.
[0189] Since the size of the concave portion 1403 of the shape
memory alloy is predetermined, the stress applied to the light
emitting element is determined depending on the size of the concave
portion 1403. The concave portion 1403 enlarged to receive the
light emitting element resumes the original shape only by heating.
Accordingly, the mounting of the light emitting element is easy.
The resultant structure with the light emitting element fitted in
the shape memory alloy is a semiconductor light emitting device
1401.
[0190] The semiconductor light emitting element 1402 is fabricated
by MOVPE in the following manner.
[0191] First, a well cleaned (0001) sapphire substrate (c plane) is
placed on a susceptor in a reaction chamber. After a hydrogen
atmosphere is established in the reaction chamber, the substrate is
heated to 1080.degree. C. to clean the substrate.
[0192] The substrate is then cooled to 505.degree. C. Four
liters/min. of ammonia and 30.times.10.sup.-6 mols/min. of
trimethyl gallium as the material gas and 2 liters/min. of hydrogen
as the carrier gas are supplied, so as to form a GaN buffer layer
on the substrate.
[0193] The supply of trimethyl gallium is then stopped. The
substrate temperature is raised to 1080.degree. C. Then,
50.times.10.sup.-6 mols/min, of trimethyl gallium and
2.times.10.sup.-9 mols/min. of silane gas are supplied, so as to
form a silicon-doped n-type GaN layer.
[0194] Then, the supply of the material gas is stopped. The
substrate temperature is lowered to 800.degree. C. The carrier gas
is switched from hydrogen to nitrogen. Then, 2 liters/min. of
nitrogen and 2.times.10.sup.-6 mols/min. of trimethyl gallium,
1.times.10.sup.-5 mols/min. of trimethyl indium, 2.times.10.sup.-6
mols/min. of diethyl cadmium, and 4 liters/min. of ammonia as the
material gas are supplied, so as to form a cadmium-doped
In.sub.0.14Ga.sub.0.86N layer.
[0195] The supply of the material gas is then stopped.
[0196] The substrate temperature is raised to 1080.degree. C. Then,
50.times.10.sup.-6 mols/min. of trimethyl gallium,
3.6.times.10.sup.-6 mols/min. of cyclopentadienyl magnesium, and 4
liters/min. of ammonia are supplied, so as to form a p-type GaN
layer.
[0197] The p-type GaN layer and the n-type InGaN layer of the
semiconductor light emitting element are partly etched to expose
the n-type GaN layer. P-type and n-type ohmic electrodes are formed
on the p-type GaN layer and the n-type GaN layer, respectively. The
semiconductor light emitting device of this example is obtained by
mounting the thus-fabricated semiconductor light emitting
element.
Example 15
[0198] FIGS. 33A and 33B show a semiconductor light emitting device
using the semiconductor light emitting element shown in Example 14.
Stress is mechanically applied to the light emitting element from
the sides thereof.
[0199] Referring to FIG. 33A, the semiconductor light emitting
element is formed by growing. AlGaInN crystal which is a
hexagonal-system compound semiconductor on the (0001) plane (c
plane). Accordingly, stress is applied to the semiconductor light
emitting element from the sides thereof so as to generate strain
uniaxially (in the y direction) in the c plane.
[0200] A semiconductor light emitting element 1502 is placed in a
vessel 1503 for stress application, and stress is gradually applied
to the semiconductor light emitting element 1502 from the sides
thereof. The magnitude of the stress is adjustable by turning a
handle 1504.
[0201] In FIG. 33A, the (0001) plane sapphire substrate is used for
the substrate of the semiconductor light emitting element 1502.
Instead, as shown in FIG. 33B, by using the R plane for the
substrate and growing AlGaInN crystal on the substrate, stress can
be applied in a direction vertical to the substrate (y direction).
This is because, though the state density of the valence band
cannot be reduced by generating strain in the c-axis direction, it
can be drastically reduced by generating anisotropic strain in the
c plane. Using the R plane for the substrate, the c axis is
directed as shown in FIG. 33B. Accordingly, anisotropic strain can
be generated in the c plane when stress is applied in the direction
vertical to the substrate (y direction). Naturally, as described
above with reference to FIG. 33A, anisotropic strain can also be
generated in the c plane by applying stress from the sides of the
semiconductor light emitting element grown on the R plane.
[0202] Thus, according to the method of this example, anisotropic
strain can be mechanically generated in the c plane of the
semiconductor light emitting element. Accordingly, the state
density of the valence band can be reduced, and thus the threshold
current for laser oscillation can be drastically reduced.
[0203] According to the present invention, based on the fact that,
in the case of applying anisotropic strain in the c plane of a
hexagonal-system compound semiconductor, the effective mass of
holes near the top of the valence band is lowered, a semiconductor
light emitting element with a reduced threshold current can be
realized by generating anisotropic strain in the c plane of an
active layer composed of a hexagonal-system compound semiconductor
grown in the c-axis direction.
[0204] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended
that the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *