U.S. patent application number 11/910609 was filed with the patent office on 2008-10-23 for method for growth of gan single crystal, method for preparation of gan substrate, process for producing gan-based element, and gan-based element.
This patent application is currently assigned to TOHOKU TECHNO ARCH CO., LTD.. Invention is credited to Meoung-Whan Cho, Takafumi Yao.
Application Number | 20080261378 11/910609 |
Document ID | / |
Family ID | 37451762 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080261378 |
Kind Code |
A1 |
Yao; Takafumi ; et
al. |
October 23, 2008 |
Method for Growth of Gan Single Crystal, Method for Preparation of
Gan Substrate, Process for Producing Gan-Based Element, and
Gan-Based Element
Abstract
A GaN-based thin film (thick film) is grown using a metal buffer
layer grown on a substrate. (a) A metal buffer layer (210) made of,
for example, Cr or Cu is vapor-deposited on a sapphire substrate
(120). (b) A substrate obtained by vapor-depositing the metal
buffer layer (210) on the sapphire substrate (120) is nitrided in
an ammonia gas ambient, thereby forming a metal nitride layer
(212). (c) A GaN buffer layer (222) is grown on the nitrided metal
buffer layers (210, 212). (d) Finally, a GaN single-crystal layer
(220) is grown. This GaN single-crystal layer (220) can be grown to
have various thicknesses depending on the objects. A freestanding
substrate can be fabricated by selective chemical etching of the
substrate fabricated by the above steps. It is also possible to use
the substrate fabricated by the above steps as a GaN template
substrate for fabricating a GaN-based light emitting diode or laser
diode.
Inventors: |
Yao; Takafumi; (Miyagi-ken,
JP) ; Cho; Meoung-Whan; (Miyagi-ken, JP) |
Correspondence
Address: |
COWAN LIEBOWITZ & LATMAN P.C
1133 AVENUE OF THE AMERICAS, 1133 AVENUE OF THE AMERICAS
NEW YORK
NY
10036
US
|
Assignee: |
TOHOKU TECHNO ARCH CO.,
LTD.
Miyagi-ken
JP
|
Family ID: |
37451762 |
Appl. No.: |
11/910609 |
Filed: |
March 31, 2006 |
PCT Filed: |
March 31, 2006 |
PCT NO: |
PCT/JP2006/306958 |
371 Date: |
June 16, 2008 |
Current U.S.
Class: |
438/458 ;
257/E21.108; 257/E21.121; 257/E21.13; 438/483 |
Current CPC
Class: |
H01L 21/0254 20130101;
C30B 25/02 20130101; H01L 21/02458 20130101; H01L 33/007 20130101;
H01L 21/02614 20130101; C30B 29/406 20130101; H01L 29/2003
20130101; H01L 21/0237 20130101; H01L 21/02491 20130101; C30B
25/183 20130101; H01L 21/02488 20130101; H01L 21/02502 20130101;
H01L 33/12 20130101 |
Class at
Publication: |
438/458 ;
438/483; 257/E21.108 |
International
Class: |
H01L 21/30 20060101
H01L021/30; H01L 31/20 20060101 H01L031/20 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2005 |
JP |
2005-108072 |
Claims
1: A GaN single crystal growth method comprising: a Cu layer growth
process of growing a Cu layer on an underlying substrate; a
nitridation process of forming a Cu nitride layer by nitriding at
least a surface of the Cu layer; a GaN buffer layer growth process
of growing a GaN buffer layer on the Cu nitride layer; and a GaN
layer growth process of growing a single-crystal GaN layer on the
GaN buffer layer.
2: A GaN single crystal growth method comprising: a chromium layer
growth process of growing a chromium layer on an underlying
substrate; a nitridation process of forming a chromium nitride
layer by nitriding at least a surface of the chromium layer; a GaN
buffer layer growth process of growing a GaN buffer layer on the
chromium nitride layer; and a GaN layer growth process of growing a
single-crystal GaN layer on the GaN buffer layer, wherein the
chromium nitride layer has (111) orientation.
3: A GaN single crystal growth method comprising: a nitridation
process of forming a Cu nitride layer by nitriding a surface of a
Cu underlying substrate; a GaN buffer layer growth process of
growing a GaN buffer layer on the Cu nitride layer; and a GaN layer
growth process of growing a single-crystal GaN layer on the GaN
buffer layer.
4: A GaN single crystal growth method comprising: a nitridation
process of forming a chromium nitride layer by nitriding a surface
of a chromium underlying substrate; a GaN buffer layer growth
process of growing a GaN buffer layer on the chromium nitride
layer; and a GaN layer growth process of growing a single-crystal
GaN layer on the GaN buffer layer, wherein the chromium nitride
layer has (111) orientation.
5: A GaN single crystal growth method according to claim 1, wherein
nitridation is performed by a gas containing ammonia in the
nitridation process.
6: A GaN single crystal growth method according to claim 1, wherein
nitridation is performed at a temperature of 500.degree. C. to
1,000.degree. C. in the nitridation process.
7: A GaN single crystal growth method according to claim 1, wherein
the GaN buffer layer is grown at a temperature of 800.degree. C. to
1,100.degree. C. in the GaN buffer layer growth process.
8: A GaN single crystal growth method according to claim 1, wherein
the GaN buffer layer is grown to have a thickness of 50 nm to 30
.mu.m in the GaN buffer layer growth process.
9: A GaN single crystal growth method according to claim 1, wherein
the underlying substrate has a metal layer on a surface.
10: A GaN single crystal growth method according to claim 1, the
method further comprising a separation process of separating the
single-crystal GaN layer from the underlying substrate by selective
chemical etching.
11: A GaN-based element fabrication method comprising: a growing
step of sequentially growing a metal buffer layer, a metal nitride
layer and a single-crystal GaN layer on an underlying substrate; an
element structure fabrication step of fabricating a GaN-based
element structure on the GaN single-crystal layer; and a chip
separation step of separating a stacking structure including the
underlying substrate, the metal buffer layer, the metal nitride
layer, the single-crystal GaN layer, and the GaN-based element
structure, into a plurality of chips wherein the metal buffer layer
and the metal nitride layer respectively include a CU layer and a
Cu nitride layer, alternatively the metal buffer layer and the
metal nitride layer respectively include a chromium layer and a
chromium nitride layer; and the chip separation step includes: a
bonding step of bonding a conductive support substrate to the
GaN-based element structure through a conductive junction; a
primary subscribing step of scribing the stacking structure
supported by the conductive support substrate, such that the
stacking structure is divided into a plurality of stacking bodies;
an etching step of etching the metal buffer layer and the metal
nitride layer by selective chemical etching to remove the
underlying substrate from each of the stacking bodies; a secondary
scribing step of scribing the conductive support substrate at
spaces between the stacking bodies, such that the stacking bodies
are separated into the plurality of chips.
12: The method according to claim 11, wherein, in the etching step,
a chemical solution is supplied to the metal buffer layer and the
metal nitride layer through the spaces between the stacking bodies
formed in the primary scribing step.
13: A GaN-based element fabrication method comprising: a growing
step of sequentially growing a metal buffer layer, a metal nitride
layer and a single-crystal GaN layer on an underlying substrate; an
element structure fabrication step of fabricating a GaN-based
element structure on the GaN single-crystal layer; and a chip
separation step of separating a stacking structure including the
metal buffer layer, the metal nitride layer, the single-crystal GaN
layer, and the GaN-based element structure, into a plurality of
chips; wherein the metal buffer layer and the metal nitride layer
respectively include a Cu layer and a Cu nitride layer,
alternatively the metal buffer layer and the metal nitride layer
respectively include a chromium layer and a chromium nitride layer,
the chip separation step includes: a primary subscribing step of
scribing the stacking structure supported by the underlying
substrate, such that the stacking structure is divided into a
plurality of stacking bodies; a bonding step of bonding a
conductive support substrate to the GaN-based element structure
through a conductive junction layer; an etching step of etching the
metal buffer layer and the metal nitride layer by selective
chemical etching to remove the underlying substrate from each of
the stacking bodies; and a secondary scribing step of scribing the
conductive support substrate at spaces between the stacking bodies,
such that the stacking bodies are separated into the plurality of
chips.
14: The method according to claim 13, wherein in the etching step,
a chemical solution is supplied to the metal buffer layer and the
metal nitride layer through the spaces between the stacking bodies
formed in the primary scribing step.
15. (canceled)
16: A GaN single crystal growth method according to claim 2,
wherein nitridation is performed by a gas containing ammonia in the
nitridation process.
17: A GaN single crystal growth method according to any one of
claim 2, wherein nitridation is performed at a temperature of
500.degree. C. to 1,000.degree. C. in the nitridation process.
18: A GaN single crystal growth method according to claim 2,
wherein the GaN buffer layer is grown at a temperature of
800.degree. C. to 1,100.degree. C. in the GaN buffer layer growth
process.
19: A GaN single crystal growth method according to claim 2,
wherein the GaN buffer layer is grown to have a thickness of 50 nm
to 30 .mu.m in the GaN buffer layer growth process.
20: A GaN single crystal growth method according to claim 2,
wherein the underlying substrate has a metal layer on a
surface.
21: A GaN single crystal growth method according to claim 2, the
method further comprising a separation process of separating the
single-crystal GaN layer from the underlying substrate by selective
chemical etching.
22: A GaN single crystal growth method according to claim 3,
wherein nitridation is performed by a gas containing ammonia in the
nitridation process.
23: A GaN single crystal growth method according to claim 3,
wherein nitridation is performed at a temperature of 500.degree. C.
to 1,000.degree. C. in the nitridation process.
24: A GaN single crystal growth method according to claim 3,
wherein the GaN buffer layer is grown at a temperature of
800.degree. C. to 1,100.degree. C. in the GaN buffer layer growth
process.
25: A GaN single crystal growth method according to claim 3,
wherein the GaN buffer layer is grown to have a thickness of 50 nm
to 30 .mu.m in the GaN buffer layer growth process.
26: A GaN single crystal growth method according to claim 3,
wherein the underlying substrate has a metal layer on a
surface.
27: A GaN single crystal growth method according to claim 3, the
method further comprising a separation process of separating the
single-crystal GaN layer from the underlying substrate by selective
chemical etching.
28: A GaN single crystal growth method according to claim 4,
wherein nitridation is performed by a gas containing ammonia in the
nitridation process.
29: A GaN single crystal growth method according to claim 4,
wherein nitridation is performed at a temperature of 500.degree. C.
to 1,000.degree. C. in the nitridation process.
30: A GaN single crystal growth method according to claim 4,
wherein the GaN buffer layer is grown at a temperature of
800.degree. C. to 1,100.degree. C. in the GaN buffer layer growth
process.
31: A GaN single crystal growth method according to claim 4,
wherein the GaN buffer layer is grown to have a thickness of 50 nm
to 30 .mu.m in the GaN buffer layer growth process.
32: A GaN single crystal growth method according to claim 4,
wherein the underlying substrate has a metal layer on a
surface.
33: A GaN single crystal growth method according to claim 4, the
method further comprising a separation process of separating the
single-crystal GaN layer from the underlying substrate by selective
chemical etching.
Description
TECHNICAL FIELD
[0001] The present invention relates to the fabrication of a GaN
freestanding substrate or GaN template substrate and a method of
fabricating a GaN-based element using the GaN template substrate
and including a light-emitting element such as a light-emitting
diode or laser diode or an electronic element and, more
particularly, to the fabrication of a high-efficiency light
emitting element or the like performed by a GaN single crystal
growth method using a metal buffer layer.
BACKGROUND ART
[0002] Nichia Corporation, Japan and Lumi LED, U.S.A. are going
ahead in the fields of the development and production of blue and
white light emitting diodes and laser diodes using GaN-based
compound semiconductors. Recently, various high-luminance light
emitting element structures to be applied to the fields of
illumination such as household fluorescent lamps and LCD (Liquid
Crystal Display) backlights have been proposed and produced.
GaN-based materials have well exhibited their possibilities not
only as optical elements but also as high-power, high-temperature
electronic elements. Presently, high-quality GaN crystals can be
grown on a sapphire substrate by using the MOCVD growth method.
[0003] An example of the principal core techniques is the
development of a low-temperature buffer layer. It is possible by
using the MOCVD growth method to grow an amorphous or
polycrystalline AlN or GaN buffer layer on a sapphire substrate at
a low growth temperature of 400.degree. C. to 700.degree. C., and
grow high-quality GaN crystals at a high temperature of
1,000.degree. C. or more. That is, the technical development of the
low-temperature buffer layer is presently the principal technique
reaching the production of light emitting elements.
[0004] At present, however, the important subjects of the GaN-based
light emitting elements are a high efficiency, a high output, and a
short wavelength in the ultraviolet region. GaN-based thin films
and thick films can be grown by methods such as MOCVD (Metal
Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy),
and HVPE (Hydride Vapor Epitaxy), in accordance with the purposes
of the growth, and optical elements or electronic elements are
implemented by using these methods. In particular, the HVPE growth
method is mainly used in the fabrication of a GaN GaN freestanding
substrate obtained by growing a thick GaN film on a sapphire
substrate at a high growth rate of 100 .mu.m/hr or more, and
separating the substrate and thick GaN film by the laser lift-off
method. In the fabrication of GaN-based optical or electronic
elements, sapphire or SiC is presently mainly used as a substrate
for crystal growth. However, a large lattice mismatching and a
large thermal expansion coefficient difference cause a high defect
density, that is, a dislocation density of about
10.sup.10/cm.sup.2, thereby posing many problems such as element
characteristic deterioration and the difficulty in element
processing caused by the chemical resistance characteristic.
Low-defect thin films can be grown by using various buffer layers
to decrease the dislocation density, and by using the selective
growth or lateral growth technique such as LEO (Lateral Epitaxial
Overgrowth) or the PENDEO epitaxy method (non-patent reference 1).
However, these growth techniques increase the unit cost of
production because a number of steps are necessary to fabricate a
substrate before the growth, and also have problems in
reproducibility and yield.
[0005] A practical conventional technique of fabricating a
high-luminance, high-output, blue light emitting diode will be
described below. When fabricating a GaN-based light emitting diode
on an insulator sapphire substrate, the TOP emission LED method as
shown in FIG. 1 that emits light in the direction of the upper
portion of a thin film is conventionally mainly used. Recently,
however, as shown in FIG. 2(a), the light emission output can be
increased to be about twice that of the conventional TOP emission
LED method by using the LED-chip method (or the flip-chip method)
that emits light in the direction of a sapphire substrate. Also, a
high heat dissipating effect can be obtained because it is possible
to perform a packing step in which a submount 110 having high
conductivity and a thin film that generates heat are packaged close
to each other. The output increases because the LED upper metal
electrode does not physically limit the light. Also, as shown in
FIG. 2(b), a mirror-coating 180 of the submount 110 can further
increase the light emission efficiency.
[0006] Recently, as shown in FIG. 3, a new LED structure (FIG.
3(d)) having a top-down electrode is proposed which is fabricated
by connecting an LED structure formed by growing a thin film on a
sapphire substrate 120 by MOCVD (FIG. 3(a)) to a Si substrate 190
by using a metal junction layer 182 (FIG. 3(b)), and separating the
sapphire substrate 120 and thin film by using the laser lift-off
technique (FIG. 3(c)).
[0007] As another method of a high-output, high-efficiency light
emitting diode, a patterned sapphire substrate 122 is sometimes
used as shown in FIG. 4. This is a method in which fine patterns
formed on the sapphire substrate 122 cause irregular reflection of
light generated from an active layer of a light emitting element,
and increase the amount of light emitted from the surface by
suppressing the transmittance of light through the sapphire
substrate, thereby increasing the light emission efficiency of the
element.
[0008] As described above, the flip-chip technique, the use of the
patterned sapphire substrate, the technique that increases the
efficiency by using a reflecting electrode metal, and the like have
been proposed to fabricate high-luminance, blue and ultraviolet
light emitting diodes and laser diodes, but various problems such
as the complexity of fabrication steps and the inefficiency of
production have arisen. When growing a thin GaN film by using the
conventional techniques, it is essential to form a seed layer of a
low-temperature GaN or AlN buffer layer in order to grow a
high-quality thin film because the growth is hetero growth on a
substrate made of different material, such as a sapphire. However,
even when this buffer layer exists, a large lattice mismatching and
a large thermal expansion coefficient difference cause a high
defect density, that is, a dislocation density of about
10.sup.10/cm.sup.2.
[0009] Also, an electrode is difficult to form on the sapphire
substrate because the sapphire substrate has an insulation
property. Therefore, complicated steps including a step of
dry-etching a grown thin film by about a few .mu.m is necessary to
form an electrode for a device.
[0010] Note that the fabrication of a device on a GaN substrate,
instead of a sapphire substrate, has been regarded as most
promising in order to greatly increase the LED light emission
efficiency, achieve a high-current operation and high luminance,
and fabricate a high-output ultraviolet laser. However, GaN bulk
growth is technically difficult in the conventional GaN substrate
fabrication. Instead, therefore, a thick GaN film is grown on a
sapphire substrate by using the HVPE method, and separated from the
substrate by mechanical polishing or the laser lift-off method,
thereby fabricating a GaN freestanding substrate. Since, however,
these methods require a high process cost after the growth of the
thick GaN film, the development of a low-cost process has been
desired.
[0011] The present inventors have grown a GaN layer on a CrN layer
directly formed on a substrate by the MBE method (non-patent
references 2, 3, and 4). To increase the area and throughput,
however, it is possible to stack Cr by a method such as sputtering
suited to mass-production, instead of stacking a CrN layer by the
MBE method or the like, and form a Cr nitride layer by nitriding Cr
in an HVPE apparatus capable of high-speed film formation and
mass-production, thereby forming a template for GaN growth.
Unfortunately, even when Cr is stacked on a sapphire substrate,
this Cr forms a polycrystalline or multi-domain layer. A single
crystal is difficult to grow on a polycrystalline or multi-domain
layer. In addition, Cr forms an extremely stable Cr oxide
(passivity) as is well known (a Cr oxide layer naturally forms on
the surface of stainless steel, and protects the interior of
stainless steel against corrosion). Since the substrate is moved
from the sputtering apparatus to the HVPE apparatus by batch
processing, the substrate must be transferred in the air, and Cr
surface oxidation occurs during the process. The existence of this
oxide layer interferes with the growth of a GaN single crystal. To
epitaxially grow single-crystal GaN on a Cr nitride as described in
non-patent references 2, 3, and 4, it is necessary to further form
the nitrided Cr nitride layer into a single crystal. It is of
course also possible to stack another metal by sputtering, nitride
the metal, and epitaxially grow single-crystal GaN on the nitrided
metal, but the above-mentioned difficulties (a metal film stacks as
a polycrystalline layer, causes surface oxidation, and makes the
formation of a single crystal difficult) still exist. Accordingly,
a demand has arisen for the development of the process of GaN
growth on a metal stacked film.
[0012] Note that patent references 1 and 2 also describe the growth
of a GaN layer on a metal film. However, as the references
disclose, although a GaN layer is formed after the formation of
AlN, Al is unfavorable to the subsequent GaN growth process because
the melting point of Al is low as a metal buffer layer (see patent
reference 1). Also, although titanium is used as a metal film to
form air spaces in a GaN layer by a Ti film and TiN film and then
to detach the GaN layer, the air spaces may deteriorate the
crystallinity of the GaN layer (see patent reference 2).
[0013] Non-patent reference 1: Pendeo-epitaxy versus Lateral
Epitaxial Overgrowth of GaN: A comparative study via finite element
Analysis, Zheleva, W. M. Ashmawi, K. A. Jones, phys. Stat. sol. (a)
176, 545 (1999)
[0014] Non-patent reference 2: Low-Temperature CrN Buffer Layers
for GaN Growth Using Molecular Beam Epitaxy (31.sup.st
International Symposium on Compound Semiconductors: announced in
Sep. 12 to 16, 2004)
[0015] Non-patent reference 3: Growth and Characterization of HVPE
GaN on c-sapphire with CrN Buffer Layer (31.sup.st International
Symposium on Compound Semiconductors: September, 2004).
[0016] Non-patent reference 4: CrN Buffer Layer Study For GaN
Growth Using Molecular Beam Epitaxy (22.sup.nd North American
Conference on Molecular Beam epitaxy: October, 2004).
[0017] Patent reference 1: Japanese Patent Laid-Open No.
2002-284600
[0018] Patent reference 2: Japanese Patent Laid-Open No.
2004-39810
DISCLOSURE OF INVENTION
Problems that the Invention is to Solve
[0019] As described in the prior art, a high-quality GaN GaN
freestanding substrate fabrication technique is desirable to truly
put GaN-based elements into practical use. This requires both a
technique that forms a high-quality GaN layer on a substrate made
of, for example, sapphire, and a technique that separates the GaN
layer from the substrate. The present inventors have shown that,
although MBE growth method is used, a GaN film having high
crystallinity is obtained by epitaxially growing CrN on a sapphire
substrate, and subsequently growing GaN layer.
[0020] Unfortunately, it is difficult to obtain a thick GaN layer
by this MBE growth method, so the method is difficult to apply a
technique to fabricate the GaN GaN freestanding substrate. To
increase the area and throughput, it is necessary to stack Cr on a
sapphire substrate or the like by, for example, sputtering method,
to form an epitaxial Cr nitride film by nitriding stacked Cr in an
HVPE apparatus capable of high-speed growth and mass-production,
and subsequently form a thick GaN film on the Cr nitride film.
[0021] Problems to be solved are to make the Cr nitride film, which
is obtained by nitriding stacked Cr in the HVPE apparatus and
serves as at least a CaN growth interface, be a single-crystal film
similar to a CrN film formed by MBE growth method, and to implement
a technique that detaches a GaN layer grown on the Cr nitride film
by, for example, dissolving a Cr-containing layer forming the
interface between the GaN layer and a substrate made of sapphire or
the like.
[0022] It is an object of the present invention to obtain an
industrially practical technique on the basis of the findings on
MBE growth method obtained by the present inventors, thereby
providing a GaN GaN freestanding substrate fabrication
technique.
[0023] It is another object of the present invention to fabricate a
GaN-based element using the fabricated GaN template substrate and
including a light emitting element such as a light emitting diode
or laser diode or an electronic element.
Means of Solving the Problems
[0024] To achieve the objects of the present invention, the present
invention is a GaN single crystal growth method characterized by
comprising a growth process of growing a metal buffer layer on a
substrate, a nitridation process of forming a metal nitride layer
by nitriding the surface or the whole of the metal buffer layer, a
GaN buffer layer growth process of growing a GaN buffer layer on
the metal nitride layer, and a GaN layer growth process of growing
a single-crystal GaN layer on the GaN buffer layer, wherein the
metal buffer layer is made of Cr or Cu.
[0025] Also, the present invention is a GaN single crystal growth
method characterized by comprising a growth process of growing a
metal buffer layer on a substrate, a nitridation process of forming
a metal nitride layer by nitriding the surface or the whole of the
metal buffer layer, a GaN buffer layer growth process of growing a
GaN buffer layer on the metal nitride layer, and a GaN layer growth
process of growing a single-crystal GaN layer on the GaN buffer
layer, wherein the metal nitride layer has (111) orientation.
[0026] It is also possible to use a metal substrate as the
substrate, and form a metal nitride layer by nitriding the surface
of the metal substrate in the nitridation process.
[0027] In the GaN single crystal growth method described above,
when the substrate is a substrate having a metal layer, it is
possible to nitride the surface of the metal layer by using it as a
metal buffer layer, or grow a metal buffer layer on the metal
layer.
[0028] Note that the nitridation process is preferably performed by
a gas containing ammonia. Note also that the nitriding temperature
range is preferably 500.degree. C. to 1,000.degree. C.
[0029] It is also desirable to set the growth temperature range in
the GaN buffer layer growth process at 800.degree. C. to
1,100.degree. C., and the thickness of the GaN buffer layer at 50
nm to 30 .mu.m.
[0030] The present invention is also a GaN-based element
fabrication method of fabricating a GaN-based element,
characterized by comprising a step of fabricating a GaN-based
element structure on a GaN single-crystal layer obtained by a
method of growing a GaN single crystal on a metal buffer layer or a
metal nitride layer, and a chip separation step of separating
chips.
[0031] The method can further comprise a step of forming a
conductive junction layer and a conductive substrate layer on the
GaN-based element structure, the chip separation step performs a
primary separation which separates the structure to the brink of
the conductive substrate layer, removes the metal buffer layer or
the metal nitride layer by selective chemical etching, and performs
a secondary separation which separates the conductive substrate
layer. With this method, it is possible to fabricate an up-down
electrode type light emitting element having an upper electrode and
a lower electrode.
[0032] Otherwise, as the chip separation step, at first a primary
separation is performed which separates the metal nitride layer or
the metal buffer layer, then a step of forming a conductive
junction layer and a conductive substrate layer on the GaN-based
element structure, the metal buffer layer or the metal nitride
layer is removed by selective chemical etching, and a secondary
separation is performed which separates the conductive substrate
layer. With this method, it is also possible to fabricate an
up-down electrode type light emitting element having an upper
electrode and a lower electrode.
[0033] In a GaN-based element fabricated by the method of
fabricating a GaN-based element on a metal buffer layer described
above, an electrode may also be formed on the metal buffer
layer.
[0034] Also, in the light emitting element fabricated by the method
of fabricating a GaN-based element on a metal buffer layer, emitted
light is reflected by the metal buffer layer. With this structure,
the light emission efficiency can be increased.
[0035] In the above-mentioned GaN single crystal growth method, a
GaN freestanding substrate can be obtained by first growing a thick
GaN single-crystal layer, and then performing a separation step of
separating the GaN single-crystal layer by selective chemical
etching.
EFFECTS OF THE INVENTION
[0036] The arrangement of the present invention can grow a
low-defect, high-quality GaN-based thin film (thick film) by using
a metal buffer layer on a substrate made of a different kind of
single crystal, a polycrystal, an amorphous semiconductor, or a
metal. This GaN-based thin film (thick film) can be formed as an
n-type, p-type, or undoped film.
[0037] Since a GaN template substrate including a metal buffer
layer can be fabricated, a light emitting element (e.g., a light
emitting diode or laser diode) or electronic element can be
fabricated on the substrate.
[0038] It is also possible to fabricate a high-output,
high-luminance light emitting diode by reflection of the metal
buffer layer.
[0039] The GaN-based element fabrication method of the present
invention makes it possible not only to improve the performance of
a GaN-based element, but also to greatly improve the GaN-based
element fabrication steps, thereby largely reducing the GaN-based
element fabrication cost.
[0040] Since a GaN GaN freestanding substrate is fabricated by
selective chemical etching of a metal buffer layer or/and a metal
nitride layer, it is possible to greatly improve the fabrication
process after lift-off, thereby increasing the throughput and
largely reducing the fabrication process cost.
BRIEF DESCRIPTION OF DRAWINGS
[0041] FIG. 1 is a schematic view of a conventional CaN light
emitting element;
[0042] FIG. 2(a) is a schematic view of a conventional flip-chip
type GaN light emitting element, and FIG. 2(b) is a schematic view
of a mirror-coated GaN light emitting element;
[0043] FIG. 3 shows views each showing an LED structure having a
top-down electrode using a metal junction layer, in which FIG. 3(a)
shows an LED (Light Emitting Diode) structure grown as a thin film
on a sapphire substrate, FIG. 3(b) shows the way a Si substrate is
connected by using the metal junction layer, FIG. 3(c) shows the
way the sapphire substrate and thin film are separated, and FIG.
3(d) shows a top-down electrode type, high-luminance LED (Light
Emitting Diode) structure;
[0044] FIG. 4 is a schematic view showing the structure of a light
emitting element in which fine patterns of a sapphire substrate
increase the amount of light emitted from the element by irregular
reflection, thereby increasing the light emission efficiency;
[0045] FIG. 5 shows views of a method of forming a GaN
single-crystal layer by using a metal buffer layer of the present
invention;
[0046] FIG. 5-1 shows a table of the physical properties of a
buffer layer and the like used when growing a GaN layer, that is,
shows the interatomic distances and thermal expansion coefficients
of GaN growth buffer layers/substrate materials;
[0047] FIG. 5-2 is a schematic view showing crystal growing
mechanisms on sapphire and CrN;
[0048] FIG. 6 shows views of X-ray diffraction data of Cr films
formed by various methods;
[0049] FIG. 7 is a photograph showing a Cr metal buffer layer
vapor-deposited on a 2-inch sapphire substrate by sputtering;
[0050] FIG. 7-1 shows photographs each showing the surface
morphology of a GaN layer grown on Cr when the nitriding
temperature of Cr was changed, that is, FIG. 7-1(a) shows the case
that the nitriding temperature was 480.degree. C., FIG. 7-1(b)
shows the case that the nitriding temperature was 520.degree. C.,
FIG. 7-1(c) shows the case that the nitriding temperature was
800.degree. C., FIG. 7-1(d) shows the case that the nitriding
temperature was 980.degree. C., and FIG. 7-1(e) shows the case that
the nitriding temperature was 1,040.degree. C.;
[0051] FIG. 7-2 is a table showing the relative evaluations
concerning the surface morphology and crystallinity of a
high-temperature GaN layer with respect to the nitriding
temperature of Cr;
[0052] FIG. 8 is an SEM sectional photograph of a sample obtained
by growing a 5-.mu.m thick GaN buffer layer at a temperature of
800.degree. C. to 1,000.degree. C. after a metal buffer layer was
nitrided;
[0053] FIG. 8-1 shows photographs each showing the surface
morphology of a GaN layer grown on a GaN buffer layer with respect
to the growth temperature of the GaN buffer layer, that is, FIG.
8-1(a) shows the case that the nitriding temperature was
650.degree. C., FIG. 8-1(b) shows the case that the nitriding
temperature was 700.degree. C., FIG. 8-1(c) shows the case that the
nitriding temperature was 800.degree. C., FIG. 8-1(d) shows the
case that the nitriding temperature was 900.degree. C., FIG. 8-1(e)
shows the case that the nitriding temperature was 1,000.degree. C.,
FIG. 8-1(f) shows the case that the nitriding temperature was
1,100.degree. C., FIG. 8-1(g) shows the case that the nitriding
temperature was 1,150.degree. C., and FIG. 8-1(h) shows the case
that the nitriding temperature was 1,200.degree. C.;
[0054] FIG. 8-2 shows the relative comparison between the
crystallinity of GaN layers with respect to the growth temperature
of a GaN buffer layer;
[0055] FIG. 8-3 is a table showing the relative evaluations of GaN
layers grown on GaN buffer layers with respect to the growth
temperatures of the GaN buffer layers;
[0056] FIG. 8-4 is a graph showing the relative comparison between
the crystallinity of GaN layers with respect to the thickness of a
GaN buffer layer;
[0057] FIG. 9 shows graphs of the X-ray rocking curves of GaN
layers;
[0058] FIG. 10 is a photograph showing a GaN single-crystal film
grown after nitriding a Cr metal buffer layer, and growing a GaN
buffer layer on the Cr metal buffer layer, that is, a photograph
showing a GaN single-crystal film grown on a Cr metal buffer layer
by HVPE method;
[0059] FIG. 11 is an SEM sectional photograph of a sample obtained
by growing a 14-.mu.m thick GaN buffer layer and 30-.mu.m thick GaN
single-crystal layer on a sapphire substrate in the process of
forming and nitriding a Cr metal buffer layer;
[0060] FIG. 12(a) is a sectional view showing the structure of a
GaN-based light emitting element, FIG. 12(b) is a sectional view
showing the structure of a GaN-based light emitting element, and
FIG. 12(c) is a sectional view showing the structure of a GaN-based
light emitting element;
[0061] FIG. 13a is a sectional view showing a light emitting
element grown on a GaN template substrate including a metal buffer
layer;
[0062] FIG. 13b is a sectional view showing a structure obtained by
connecting a conductive substrate on a GaN-based light emitting
element structure by using a conductive junction layer;
[0063] FIG. 13c-(1) is a sectional view showing chip separation in
a primary scribing step, and FIG. 13c-(2) is a plan view of a
sapphire substrate;
[0064] FIG. 13d is a sectional view showing separation of a
sapphire substrate by selective chemical etching of a metal buffer
layer;
[0065] FIG. 13e is a sectional view showing chip separation in a
secondary scribing step;
[0066] FIG. 13f is a schematic view showing a structure in which a
top-down electrode type, high-output light emitting element is
mounted on a submount layer after electrodes are formed in the
upper and lower portions;
[0067] FIG. 14 shows views for explaining another method of
fabricating a top-down electrode type light emitting element or
electronic element, that is, FIG. 14(a) is a sectional view showing
chip separation by primary scribing or dry etching, FIG. 14(b) is a
sectional view showing a structure obtained by connecting a
conductive substrate on the surfaces of a light emitting element
and electronic element having undergone chip separation by using a
conductive junction layer, and FIG. 14(c) is a sectional view
showing separation of a sapphire substrate by selective chemical
etching of a metal buffer layer;
[0068] FIG. 15 shows views for explaining the way the light
emission efficiency is increased by using reflection of a metal
buffer layer, that is, FIG. 15(a) is a sectional view of a
high-output, high-efficiency light emitting element obtained by
reflection of the metal buffer layer, and FIG. 15(b) is a sectional
view of another high-output, high-efficiency light emitting element
obtained by reflection of the metal buffer layer;
[0069] FIG. 16 is a graph showing the results (calculated values)
of simulation of the reflectance and transmittance obtained by the
change in thickness of a metal buffer layer when the light emission
wavelength is 460 nm;
[0070] FIG. 17 is a graph showing the result (calculated value) of
calculation of the reflectivity (reflectance) with respect to the
light emission wavelength when the thickness of a Cr metal buffer
layer is 100 .ANG. (10 nm);
[0071] FIG. 18 is a graph showing the measurement value
(experimental value) of the reflectivity (reflectance) with respect
to the light emission wavelength in a sample formed by setting the
thickness of a Cr metal buffer layer to 100 .ANG. (10 nm);
[0072] FIG. 19 shows views of the fabrication of a GaN freestanding
substrate by selective chemical etching of a metal buffer layer, in
which FIG. 19(a) is a sectional view when a thick GaN film is grown
on the metal buffer layer by HVPE method, and FIG. 19(b) is a
sectional view showing the way a sapphire substrate and the thick
GaN layer (freestanding substrate) are separated by selective
chemical etching of the metal buffer layer;
[0073] FIG. 20 is a sectional SEM photograph of a GaN freestanding
substrate obtained by selective chemical etching of a Cr metal
buffer layer; and
[0074] FIG. 21 shows views of a method of growing a thick GaN layer
on a Cu substrate, in which FIG. 21(a) is a sectional view showing
a structure in which a GaN template substrate including no Cu metal
buffer layer is formed on the Cu substrate, and a light emitting
element structure is formed on the GaN template substrate, and FIG.
21(b) is a sectional view showing a structure in which a GaN
template substrate including a Cu metal buffer layer is formed on
the Cu substrate, and a light emitting element structure is formed
on the GaN template substrate.
EXPLANATION OF REFERENCE NUMERALS
[0075] 100: GaN-based element [0076] 110: Submount layer [0077]
120: Sapphire substrate [0078] 122: Patterned sapphire substrate
[0079] 125: Cu substrate [0080] 130: N-GaN layer [0081] 142: N-type
electrode [0082] 144: P-type electrode [0083] 150: Active layer
(InGaN QW layer) [0084] 160: P-GaN layer [0085] 172, 174:
Interconnection [0086] 176, 178: Au bump [0087] 180: Metal mirror
coating layer [0088] 182: Metal junction layer [0089] 190: Si
substrate [0090] 210: Metal buffer layer [0091] 212: Metal nitride
layer [0092] 220: GaN single-crystal layer [0093] 222: GaN buffer
layer [0094] 224: Thick GaN layer [0095] 230: Conductive substrate
[0096] 232: Conductive junction layer [0097] 300: Light emitting
element structure [0098] 400: GaN template substrate [0099] 501:
Chip [0100] 502: Trench formed by scribing [0101] 600: Region to be
etched
BEST MODE FOR CARRYING OUT THE INVENTION
[0102] The present invention forms a metal buffer layer on a
different kind of single-crystal substrate, a polycrystalline
substrate, an amorphous substrate, or a metal substrate, in
addition to a sapphire substrate and SiC substrate, by electron
beam evaporation method (E-beam evaporator), thermal evaporation
method (Thermal evaporator), sputtering method (Sputter), chemical
vapor deposition method (Chemical Vapor Deposition), or metal
organic chemical vapor deposition (MOCVD), and then grows
single-crystal GaN on the metal buffer layer. It is the first
attempt to use a metal as a buffer layer on various substrates, and
fabricate a light emitting element or electronic element on the
metal buffer layer. This makes it possible to provide various
structures and various substrates of GaN-based light emitting
elements in the future. Since the metal buffer layer is inserted in
the interface between any of various substrates such as a sapphire
substrate or SiC substrate and a GaN-based single-crystal thin film
(thick film), it is possible to prevent light generated by an
active layer of a light emitting element from transmitting through
the sapphire substrate, and increase the extraction efficiency of
light emission by reflection of the metal buffer layer in the
interface. It is also possible to efficiently separate the light
emitting element or an electronic element from the substrate by
selective chemical etching of the metal buffer layer or a metal
nitride layer in the interface, thereby forming a top-down
electrode.
[0103] The following can be pointed out as the important elemental
techniques of the present invention.
[0104] 1) The technique that forms a nitridable metal layer.
[0105] 2) The nitridation technique that forms GaN growth nuclei on
the metal buffer layer in a reaction tube of HVPE or in a chamber
of MOCVD or MBE.
[0106] 3) The technique that grows a GaN buffer layer on the metal
buffer layer.
[0107] 4) The technique that grows a GaN single-crystal layer.
[0108] 5) The technique that separates the substrate and GaN layer
by selective chemical etching of the metal buffer layer or a metal
nitride layer.
[0109] 6) The technique that, on the GaN single-crystal thin film
(thick film) thus formed, forms a light emitting element such as an
InGaN/GaN blue light emitting diode, GaN/AlGaN ultraviolet light
emitting diode, or laser diode, or an electronic element.
[0110] 7) The technique that fabricates various optical elements,
such as a high-luminance light emitting diode and flip-chip using
reflection of the metal interface layer, and a top-down electrode
type light emitting element obtained by selective chemical etching
of the metal buffer layer.
[0111] 8) The technique that performs primary chip separation in a
scribing step after formation of the optical or electronic element
structure, and then performs selective chemical etching of the
metal buffer layer. This makes it possible to suppress the
generation of cracks, and significantly increase the throughput of
the element.
[0112] 9) The technique that fabricates a GaN freestanding
substrate by selective chemical etching of the metal buffer
layer.
[0113] Embodiments of the present invention will be explained in
detail below with reference to the accompanying drawings.
[0114] First, an outline of a method of forming a GaN
single-crystal layer by using a metal buffer of the present
invention will be explained with reference to FIG. 5.
[0115] 1) A nitridable metal layer (metal buffer layer) 210 made
of, for example, Cr or Cu is formed to have a predetermined
thickness (a few nm to a few .mu.m) on a sapphire substrate 120 by
vacuum vapor deposition method (E-beam evaporator or thermal
evaporator), sputtering method (Sputter), metal organic chemical
vapor deposition method (MOCVD), chemical vapor deposition method
(Chemical Vapor Deposition), or MBE method (Molecular Beam Epitaxy)
(FIG. 5(a)).
[0116] In case of vacuum vapor deposition method, the substrate
temperature is room temperature or less to 1,000.degree. C., and a
gas to be used in sputtering method is preferably Ar or nitrogen.
In case of chemical vapor deposition method, it is possible to use
an alkyl compound or chloride containing a predetermined metal.
[0117] 2) A substrate in which the metal buffer layer 210 is
vapor-deposited on the sapphire substrate 120 is nitrided to form
nuclei of GaN crystal growth.
[0118] Ammonia gas can be used when using MOCVD method and HVPE
method as GAN crystal growth methods, and ammonia or nitrogen
plasma can be used when using MBE method.
[0119] Nitridation process is preferably performed in an ammonia
gas ambient or an ammonia gas-containing hydrogen or nitrogen gas
ambient at a substrate temperature of 500.degree. C. to
1,000.degree. C. A strong reducing function of ammonia can nitride
the surface of the metal buffer layer even when the metal buffer
layer has a surface oxide layer. Optimum nitriding conditions for
forming a uniform metal nitride on the surface of the metal buffer
layer, that is, the flow rate of ammonia and the nitriding
temperature are determined.
[0120] This nitridation can form a metal nitride, that is,
Cr.sub.xN.sub.y or Cu.sub.xN.sub.y in the case of Cr or Cu used in
the embodiment, on the surface of the metal buffer layer by a
predetermined thickness in accordance with the conditions of
nitriding the surface of the metal buffer layer vapor-deposited in
step 1). In this case, the whole metal buffer layer can be formed
into a metal nitride by controlling the nitriding conditions.
[0121] This process can be performed in a reaction tube of an HVPE
growth system or MOCVD growth system, or in a MBE chamber.
[0122] The metal nitride thus formed can function as a nucleus of
GaN crystal growth. This will be explained in detail below with
reference to a table of FIG. 5-1 showing the physical properties of
buffer layers and the like used to grow GaN layers, and a schematic
view of FIG. 5-2 showing the growth of crystals on sapphire and
CrN.
[0123] It is well known that when a GaN film is grown on the (0001)
plane of sapphire, the GaN film grows with a domain in which the
crystal axis of the GaN film rotates 30.degree. in the (0001) plane
with respect to the corresponding crystal axis of sapphire. Because
the 30.degree. rotation described above reduces the lattice
mismatching between sapphire and GaN. Even in this case, however,
the lattice mismatching is as large as 16.1% and causes crystal
defects in the GaN layer (see FIG. 5-1).
[0124] When Cr is vapor-deposited on the (0001) plane of sapphire,
Cr having a face-centered cubic structure shows (110) orientation.
Cr forms CrN having a rock salt structure by nitriding, and shows
(111) orientation. When GaN is grown on this (111) plane of CrN,
the lattice constant of the (111) plane of CrN has an intermediate
value between the lattice constant of the (0001) plane of GaN and
the lattice constant of the (0001) plane of sapphire that has
rotated 30.degree. (see FIG. 5-1).
[0125] That is, as shown in FIG. 5-2, when GaN is grown on an ideal
(111) plane of CrN formed on a c-plane sapphire substrate, the
lattice mismatching is 6.6% between the CrN (111) plane and c-plane
sapphire, and 8.9% between the GaN (0001) plane and CrN (111)
plane, that is, the lattice mismatching can be reduced step by step
compared to the case that GaN is directly grown on c-plane sapphire
(the lattice mismatching is 16.1%). This suppresses the formation
of crystal defects compared to the case that GaN is directly grown.
In addition, CrN has a thermal expansion coefficient of
6.00.times.10.sup.-6 [/K], and this value is also an intermediate
value between GaN and sapphire. A difference in thermal expansion
at GaN(/a buffer layer)/the substrate interface(s) generates cracks
in a thick GaN film on a sapphire substrate when the temperature
decreases. However, the use of CrN as the buffer layer can
presumably reduce the cracks because the thermal expansion
coefficient difference can be reduced stepwise. On the other hand,
as shown in FIG. 5-1, the relationship between the thermal
expansion coefficients of AlN and TiN is AlN (0001)<GaN
(0001)<CrN (111)<Al.sub.2O.sub.3 (0001)<TiN (111), so AlN
and TiN cannot reduce the thermal expansion coefficient difference
stepwise.
[0126] From the foregoing, the crack reducing effect obtained by
the stepwise reductions in lattice mismatching and thermal
expansion coefficient difference cannot be obtained by the
conventional metal nitride film of Al and Ti (AlN and TiN)
described in patent references 1 and 2; the crack reducing effect
is one merit of using CrN as a buffer layer.
[0127] 3) A GaN buffer layer 222 is grown on the nitrided metal
buffer layers 210 and 212 (FIG. 5(c)). Note that, when the whole
metal buffer layer is nitrided, 210 is entirely replaced with
212.
[0128] 4) Finally, a GaN single-crystal layer 220 is grown (FIG.
5(d)).
[0129] The GaN single-crystal layer 220 can be grown to have
various thicknesses in accordance with the purposes. The substrate
fabricated by the above-mentioned steps can be used as a GaN
template substrate for fabricating a GaN-based light emitting diode
or laser diode.
[0130] As a substrate for growing the metal buffer layer, it is
possible to use a single-crystal or polycrystalline semiconductor
substrate such as Al.sub.2O.sub.3, Si, SiC, or GaAs, or a
single-crystal or amorphous substrate such as Nb, V, Ta, Zr, Hf,
Ti, Al, Cr, Mo, W, Cu, Fe, or C.
[0131] Also, as a metal layer used as the metal buffer layer, it is
possible to use a nitridable metal such as Ga, Nb, V, Ta, Zr, Hf,
Ti, Al, Cr, Mo, W, or Cu.
[0132] As described above, various metals can be used as the metal
buffer layer. However, the nitriding conditions such as the
temperature conditions are narrow depending on the type of metal,
and this poses the problem of reproducibility. For example, the
reproducibility of Ti is low because hydrogen absorption occurs
during nitriding using NH.sub.3, and desorption of hydrogen
absorbed during nitriding and during later GaN growth occurs. This
probably makes it impossible to obtain a flat nitride layer
necessary for the present invention with high reproducibility.
[0133] From the viewpoint of reproducibility, Cr and Cu described
in the embodiment are most favorable as the metal buffer layer.
This result corresponds to the fact that the metal buffer layer
made of Cr or Cu is always flat as seen in an SEM sectional
photograph shown in FIG. 8.
[0134] From the foregoing, the metal buffer layer is preferably Cr
or Cu, and more preferably, Cr.
[0135] As a GaN-based single crystal, it is possible to grow a
GaN-based thin film (thick film), that is, GaN,
In.sub.xGa.sub.1-xN, Al.sub.xGa.sub.1-xN, or
Al.sub.xIn.sub.yGa.sub.1-x-yN. This GaN-based thin film (thick
film) can be formed as an n-type, p-type, or undoped film.
[0136] Also, MOCVD method, MBE method, or HVPE method can be used
as a method of growing a GaN buffer layer and GaN single crystal
via nitridation process after a nitridable metal layer (metal
buffer layer) is formed on a different kind of substrate. After the
GaN single-crystal layer is formed, GaN layer can be separated from
the substrate by selective chemical etching of the metal buffer
layer.
[0137] The fabrication process will be explained in detail below as
an embodiment practiced by using the HVPE growth technique. Note
that it is of course also possible to perform the following growth
process by using MOCVD method or the like by appropriately changing
the growth conditions.
[0138] 1) Vapor Deposition of Metal Buffer Layer on Sapphire
Substrate (FIG. 5(a))
[0139] Cr or Cu as a metal buffer layer is formed to have a
predetermined thickness (a few nm to a few .mu.m) on a sapphire
substrate by vacuum vapor deposition method (E-beam evaporator or
thermal evaporator), sputtering method (Sputter), metal organic
chemical vapor deposition method (MOCVD), or chemical vapor
deposition method (CVD). The thickness of the formed metal buffer
layer has a close relationship with the application to a light
emitting element and the selective chemical etching rate. In this
embodiment, Cr or Cu having a thickness of a few hundred .ANG. (a
few ten nm) was used as the metal buffer layer. To increase the
connecting force between the substrate and metal buffer layer and
improve the surface flatness, the substrate temperature during the
formation was room temperature to 1,000.degree. C. The thickness of
the formed metal was about 10 to 1,000 .ANG. (1 to 100 nm).
[0140] For example, a Cr film formed by sputtering method is
polycrystalline as indicated by X-ray diffraction data (Sputtered
Cr metal) shown in FIG. 6(a). Note that the Cr surface oxide layer
is very thin and amorphous, and hence cannot be sensed by X-ray
diffraction. Electron microscopic observation shows that when the
Cr film thickness is decreased to about 4 nm, a crystalline Cr film
grows, but a multi-domain structure is obtained as a whole. Note
also that a single-crystal Cr film grows when Cr is stacked in an
ultrahigh vacuum by using MBE method.
[0141] FIG. 7 shows a photograph of Cr vapor-deposited on a 2-inch
sapphire substrate.
[0142] 2) Nitridation of Metal Buffer Layer (FIG. 5(b))
[0143] The formed metal buffer layer was nitrided in a quartz
reaction tube by using the HVPE growth method.
[0144] A metal nitride layer 212 was formed by performing
nitridation in an ammonia gas ambient at a temperature of
600.degree. C. or more. FIG. 6(a) shows X-ray diffraction data (CrN
in N and CrN in Ar) including this result. That is, FIG. 6(a) shows
the data of a sputtered Cr film, a nitrided Cr film (CrN in Ar), a
nitrided Cr film (CrN in N), and a CrN film grown by MBE method in
this order from below. As shown in FIG. 6(a), this nitridation
forms (111) CrN. That is, a strong reducing function of ammonia gas
reduced and nitrided the Cr oxide on the Cr surface, thereby
growing CrN. FIG. 6(b) is a schematic view showing the nitrided
structure.
[0145] Note that CrN in N and CrN in Ar shown in FIG. 6(a) compare
the results of nitridation of Cr films formed by sputtering by
using nitrogen and Ar as sputtering gases. The uppermost X-ray
diffraction data (MBE-CrN) is the X-ray diffraction data of a CrN
film formed on a sapphire substrate by MBE method, and indicates
the growth of a (111) single crystal.
[0146] The X-ray diffraction data shown in FIG. 6 of this CrN layer
formed by nitriding has no peak except for (111) orientation. It is
possible to determine from the diffraction data that the
<111> axis was oriented in the CrN layer. This result agrees
with the SEM sectional image shown in FIG. 8 in which the interface
between GaN and the metal nitride layer is flat. That is, this
proves that the fabrication method of the present invention can
form a metal nitride layer without forming any multi-domain.
[0147] Note that (111) orientation described above means that the
<11> axis is oriented.
[0148] A comparison of CrN grown by MBE method with nitrided Cr
reveals that diffraction peaks from not only CrN but also Cr are
observed from nitrided Cr, indicating that CrN is formed on the Cr
film by nitridation.
[0149] Various Cr nitriding temperature conditions will be
explained in detail below.
[0150] First, when the temperature was 500.degree. C. or less, the
ratio of decomposition of ammonia gas into nitrogen and hydrogen
was low, so the formation of CrN by nitriding of Cr was difficult.
Consequently, no GaN buffer layer grew on the surface of Cr
nitrided at 500.degree. C. or less.
[0151] In addition, when GaN was grown on the Cr surface at a high
temperature, the surface morphology was rough in a
three-dimensional growth mode. At a temperature of 1,000.degree. C.
or more, Cr attached on the sapphire substrate started evaporating,
and a GaN layer grown on Cr abnormally grew to form giant pits.
This deteriorated the crystallinity of a GaN film formed on the GaN
layer.
[0152] FIG. 7-1 shows photographs of the surface morphologies of
GaN layers grown on Cr when the nitriding temperature was changed.
FIGS. 7-1(a), 7-1(b), 7-1(c), 7-1(d), and 7-1(e) respectively
illustrate the results of nitriding at 480.degree. C., 520.degree.
C., 800.degree. C., 980.degree. C., and 1,040.degree. C. FIG. 7-2
is a table relatively comparing the surface morphology and
crystallinity (indicated by the half-width of (0002) XRD) of the
GaN layer with the nitriding temperature of Cr. As shown in FIG.
7-2, the surface morphology and crystallinity of the GaN buffer
layer were bad at about 500.degree. C. or less, were optimum at
980.degree. C., and deteriorated at 1,000.degree. C. or more.
[0153] As is also apparent from the photographs of the surfaces
shown in FIGS. 7-1, a flat, mirror-surface GaN film can be formed
on a metal film nitrided under good conditions.
[0154] From the foregoing, the nitriding temperature of the metal
buffer layer is preferably 500.degree. C. to 1,000.degree. C., and
more preferably, 800.degree. C. to 1,000.degree. C.
[0155] A favorable GaN film can be formed on the surface morphology
of the nitrided metal buffer only when an almost continuous metal
nitride film is formed, although that depends upon the metal growth
conditions, metal film thickness, and nitriding conditions. "An
almost continuous metal nitride film" is a continuous film having a
domain-like surface morphology in which voids are formed between
domains when observed with an SEM or the like, but the area of the
uppermost surface of the domain is larger than that of the void,
and the domain surface is flat. This film looks like a continuous
film on the optical microscopic level.
[0156] Next, the conditions under which this mirror surface is
obtained by nitridation will be described below. When a gas
containing ammonia is used in nitridation process, H atoms are
generated during the nitridation process, and a hydrogen storing
metal (representative examples are Ti and Ta) forms voids inside
crystals as a result of desorption of hydrogen. This causes large
three-dimensional surface roughness after nitridation. That is, it
is difficult for a hydrogen storing metal to form "ran almost
continuous metal nitride film" described above after the metal
buffer layer is nitrided. Therefore, this embodiment uses Cr that
is not a hydrogen storing metal.
[0157] 3) Growth of GaN Buffer Layer in HVPE Reaction Tube (FIG.
5(c))
[0158] As described above in step 2), a GaN buffer layer was grown
on the nitrided metal buffer layer in the HVPE reaction tube. In
this embodiment, the growth temperature range of the GaN buffer
layer was 800.degree. C. to 1,000.degree. C. higher than the growth
temperature of the conventional low-temperature GaN buffer layer.
The thickness of the GaN buffer layer was set at a few nm to a few
ten .mu.m in accordance with the growth conditions.
[0159] FIG. 8 is an SEM sectional photograph of a sample obtained
by growing a 5-.mu.m thick GaN buffer layer at 800.degree. C. to
1,000.degree. C. after nitridation of the metal buffer layer. It
was possible to grow columnar crystals.
[0160] When the MOCVD growth method was used, the growth
temperature range was set at 500.degree. C. to 1,000.degree. C.,
and the thickness was set at a few nm to a few .mu.m.
[0161] Note that the GaN buffer layer can be any of n-type, p-type,
and undoped GaN.
[0162] Practical GaN buffer layer formation conditions will be
explained below.
[0163] a) Growth Temperature of GaN Buffer Layer
[0164] The growth temperature of the GaN buffer layer had large
influence on the surface morphology and crystallinity of the GaN
layer.
[0165] FIG. 8-1 shows photographs of the surface morphologies of
GaN buffer layers when the growth temperatures were 650.degree. C.,
700.degree. C., 800.degree. C., 900.degree. C., 1,100.degree. C.,
1,150.degree. C., and 1,200.degree. C. These photographs indicate
that the surface morphology exhibited no compatibility and had a
polycrystalline shape when the temperature was 650.degree. C., and
had a large number of pits when the temperature was 1,100.degree.
C.
[0166] FIG. 8-2 is a graph comparing between the crystallinity of
the GaN layers with respect to the growth temperature of the GaN
buffer layer. FIG. 8-2 shows that the crystallinity was relatively
high when the growth temperature was 800.degree. C. to
1,100.degree. C. Conventionally, a method of growing a
low-temperature GaN buffer layer at a temperature of 500.degree. C.
to 700.degree. C. in order to grow GaN crystals is generally known.
By contrast, this embodiment is characterized by using a GaN buffer
layer at 800.degree. C. or more.
[0167] FIG. 8-3 shows a table relatively comparing between the
surface morphology and crystallinity of the GaN layers with respect
to the growth temperature of the CaN buffer layer. As shown in FIG.
8-3, it is favorable to grow the GaN buffer layer at about
800.degree. C. to 1,000.degree. C., particularly, about 900.degree.
C.
[0168] From the foregoing, the growth temperature range of the GaN
buffer layer growth process is preferably 800.degree. C. to
1,100.degree. C., and more preferably, 800.degree. C. to
1,000.degree. C.
[0169] b) Thickness of GaN Buffer Layer
[0170] The thickness of the GaN buffer layer can be widely selected
by the growth method. When MOCVD method is used, good GaN crystals
can be obtained even when the thickness is a few ten nm or more.
When HVPE method enabling a growth rate higher than MOCVD method is
used, a GaN buffer can be grown from a few .mu.m to a few ten
.mu.m. In reality, however, it is difficult for HVPE method having
a growth rate of 100 .mu.m/hr or more to control the thickness of
the GaN buffer layer to 10 nm or less. Also, if the thickness of
the GaN buffer layer is as small as a few ten nm or less, many pits
are formed in the surface of the GaN layer, and the crystallinity
deteriorates as well, so the thickness is preferably 50 nm or more.
When HVPE method was used, the surface morphology and crystallinity
of the GaN layer changed in accordance with the thickness of the
GaN buffer layer.
[0171] FIG. 8-4 shows the crystallinity of the GaN layer as a
function of the thickness of the GaN buffer layer grown by HVPE
method. The smallest thickness (the leftmost point) of the measured
GaN buffer layer was 20 nm, and the next film thickness (the next
point) was 50 nm.
[0172] This graph reveals that the crystallinity was relatively
stable when the thickness of the GaN buffer layer was 50 nm to 30
.mu.m. When the GaN buffer layer thickness was less than 50 nm or
larger than 30 .mu.m, a uniform GaN buffer layer was difficult to
form, and the crystallinity deteriorated.
[0173] From the foregoing, the thickness of the GaN buffer layer
grown in the GaN buffer layer growth process is preferably 50 nm to
30 .mu.m.
[0174] In this embodiment, a metal such as Cr or Cu that hardly
absorbs hydrogen is used as the metal buffer layer, so voids (air
holes) as described in patent reference 2 are not formed in the
interface between the metal buffer layer and GaN buffer layer, and
the GaN buffer layer is flat. Note that in patent reference 2, Ti
that easily occludes hydrogen is used as the metal buffer
layer.
[0175] 4) Growth of GaN Single-Crystal Film in HVPE Reaction Tube
(FIG. 5(d)).
[0176] After the growth of the GaN buffer layer obtained in step 3)
described above, a high-temperature GaN single crystal was
successively grown at 1,000.degree. C. or higher than the growth
temperature of the GaN buffer layer. This high-temperature GaN
single-crystal film can be grown within a wide thickness range in
accordance with the purpose. When the object is a GaN freestanding
substrate, a thick film having a thickness of 100 .mu.m or more can
be grown. For a flip-chip type or top-down electrode type light
emitting element, the film thickness is preferably a few nm to a
few ten .mu.m.
[0177] FIG. 9 illustrates the X-ray diffraction data of a grown
25-.mu.m thick GaN layer. FIG. 9(a) shows the (0002) rocking
curves, and FIG. 9(b) shows the (10-11) rocking curves, in each of
which the Cr buffer layer film thickness was changed. The (0002)
peak reflects spiral dislocation, and the (10-11) peak reflects
spiral dislocation+edge dislocation.
[0178] Under the nitriding conditions (ammonia was supplied to the
reaction tube at 620.degree. C., and the temperature was raised to
the GaN crystal growth temperature (in this example, 1,040.degree.
C.) over about 30 min, and kept at the growth temperature for 30
min) of the example shown in FIG. 9, the optimum Cr buffer film
thickness was found to be 10 to 20 nm.
[0179] The half-width of this X-ray diffraction was equivalent to
that of a GaN film similarly grown by HVPE method on a GaN template
formed on a sapphire substrate by MOCVD method, indicating the
growth of a high-quality, single-crystal film. Note that the
optimum nitriding conditions change in accordance with the film
thickness of Cr.
[0180] FIG. 10 is a photograph of a structure in which a Cr metal
buffer layer was nitrided on a 2-inch sapphire substrate, a GaN
buffer layer was grown by HVPE method on the nitrided Cr metal
buffer layer, and a high-temperature GaN single-crystal film was
grown on the GaN buffer layer. In this case, it was possible to
obtain a flat mirror surface.
[0181] Note that similar results were obtained even when MOCVD
method using ammonia and TMG as materials was used in the growth
process of a high-temperature GaN single crystal. In this case, the
growth temperature was 1,000.degree. C. or more.
[0182] Also, FIG. 11 shows an SEM sectional photograph of a
structure obtained by growing a 14-.mu.m thick GaN buffer layer and
30-.mu.m thick high-temperature GaN single-crystal layer. This
photograph clearly shows the interface between the GaN buffer layer
and high-temperature GaN single-crystal layer. The high-temperature
GaN single-crystal layer can be variously grown to have a thickness
of a few .mu.m to a few hundred .mu.m in accordance with the object
of application. It is also possible to grow n-type, undoped, or
p-type GaN.
[0183] A GaN substrate obtained by vapor-depositing a metal buffer
layer on a sapphire substrate and growing high-temperature,
high-quality, single-crystal GaN by HVPE method in the fabrication
process from steps 1) to 4) described above can be used as a GaN
template substrate for fabricating a GaN-based light emitting diode
or laser diode.
[0184] <Fabrication of Light Emitting Element or Electronic
Element on GaN Template Substrate>
[0185] Various element structures such as a blue InGaN/GaN light
emitting diode, ultraviolet GaN/AlGaN light emitting diode, laser
diode, and electronic element can be formed on a GaN template
substrate fabricated in steps 1) to 4) described above. For
example, when a GaN template substrate is fabricated by using HVPE
method, an element structure can be formed on the substrate by the
MOCVD crystal growth method that is presently most widely used.
After that, individual light emitting elements or electronic
elements are obtained by separating the element structure into
chips.
[0186] FIG. 12(a) shows an example of the fabrication of a
representative light emitting element structure. This is an example
in which a GaN-based light emitting element structure or electronic
element structure is formed on a GaN single-crystal layer 220 by
MOCVD method or MBE method. It is also possible to fabricate other
various light emitting element structures and electronic element
structures.
[0187] FIG. 12(b) shows the formation of an electrode when a GaN
buffer layer 222 and GaN single-crystal layer 220 are undoped. An
n-type electrode 142 is formed by partially etching an n-GaN layer
130 by dry etching.
[0188] FIG. 12(c) shows the case that dry etching is advanced to a
GaN buffer layer 222 and GaN single-crystal layer 220, and a metal
buffer layer 210 is used as an n-type electrode 142. When an
electrode is formed as shown in FIG. 12(b), it is possible to
expand the effective area of the element, and increase the chip
yield. Also, the element electrical characteristics can be improved
when a metal buffer layer having high electrical conductivity is
used as an electrode.
[0189] <Fabrication of Top-Down Electrode Type Light Emitting
Element>
[0190] A top-down electrode type light emitting element can be
fabricated by removing the metal buffer layer 210 or metal nitride
layer 212 in the interface between the sapphire substrate and GaN
by selective chemical etching from the light emitting element
structure or electronic element structure fabricated by the above
steps, thereby separating the sapphire substrate 120 and the chip
of the light emitting element or electronic element. The process of
fabricating this element will be explained below with reference to
FIG. 13.
[0191] 1) First, as shown in FIG. 13a, a light emitting element or
electronic element structure is fabricated by using MOCVD method or
MBE method on a GaN template substrate including a GaN
single-crystal layer 220 formed by HVPE method or MOCVD method. It
is also possible to form a template substrate by forming a
single-crystal layer of In.sub.xGa.sub.1-xN, Al.sub.xGa.sub.1-xN,
or Al.sub.xIn.sub.yGa.sub.1-x-yN, instead of the GaN single-crystal
layer 220. A light emitting element structure or electronic element
structure is fabricated on this template substrate by using MOCVD
method or MBE method. FIG. 13a shows the case that a light emitting
element is formed, so the fabrication of this light emitting
element will be explained below with reference to FIG. 13.
[0192] 2) Another conductive support substrate (e.g., an Si
substrate) 230 is connected on the uppermost layer of the GaN-based
light emitting element structure or electronic element structure by
using a junction layer 232 having high conductivity (FIG. 13b).
This facilitates handling when performing selective chemical
etching of the metal buffer layer to be explained later.
[0193] 3) After the sapphire substrate 120 is polished, a primary
scribing step is performed in which the structure is separated from
the sapphire substrate 120 to the brink of the conductive substrate
(FIG. 13c). In this step, individual chips can be separated. FIG.
13c(1) is a schematic sectional view, and FIG. 13c(2) is a plan
view viewed from the surface of the sapphire substrate 120.
[0194] 4) The sapphire substrate 120 is separated by selective
chemical etching of the metal buffer layer 210 (FIG. 13d). In this
step, the sapphire substrate 120 can be separated by supplying an
etching solution through spaces between the chips. With this
process it is possible to suppress the formation of cracks.
[0195] 5) In the next step, a secondary scribing step is performed
in which the conductive substrate 230 is separated into individual
chips (FIG. 13e).
[0196] 6) To form electrodes in the upper and lower portions of the
chip of the light emitting element, the GaN buffer layer 222 and
GaN single-crystal layer 220 are formed into p-type or n-type GaN
layers.
[0197] FIG. 13f is a schematic view showing a structure in which
electrodes 142 and 144 are formed in the upper and lower portions,
and the top-down electrode type light emitting element is mounted
on a submount layer 110.
[0198] The primary scribing step described above is a preparation
step for chip separation, and the sapphire substrate is separated
by the subsequent chemical etching. The secondary scribing is a
chip separation step that separates the conductive support
substrate. Note that the primary scribing has the effect of
reducing the stress generated when the metal buffer layer is
etched, thereby suppressing the formation of cracks during the
etching.
[0199] The object of the second scribing step is to completely
separate the chips in the second scribing step for the adhered
conductive substrate.
[0200] <Another Element Fabrication Method>
[0201] Another method of fabricating the top-down electrode type
light emitting element or electronic element will be explained
below with reference to FIG. 14. As an example, FIG. 14 illustrates
a light emitting element.
[0202] 1) As shown in FIG. 14(a), primary scribing step or dry
etching step is performed in which a light emitting element
structure fabricated on a GaN template substrate formed by HVPE
method or MOCVD method is separated in the brink of a metal nitride
layer 212 or metal buffer layer 210 into chips (FIG. 14(a) shows
the case where the structure is separated to the metal nitride
layer 212).
[0203] 2) Then, to facilitate handling, a conductive support
substrate (e.g., an Si substrate) 230 is adhered on the uppermost
layer of the GaN-based light emitting element structure or
electronic element structure by using a junction layer 232 having
high conductivity (FIG. 14(b). As the junction layer, it is
possible to use, for example, Cu, Au, or Ag.
[0204] 3) The metal buffer layer 210 or metal nitride layer 212 is
separated from a sapphire substrate 120 by selective chemical
etching (FIG. 14(c)).
[0205] 4) Fabrication steps after that are performed in the same
manner as shown in FIGS. 13(e) and 13(f) described above, thereby
fabricating the top-down electrode type light emitting element or
electronic element.
[0206] <Etching of Metal Buffer Layer and Metal Nitride
Layer>
[0207] The above-mentioned selective chemical etching uses an
etching solution corresponding to the types of the nitrided metal
buffer layer and metal nitride layer. Many types of etching
solutions can be selected for one metal buffer layer and one metal
nitride layer. The present invention is thus characterized by
selectively etching the metal buffer layer and metal nitride layer
at the same time by means of the same etching solution.
[0208] Cr can be etched by either wet etching or dry etching. The
embodiment used wet etching because etching was performed from the
side surfaces. Known etching solutions are, for example, HCl,
HNO.sub.3, HClO.sub.4, and CAN (Ceric Ammonium Nitrate).
[0209] It is well known that an N-polarity GaN surface is etched by
a chemical solution more easily than a Ga-polarity GaN surface. An
N-polarity GaN surface appears after Cr and CrN of the metal buffer
layer are etched, and this N-polarity GaN surface may be a rough
surface because it is etched with an etching solution. Therefore, a
solution mixture of
Ce(NH.sub.4).sub.2(NO.sub.3).sub.6+HClO.sub.4+H.sub.2O having
little influence on the N-polarity GaN surface is suitable.
[0210] Note that when Cu is used as a metal buffer layer, nitric
acid (HNO.sub.3)+water (H.sub.2O) can be used as an etching
solution.
[0211] When chips can be separated from the substrate by selective
chemical etching of the metal buffer layer and metal nitride layer
as described above, an electrode can be formed on the surface of
the GaN-based element structure (normally, the GaN-based
semiconductor surface) of the separated chip.
[0212] It is possible to control the etching rate by the
temperature and concentration of the solution, and suppress cracks
produced upon separation of light emitting element or electronic
element from the sapphire substrate by adjusting the etching
rate.
[0213] <Fabrication of High-Output, High-Luminance Light
Emitting Diode>
[0214] When forming an element after an electrode is formed on the
fabricated light emitting element chip, the metal buffer layer 210
existing in the interface between the sapphire substrate and light
emitting element reflects light emitted from the active layer 150
by injection of an electric current. In a surface emission type
light emitting element, this reflected light from the metal buffer
layer 210 increases the light emission efficiency. FIGS. 15(a) and
15(b) are schematic views showing this.
[0215] FIG. 15(a) shows electrode formation when the GaN buffer
layer 222 and GaN single-crystal layer 220 are undoped. In this
arrangement, an n-type electrode 142 is formed by partially etching
the n-GaN layer by dry etching. FIG. 15(b) shows the case where dry
etching is advanced to the GaN buffer layer 222 and GaN
single-crystal layer 220, and the metal buffer layer 210 is used as
an n-type electrode 142.
[0216] To increase the efficiency of the light emitting element by
the reflection of the metal buffer layer 210, the thickness of the
metal buffer layer was determined based on simulation. FIG. 16
shows examples of the calculations of reflection, including
absorption to a wavelength of 460 nm, performed by changing the
thicknesses of the metal buffer layers of Cr and Cu. A maximum
reflectance was about 40% when the thickness of Cr was about 200
.ANG. (about 20 nm), and about 35% when the thickness of Cu was
about 400 .ANG. (about 40 nm). The light emission efficiency of the
light emitting element can be increased by this reflection in the
interface.
[0217] A metal buffer layer and its thickness optimum for the light
emission wavelength of a light emitting diode were determined by
using simulation of the reflectance with respect to the thickness
of the metal buffer layer. FIG. 17 shows the result of the
calculation of the reflectance to the wavelength of incident light
when the thickness of the Cr metal buffer layer was 100 .ANG. (10
nm) and the wavelength was 460 nm. The result shown in FIG. 17
indicates that a constant reflectance was maintained regardless of
the wavelength, so the present invention is also applicable to an
ultraviolet light emitting diode.
[0218] FIG. 18 shows the measurement values of a sample fabricated
by setting the thickness of the Cr metal buffer layer to 100 .ANG.
(10 nm). FIGS. 17 and 18 reveal that the calculated values and
measurement values match at a ratio of 19% to 20%. That is, to
obtain a high-luminance light emitting diode using the reflectance
of the metal buffer layer, an optimum thickness must be set by
taking account of absorption, reflection, and transmission at the
light emission wavelength. The optimum thickness of Cr or Cu can be
determined based on simulation.
[0219] <Fabrication of GaN Freestanding Substrate>
[0220] A GaN freestanding substrate can be fabricated by selective
chemical etching of the metal buffer layer or/and the metal nitride
layer.
[0221] As described above, a high-temperature GaN single-crystal
layer was grown on a metal buffer layer on a sapphire substrate,
and the thickness of the grown high-temperature GaN single-crystal
layer was finally set at 100 .mu.m or more. A GaN freestanding
substrate can be fabricated after the growth by separating thick
GaN film from the sapphire substrate by selective chemical etching
of the metal buffer layer or a metal nitride layer.
[0222] FIG. 19(a) is a schematic sectional view showing a structure
in which a thick GaN film 224 is grown on a metal buffer layer 210
by HVPE method, and FIG. 19(b) is a schematic view showing a GaN
freestanding substrate fabricated by selective chemical etching of
the metal buffer layer 210.
[0223] FIG. 20 shows an SEM sectional photograph of a GaN
freestanding substrate obtained by growing a 122-.mu.m thick
high-temperature GaN single-crystal layer to be used as a GaN
freestanding substrate, and then performing selective chemical
etching of the metal buffer layer. The etching was performed under
the same etching conditions as described above. In this etching,
the metal buffer layer and a metal nitride layer can be
simultaneously selectively etched with almost the same etching
solution.
[0224] When the GaN freestanding substrate can be separated from
the sapphire substrate by selective chemical etching of the metal
buffer layer and metal nitride layer as described above, the
separated surface of the GaN freestanding substrate is also flat.
This obviates the need for an additional polishing step of removing
or planarizing the metal nitride layer remaining on the separated
side, unlike in patent references 1 and 2.
[0225] <Fabrication of GaN Template Substrate on Metal
Substrate>
[0226] A GaN layer can be grown after directly nitriding the
surface of a Cu substrate capable of providing a metal
single-crystal substrate as a presently commercially available
substrate, and forming a GaN buffer layer on the Cu metal nitride
layer, or after nitriding the surface of the Cu substrate by the
above-mentioned method, and forming a GaN buffer layer on the Cu
metal nitride layer.
[0227] When a Cu substrate having good conduction properties and a
high thermal conductivity is used, a GaN-based light emitting diode
can be formed on the grown GaN layer without using any submount in
the light emitting diode packaging step.
[0228] It is also possible to increase the chip yield and simplify
the steps because the top-down electrode is used as an electrode.
Metal substrates having various metal layers can also be used
instead of the Cu substrate, and the surface of this metal layer
can also be nitrided. FIG. 21 illustrate fabricated structures.
[0229] FIG. 21(a) shows the case where a CaN layer 220 was grown on
a Cu substrate 125 through nitridation process, and light emitting
diode structures 130, 150, and 160 were directly grown on the GaN
layer 220.
[0230] FIG. 21(b) shows a structure obtained by forming a Cu layer
210 on a Cu substrate 125 by vacuum vapor deposition method,
sputtering method, or chemical vapor deposition method, performing
nitridation process similar to that shown in FIG. 21(a), growing a
GaN buffer layer 222 and high-temperature GaN single-crystal layer
220, and finally fabricating a light emitting element on top of the
structure.
[0231] The methods of fabricating high-quality GaN freestanding
substrate with high reproducibility have been described above.
[0232] Patent reference 2 discloses that Ti is used as a metal
buffer layer, and GaN is grown on nitrided Ti and removed by using
the decrease in mechanical strength caused by air spaces formed by
desorption of hydrogen contained in the region. The disclosed metal
nitride layer is a multi-domain layer and has low crystal
orientation, and the crystallinity of the GaN layer on this metal
nitride layer is also insufficient, presumably because the method
as described above is used. The method of the present invention
keeps flatness with which no air spaces form in the metal buffer
layer region, and can also improve the orientation of the metal
nitride layer, so the GaN layer on this metal nitride layer well
improves immediately after the start of growth. In addition, this
metal buffer layer region can be chemically etched and hence
readily removed from the underlying substrate such as sapphire, so
a high-quality GaN freestanding substrate can be obtained. Also,
unlike in the description of patent reference 2, the flatness of
the removed GaN and the removal surface of the underlying substrate
is maintained, and this achieves the additional effect that at
least the underlying substrate can be used any number of times.
Note that in the method described in patent reference 2, the
reaction probably occurs between the underlying substrate and metal
buffer layer as well, so the removed underlying substrate must be
polished before being reused because the surface is roughened.
Also, this reaction presumably makes removal by chemical etching
difficult in the method described in patent reference 2.
[0233] Conventionally, GaN or AlN was mainly used as a
low-temperature buffer layer in order to implement a GaN-based
light emitting element on a sapphire substrate. The present
invention has successfully grown a high-quality GaN single crystal
by using a metal buffer layer, metal nitride layer, and GaN buffer
layer as new buffer layers.
[0234] Furthermore, a new amorphous metal can be used as a GaN
buffer layer by the technique proposed by the present invention.
This makes various element applications feasible, and makes growth
on a metal, semiconductor, and dielectric material in an amorphous
state possible. Although GaN crystal growth is presently limited on
a sapphire substrate and SiC substrate, the present invention is an
important technique capable of increasing the number of types of
substrates usable in GaN crystal growth.
[0235] The technique provided by the present invention provides an
important technique for next-generation GaN-based element
applications. From the viewpoint of an element application, a
high-efficiency light emitting diode can be fabricated by using a
metal buffer layer. It is also possible to, for example, reduce the
number of fabrication steps of a top-down electrode type light
emitting element obtained by the conventional laser lift-off
method, provide various types of metal buffer layers, and provide
various substrates. Practical economical and technical effects are
as follows.
[0236] 1) Selective chemical etching of a metal buffer layer
proposed by the present invention can simply separate a fabricated
GaN-based element from a substrate. This separation achieves the
effects of simplifying the fabrication steps of the top-down
electrode type, high-luminance light emitting element, increasing
the productivity of the element, and reducing the cost of the
element.
[0237] 2) The use of a sapphire substrate makes it possible to
fabricate a high-output, high-luminance light emitting diode using
the reflectance of a metal buffer layer, and simply control the
complexity and reproducibility of the fabrication steps when
compared to the case that the conventional patterned sapphire
substrate or the flip-chip is used.
[0238] 3) The conventional GaN-based light emitting diode and laser
diode mainly use a sapphire substrate or SiC substrate. However,
the present invention makes it possible to use an Si substrate,
GaAs substrate, metal single-crystal substrate, and amorphous
substrate, and use various metal buffer layers.
[0239] 4) A GaN freestanding substrate can be fabricated by growing
a thick GaN film on a sapphire substrate including a metal buffer
layer, and separating the thick CaN film from the substrate by
selective chemical etching of the metal buffer layer or a metal
nitride layer.
* * * * *