U.S. patent application number 12/524811 was filed with the patent office on 2011-01-20 for group-iii element nitride crystal producing method and group-iii element nitride crystal.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Takeshi HATAKEYAMA, Kouichi HIRANAKA, Fumio KAWAMURA, Yasuo KITAOKA, Hisashi MINEMOTO, Yusuke MORI, Takatomo SASAKI, Osamu YAMADA.
Application Number | 20110012070 12/524811 |
Document ID | / |
Family ID | 39759233 |
Filed Date | 2011-01-20 |
United States Patent
Application |
20110012070 |
Kind Code |
A2 |
YAMADA; Osamu ; et
al. |
January 20, 2011 |
GROUP-III ELEMENT NITRIDE CRYSTAL PRODUCING METHOD AND GROUP-III
ELEMENT NITRIDE CRYSTAL
Abstract
A method for producing a high-quality group-III element nitride
crystal at a high crystal growth rate, and a group-III element
nitride crystal are provided. The method includes the steps of
placing a group-III element, an alkali metal, and a seed crystal of
group-III element nitride in a crystal growth vessel, pressurizing
and heating the crystal growth vessel in an atmosphere of
nitrogen-containing gas, and causing the group-III element and
nitrogen to react with each other in a melt of the group-III
element, the alkali metal and the nitrogen so that a group-III
element nitride crystal is grown using the seed crystal as a
nucleus. A hydrocarbon having a boiling point higher than the
melting point of the alkali metal is added before the
pressurization and heating of the crystal growth vessel.
Inventors: |
YAMADA; Osamu; (Ehime,
JP) ; MINEMOTO; Hisashi; (Ehime, JP) ;
HIRANAKA; Kouichi; (Ehime, JP) ; HATAKEYAMA;
Takeshi; (Ehime, JP) ; SASAKI; Takatomo;
(Osaka, JP) ; MORI; Yusuke; (Osaka, JP) ;
KAWAMURA; Fumio; (Osaka, JP) ; KITAOKA; Yasuo;
(Osaka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
1030 15th Street, N.W.
Suite 400 East
Washington
DC
20005-1503
UNITED STATES
202-721-8200
202-721-8250
eoa@wenderoth.com
|
Assignee: |
PANASONIC CORPORATION
1006, Oaza Kadoma, Kadoma-shi
Osaka
JP
571-8501
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20100078606 A1 |
April 1, 2010 |
|
|
Family ID: |
39759233 |
Appl. No.: |
12/524811 |
Filed: |
July 28, 2009 |
Current U.S.
Class: |
252/521.5;
117/77; 252/519.14; 423/409 |
Current CPC
Class: |
C30B 29/403 20130101;
C30B 29/406 20130101; C30B 11/12 20130101; H01L 31/03044 20130101;
C30B 19/02 20130101; G02B 1/02 20130101; C30B 9/10 20130101 |
Class at
Publication: |
252/521.5;
423/409; 252/519.14; 117/077 |
International
Class: |
H01B 1/06 20060101
H01B001/06; C01B 21/06 20060101 C01B021/06; C30B 17/00 20060101
C30B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2007 |
JP |
2007-065845 |
Claims
1. A method for producing a group-III element nitride crystal,
comprising the steps of: placing a group-III element, an alkali
metal, and a seed crystal of group-III element nitride in a crystal
growth vessel; pressurizing and heating the crystal growth vessel
in an atmosphere of nitrogen-containing gas; causing the group-III
element and nitrogen to react with each other in a melt of the
group-III element, the alkali metal and the nitrogen so that a
group-III element nitride crystal is grown using the seed crystal
as a nucleus; and adding a hydrocarbon having a boiling point
higher than a melting point of the alkali metal is added before the
pressurization and heating of the crystal growth vessel.
2. The method according to claim 1, wherein the alkali metal is
coated with a hydrocarbon, and the coated alkali metal is added to
the crystal growth vessel.
3. The method according to claim 1 or 2, further comprising:
adjusting an amount of the hydrocarbon in the crystal growth
vessel.
4. The method according to any one of claims 1 to 3, wherein the
hydrocarbon added in the crystal growth vessel is in at least
either of a solid state and a liquid state at room temperature.
5. The method according to any one of claims 1 to 4, wherein the
hydrocarbon has a boiling point of 150.degree. C. or more.
6. The method according to any one of claims 1 to 5, wherein the
hydrocarbon is at least one compound selected from the group
consisting of chain saturated hydrocarbons, chain unsaturated
hydrocarbons, alicyclic hydrocarbons and aromatic hydrocarbons.
7. The method according to any one of claims 1 to 6, wherein an
amount of the hydrocarbon added is 0.03 parts by mass or more per
100 parts by mass of the alkali metal.
8. The method according to any one of claims 1 to 7, wherein when
the seed crystal is a group-III element nitride single crystal, an
amount of the hydrocarbon added is one part by mass or less per 100
parts by mass of the alkali metal.
9. The method according to any one of claims 1 to 7, wherein the
seed crystal is a thin film of group-III element nitride formed on
a substrate, and an amount of the hydrocarbon added is 0.6 parts by
mass or less per 100 parts by mass of the alkali metal.
10. The method according to any one of claims 1 to 9, wherein the
group-III element is at least one element selected from Al, Ga and
In, and the group-III element nitride is a compound represented by
Al.sub.sGa.sub.tIn.sub.(1-s-t)N, where 0.ltoreq.s.ltoreq.1,
0.ltoreq.t.ltoreq.1, and s+t.ltoreq.1.
11. The method according to any one of claims 1 to 10, wherein the
group-III element is Ga, the group-III element nitride is GaN, and
the alkali metal includes Na.
12. The method according to claim 11, wherein the hydrocarbon has a
boiling point higher than the melting point of Na.
13. A group-III element nitride crystal produced by the method
according to any one of claims 1 to 12, wherein an optical
absorption coefficient thereof with respect to light having a
wavelength of 400 nm or more and 620 nm or less is 10 cm.sup.-1 or
less.
14. The group-III element nitride crystal according to claim 13,
wherein the group-III element is at least one element selected from
Al, Ga and In, and the group-III element nitride is a compound
represented by Al.sub.sGa.sub.tIn.sub.(1-s-t)N, where
0.ltoreq.s.ltoreq.1, 0.ltoreq.t.ltoreq.1, and s+t.ltoreq.1.
15. The group-III element nitride crystal according to claim 14,
wherein the group-III element is Ga, and the group-III element
nitride is GaN.
16. A substrate for forming a semiconductor device including a
group-III element nitride crystal, wherein the group-III element
nitride crystal is the group-III element nitride crystal according
to any one of claims 13 to 15.
17. A semiconductor device, wherein a semiconductor layer is formed
on a substrate, and the substrate is the substrate according to
claim 16.
Description
TECHNICAL FIELD
[0001] The present invention relates to a group-III element nitride
crystal producing method and a group-III element nitride
crystal.
BACKGROUND ART
[0002] Group-III element nitride compound semiconductors, such as
gallium nitride (GaN) and the like, have attracted attention as
materials for semiconductor devices that emit blue or ultraviolet
light. Blue laser diodes (LDs) are applied to high-density optical
discs and displays, and blue light emitting diodes (LEDs) are
applied to displays, lights and the like. Ultraviolet LDs are
expected to be applied to biotechnology and the like, and
ultraviolet LEDs are expected to provide ultraviolet light for
fluorescent lamps.
[0003] A substrate made of a group-III element nitride compound
semiconductor, such as gallium nitride (GaN) or the like, for an LD
or an LED is typically produced by heteroepitaxially growing a
group-III element nitride crystal on a sapphire substrate using
vapor phase epitaxy. Examples of vapor phase epitaxy include Metal
Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase
Epitaxy (HVPE), Molecular Beam Epitaxy (MBE), and the like.
However, the dislocation density of a gallium nitride crystal
obtained by these vapor phase epitaxy methods is 10.sup.8 cm.sup.-2
to 10.sup.9 cm.sup.-2, which is poor crystal quality. To avoid this
problem, ELOG (Epitaxial Lateral Overgrowth) has been developed,
for example. This method can reduce the dislocation density to
about 10.sup.5 cm.sup.-2 to 10.sup.6 cm.sup.-2. However, this
method disadvantageously includes a complicated step.
[0004] On the other hand, crystal growth may be carried out in
liquid phase instead of vapor phase epitaxy. Since the nitrogen
equilibrium vapor pressure at the melting point of a group-III
element nitride single crystal, such as gallium nitride (GaN),
aluminum nitride (AlN) or the like, is 10000 atm or more, liquid
phase growth of a group-III element, such as a gallium nitride
crystal, an aluminum nitride crystal or the like, needs to be
conducted under severe conditions, such as at 1200.degree. C. at
8000 atm (800.times.1.013.times.10.sup.5 Pa).
[0005] To solve this problem, a method of using an alkali metal,
such as sodium (Na) or the like, as a flux has been recently
developed. This method allows a group-III element nitride crystal,
such as a gallium nitride crystal, an aluminum nitride crystal or
the like, to be obtained under relatively mild conditions. As an
example, in a nitrogen gas atmosphere containing ammonia, sodium
(alkali metal) and gallium (group-III element) are melted by
application of pressure and heat, and the melt (sodium flux) is
used to conduct crystal growth for 96 hours to obtain a gallium
nitride crystal having a maximum crystal size of about 1.2 mm (see,
for example, Patent Citation 1). Also, a method in which a reaction
vessel and a crystal growth vessel are separated and a large
crystal is grown while suppressing spontaneous nucleation has been
proposed (see, for example, Patent Citation 2). Also, a method of
growing a high-quality bulk crystal, where an alkaline earth metal
or the like is added to sodium, has been proposed (see, for
example, Patent Citation 3).
[0006] A gallium nitride crystal obtained by the method of using a
sodium flux has a low disclocation density (i.e., high quality),
but a low growth rate (i.e., poor productivity) as compared to when
vapor phase epitaxy is employed. Therefore, an improvement in
growth rate is required for the method of producing a gallium
nitride crystal in liquid phase, where an alkali metal, such as
sodium or the like, is used as a flux.
[0007] Patent Citation 1: JP 2002-293696 A
[0008] Patent Citation 2: JP 2003-300798 A
[0009] Patent Citation 3: WO04/013385
DISCLOSURE OF INVENTION
[0010] In the method of producing a group-III element nitride
crystal using an alkali metal as a flux, it is important to
efficiently dissolve nitrogen in the flux so as to improve the
growth rate. To achieve this, it is necessary to cause both the
temperature of the flux and the pressure of nitrogen-containing gas
(atmosphere gas) to be higher so as to increase the amount of
nitrogen dissolved in flux. However, when the temperature and the
pressure are high, the supersaturation degree of nitrogen in the
flux increases near the gas-liquid interface between the atmosphere
gas and the flux, so that nonuniform nucleation easily occurs. If
nonuniform nucleation occurs at the gas-liquid interface, a
polycrystal of group-III element nitride is grown as miscellaneous
crystals (so-called nonuniform nucleation) based on the nuclei.
Therefore, crystal growth that would otherwise occur on a seed
crystal is suppressed, disadvantageously resulting in a reduction
in growth rate.
[0011] Therefore, an object of the present invention is to provide
a method for producing a group-III element nitride crystal, in
which a crystal growth rate can be improved by suppressing the
occurrence of miscellaneous crystals.
[0012] To achieve the object, a method for producing a group-III
element nitride crystal according to the present invention includes
the steps of adding a hydrocarbon having a boiling point higher
than the melting point of an alkali metal to a crystal growth
vessel containing a group-III element, an alkali metal, and a seed
crystal of group-III element nitride, pressurizing and heating the
crystal growth vessel in an atmosphere of nitrogen-containing gas,
and causing the group-III element and nitrogen to react with each
other in a melt of the group-III element, the alkali metal and the
nitrogen so that a group-III element nitride crystal is grown using
the seed crystal as a nucleus.
[0013] Also, a group-III element nitride crystal according to the
present invention is one that has an optical absorption coefficient
of 10 cm.sup.-1 or less with respect to light having a wavelength
of 400 nm or more and 620 nm or less and is produced by the
above-described method.
[0014] A semiconductor device formation substrate according to the
present invention includes the group-III element nitride crystal of
the present invention.
[0015] Moreover, a semiconductor device according to the present
invention is such that a semiconductor layer is formed on the
substrate of the present invention.
[0016] By adding a hydrocarbon having a specific boiling point to
the flux during the growth of a group-III element nitride crystal
using an alkali metal flux, nonuniform nucleation can be
suppressed. Thereby, it is possible to efficiently grow a group-III
element nitride crystal at higher temperatures and higher
pressures. As a result, the crystal growth rate can be improved.
Also, by coating the alkali metal with a hydrocarbon, it is
possible to suppress reaction of the alkali metal with oxygen and
water in an atmosphere. Further, by adding a hydrocarbon, the film
thickness of a grown crystal can be uniform. As a result, the
quality of the group-III element nitride crystal can be improved as
well. The production method of the present invention is effective
to general group-III element nitride crystals, and is particularly
effective when the alkali metal is sodium and the group-III element
nitride crystal is a gallium nitride crystal.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1(a) is a diagram showing an exemplary configuration of
a production apparatus for use in a production method according to
the present invention, and FIG. 1(b) is a diagram showing an
exemplary closed pressure-resistant and heat-resistant vessel for
use in the production method of the present invention.
[0018] FIG. 2(a) shows a gallium nitride crystal for production
number 3 of Example 1, and FIG. 2(b) shows a gallium nitride
crystal for production number 5 of Example 1.
[0019] FIG. 3(a) shows a gallium nitride crystal for production
number 9 of Example 2, and FIG. 3(b) shows a gallium nitride
crystal for production number 11 of Example 2.
[0020] FIG. 4 shows a gallium nitride crystal for production number
12 of Example 3.
[0021] FIG. 5 is a diagram showing an exemplary configuration of a
production apparatus used in Example 4.
[0022] FIGS. 6(a) to 6(c) are schematic diagrams showing exemplary
states of a substrate and a crystal of Example 4.
[0023] FIGS. 7(a) to 7(c) are schematic diagrams showing exemplary
states of a substrate and a crystal of Example 5.
EXPLANATION OF REFERENCE
[0024] 1 gas supply apparatus [0025] 2 pipe [0026] 3 pressure
adjuster [0027] 4 pipe [0028] 5 valve [0029] 7 joint [0030] 9 valve
[0031] 10 heat-resistant pipe [0032] 11 pipe [0033] 12 pipe [0034]
13 valve [0035] 14 exhaust apparatus [0036] 15 closed
pressure-resistant and heat-resistant vessel [0037] 16 heating
apparatus [0038] 17 reaction vessel [0039] 18 crystal growth vessel
[0040] 20 seed crystal [0041] 21 flux [0042] 30 gallium nitride
crystal [0043] 31 miscellaneous crystals [0044] 32 gallium nitride
crystal [0045] 60 high-pressure chamber [0046] 61 high-pressure
chamber lid [0047] 62 gas flow rate adjuster [0048] 64 joint [0049]
65 gas inlet-side valve [0050] 66 gas outlet-side valve [0051] 68
pressure adjuster [0052] 70 heater [0053] 72 heat insulator [0054]
80 reaction vessel [0055] 82 crystal growth vessel [0056] 84 sodium
[0057] 86 gallium [0058] 88 seed [0059] 100 holding substrate
[0060] 102 seed layer [0061] 104 grown crystal [0062] 106
self-sustaining substrate [0063] 108 grown crystal [0064] 110
self-sustaining substrate
BEST MODE FOR CARRYING OUT THE INVENTION
[0065] A method for producing a group-III element nitride crystal
according to the present invention is performed as follows. A
group-III element, an alkali metal, and a seed crystal of group-III
element nitride are placed in a crystal growth vessel, to which a
hydrocarbon having a boiling point higher than the melting point of
the alkali metal is then added. The crystal growth vessel is
pressurized and heated in a nitrogen-containing gas atmosphere so
that the group-III element and nitrogen are caused to react with
each other in the melt containing the group-III element, the alkali
metal and nitrogen. As a result, a group-III element nitride
crystal is grown on the seed crystal as a nucleus. In particular,
the alkali metal is preferably coated with a hydrocarbon, and the
alkali metal coated with the hydrocarbon is preferably added to the
crystal growth vessel.
[0066] In the present invention, the group-III element is gallium
(Ga), aluminum (Al) or indium (In), particularly preferably gallium
(Ga). The group-III element nitride is, for example, aluminum
nitride (AlN), indium nitride (InN), gallium nitride (GaN) or the
like, particularly desirably gallium nitride (GaN). The alkali
metal is lithium (Li), sodium (Na), potassium (K), rubidium (Rb),
cesium (Cs) or francium (Fr), particularly desirably sodium (Na).
These alkali metals may be used singly or in combination of two or
more. In this case, the major alkali metal component is desirably
sodium. As a flux component, an alkaline earth metal may be used.
The alkaline earth metal is, for example, Mg, Ca, Sr or Ba. A
dopant may also be added to the flux. An n-type dopant is, for
example, Si, Ge, Sn or O. A p-type dopant is, for example, Mg, Ca,
Sr, Ba or Zn. The amount of the dopant in the obtained crystal
fails within the range of 1.times.10.sup.15 to 1.times.10.sup.9
cm.sup.-3, for example.
[0067] Hereinafter, an embodiment of the production method of the
present invention will be described, assuming that the alkali metal
is sodium (Na), the group-III element is gallium (Ga), and the
group-III element nitride crystal is gallium nitride (GaN). Note
that a group-III element nitride crystal other than gallium nitride
may also be used and may be produced with reference to the
following description.
Embodiment 1
[0068] In this embodiment, a method for producing a gallium nitride
crystal will be described in which a seed crystal, sodium, gallium,
and a hydrocarbon are placed in a crystal growth vessel for holding
a sodium flux, and thereafter, nitrogen-containing gas is
pressurized and the crystal growth vessel is heated to melt sodium
and gallium, thereby generating a sodium flux.
[0069] An exemplary configuration of a production apparatus that is
employed in the production method of the present invention is shown
in FIG. 1(a). An exemplary closed pressure-resistant and
heat-resistant vessel for use in the production method of the
present invention is shown in FIG. 1(b).
[0070] The apparatus of FIG. 1(a) comprises a gas supply apparatus
1 for supplying material gas, a pressure adjuster 3 for adjusting
the pressure of the material gas, a closed pressure-resistant and
heat-resistant vessel 15 for conducting crystal growth, a heating
apparatus 16 for heating, and an exhaust apparatus 14.
[0071] The gas supply apparatus 1, which is filled with
nitrogen-containing gas as the material gas, is connected via a
pipe 2 to a pressure adjuster 3. The pressure adjuster 3, which has
a function of adjusting the material gas into an optimal gas
pressure, is connected via a pipe 4 and a valve 5 to a detachable
joint 7. Also, a portion of the pipe 4 (portion indicated with a
wavy line) is formed of a pressure-resistant flexible hose, thereby
enabling it to freely change the position and the direction of the
joint 7. Moreover, the pipe 4 branches into a pipe 11 at an
intermediate portion thereof. The pipe 11 is connected via a valve
13 and a pipe 12 to the exhaust apparatus 14.
[0072] On the other hand, a reaction vessel 17 having a valve 9, a
heat-resistant pipe 10 and the closed pressure-resistant and
heat-resistant vessel 15 is connected to the joint 7. Note that the
reaction vessel 17 is detachably connected to the joint 7.
[0073] As the heating apparatus 16, an electric furnace including a
heat insulator and a heater can be used, for example. Also, the
heating apparatus 16 preferably performs a temperature control so
that the temperature of the closed pressure-resistant and
heat-resistant vessel 15 and a portion within the heating apparatus
16 of the heat-resistant pipe 10 is maintained uniform,
particularly in terms of prevention of aggregation of the sodium
flux. Temperature in the heating apparatus 16 can be controlled to,
for example, 600.degree. C. (873 K) to 1100.degree. C. (1373 K).
The pressure adjuster 3 can control the nitrogen-containing gas
within the range of 100 atm (10.times.1.01325.times.10.sup.5 Pa) or
less. The heating apparatus 16 also has a shaking function. The
closed pressure-resistant and heat-resistant vessel 15 can be fixed
to the heating apparatus 16 so as to be shaken.
[0074] FIG. 1(b) shows a configuration of the closed
pressure-resistant and heat-resistant vessel 15. A crystal growth
vessel 18 is provided in the closed pressure-resistant and
heat-resistant vessel 15. A seed crystal 20 is vertically arranged
in the crystal growth vessel 18. The crystal growth vessel 18 is
filled with a flux 21 of melted gallium (Ga) and sodium (Na). The
seed crystal 20 may be horizontally arranged on a bottom of the
crystal growth vessel 18 instead of the vertical arrangement as
shown in the figure. Note that, in the case of the bottom
arrangement, a surface of the seed crystal 20 on which crystal
growth is conducted needs to be caused to face upward.
[0075] Examples of a material for the crystal growth vessel 18
include, but are not particularly limited to, alumina
(Al.sub.2O.sub.3), yttria (Y.sub.2O.sub.3), BN, PBN, MgO, CaO, W,
SiC, carbon materials (graphite, diamond-like carbon, etc.), and
the like. Particularly, yttria or alumina, which hinders
dissolution of oxygen and aluminum into the flux even at high
temperatures, preferably enables growth of a gallium nitride
crystal including less impurities.
[0076] Examples of a material for the closed pressure-resistant and
heat-resistant vessel 15 include SUS materials (SUS316, etc.),
nickel alloys (Inconel, Hastelloy, Incoloy, etc.), and the like,
which are resistant to high temperatures. In particular, materials,
such as Inconel, Hastelloy, Incoloy and the like, are resistant to
oxidation at high temperatures and high pressures, and can also be
used in atmospheres other than inert gas, and are preferable in
terms of reusability and durability.
[0077] Next, production of a gallium nitride crystal using the
production apparatus described above will be described.
[0078] Initially, sodium and gallium as materials and the seed
crystal 20 as a nucleus (template) for crystal growth are placed in
the crystal growth vessel 18.
[0079] In the present invention, sodium as a material is desirably
of high purity in terms of the suppression of nonuniform nucleation
and the high quality of a crystal. Specifically, the purity is 99%
or more, more preferably 99.95% or more. The purity of gallium is
similarly preferably 99% or more, more preferably 99.9% or more.
The mass ratio (Na:Ga) of sodium (Na) and gallium (Ga) is
preferably within the range of Na:Ga=4:1 to 1:4, more preferably
Na:Ga=2:1 to 1:2, taking the solubility of nitrogen into
consideration. The total amount of sodium and gallium is set such
that the range of the depth of the seed crystal 20 from a liquid
surface of the flux 21 in the crystal growth vessel 18 has a
predetermined value. The range of the depth from the liquid surface
of the flux 21 to the seed crystal 20 is preferably 0 mm to 40 mm,
more preferably 2 mm to 20 mm.
[0080] In the present invention, the seed crystal may be any of a
single crystal, a polycrystal, and an amorphous substance, though a
single crystal or an amorphous substance is desirable. The form of
the nucleus is not particularly limited and is desirably a
single-crystal substrate of gallium nitride or a thin film
substrate of gallium nitride, for example. The thin film substrate
of gallium nitride is provided by forming a gallium nitride thin
film on a substrate made of sapphire using, for example, Metal
Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase
Epitaxy (HVPE), Molecular Beam Epitaxy (MBE), or the like. The
gallium nitride thin film preferably has a thickness of 5 .mu.m or
more, more preferably 10 .mu.m or more. This is because when the
temperature of the flux exceeds 700.degree. C. as temperature
increases, the amount of nitrogen that can be dissolved rapidly
increases and surpasses the supply of nitrogen from the atmosphere
gas. As a result, there is an extreme shortage of nitrogen in the
flux, so that gallium nitride of the seed crystal is extremely
dissolved (excessive meltback). Therefore, the gallium nitride thin
film preferably has a large thickness.
[0081] Next, a hydrocarbon is added to the crystal growth vessel
18. The hydrocarbon is preferably liquid, solid or a mixture of
liquid and solid at room temperature (e.g., 25.degree. C.). In the
present invention, the hydrocarbon is preferably, for example, a
chain saturated hydrocarbon, a chain unsaturated hydrocarbon, an
alicyclic hydrocarbon, an aromatic hydrocarbon or the like, or a
mixture thereof. The hydrocarbon also preferably does not contain
oxygen, so as to prevent oxidation of an alkali metal. Note that
since the hydrocarbon evaporates in the step of heating the crystal
growth vessel 18 to generate a flux, the hydrocarbon preferably has
a high boiling point. The boiling point of the hydrocarbon is
suitably higher than or equal to the boiling point of the alkali
metal. As used herein, "the boiling point of the hydrocarbon is
higher than or equal to the melting point of the alkali metal"
means that when there are two or more alkali metals, the boiling
point of the hydrocarbon is higher than or equal to the melting
point of at least one of the two or more alkali metals.
[0082] For example, the boiling point of the hydrocarbon is higher
than or equal to 97.7.degree. C., which is the melting point of
sodium, preferably 150.degree. C. or more, and more preferably
300.degree. C. or more. In the present invention, the hydrocarbon
is, for example, kerosenes having a boiling point of 150.degree. C.
or more, paraffins having a boiling point of 300.degree. C. or more
(e.g., heptadecane (boiling point: 302.degree. C.), octadecane
(boiling point: 317.degree. C.), nanodecane (boiling point:
330.degree. C.), icosane (boiling point: 342.degree. C.),
triacontane (boiling point: 449.8.degree. C.), vaseline (boiling
point: 302.degree. C.), liquid paraffins (boiling point: 170 to
340.degree. C.), and solid paraffins (boiling point: 300.degree. C.
or more)), lamp oils having a boiling point of 150 to 250.degree.
C., biphenyl (boiling point: 254.degree. C.), o-xylene (boiling
point: 144.degree. C.), m-xylene (boiling point: 139.degree. C.),
p-xylene (boiling point: 138.degree. C.), cumene (boiling point:
153.degree. C.), ethyltoluenes (boiling point: 161 to 165.degree.
C.), cymene (boiling point: 177.degree. C.), tetralin (boiling
point: 208.degree. C.), or the like. These may be used singly or in
combination of two or more. Of these hydrocarbons, octadecane,
nanodecane, icosane, triacontane, solid paraffin and biphenyl are
solid at room temperature. Heptadecane having a boiling point of
300.degree. C. or more, liquid paraffins, kerosines having a
boiling point of 150.degree. C. or more, lamp oils having a boiling
point of 150 to 250.degree. C., o-xylene, m-xylene, p-xylene,
cumene, ethyltoluene, cymene, and tetralin are liquid at room
temperature. Of these hydrocarbons, liquid paraffins are
preferable, which have a low vapor pressure at room temperature and
are liquid and therefore easy to weigh.
[0083] The lower limit of the amount of the hydrocarbon added is
preferably 0.03 parts by mass (%) or more, more preferably 0.05
parts by mass (%) or more, per 100 parts by mass of an alkali metal
(e.g., sodium).
[0084] The upper limit of the amount of the hydrocarbon added is
preferably determined as appropriate, when a single crystal of
gallium nitride is used as a seed crystal and when a thin film
substrate of gallium nitride is used.
[0085] When a single crystal of gallium nitride is used as a seed
crystal, the amount of the hydrocarbon added is preferably, but is
not particularly limited to, one part by mass (%) or less per 100
parts by mass of sodium. If the amount of the hydrocarbon added
exceeds 1%, a reduction in quality, such as an increase in defect
or coloration of gallium nitride crystal or the like, is likely to
occur.
[0086] On the other hand, when a thin film substrate is used as a
seed crystal (e.g., a thin film substrate made of gallium nitride
having a thickness of about 10 .mu.m is used, the upper limit of
the amount of the hydrocarbon added is preferably 0.6 parts by mass
(%) or less, more preferably 0.4 parts by mass (%) or less, per 100
parts by mass of an alkali metal (e.g., sodium). This is because
the hydrocarbon promotes meltback. Note that meltback means that
when the temperature of the sodium flux increases during an initial
period of crystal growth, the amount of nitrogen that can be
dissolved in the sodium flux rapidly increases, whereas the
dissolution of nitrogen into the sodium flux is slowed down, so
that gallium nitride is dissolved from the seed crystal. If the
amount of dissolved nitrogen is thus large, then when, for example,
a thin film crystal is used as a seed crystal, the seed may be
dissolved and lost (excessive dissolution or excessive meltback).
Therefore, when a thin film substrate of gallium nitride is used as
a seed crystal, the amount of the hydrocarbon is preferably set to
fall within the range described above. Note that when the gallium
nitride thin film has a thickness of more than 10 .mu.m, the upper
limit of the amount of the hydrocarbon added is preferably
increased in substantially proportion to the thickness of the thin
film. When the gallium nitride has a thickness of less than 10
.mu.m, the upper limit is preferably decreased in proportion to the
thickness.
[0087] Next, the crystal growth vessel 18 is placed in the closed
pressure-resistant and heat-resistant vessel 15, and then the
closed pressure-resistant and heat-resistant vessel 15, the valve 9
and the pipe 10 are assembled into the reaction vessel 17, which is
then connected to the joint 7. Thereafter, the valve 9 is closed so
as to prevent entry of atmosphere gas. These preliminary steps are
preferably conducted in an atmosphere of inert gas (e.g., nitrogen
gas, argon, etc.) whose oxygen concentration and water content are
controlled, so as to suppress oxidation and hydroxylation of
sodium. The oxygen concentration of the inert gas is preferably 5
ppm or less, more preferably 1 ppm or less. The water content (by
volume) of the inert gas is preferably 3 ppm or less, more
preferably 0.5 ppm or less. Particularly, more preferably, the gas
atmosphere has an oxygen concentration of 1 ppm or less and a water
content (by volume) of 0.5 ppm or less. In this case, oxidation and
hydroxylation of sodium surface can be hindered for several
hours.
[0088] Next, the reaction vessel 17 is removed from the inert gas
atmosphere, the closed pressure-resistant and heat-resistant vessel
15 is placed in the heating apparatus 16, and the joint 7 is
connected to the pipe 4. Thereafter, the valve 13 is opened, and
the exhaust apparatus 14 is used to remove gas from the closed
pressure-resistant and heat-resistant vessel 15 through the pipe 4.
After the end of the gas removal, the valve 13 is closed and the
valve 5 and the valve 9 are opened, so that the inside of the
closed pressure-resistant and heat-resistant vessel 15 is
pressurized with nitrogen-containing gas from the gas supply
apparatus 1. Note that the applied pressure is adjusted by the
pressure adjuster 3.
[0089] Next, the closed pressure-resistant and heat-resistant
vessel 15 is heated by the heating apparatus 16 to melt sodium and
gallium in the crystal growth vessel 18, thereby generating a
sodium flux. Thereafter, after a lapse of about 10 to 30 hours,
dissolution of nitrogen in the sodium flux reaches supersaturation.
Thereby, gallium nitride crystal is deposited on the seed crystal
20. Further, by continuing heating and pressurization for a
predetermined time, gallium nitride crystal is further grown. When
about 70% to 95% of gallium supplied as a material has been
deposited as gallium nitride crystal, the seed crystal is removed
from the crystal growth vessel 18.
[0090] Here, the heating conditions are appropriately determined,
depending on a component of the flux and a component of the
atmosphere gas and its pressure. For example, the heating
temperature is within the range of 700.degree. C. (973 K) to
1100.degree. C. (1373 K), preferably the range of 800.degree. C.
(1073 K) to 1000.degree. C. (1273 K). For example, the
pressurization conditions are 2 atm (2.times.1.01325.times.10.sup.5
Pa) or more, preferably 20 atm (20.times.1.01325.times.10 Pa) or
more. The upper limit of the pressurization conditions is
preferably 100 atm (100.times.1.01325.times.10.sup.5 Pa) or
less.
[0091] The nitrogen-containing gas is, for example, nitrogen gas
(N.sub.2), ammonia gas (NH.sub.3) or the like, which may or may not
be mixed (the mixture ratio is not limited). In particular, the use
of ammonia gas is preferable since a reaction pressure can be
reduced. Also, the nitrogen-containing gas preferably has a low
oxygen concentration and a low water content so as to suppress
oxidation and hydroxylation of the flux. The oxygen concentration
of the nitrogen-containing gas is preferably 5 ppm or less, more
preferably 1 ppm or less. The water content (by volume) of the
nitrogen-containing gas is preferably 3 ppm or less, more
preferably 0.1 ppm or less.
[0092] Note that the principle of the effect of suppressing
nonuniform nucleation by addition of a hydrocarbon is as follows.
The hydrocarbon is broken down to carbon and hydrogen in the flux.
Carbon in the flux is easily coupled with nitrogen to form a
cyanide ion. Therefore, the amount of nitrogen that can be
dissolved in the flux increases, and nitrogen is diffused due to
convection in the flux near the gas-liquid interface without
reaching excessive supersaturation. On the other hand, hydrogen in
the flux acts to break down gallium nitride at high temperatures as
is similar to when it is in gas. Therefore, hydrogen invariably
breaks down the seed crystal of gallium nitride or nucleated
gallium nitride. In particular, a small gallium nitride immediately
after nucleation has an excessively large surface area as compared
to its volume, and therefore, is more effectively broken down or
extinguished. Thus, carbon and hydrogen allow relaxation of
excessive supersaturation near the gas-liquid interface of the flux
or the like. Moreover, due to hydrogen, gallium nitride immediately
after nucleation cannot grow and is broken down. It can be
considered that, as their synergetic effect, the occurrence of
miscellaneous crystals near the gas-liquid interface can be
suppressed, and in addition, the film thickness of gallium nitride
grown on the seed crystal is caused to be more uniform.
Embodiment 2
[0093] In this embodiment, a method for producing gallium nitride
will be described in which a seed crystal, sodium (alkali metal)
coated with a hydrocarbon, gallium (group-III element), and a
hydrocarbon are placed in a crystal growth vessel for holding a
sodium flux that is an alkali metal, and thereafter, a portion of
the hydrocarbon is removed, followed by heating of the crystal
growth vessel.
[0094] In this production method, further, the amount of the
hydrocarbon in the crystal growth vessel is preferably adjusted.
For example, the suppression of nucleation and the coating of the
alkali metal require different amounts of the hydrocarbon.
Therefore, the hydrocarbon may be added in amount required for the
coating of the alkali metal in a preliminary step, and the amount
of the hydrocarbon may be adjusted within an appropriate range so
as to prevent dissolution of the seed crystal in an adjustment
step. The adjustment of the amount of the hydrocarbon includes, for
example, removal or addition of the hydrocarbon.
[0095] An exemplary production apparatus for use in the production
method of the present invention is similar to that of FIGS. 1(a)
and 1(b) described in Embodiment 1. A procedure for producing a
gallium nitride crystal using this production apparatus will be
described.
[0096] Initially, sodium and gallium as materials and a seed
crystal as a nucleus (template) for crystal growth are placed in
the crystal growth vessel 18. Here, sodium that is coated in
advance with a hydrocarbon is used. Sodium is coated with a
hydrocarbon as follows. For example, sodium from which oxidized and
hydroxylated portions thereof have been removed may be immersed in
a liquid hydrocarbon and then pulled out. Alternatively, a
hydrocarbon that is solid at room temperature may be heated in
advance to be melted, and then sodium may be immersed in the melt
and then pulled out, and the temperature of the hydrocarbon may be
then returned to room temperature so that the hydrocarbon is
solidified. Also, a liquid hydrocarbon and a solid hydrocarbon may
be mixed.
[0097] In the present invention, the boiling point of a hydrocarbon
used for coating is preferably sufficiently higher than room
temperature so as to maintain the effect of preventing oxidation
and hydroxylation of sodium at room temperature in addition to the
reason of Embodiment 1. Specifically, the boiling point of the
hydrocarbon is higher than or equal to 97.7.degree. C. that is the
melting point of sodium, more preferably 150.degree. C. or more,
and even more preferably 300.degree. C. or more. Examples of the
hydrocarbon include hydrocarbons similar to those described in
Embodiment 1.
[0098] If the mass of the hydrocarbon for coating sodium is
excessively large, a reduction in quality, such as an increase in
defect, coloration of gallium nitride crystal or the like, is
likely to occur. As a guideline, one part by mass (%) or less of
the hydrocarbon is preferably added per 100 parts by mass of
sodium. When the seed crystal is a thin film substrate made of
gallium nitride having a thickness of about 10 .mu.m, the amount of
the hydrocarbon is preferably 0.6 parts by mass (%) or less, more
preferably 0.4 parts by mass (%) or less, per 100 parts by mass of
sodium. However, the mass of the hydrocarbon for coating sodium may
often exceed the above-described upper limit, depending on the
shape of sodium or the thickness of the coating film of the
hydrocarbon. In this case, a portion of the hydrocarbon is
preferably removed in an adjustment step described below so as to
obtain an appropriate amount of the hydrocarbon to be left in the
crystal growth vessel 18.
[0099] Next, the crystal growth vessel 18 is placed in the closed
pressure-resistant and heat-resistant vessel 15, and then the
closed pressure-resistant and heat-resistant vessel 15, the valve 9
and the pipe 10 are assembled into the reaction vessel 17, which is
then connected to the joint 7. Thereafter, the valve 9 is closed so
as to prevent entry of atmosphere gas. These preliminary steps are
preferably conducted in an atmosphere of inert gas (e.g., nitrogen
gas, argon, etc.) whose oxygen concentration and water content are
controlled, so as to suppress oxidation and hydroxylation of
sodium. When sodium coated with a hydrocarbon is used, the gas
atmosphere preferably has an oxygen concentration of 10 ppm or less
and a water content (by volume) of 10 ppm or less. This is because
oxidation and hydroxylation can be hindered for several hours.
[0100] Next, the reaction vessel 17 is removed from the inert gas
atmosphere, the closed pressure-resistant and heat-resistant vessel
15 is placed in the heating apparatus 16, and the joint 7 is
connected to the pipe 4. Thereafter, the valve 13 is opened, and
the exhaust apparatus 14 is used to remove gas from the closed
pressure-resistant and heat-resistant vessel 15 through the pipe 4,
and optionally, perform an adjustment step of removing a portion of
the hydrocarbon coating the sodium. The removal of a portion of the
hydrocarbon to obtain an appropriate amount of remainder thereof
may be, for example, carried out by adjusting the time length of
the gas removal (exhaust time length). Since the exhaust time
length varies from hydrocarbon to hydrocarbon, the exhaust time
length is desirably determined by previously measuring the exhaust
time length and the remaining amount of the hydrocarbon in the
crystal growth vessel 18, or may be desirably determined based on
the state of meltback or the state of a crystal after growth of a
gallium nitride crystal. The hydrocarbon that coats the sodium may
be solid or a mixture of solid and liquid at room temperature. When
the vapor pressure is about 1000 Pa or less, the exhaust time may
be considerably long. In this case, the closed pressure-resistant
and heat-resistant vessel 15 is desirably heated while the gas is
being removed. The heating temperature is preferably higher than
room temperature and lower than 700.degree. C. that is the crystal
growth temperature. In view of the removal of water or the like
attached by the time the reaction vessel 17 is assembled, the
heating temperature is preferably higher than or equal to
100.degree. C. and lower than or equal to 490.degree. C. that is a
temperature at which sodium and gallium form an alloy.
Alternatively, a portion of the hydrocarbon coating the sodium can
be removed, for example, by performing pressure application and
pressurized cleaning purge with respect to the closed
pressure-resistant and heat-resistant vessel 15 a predetermined
number of times, or by providing an exhaust outlet (not shown) in
the closed pressure-resistant and heat-resistant vessel 15 and
causing inert gas, such as nitrogen gas or the like, to flow near
the crystal growth vessel for a predetermined time. These methods
can also be combined with heating of the closed pressure-resistant
and heat-resistant vessel 15.
[0101] Next, the inside of the closed pressure-resistant and
heat-resistant vessel 15 is pressurized with nitrogen-containing
gas from the gas supply apparatus 1. Further, the closed
pressure-resistant and heat-resistant vessel 15 is heated to a
crystal growth temperature by the heating apparatus 16, thereby
growing a gallium nitride crystal. Thereafter, when about 70% to
95% of gallium supplied as a material has been deposited as gallium
nitride crystal, the seed crystal 20 is removed from the crystal
growth vessel 18. In this case, gallium nitride crystal has been
grown on the seed crystal 20.
Embodiment 3
[0102] A group-III element nitride crystal of the present invention
is produced by the production method of the present invention
described above.
[0103] This crystal preferably has an optical absorption
coefficient of 10 cm.sup.-1 or less with respect to light having a
wavelength of 400 nm or more and 620 nm or less. The optical
absorption coefficient is preferably 5 cm.sup.-1 or less. Note that
the lower limit of the optical absorption coefficient is a value
exceeding zero.
[0104] The group-III element nitride crystal produced by the
production method of the present invention may contain carbon. For
example, the group-III element nitride crystal of the present
invention may contain 5.times.10.sup.17 (cm.sup.-3) or less carbon
atoms as a result of analysis by SIMS or the like.
[0105] In such a crystal, the group-III element is at least one
element selected from Al, Ga and In. The group-III element nitride
is preferably a compound that is represented by
Al.sub.sGa.sub.tIn.sub.(1-s-t)N, where 0.ltoreq.s.ltoreq.1,
0.ltoreq.t.ltoreq.1, and s+t.ltoreq.1.
[0106] In particular, the group-III element is preferably gallium
(Ga), and the group-III element nitride is preferably gallium
nitride (GaN).
Embodiment 4
[0107] A substrate of the present invention includes a group-III
element nitride crystal produced by the production method of the
present invention described above.
[0108] A semiconductor device of the present invention is a
semiconductor device in which a semiconductor layer is formed on
the substrate.
[0109] The semiconductor layer is not particularly limited and may
be any compound semiconductor, such as
Al.sub.sGa.sub.tIn.sub.(1-s-t)N or the like. The semiconductor
layer may have either a single-layer structure or a multilayer
structure.
[0110] Examples of a semiconductor device formed using such a
substrate include a laser diode (LD), a light emitting diode (LED)
and the like.
[0111] Hereinafter, examples of the present invention will be
described.
Example 1
[0112] Gallium nitride crystals were produced using the apparatus
of FIG. 1 under six sets of conditions in accordance with the
description of Embodiment 1 above. Generation of the gallium
nitride crystals was confirmed and the effect of suppressing
nonuniform nucleation by addition of hydrocarbons was confirmed.
Hereinafter, the production condition will be described.
[0113] (Production Conditions)
[0114] Seed crystal: gallium nitride thin film substrate
[0115] Dimension: 14 mm.times.15 mm gallium nitride thin film (film
thickness: 10 .mu.m)
[0116] Gas species: nitrogen gas (N.sub.2), purity 5 N
[0117] Sodium: purity 99.9 to 99.99%
[0118] Gallium: purity 99.999 to 99.99999%
[0119] Hydrocarbons: [0120] Liquid paraffin, specific gravity 0.82,
boiling point: 170 to 340.degree. C. [0121] Solid paraffin,
specific gravity 0.90, boiling point: 300.degree. C. or more [0122]
Lamp oil, specific gravity 0.80, boiling point: 170.degree. C. to
250.degree. C.
[0123] Crucible used: Al.sub.2O.sub.3 (alumina), purity 99.9 to
99.99%
[0124] Growth time: 144 hours
[0125] Direction in which seed crystal is placed: vertical
[0126] Table 1 below shows production conditions, i.e., the mass of
sodium (Na), the mass of gallium (Ga), the type and mass of a
hydrocarbon (HC), and a pressure and a temperature during growth.
Note that production numbers 1 to 3 indicate comparative examples
and production numbers 4 to 6 indicate Example 1. The results of
production under the six sets of conditions are shown in Table 2
below. Note that measurement and evaluation in Table 2 were carried
out by a method described below.
[0127] Generation of gallium nitride was confirmed by conducting
element analysis (EDX) and photoluminescence measurement (PL).
Element analysis was conducted by electron irradiation having an
acceleration voltage of 15 kV while confirming the position of a
sample using an electron microscope. A photoluminescence
measurement was conducted by helium-cadmium laser irradiation at
room temperature.
[0128] The amount of gallium nitride was measured by separately
measuring the amount of a grown crystal and the amount of
miscellaneous crystals grown using a method below. The crystal
growth amount was obtained by subtracting the mass of a crystal
seed alone previously measured from the mass of a seed crystal
(including a crystal growth portion) after crystal growth. The
amount of miscellaneous crystals was obtained by collecting
crystals attached to the inner surface of the crystal growth vessel
(crucible) and measuring the mass of the collected crystals.
[0129] The yield of gallium nitride (GaN) was obtained as the
proportion of a mass corresponding to gallium of an amount
(=crystal growth amount) grown on the seed crystal substrate with
respect to the mass of gallium originally supplied.
[0130] An upper-lower film thickness ratio was obtained as a lower
thickness/an upper thickness, where the upper thickness is the
thickness of crystal growth at a portion located 5 mm from the
upper end of the vertically arranged seed crystal and the lower
thickness is the thickness of crystal growth at a portion located 5
mm from the lower end thereof. TABLE-US-00001 TABLE 1 Pro- Growth
Growth duction Na Ga Hydrocarbon HC/NA temperature pressure No. (g)
(g) (HC) (mg) (wt %) (.degree. C.) (MPa) 1 2.3 2.0 None -- 0 850
3.4 2 2.3 2.0 None -- 0 860 3.6 3 2.3 2.0 None -- 0 870 3.8 4 2.3
2.0 Lamp 7.9 0.34 870 3.8 oil 5 2.3 2.0 Liquid 8.1 0.35 870 3.8
paraffin 6 2.3 2.0 Solid 9.0 0.39 870 3.8 paraffin
[0131] TABLE-US-00002 TABLE 2 Miscellaneous Upper-lower Crystal
crystals film growth growth GaN thickness Production Confirmation
amount amount yield ratio No. of generation (g) (g) (%)
(lower/upper) 1 Confirmed 1.30 0.78 54 0.3 2 Confirmed 0.72 1.30 30
0.3 3 Confirmed 0.56 1.39 27 0.4 4 Confirmed 1.72 0.17 72 0.8 5
Confirmed 2.06 0.00 87 1.1 6 Confirmed 2.02 0.00 84 1.2
[0132] As shown in Table 2, for all production numbers 1 to 6,
generation of gallium nitride was confirmed. For production numbers
1 to 3 in which no hydrocarbon was added, a large amount of
miscellaneous crystals that were considered to be caused by
nonuniform nucleation were grown in the crucible. Particularly,
when the temperature and the pressure are high, the amount of
miscellaneous crystals grown increases and the amount of crystal
growth decreases. At the same time, the yield of gallium nitride
decreases. On the other hand, for production numbers 4 to 6 in
which a hydrocarbon was added, substantially no miscellaneous
crystals were generated, i.e., the effect of suppressing nonuniform
nucleation by addition of the hydrocarbon was confirmed. At the
same time, the value of the upper-lower film thickness ratio was
improved, and the effect of growth with more uniform film
thicknesses was confirmed.
[0133] FIGS. 2(a) and 2(b) show photographs of gallium nitride
crystals 30 of production numbers 3 and 5. As shown in FIG. 2(a),
when no hydrocarbon was added, a considerably large amount of
miscellaneous crystals 31 was generated. On the other hand, as
shown in FIG. 2(b), when a hydrocarbon (lamp oil) was added,
generation of the miscellaneous crystals 31 was suppressed.
Example 2
[0134] Gallium nitride crystals were produced under five sets of
conditions (production numbers 7 to 11) using the apparatus of FIG.
1 in a manner similar to that of Embodiment 1 described above,
where a gallium nitride thin film was used as a seed crystal.
Hereinafter, the production conditions will be described.
[0135] (Production Conditions)
[0136] Seed crystal: gallium nitride thin film substrate
[0137] Dimension: 14 mm.times.15 mm gallium nitride thin film (film
thickness: 10 .mu.m)
[0138] Gas species: nitrogen gas (N.sub.2), purity 99.999%
[0139] Sodium: purity 99.9 to 99.99%
[0140] Gallium: purity 99.999 to 99.99999%
[0141] Hydrocarbon: Liquid paraffin, specific gravity 0.82, boiling
point: 170 to 340.degree. C.
[0142] Crucible used: Al.sub.2O.sub.3 (alumina), purity 99.9 to
99.99%
[0143] Growth time: 144 hours
[0144] Direction in which seed crystal is placed: vertical
[0145] Table 3 described below shows production conditions, i.e.,
the mass of sodium (Na), the mass of gallium (Ga), the type and
mass of a hydrocarbon (HC), and a pressure and a temperature during
growth. The results of production under the five sets of conditions
are also shown in Table 3 below. Here, conformation of generation
of gallium nitride, the amount of a grown crystal, the amount of
miscellaneous crystals grown, and the yield of gallium were
measured by a method similar to that of Example 1. A growth area
ratio indicates the proportion of a grown gallium nitride crystal
with respect to the area of a gallium nitride thin film as a seed
crystal. TABLE-US-00003 TABLE 3 Pro- Growth Growth duction Na Ga
Hydrocarbon HC/Na temperature pressure No. (g) (g) (HC) (mg) (wt %)
(.degree. C.) (MPa) 7 2.3 2.0 Liquid 0.41 0.018 865 3.6 paraffin 8
2.3 2.0 Liquid 0.82 0.036 865 3.6 paraffin 9 2.3 2.0 Liquid 4.06
0.18 865 3.6 paraffin 10 2.3 2.0 Liquid 8.12 0.35 865 3.6 paraffin
11 2.3 2.0 Liquid 16.2 0.70 865 3.6 paraffin
[0146] TABLE-US-00004 TABLE 4 Miscellaneous Crystal crystals
Production Confirmation growth growth GaN Growth area No. of
generation amount (g) amount (g) yield (%) ratio (%) 7 Confirmed
1.64 0.23 68 100 8 Confirmed 1.99 0.02 83 98 9 Confirmed 2.00 0.00
84 98 10 Confirmed 1.63 0.02 70 95 11 Confirmed 0.59 0.00 24 53
[0147] Initially, it was confirmed that a gallium nitride crystal
was generated under all the sets of conditions (i.e., production
numbers 7 to 11). Whereas the effect of suppressing generation of
miscellaneous crystals by addition of a hydrocarbon was confirmed
for all production numbers 7 to 11, generation of miscellaneous
crystals was more effectively suppressed for production numbers 8
to 11 in which a larger amount of a hydrocarbon was added.
According to these results, the amount of a hydrocarbon added is
more preferably about 0.03% or more by mass with respect to the
amount of sodium in terms of the effect of suppressing the
occurrence of miscellaneous crystals. On the other hand, a
satisfactory growth area ratio was obtained when the amount of a
hydrocarbon added was less than 0.7 wt %, and therefore, it can be
said that the melting during an initial state of crystal growth of
a nitrogen gallium thin film (excessive meltback) was suppressed.
If the amount of a hydrocarbon added was 0.35 wt % or less, the
growth area ratio was 95% or more, and the gallium nitride thin
film was substantially not melted. According to these results, when
a gallium nitride thin film is used as a seed crystal, the
appropriate range of the amount of a hydrocarbon added is more
preferably 0.4% or less by mass with respect to the amount of
sodium in terms of prevention of excessive meltback.
[0148] FIGS. 3(a) and 3(b) show photographs of gallium nitride
crystals 32 for production numbers 9 and 11. FIG. 3(a) shows a case
where a hydrocarbon for production number 9 was added in an amount
of 0.18 wt %. In this case, gallium nitride was grown on an entire
surface of the seed substrate, and there was substantially no
excessive meltback of the gallium nitride thin film. FIG. 3(b)
shows a case where a hydrocarbon for production number 11 was added
in an amount of 0.7 wt %. In this case, the occurrence of
miscellaneous crystals was suppressed, and the area ratio of a
melted portion 33 of the grown gallium nitride thin film with
respect to the seed substrate was about 53%.
Example 3
[0149] Gallium nitride crystals were produced under two sets of
conditions (production numbers 12 and 13) shown in Table 5 using
the apparatus of FIG. 1 in a manner similar to that of Embodiment 2
described above. Hereinafter, the production conditions will be
described.
[0150] (Production Conditions)
[0151] Seed crystal: gallium nitride thin film substrate
[0152] Dimension: 14 mm.times.15 mm gallium nitride thin film (film
thickness: 10 .mu.m)
[0153] Gas species: nitrogen gas (N.sub.2), purity 99.999%
[0154] Sodium: purity 99.9 to 99.99%
[0155] Gallium: purity 99.999 to 99.99999%
[0156] Hydrocarbon: Lamp oil, boiling point: 150.degree. C. to
250.degree. C.
[0157] Crucible used: Al.sub.2O.sub.3 (alumina), purity 99.9 to
99.99%
[0158] Direction in which seed crystal is placed: vertical
[0159] Growth time: 144 hours
[0160] Conditions, such as sodium, gallium, the type of a
hydrocarbon that coats sodium, and an exhaust time and a heating
time of the step of removing a portion of the hydrocarbon, are
shown in Table 5 below. The results from these two sets of
production conditions are shown in Table 6. Confirmation of
generation of gallium nitride, the amount of a grown crystal, the
amount of miscellaneous crystals grown, and the yield of gallium
are measured by a method similar to that of Example 1.
TABLE-US-00005 TABLE 5 Exhaust Heating Growth Growth Production Na
Ga time temperature temperature pressure No. (g) (g) Hydrocarbon
(min) (.degree. C.) (.degree. C.) (MPa) 12 2.3 2.0 Lamp oil 120
25.degree. C. 865 3.6 13 2.3 2.0 Lamp oil 20 120.degree. C. 865
3.6
[0161] TABLE-US-00006 TABLE 6 Miscellaneous Crystal crystals
Production Confirmation growth growth GaN Growth area No. of
generation amount (g) amount (g) yield (%) ratio (%) 12 Confirmed
1.71 0.12 71 82 13 Confirmed 1.81 0.02 75 92
[0162] Firstly, as shown in Table 6, a gallium nitride crystal was
confirmed for all production numbers 12 and 13. The amount of
miscellaneous crystals grown was 0.12 g or less for all production
numbers 12 and 13, which was smaller than the amount of
miscellaneous crystals grown when no hydrocarbon was added in
Example 1, so that the effect of suppressing nonuniform nucleation
was confirmed. Also, since sodium was coated with a hydrocarbon
(lamp oil), oxidation of sodium or the like was prevented, so that
the resultant crystal had high quality.
[0163] FIG. 4 shows a photograph for production number 12. For
production number 12, sodium was coated with a lamp oil, and gas
was removed at 25.degree. C. for 120 minutes in an adjustment step.
A gallium nitride crystal was satisfactorily grown on substantially
an entire surface of the gallium nitride thin film at a growth area
ratio of 82%, though a portion of the gallium nitride thin film was
melted (melted portion 33). For production number 13 (not shown),
sodium was coated with a lamp oil, and gas was removed at
120.degree. C. for 20 minutes. In this case, the growth area was
92%, i.e., more satisfactory growth.
Example 4
[0164] A gallium nitride crystal was produced using the apparatus
of FIG. 5 by the method of Embodiment 2 described above. The
production conditions are described below, and the results are
shown in Table 7 below. Note that, in Table 7, With carbon coating
refers to this example and Without carbon coating refers to a
comparative example.
[0165] (Production Conditions)
[0166] Seed crystal: gallium nitride thin film substrate
[0167] Dimension: 2-inch gallium nitride thin film (film thickness:
10 .mu.m)
[0168] Gas species: nitrogen gas (N.sub.2), purity 99.999%
[0169] Sodium: purity 99.9 to 99.99%
[0170] Gallium: purity 99.999 to 99.99999%
[0171] Hydrocarbons: [0172] Lamp oil: boiling point 150.degree. C.
to 250.degree. C. (for coating sodium) [0173] Liquid paraffin,
boiling point: 170.degree. C. to 340.degree. C. (for coating
sodium)
[0174] Crucible used: Al.sub.2O.sub.3 (alumina), purity 99.9 to
99.99%
[0175] Direction in which seed crystal is placed: fixed to bottom
of crucible
[0176] Growth time: 144 hours
[0177] A reaction vessel 80, a crystal growth vessel (crucible) 82,
gallium 86, and sodium 84 were placed in a glove box (not shown),
followed by charging of materials. Initially, a seed 88 was set in
the crystal growth vessel 82. Next, the sodium 84 and the gallium
86 were weighed and then placed in the crucible 82. The sodium 84
was coated with a hydrocarbon (a lamp oil or a liquid paraffin) as
described in Embodiment 2 after removal of oxides or impurities on
the surface, and was then placed in the crucible 82. Thereafter,
the crucible 82 in which the materials were set in the glove box
was set in the reaction vessel 80. Also, in the glove box, a valve
65 at a gas inlet side and a valve 66 at a gas outlet side were
closed so that the sodium 84 in the crucible was not oxidized even
if the reaction vessel was removed into the air. Further, a
high-pressure chamber lid 61 was opened, and the reaction vessel
was set in the crystal growth apparatus via a joint 64. Thereafter,
reaction gas (here, nitrogen gas) was caused to flow via a gas flow
rate adjuster 62. In this case, the valve 65 and the valve 66 were
opened to allow the nitrogen gas to flow into the reaction vessel.
In this situation, the high-pressure chamber lid 61 was closed and
the high-pressure chamber 60 was evacuated. When a predetermined
degree of vacuum was reached, the gas flow rate adjuster 62 was
temporarily closed and the chamber 60 was evacuated to high vacuum.
Thereafter, evacuation was performed while causing the reaction gas
to flow again, thereby removing a desired amount of the hydrocarbon
(a lamp oil, a liquid paraffin, etc.) in the crucible in the
reaction vessel.
[0178] Although a hydrocarbon, such as a lamp oil or the like, that
is relatively easily evaporated, can be removed by gas flow or
evacuation even at room temperature, a time required to evaporate
the hydrocarbon can be effectively reduced by holding the
hydrocarbon, for example, at about 100 to 200.degree. C. for about
30 min to 2 hr while causing nitrogen gas to flow. When the
hydrocarbon is a liquid paraffin or the like, which has a high
boiling point, the hydrocarbon may not be sufficiently removed at
room temperature by evacuation while causing nitrogen gas to flow.
In this case, the reaction vessel may be temporarily held at
200.degree. C. to 300.degree. C. (e.g., 1 to 2 hours), and the flow
of nitrogen gas may be further continued, thereby making it
possible to adjust the amo the amount of the hydrocarbon remaining
in the crucible. Note that, when completely no hydrocarbon is used,
the sodium 84 may react with oxygen or water due to an influence of
a slight amount of a residual impurity or the like in a pipe or the
like remaining in the joint 64 and the valve 65.
[0179] After leaving an appropriate amount of the hydrocarbon, the
crystal growth temperature and the growth pressure were adjusted
into predetermined values. Conditions for crystal growth used
herein are a growth temperature of 850 to 870.degree. C. and a
growth pressure of 3.4 to 3.8 MPa.
[0180] As shown in FIG. 6(a), a crystal having a seed layer 102 of
about 10 .mu.m that was TABLE-US-00007 TABLE 7 Film Presence of
thickness miscellaneous (mm) crystals Coloration Yield Without 1 to
1.5 Yes Yes 2/5 hydrocarbon coating With 2 to 2.5 Substantially no
Substantially no 4/5 hydrocarbon coating
[0181] As shown in Table 7, under the conditions that a hydrocarbon
was added, a grown crystal 104 having a thickness of about 2 to 2.5
mm that was LPE-grown on a seed was able to be grown with high
reproducibility. On the other hand, when no hydrocarbon was used
for coating of sodium, the probability (yield) of crystal growth
was lower than when a hydrocarbon was used, and the film thickness
that was able to be achieved was as thin as 1 to 1.5 mm. This is a
main reason why nonuniform nucleation occurs (so-called occurrence
of miscellaneous crystals) on a wall of the crystal growth vessel
other than the seed crystal even if growth is carried out under the
same conditions. Further, when a surface of sodium was coated with
a hydrocarbon, a crystal with substantially no coloration was able
to be grown with high reproducibility. On the other hand, when no
hydrocarbon was used, a crystal with deep coloration was frequently
observed. In some cases, a case where only a black thin film
crystal was obtained was frequently observed.
[0182] As described above, when sodium was coated with a
hydrocarbon, not only miscellaneous crystals occurred less, but
also the yield of crystal growth was able to be improved and a
highly transparent crystal was able to be grown with high
reproducibility. Moreover, when sodium is coated with a
hydrocarbon, the amount of the hydrocarbon tends to be larger than
the optimal value of Example 1. Therefore, a portion of the
hydrocarbon was removed by evaporation, so that a GaN crystal was
able to be grown on an entire surface of a 2-inch seed crystal
without excessive melting (excessive meltback) of the seed
layer.
[0183] Next, the holding substrate 100 was removed by polishing,
and thereafter, a growth surface of the LPE-grown crystal 104 (FIG.
6(b)) was mechanically or chemical-mechanically polished, so that a
2-inch self-sustaining GaN substrate 106 (FIG. 6(c)) was able to be
obtained.
Example 5
[0184] As Example 5, a case where a large number of self-sustaining
GaN substrates are obtained using an LPE-grown substrate 106 as a
seed will be described with reference to FIG. 7. As shown in FIGS.
7(a) and 7(b), a GaN crystal 108 of about 4 mm was able to be grown
on the seed crystal 106 under the same growth conditions as those
of Example 4, except that the growth time was two times longer than
that of Example 4. Here, as in Example 4, by adding a hydrocarbon,
growth was able to be carried out with substantially no occurrence
of miscellaneous crystals, even though the growth time was as long
as 288 hours.
[0185] A seed portion of the obtained crystal of FIG. 7(b) was
removed. The crystal was cut into slices each having a thickness of
about 1 mmn using a wire saw, followed by polishing of the rear and
front surfaces thereof. Thereby, crystals 110 each having a
thickness of 400 .mu.m (FIG. 7(c)) were able to be extracted from
about 2-inch crystal. These crystals were obtained from an
LPE-grown crystal as a seed. Therefore, a GaN self-sustaining
crystal substrate having excellent crystallinity, an EPD density of
1.times.10.sup.4 to 5.times.10.sup.5 (cm.sup.2), and a low
dislocation degree was able to be obtained. Also, the transparency
of the crystal was satisfactory and the coloration of the crystal
was substantially not observed. In this case, the optical
absorption coefficient with respect to a wavelength of 400 to 620
nm was able to be 10 cm.sup.-1. Therefore, by coating sodium with a
hydrocarbon, the yield of crystal growth and the reproducibility of
the transparency can be caused to be satisfactory in addition to
the effect of suppressing miscellaneous crystals by the
hydrocarbon. The miscellaneous-crystals suppression effect was more
advantageous, particularly when long-time growth is required for
large thickness or bulk growth. Although a dopant was not added in
Examples 1 to 5, an n-type dopant (Si, O, Ge or Sn) or a p-type
dopant (Mg, Sr, Ba or Zn) may be added in an appropriate amount as
described above.
INDUSTRIAL APPLICABILITY
[0186] According to the present invention, suppression of
miscellaneous crystals, an improvement in reproducibility, and a
reduction in coloration can be carried out during the growth of a
group-III element nitride crystal using a flux made of an alkali
metal.
* * * * *