U.S. patent application number 13/847222 was filed with the patent office on 2013-08-22 for high-pressure vessel for growing group iii nitride crystals and method of growing group iii nitride crystals using high-pressure vessel and group iii nitride crystal.
This patent application is currently assigned to SixPoint Materials, Inc.. The applicant listed for this patent is SixPoint Materials, Inc.. Invention is credited to Tadao Hashimoto, Masanori Ikari, Edward Letts.
Application Number | 20130216845 13/847222 |
Document ID | / |
Family ID | 41059761 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130216845 |
Kind Code |
A1 |
Hashimoto; Tadao ; et
al. |
August 22, 2013 |
HIGH-PRESSURE VESSEL FOR GROWING GROUP III NITRIDE CRYSTALS AND
METHOD OF GROWING GROUP III NITRIDE CRYSTALS USING HIGH-PRESSURE
VESSEL AND GROUP III NITRIDE CRYSTAL
Abstract
Present invention discloses a high-pressure vessel of large size
formed with a limited size of e.g. Ni--Cr based precipitation
hardenable superalloy. Vessel may have multiple zones. For
instance, the high-pressure vessel may be divided into at least
three regions with flow-restricting devices and the crystallization
region is set higher temperature than other regions. This structure
helps to reliably seal both ends of the high-pressure vessel, at
the same time, may help to greatly reduce unfavorable precipitation
of group III nitride at the bottom of the vessel. Invention also
discloses novel procedures to grow crystals with improved purity,
transparency and structural quality. Alkali metal-containing
mineralizers are charged with minimum exposure to oxygen and
moisture until the high-pressure vessel is filled with ammonia.
Several methods to reduce oxygen contamination during the process
steps are presented. Back etching of seed crystals and a new
temperature ramping scheme to improve structural quality are
disclosed.
Inventors: |
Hashimoto; Tadao; (Santa
Barbara, CA) ; Letts; Edward; (Buellton, CA) ;
Ikari; Masanori; (Santa Barbara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SixPoint Materials, Inc.; |
|
|
US |
|
|
Assignee: |
SixPoint Materials, Inc.
Buellton
CA
|
Family ID: |
41059761 |
Appl. No.: |
13/847222 |
Filed: |
March 19, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13784210 |
Mar 4, 2013 |
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13847222 |
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13491392 |
Jun 7, 2012 |
8420041 |
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13784210 |
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12455683 |
Jun 4, 2009 |
8236267 |
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13491392 |
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61058910 |
Jun 4, 2008 |
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Current U.S.
Class: |
428/548 ;
423/269 |
Current CPC
Class: |
C30B 35/002 20130101;
C30B 29/406 20130101; Y10T 428/12028 20150115; C30B 7/105 20130101;
C30B 7/10 20130101; B32B 15/01 20130101; C30B 29/403 20130101 |
Class at
Publication: |
428/548 ;
423/269 |
International
Class: |
C30B 7/10 20060101
C30B007/10; B32B 15/01 20060101 B32B015/01 |
Claims
1. An article comprising a. a mineralizer comprising an element
that aids in ammonothermal growth of a group III-nitride crystal in
a high-pressure reactor; and b. a covering upon the mineralizer
that prevents oxygen from contaminating the mineralizer, said
covering being compatible with the ammonothermal growth of the
group III-nitride crystal.
2. An article according to claim 1 wherein the mineralizer
comprises an alkali metal mineralizer.
3. An article according to claim 2 wherein the alkali metal
mineralizer comprises at least one of Na, Li, and K.
4. An article according to claim 2 wherein the covering
encapsulates the mineralizer and is oxygen- and
water-impermeable.
5. An article according to claim 2 wherein the covering releases
the mineralizer while the high-pressure vessel is self-pressurized
by heating.
6. An article according to claim 1 wherein the mineralizer
comprises Na.
7. An article according to claim 6 wherein the mineralizer
comprises NaNH.sub.2.
8. An article according to claim 6 wherein the mineralizer
comprises elemental Na.
9. An article according to claim 1 wherein the covering comprises a
metal layer.
10. An article according to claim 9 wherein the metal layer
comprises a metal foil.
11. An article according to claim 9 wherein the metal layer
encapsulates the mineralizer.
12. An article according to claim 9 wherein the metal comprises
nickel.
13. An article according to claim 1 wherein the covering comprises
a container.
14. An article according to claim 13 wherein the article was formed
by a method of pouring molten mineralizer into the container and
solidifying the molten mineralizer.
15. An article according to claim 14 wherein a top surface of the
mineralizer is covered with a foil.
16. An article according to claim 13 wherein the container has a
yield strength that is exceeded at group III-nitride crystal
ammonothermal growth conditions.
17. An article according to claim 13 wherein the container ruptures
as the high-pressure reactor attains conditions for ammonothermal
growth of the group III-nitride crystal.
18. An article according to claim 1 and additionally comprising an
oxygen getter accompanying the mineralizer.
19. An article according to claim 1 wherein the covering
encapsulates the mineralizer and is oxygen- and
water-impermeable.
20. An article according to claim 1 wherein the covering releases
the mineralizer while the high-pressure vessel is self-pressurized
by heating.
21. An article according to claim 1 wherein the covering comprises
a metal which is stable in supercritical ammonia.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of and claims
priority to U.S. application Ser. No. 13/784,210, entitled
"High-Pressure Vessel For Growing Group III Nitride Crystals And
Method Of Growing Group III Nitride Crystals Using High-Pressure
Vessel And Group III Nitride Crystal," inventors Tadao Hashimoto,
Edward Letts, and Masanori Ikari, filed Mar. 4, 2013, which is a
continuation application of and claims priority to U.S. application
Ser. No. 13/491,392, entitled "High-Pressure Vessel For Growing
Group III Nitride Crystals And Method Of Growing Group III Nitride
Crystals Using High-Pressure Vessel And Group III Nitride Crystal,"
inventors Tadao Hashimoto, Edward Letts, and Masanori Ikari, filed
Jun. 7, 2012, which is a divisional application of and claims
priority to U.S. application Ser. No. 12/455,683, entitled
"High-Pressure Vessel For Growing Group III Nitride Crystals And
Method Of Growing Group III Nitride Crystals Using High-Pressure
Vessel And Group III Nitride Crystal," inventors Tadao Hashimoto,
Edward Letts, and Masanori Ikari, filed Jun. 4, 2009, which claims
priority to U.S. App. Ser. No. 61/058,910 entitled "High-Pressure
Vessel For Growing Group III Nitride Crystals And Method Of Growing
Group III Nitride Crystals Using High-Pressure Vessel And Group III
Nitride Crystal", inventors Tadao Hashimoto, Edward Letts, and
Masanori Ikari, filed on Jun. 4, 2008, the entire contents of each
of which are incorporated by reference herein as if put forth in
full below. This application is also related to PCT application
serial number PCT/US2009/046317 entitled "High-Pressure Vessel For
Growing Group III Nitride Crystals And Method Of Growing Group III
Nitride Crystals Using High-Pressure Vessel And Group III Nitride
Crystal", inventors Tadao Hashimoto, Edward Letts, and Masanori
Ikari, filed on Jun. 4, 2009, the entire contents of which are
incorporated by reference herein as if put forth in full below.
[0002] This application is also related to the following U.S.
patent applications:
[0003] PCT Utility Patent Application Serial No. US2005/024239,
filed on Jul. 8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji
Nakamura, entitled "METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS
IN SUPERCRITICAL AMMONIA USING AN AUTOCLAVE," attorneys' docket
number 30794.0129-WO-01 (2005-339-1); U.S. Utility patent
application Ser. No. 11/784,339, filed on Apr. 6, 2007, by Tadao
Hashimoto, Makoto Saito, and Shuji Nakamura, entitled "METHOD FOR
GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN
SUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE
CRYSTALS," attorneys docket number 30794.179-US-U1 (2006-204),
which application claims the benefit under 35 U.S.C. Section 119(e)
of U.S. Provisional Patent Application Ser. No. 60/790,310, filed
on Apr. 7, 2006, by Tadao Hashimoto, Makoto Saito, and Shuji
Nakamura, entitled "A METHOD FOR GROWING LARGE SURFACE AREA GALLIUM
NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIA AND LARGE SURFACE AREA
GALLIUM NITRIDE CRYSTALS," attorneys docket number 30794.179-US-P1
(2006-204);
[0004] U.S. Utility Patent Application Ser. No. 60/973,602, filed
on Sep. 19, 2007, by Tadao Hashimoto and Shuji Nakamura, entitled
"GALLIUM NITRIDE BULK CRYSTALS AND THEIR GROWTH METHOD," attorneys
docket number 30794.244-US-P1 (2007-809-1);
[0005] U.S. Utility patent application Ser. No. 11/977,661, filed
on Oct. 25, 2007, by Tadao Hashimoto, entitled "METHOD FOR GROWING
GROUP III-NITRIDE CRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA
AND NITROGEN, AND GROUP III-NITRIDE CRYSTALS GROWN THEREBY,"
attorneys docket number 30794.253-US-U1 (2007-774-2).
[0006] U.S. Utility Patent Application Ser. No. 61/067,117, filed
on Feb. 25, 2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari,
entitled "METHOD FOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUP
III-NITRIDE WAFERS," attorneys docket number 62158-30002.00.
[0007] all of which applications are incorporated by reference
herein.
FIELD OF THE INVENTION
[0008] The invention is related to the high-pressure vessel used to
grow group III nitride crystals expressed as
B.sub.xAl.sub.yGa.sub.zIn.sub.1-x-y-zN(0.ltoreq.x,y,z.ltoreq.1)
such as gallium nitride, boron nitride, indium nitride, aluminum
nitride, and their solid solutions in high-pressure ammonia. The
invention is also related to the method of growing group III
nitride crystals and the grown group III nitride crystals.
BACKGROUND
[0009] (Note: This patent application refers to several
publications and patents as indicated with numbers within brackets,
e.g., [x]. A list of these publications and patents can be found in
the section entitled "References.")
[0010] Gallium nitride (GaN) and its related group-III alloys are
key materials for various opto-electronic and electronic devices
such as light emitting diodes (LEDs), laser diodes (LDs), microwave
power transistors, and solar-blind photo detectors. Currently LEDs
are widely used in cell phones, indicators, displays, and LDs are
used in data storage disk drives. However, the majority of these
devices are grown epitaxially on heterogeneous substrates, such as
sapphire and silicon carbide since GaN wafers are extremely
expensive compared to these heteroepitaxial substrates. The
heteroepitaxial growth of group III-nitride causes highly defected
or even cracked films, which hinders the realization of high-end
optical and electronic devices, such as high-brightness LEDs for
general lighting or high-power microwave transistors.
[0011] To solve fundamental problems caused by heteroepitaxy, it is
useful to utilize single crystalline group III nitride wafers
sliced from bulk group III nitride crystal ingots. For the majority
of devices, single crystalline GaN wafers are favorable because it
is relatively easy to control the conductivity of the wafer and GaN
wafer will provide the smallest lattice/thermal mismatch with
device layers. However, due to the high melting point and high
nitrogen vapor pressure at elevated temperature, it has been
difficult to grow GaN crystal ingots. Growth methods using molten
Ga, such as high-pressure high-temperature synthesis [1,2] and
sodium flux [3,4], have been proposed to grow GaN crystals,
nevertheless the crystal shape grown in molten Ga becomes a thin
platelet because molten Ga has low solubility of nitrogen and a low
diffusion coefficient of nitrogen.
[0012] The ammonothermal method, which is a solvothermal method
using high-pressure ammonia as a solvent has demonstrated
successful growth of real bulk GaN [5-10]. This new technique is
able to grow large GaN crystal ingots, because high-pressure
ammonia used as a fluid medium has a high solubility of source
materials such as GaN polycrystals or metallic Ga, and high
transport speed of dissolved precursors can be achieved. There are
mainly three approaches to grow GaN in supercritical ammonia; a
method using ammonobasic solutions in single reactor with external
heating as disclosed in [6-10] and a method using ammonoacidic
solutions in Pt-lined single reactor with external heating as
disclosed in [1,1] and a method using supercritical ammonia with a
capsule and internal heaters enclosed in high-pressure reactor as
disclosed in [1,2]. The latter two methods have disadvantages in
expanding the reactor scale. For the ammonoacidic approach, it is
extremely expensive to use a Pt-liner in a large-scaled pressure
vessel. As for the internal capsule, it is structurally very
challenging to operate the capsule reactor larger than 2''
diameter. Therefore, the ammonothermal growth using basic
mineralizer is the most practical approach to mass-produce bulk
GaN. As disclosed in the literature [6, 13-16], GaN has retrograde
solubility in supercritical ammonobasic solutions. Therefore, in
the conventional ammonothermal growth using basic mineralizer, the
temperature for a nutrient zone is set lower than that for a
crystallization zone. In addition to this temperature setting,
basic ammonothermal method differs in many aspects from other
solvothermal methods such as hydrothermal growth of quartz and zinc
oxide. Because of this difference, it is not straightforward to
apply the solvothermal method to grow group III nitride crystals
and more improvements are required to realize mass production of
GaN wafers by the ammonothermal method.
[0013] First, state-of-the-art ammonothermal method [6-10] lacks
scalability into industrial production because it is quite
difficult to obtain large enough superalloy material to construct a
high-pressure vessel. Since group III nitrides have high melting
temperature or dissociation temperature, crystal growth requires
relatively higher temperature than other materials grown by the
solvothermal method. For example, both quartz and zinc oxide (ZnO)
are grown at about 300-400.degree. C. by the hydrothermal method.
On the other hand, typical growth temperature of GaN in the
ammonothermal method is 450-600.degree. C. [6-10]. Furthermore, our
experiment showed that growth at 550.degree. C. or above is
typically needed to obtain high-quality crystals. Therefore, Ni--Cr
based precipitation hardenable (or age hardenable) superalloy such
as Rene-41 (Haynes R-41 or Pyromet 41), Inconel 720 (Pyromet 720),
Inconel 718 (Pyromet 718), and Waspaloy A must be used for a vessel
material. These superalloys are forged to obtain small-sized, dense
grain structure which provides the necessary tensile strength for
conditions allowing the solvent to be supercritical. However, if
the solid dimension of the piece being worked (such as its
thickness) becomes too large, the grain structure necessary for
high-tensile strength cannot be obtained by forging. This is
because the forging pressure is always applied from the surface
during the forging process and the grain size at the inner portion
of the material tends to be unaffected if the work size exceeds a
certain size. Cracking during the forging/cooling process is also
profound for large diameter rods. These problems limit the
available size of Ni--Cr based precipitation hardenable
superalloys. In case of Rene-41, the maximum available outer
diameter for a rod is 12 inch, although the maximum outer diameter
for an as-cast (i.e. unforged) rod is larger than 12 inch.
[0014] Another obstacle to apply the ammonothermal method to
commercial production of GaN single crystals is mediocre quality of
grown crystals. Currently, their purity, transparency and
structural quality are not sufficient for commercial use. In
particular, oxygen concentration is at the order of 10.sup.20
cm.sup.-3. This high level of oxygen together with Ga vacancy is
thought to be the origin of brownish color of GaN grown by the
ammonothermal method. The grown crystals also show multiple grains
in the growth plane.
[0015] Considering above-mentioned limitations, this patent
discloses several new ideas to realize a high-pressure vessel that
is practically usable for production of group III nitride crystals
by the ammonothermal method. This patent also discloses new ways to
improve purity, transparency, and structural quality of group III
nitride crystals grown by the ammonothermal method.
SUMMARY OF THE INVENTION
[0016] The present invention discloses a new high-pressure vessel
suitable for use in ammonothermal growth of a Group III-nitride
material. The vessel is made from a raw material such as superalloy
rod or billet limited in size today when incorporated into a
high-pressure vessel. A multiple-zone high-pressure vessel, which
can attain the largest possible high-pressure vessel is
disclosed.
[0017] The vessel may have one or more clamps to seal the vessel. A
clamp may be formed of a metal or alloy having grain flow in a
radial direction in the clamp. This configuration enables the
vessel to be much greater in size than vessels today.
[0018] The high-pressure vessel may be divided into at least three
regions by flow-restricting devices such as baffles. In this
embodiment, the vessel has a nutrient region in which a nutrient
such as polycrystalline GaN or other feed material is dissolved or
supplied. The vessel also has a crystallization region in which the
group III-nitride material is crystallized upon a seed material.
The vessel also has a buffer region between the nutrient region and
the crystallization region, one or more cool zones adjacent to the
crystallization zone and/or the nutrient zone and in the vicinity
of a clamp, or both.
[0019] This invention also discloses new procedures to grow
crystals with improved purity, transparency and structural quality.
Alkali metal-containing mineralizers are charged with minimum
exposure to oxygen and moisture until the high-pressure vessel is
filled with ammonia. Several methods to reduce oxygen contamination
during the process steps are presented. Also, back etching of seed
crystals and a new temperature ramping scheme to improve structural
quality are disclosed.
[0020] In addition, the current invention discloses a method of
reducing parasitic deposition of polycrystalline GaN on the inner
surface of the pressure vessel while optionally improving
structural quality and without reducing growth rate. Because of the
retrograde solubility of GaN in the ammonothermal growth using
basic mineralizers, the temperature in the crystallization region
is conventionally set higher than the temperature in the nutrient
region. We, however, discovered that GaN can be grown by setting
the temperature of the crystallization region slightly lower than
the temperature of the nutrient region. By utilizing this "reverse"
temperature setting for growth, parasitic deposition of GaN on the
reactor wall was greatly reduced. Moreover, the structural quality
of grown crystal was improved without sacrificing growth rate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0022] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0023] FIG. 1 is a schematic drawing of the high-pressure vessel in
this invention.
[0024] In the figure each number represents the followings: [0025]
1. High-pressure vessel [0026] 2. Lid [0027] 3. Clamp [0028] 4.
Gasket [0029] 5. Heater(s) for the crystallization region [0030] 6.
Heater(s) for the dissolution region [0031] 7. Flow-restricting
baffle(s) [0032] 8. Nutrient basket [0033] 9. Nutrient [0034] 10.
Seed crystals [0035] 11. Flow-restricting baffle(s) [0036] 12.
Blast containment enclosure [0037] 13. Valve [0038] 14. Exhaust
tube [0039] 15. Device to operate valve
[0040] FIG. 2 is a photograph comparing colorations depending on
mineralizers and additives.
[0041] FIG. 3 is a photograph of GaN crystal with Mg additive.
DETAILED DESCRIPTION OF THE INVENTION
Overview
[0042] The present invention provides a design to achieve a
high-pressure vessel suitable for mass production of group
III-nitride crystals by the ammonothermal method. With the limited
size of available Ni--Cr based precipitation hardenable superalloy,
innovative designs are presented to maximize the vessel size. Also,
methods of growing improved-quality group III nitride crystals are
presented.
[0043] In one instance, the invention provides a reactor for
ammonothermal growth of a group III-nitride crystalline ingot. The
reactor has: a body defining a chamber; and a first clamp for
sealing an end of the reactor, wherein the first clamp is formed of
a metal or alloy having a grain flow in a radial direction in the
clamp.
[0044] In another instance, a reactor for ammonothermal growth of a
group III-nitride crystalline ingot has: a body defining a chamber;
a first heater for heating a nutrient region of the reactor; a
second heater for heating a crystallization region of the reactor;
a third region selected from a buffer region and an end region; and
at least one baffle separating the crystallization region from the
nutrient region or the crystallization region from an end region of
the reactor, with the qualifications that the buffer region when
present is defined by a plurality of baffles comprising a first end
baffle and an opposite end baffle with one or more optional baffles
therebetween, and a distance from the first end baffle to the
opposite end baffle of the plurality is at least 1/5 of an inner
diameter of the reactor; and the reactor further comprises a first
clamp at the end region when the end region is present.
[0045] Any of the reactors described herein may have a single
opening at an end. Alternatively, any of the reactors described
herein may have two or more openings, with one opening at each end
of the reactor body. The reactor may be generally cylindrical, or
the reactor may be formed in another shape such as spherical or
semispherical.
[0046] A reactor as discussed herein may have a buffer region
and/or one or more cooled end regions.
[0047] A clamp for the reactor, such as a clamp positioned in an
end region, may be formed of a metal or alloy having a grain flow
in a radial direction in the clamp. The clamp may be formed of a
superalloy such as a Ni--Cr superalloy. Alternatively, a clamp may
be formed of high strength steel.
[0048] A reactor may have a first heater that is configured to
maintain the nutrient region of the reactor at a temperature near
but greater than a temperature of the reactor in a crystallization
region of the reactor when the reactor is configured to grow a
gallium nitride crystal having retrograde solubility in
supercritical ammonia.
[0049] The reactor may be configured so that the first heater
maintains the nutrient region no more than about 20.degree. C.
greater than the temperature of the crystallization region.
[0050] Any reactor may have a second clamp for sealing a second end
of the reactor. The second clamp may be identical to the first
clamp discussed above.
[0051] The clamp may be formed in two or three or more pieces so
that the clamp can be disassembled and removed from the reactor.
The pieces may be held together by bolts formed of the same
material as the clamps.
[0052] A reactor for ammonothermal growth of a group III-nitride
crystalline ingot may also have a body defining a chamber; and a
mineralizer in an encapsulant, wherein the encapsulant is an
oxygen- and water-impermeable material capable of rupturing under
growth conditions in the reactor.
[0053] As discussed in further detail below, the encapsulant may be
a metal or alloy coating on the mineralizer that softens or melts
at crystal growth conditions in the reactor.
[0054] The encapsulant may instead or additionally comprise a
water- and oxygen-impermeable container that ruptures under crystal
growth conditions in the reactor.
[0055] The container may have a chamber which contains the
encapsulant and has a pressure much less than a pressure within the
reactor under crystal growth conditions, so that the pressure on
the container exceeds its yield strength when the reactor attains
operating pressure and temperature.
[0056] Any of the reactors disclosed herein may operate using an
ammonobasic supercritical solution or an ammonoacidic supercritical
solution. The mineralizer may therefore be basic or acidic. Such
mineralizers are well-known in the art.
[0057] Also disclosed herein is a method of forming a group
III-nitride crystalline material. The method includes:
[0058] providing a mineralizer substantially free of oxygen and
water to a reaction chamber of an ammonothermal growth reactor
[0059] evacuating the chamber
[0060] growing the group III-nitride crystalline material in the
chamber.
[0061] Any method disclosed herein may include providing an oxygen
getter in the reaction chamber.
[0062] A method of forming gallium nitride crystalline material may
comprise
[0063] heating a nutrient comprising polycrystalline GaN in a
nutrient region of a reactor
[0064] heating a seed material in a crystallization region of the
reactor
[0065] dissolving the polycrystalline GaN in supercritical
ammonia
[0066] depositing the dissolved GaN on the seed material to grow
the gallium nitride crystalline material
[0067] wherein the temperature of the nutrient region is greater
than but near the temperature of the crystallization region.
[0068] In such a method, the difference in temperature between the
nutrient region and the crystallization region may be no more than
about 50, 40, 30, 25, 20, 15, or 10.degree. C. during the act of
depositing the dissolved GaN on the seed material.
[0069] Any method discussed herein may further comprise
back-etching the nutrient prior to the act of growing the
crystalline material.
[0070] Reactors and methods are discussed in greater detail
below.
Technical Description
[0071] Since there is a size limitation in forged materials, it is
not possible today to obtain large diameter rod of various metals
or alloys such as precipitation hardenable Ni--Cr based superalloy
from which a pressure vessel suitable for ammonothermal growth of a
group III-nitride crystalline material can be made. For example,
the maximum diameter of as-cast R-41 billet is 17 inches, and this
billet is forged (e.g. round-forged, where the forging pressure is
applied along the radial direction) into 12-inch diameter rod
before making the reactor. This is the maximum diameter of forged
rod in state-of-the-art technology and means that the maximum outer
diameter of the high-pressure vessel can be close to 12 inches.
However, this maximum diameter cannot be realized using existing
methods because it is also necessary to form clamps for the
high-pressure vessel from the forged metal or alloy.
[0072] For a high-pressure vessel of this scale, a clamp-type
closure is safer and more practical due to the increased cover load
(i.e. the force applied to lids at the conditions present in
ammonothermal crystal growth). A screw-type closure is not
practical for a high-pressure vessel having an inner diameter
larger than 2 inches because the screw cap requires too many
threads to hold the cover load.
[0073] However, since the clamp diameter must be larger than the
outer diameter of the high-pressure vessel, the maximum outer
diameter of the high-pressure vessel is limited by the clamp
diameter. Using R-41 for example, the clamp diameter of 12 inch
results in a high-pressure vessel with 4-6 inch outer diameter and
2-3 inch inner diameter. Although one can produce 2-inch wafers of
GaN with such high-pressure vessel, the productivity is not large
enough to compete with other growth methods such as hydride vapor
phase epitaxy (HVPE).
Example 1
[0074] To solve the above-mentioned problem, a few methods to
obtain larger clamps were sought and we discovered it possible to
manufacture a large diameter disk with e.g. precipitation
hardenable Ni--Cr based superalloy or other metal or alloy by
forging as-cast billet along longitudinal direction. With this
method, a 17-inch as-cast billet is forged along the longitudinal
direction to form a 20-inch diameter, 1-foot thick disk. The
forging process usually creates a directional microstructure (grain
flow) which determines tensile strength of the material.
[0075] The grain flow of the round rod to construct the body and
lids is typically along the longitudinal direction whereas the
grain flow of the disks to construct the clamps is along the radial
direction. While one might expect a disk with radial grain flow
rather than axial grain flow to not have sufficient tensile
strength to reliably clamp a lid to a reactor body during use, this
difference does not have a significant impact on the reliability of
the high-pressure vessel as long as the high-pressure vessel is
designed with sufficient safety factor. With this method, we
designed a high-pressure vessel with 11.5'' outer diameter, 4.75''
inner diameter, and height larger than 47.5''. Since the height of
the high-pressure vessel is large, it is necessary to bore a hole
from both ends to form a chamber within the reactor vessel.
Therefore a high-pressure vessel might require two lids, one on
each end. The clamp diameter in this instance is 20'' and the
thickness is 12'', which ensures sufficient strength to hold a lid
during crystal growth. A large clamp such as this may be formed of
two or more clamp sections that are e.g. bolted together to form
the finished clamp.
[0076] A large reactor such as the one discussed above provides
greater opportunities to improve the quality of larger-diameter
crystal grown in the reactor. Hot ammonia circulates inside the
high-pressure vessel and transfers heat. To establish preferable
temperature distribution, flow-restricting baffles may be used.
Unlike the conventional ammonothermal reactor, the invented
high-pressure vessel may be equipped with a flow restricting baffle
to cool the bottom region (11 in FIG. 1). In this way, the
temperature around the seal and the clamp is maintained lower than
that of the crystallization region to improve reliability. In
addition, this temperature distribution prevents unnecessary
precipitation of GaN on the bottom lid.
[0077] The large reactor design also allows a large buffer region
to be incorporated between the crystallization region and the
nutrient region. This buffer region may comprise multiple baffles
where holes or openings in a baffle are offset from holes or
openings in adjacent baffles.
[0078] These baffles increase the average residence time within the
buffer region while providing some regions of relatively stagnant
flow and other regions of vortex flow for ammonia passing through
the regions between baffles. The average flow-rate from the
nutrient region to the crystallization region in the
larger-diameter reactor may also be less than the average flow-rate
from the nutrient region to the crystallization region in a
smaller-diameter reactor.
[0079] The buffer region may also be positioned primarily or
exclusively adjacent to the heaters for the nutrient region for a
reactor configured to provide a lower temperature in the nutrient
region and higher temperature in the crystallization region to take
advantage of the retrograde solubility of e.g. GaN in supercritical
ammonia. Or, the buffer region may extend past heaters for the
crystallization zone to provide earlier heating or cooling of the
supercritical ammonia, so that the ammonia solution is more likely
to be at the desired temperature when the solution encounters the
seed material.
[0080] The slower ammonia flow supplied in the crystallization
region with its increased residence time in the buffer zone, of the
supersaturated ammonia, may thus increase the growth rate as shown
in Example 3.
[0081] As the size of the high-pressure vessel increases, the total
internal volume becomes large. Thus, it is important to mitigate
possible blast hazards. The equivalent blast energy in this example
is estimated to be as high as 9 lbs TNT. In this example, we
designed a blast containment enclosure having 1/2 inch-thick steel
wall (12 in FIG. 1). One important thing is that the ammonia in the
high-pressure vessel can be released by remote operation of the
high-pressure valve (13 in FIG. 1). The valve can be
remote-operated via either mechanical, electrical, or pneumatic
way.
Example 2
[0082] Another method to realize a large diameter high-pressure
vessel is to use high-strength steel for the clamp material.
Although the maximum practical temperature for high-strength steel
(such as 4140) is 550.degree. C., the maximum available size is
large enough to fabricate clamps for high-pressure vessel having
12-inch outer diameter. In this case extra caution is necessary to
control the clamp temperature. An end region in the reactor that
has one or more baffles to impede ammonia flow permits ammonia to
cool in an end region, reducing the reactor temperature in that
region and allowing a clamp to be formed of a material such as high
strength steel that might not otherwise be used to form a clamp.
Also, appropriate anti-corrosion coating is advisable in case of
ammonia leak since steel is susceptible to corrosion by basic
ammonia.
Example 3
[0083] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the enhanced growth rate
by having one or more buffer regions between the crystallization
region and the nutrient region. Unlike the Examples 1, 2, this
high-pressure vessel has an open end on one side only. The chamber
of the high-pressure vessel is divided into several regions. From
bottom to top, there were a crystallization region, buffer regions,
and a nutrient region. GaN crystals were grown with 2 buffer
regions, 5 buffer regions, and 8 buffer regions and the growth rate
in each condition was compared. First, the high-pressure vessel was
loaded with 4 g of NaNH.sub.2, a seed crystal suspended from a
flow-restricting baffle having 55 mm-long legs (the first baffle).
To create buffer regions, 2 flow restricting baffles with 18 mm
legs, 5 flow restricting baffles with 18 mm legs or 8 flow
restricting baffles with 10 mm legs were set on top of the first
baffle (i.e. buffer regions with height of 18 mm were created for
the first two conditions and buffer regions with height of 10 mm
were created for the last condition). All of the baffles had 1/4''
hole at the center and the total opening was about 14%. The baffle
holes in this instance were not offset from one another. On top of
these baffles, a Ni basket containing 10 g of polycrystalline GaN
was placed and the high-pressure vessel was sealed with a lid. All
of these loading steps were carried out in a glove box filled with
nitrogen. The oxygen and moisture concentration in the glove box
was maintained to be less than 1 ppm. Then, the high-pressure
vessel was connected to a gas/vacuum system, which can pump down
the vessel as well as supply NH.sub.3 to the vessel. First, the
high-pressure vessel was pumped down with a turbo molecular pump to
achieve pressure less than 1.times.10.sup.-5 mbar.
[0084] The actual pressure in this example was 2.0.times.10.sup.-6,
2.4.times.10.sup.-6 and 1.4.times.10.sup.-6 mbar for conditions
with 2, 5, and 8 buffer regions, respectively. In this way,
residual oxygen and moisture on the inner wall of the high-pressure
vessel were removed. After this, the high-pressure vessel was
chilled with liquid nitrogen and NH.sub.3 was condensed in the
high-pressure vessel. About 40 g of NH.sub.3 was charged in the
high-pressure vessel. After closing the high-pressure valve of the
high-pressure vessel, the vessel was transferred to a two zone
furnace. The high-pressure vessel was heated to 575.degree. C. of
the crystallization zone and 510.degree. C. for the nutrient zone.
After 7 days, ammonia was released and the high-pressure vessel was
opened. The growth rate for conditions with 2, 5, and 8 buffer
regions were 65, 131, and 153 microns per day, respectively. The
growth rate was increased with increased number of buffer regions
between the crystallization region and the nutrient region. One
reason for the enhanced growth rate is that the convective flow of
ammonia became slower as the buffer regions increased. Another
reason is creating larger temperature difference by restricting
heat exchange between the nutrient region and crystallization
region. Therefore, adjusting the opening of the baffle, changing
the position of holes on the baffle, adjusting the height of the
buffer region is expected to have a similar effect on growth rate.
To enhance the growth rate effectively, it is desirable to maintain
the height of the buffer region larger than 1/5 of the inner
diameter of the high-pressure vessel so that enough space is
created to promote stagnant/vortex flow of ammonia.
Additional Technical Description
[0085] Here several methods to improve purity, transparency, and
structural quality of GaN grown by the ammonothermal method are
presented. The followings examples help illustrate further
principles of the claimed invention, as do the preceding
examples.
Example 4
[0086] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the ammonothermal growth
steps with high-vacuum pumping before ammonia filling. Unlike the
Examples 1, 2, this high-pressure vessel has an open end on one
side only. All necessary sources and internal components including
10 g of polycrystalline GaN nutrient held in a Ni basket, 0.3
mm-thick single crystalline GaN seeds, and three flow-restricting
baffles were loaded into a glove box together with the
high-pressure vessel. The glove box is filled with nitrogen and the
oxygen and moisture concentration is maintained to be less than 1
ppm. Since the mineralizers are reactive with oxygen and moisture,
the mineralizers are stored in the glove box at all times. 4 g of
NaNH.sub.2 was used as a mineralizer. After loading the mineralizer
into the high-pressure vessel, three baffles together with seeds
and nutrient were loaded. After sealing the high-pressure vessel,
it was taken out of the glove box. Then, the high-pressure vessel
was connected to a gas/vacuum system, which can pump down the
vessel as well as supply NH.sub.3 to the vessel. The high-pressure
vessel was heated at 110.degree. C. and evacuated with a turbo
molecular pump to achieve pressure less than 1.times.10.sup.-5
mbar. The actual pressure before filling ammonia was
2.0.times.10.sup.-6 mbar. In this way, residual oxygen and moisture
on the inner wall of the high-pressure vessel were removed. After
this, the high-pressure vessel was chilled with liquid nitrogen and
NH.sub.3 was condensed in the high-pressure vessel. Approximately
40 g of NH.sub.3 was charged in the high-pressure vessel. After
closing the high-pressure valve of the high-pressure vessel, it was
transferred to a two zone furnace. The high-pressure vessel was
heated up. The crystallization region was maintained at 575.degree.
C. and the nutrient region was maintained at 510.degree. C. After 7
days, ammonia was released and the high-pressure vessel was opened.
The grown crystals showed dark brownish color (the top sample in
FIG. 2) and the oxygen concentration measured with secondary ion
mass spectroscopy (SIMS) was 1.2-2.8.times.10.sup.20 cm.sup.-3. In
this example, pumping and heating of the high-pressure vessel did
not contribute to oxygen reduction. This is because the NaNH.sub.2
mineralizer contained significant amount of oxygen/moisture as
shown in the next example. After minimizing oxygen/moisture content
of mineralizers, pumping and heating of the high-pressure vessel is
necessary to minimize possible source of oxygen in the process.
Example 5
[0087] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the ammonothermal growth
steps using Na mineralizer with high-vacuum pumping before ammonia
filling. Unlike the Examples 1, 2, this high-pressure vessel has an
open end on one side. All necessary sources and internal components
including 10 g of polycrystalline GaN nutrient held in a Ni basket,
0.3 mm-thick single crystalline GaN seeds, and six flow-restricting
baffles were loaded into a glove box together with the
high-pressure vessel. The glove box is filled with nitrogen and the
oxygen and moisture concentration is maintained to be less than 1
ppm. Since the mineralizers are reactive with oxygen and moisture,
the mineralizers are stored in the glove box at all times. 2.3 g of
Na cube was used as a mineralizer. The surface of the Na was
"peeled" to remove oxide layer. However, even in the glovebox, the
fresh surface changed color in seconds, which represented
instantaneous oxidation of the Na surface. After loading the
mineralizer into the high-pressure vessel, six baffles together
with seeds and nutrient were loaded. After sealing the
high-pressure vessel, it was taken out of the glove box. Then, the
high-pressure vessel was connected to a gas/vacuum system, which
can pump down the vessel as well as supply NH.sub.3 to the vessel.
The high-pressure vessel was evacuated at room temperature with a
turbo molecular pump to achieve pressure less than
1.times.10.sup.-5 mbar. The actual pressure before filling ammonia
was 1.5.times.10.sup.-6 mbar. In this way, residual oxygen and
moisture on the inner wall of the high-pressure vessel were
removed. After this, the high-pressure vessel was chilled with
liquid nitrogen and NH.sub.3 was condensed in the high-pressure
vessel. Approximately 40 g of NH.sub.3 was charged in the
high-pressure vessel. After closing the high-pressure valve of the
high-pressure vessel, it was transferred to a two zone furnace. The
high-pressure vessel was heated up. The crystallization region was
maintained at 575.degree. C. and the nutrient region was maintained
at 510.degree. C. After 7 days, ammonia was released and the
high-pressure vessel was opened. The coloration of the grown
crystals was improved (the second sample from the top in FIG. 2).
The oxygen concentration measured by secondary ion mass
spectroscopy (SIMS) was 0.7-2.0.times.10.sup.19 cm.sup.-3, which
was more than one order of magnitude lower level than the sample in
Example 4. Using mineralizer with low oxygen/moisture content
together with pumping the high-pressure vessel is shown to be
important to minimize oxygen concentration in the crystal.
Example 6
[0088] As shown in Example 5, reducing oxygen/moisture content of
mineralizers is vital for improving purity of GaN. As explained in
Example 5, Na surface was oxidized quickly even in the glove box.
Therefore, it is important to reduce exposed surface of Na or
alkali metal mineralizers. Oxidation of Na was reduced by melting
the Na in Ni crucible. A hot plate was introduced in the glovebox
mentioned in the Example 5 to heat Ni crucible under oxygen and
moisture controlled environment. After removing surface oxide layer
of Na, it was placed in a Ni crucible heated on the hot plate. When
the Na melts, its surface fully contacts on the bottom and wall of
the crucible. Ni foil is placed on the top surface of the Na to
minimize exposed surface of Na. Since the Ni crucible is small
enough to fit the autoclave and Ni is stable in the growth
environment, the Ni crucible containing Na can be loaded in the
high-pressure vessel directly. In this way, most of surface of Na
is covered with Ni, therefore oxidation of Na is minimized. Any
metal which is stable in the supercritical ammonia can be used for
crucible material. Typical metal includes Ni, V, Mo, Ni--Cr
superalloy, etc.
Example 7
[0089] Another method to cover fresh surfaces of alkali metal is to
punch through alkali metal cake with hollow metal tubing. This way,
the sidewall of the alkali metal is automatically covered with the
inner wall of the tubing, thus reduces surface area exposed to the
environment. For example, a slab-shaped Na sandwiched with Ni foil
can be prepared by attaching Ni foil immediately after cutting top
or bottom surface of the slab. Although the sidewall of the slab is
not covered, punching through the Na slab with Ni tubing can create
Na cake with sidewall attaching to Ni and top and bottom surface
covered with Ni foil. In this way, oxygen/moisture content of
alkali metal mineralizer can be greatly reduced.
Example 8
[0090] When the high-pressure vessel becomes large, it is
impossible to load mineralizers to the high-pressure vessel in a
glove box. Therefore it is necessary to take alternative procedure
to avoid exposure of mineralizers to oxygen/moisture. One method is
to use an airtight container which cracks under high pressure. Here
is one example of the procedure. Mineralizer is loaded into the
airtight container in the glove box. If alkali metal is used, it
can be melted and solidified in the way explained in Example 6.
Then, the high pressure vessel is loaded with the airtight
container and all other necessary parts and materials under
atmosphere. After sealing the high-pressure vessel, it is pumped
down and heated to evacuate residual oxygen/moisture. Ammonia is
charged in the high-pressure vessel and the high-pressure vessel is
heated to grow crystals. When internal ammonia pressure exceeds
cracking pressure of the airtight container, the mineralizer is
released into the ammonia. In this way, mineralizers can be added
to ammonia without exposing them to oxygen and moisture.
Example 9
[0091] To further reduce oxygen concentration, it is effective to
remove oxygen in the high-pressure vessel after the high-pressure
vessel is sealed and before the ammonia is charged. One practical
procedure is to pump and backfill reducing gas in the high-pressure
vessel. The reducing gas such as ammonia and hydrogen reacts with
residual oxygen in the high-pressure vessel and form water vapor.
Therefore, it is further effective to heat the high-pressure vessel
to enhance the reduction process.
Example 10
[0092] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the ammonothermal growth
steps with Ce addition as an oxygen getter. Unlike the Examples 1,
2, this high-pressure vessel has an open end on one side. All
necessary sources and internal components including 5 g of
polycrystalline GaN nutrient held in a Ni basket, 0.4 mm-thick
single crystalline GaN seeds, and six flow-restricting baffles were
loaded into a glove box together with the high-pressure vessel. The
glove box is filled with nitrogen and the oxygen and moisture
concentration is maintained to be less than 1 ppm. Since the
mineralizers are reactive with oxygen and moisture, the
mineralizers are stored in the glove box at all times. 2.4 g of Na
was used as a mineralizer. After loading the mineralizer into the
high-pressure vessel, six baffles together with seeds and nutrient
were loaded. Then, 0.4 g of Ce powder was added. After sealing the
high-pressure vessel, it was taken out of the glove box. The
high-pressure vessel was connected to a gas/vacuum system, which
can pump down the vessel as well as supply NH.sub.3 to the vessel.
The high-pressure vessel was evacuated with a turbo molecular pump
to achieve pressure less than 1.times.10.sup.-5 mbar. The actual
pressure before filling ammonia was 2.6.times.10.sup.-6 mbar. In
this way, residual oxygen and moisture on the inner wall of the
high-pressure vessel were removed. After this, the high-pressure
vessel was chilled with liquid nitrogen and NH.sub.3 was condensed
in the high-pressure vessel. Approximately 40 g of NH.sub.3 was
charged in the high-pressure vessel. After closing the
high-pressure valve of the high-pressure vessel, it was transferred
to a two zone furnace. The high-pressure vessel was heated up.
First, the furnace for the crystallization region was maintained at
510.degree. C. and the nutrient region was maintained at
550.degree. C. for 24 hours. This reverse temperature setting was
discovered to be beneficial for improving crystal quality as shown
in Example 15. Then, the temperatures for the crystallization
region and the nutrient region were set to 575 and 510.degree. C.
for growth. After 4 days, ammonia was released and the
high-pressure vessel was opened. The grown crystal showed yellowish
color (the middle sample in FIG. 2), which represented improvement
of transparency.
[0093] When 0.1 g of Ca lump was added instead of Ce, the grown
crystal was semi transparent with slight tan color (the bottom
sample in FIG. 2), which represented improvement of transparency.
Similar result can be expected with adding oxygen getter containing
Al, Mn, or Fe.
Example 11
[0094] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the ammonothermal growth
steps with B addition as surfactant. Unlike the Examples 1, 2, this
high-pressure vessel has an open end on one side. All necessary
sources and internal components including 5 g of polycrystalline
GaN nutrient held in a Ni basket, one 0.4 mm-thick single
crystalline GaN seed, and six flow-restricting baffles were loaded
into a glove box together with the high-pressure vessel. The glove
box is filled with nitrogen and the oxygen and moisture
concentration is maintained to be less than 1 ppm. Since the
mineralizers are reactive with oxygen and moisture, the
mineralizers are stored in the glove box at all times. 2.4 g of Na
was used as a mineralizer. After loading the mineralizer into the
high-pressure vessel, six baffles together with the seed and
nutrient were loaded. Then, 0.1 g of BN platelet was added. After
sealing the high-pressure vessel, it was taken out of the glove
box. The high-pressure vessel was connected to a gas/vacuum system,
which can pump down the vessel as well as supply NH.sub.3 to the
vessel. The high-pressure vessel was evacuated with a turbo
molecular pump to achieve pressure less than 1.times.10.sup.-5
mbar. The actual pressure before filling ammonia was
1.8.times.10.sup.-6 mbar. In this way, residual oxygen and moisture
on the inner wall of the high-pressure vessel were removed. After
this, the high-pressure vessel was chilled with liquid nitrogen and
NH.sub.3 was condensed in the high-pressure vessel. Approximately
40 g of NH.sub.3 was charged in the high-pressure vessel. After
closing the high-pressure valve of the high-pressure vessel, it was
transferred to a two zone furnace. The high-pressure vessel was
heated up. First, the furnace for the crystallization region was
maintained at 510.degree. C. and the nutrient region was maintained
at 550.degree. C. for 24 hours. This reverse temperature setting
was discovered to be beneficial for improving crystal quality as
shown in Example 15. Then, the temperatures for the crystallization
region and the nutrient region were set to 575 and 510.degree. C.
for growth. After 4 days, ammonia was released and the
high-pressure vessel was opened. The grown crystal showed less
brownish color (the second sample from the bottom in FIG. 2), which
showed improvement of transparency. Also, the structural quality
was improved compared to one with conventional ammonothermal growth
[5]. The full width half maximum (FWHM) of X-ray rocking curve from
(002) and (201) planes were 295 and 103 arcsec, respectively. These
numbers are about 5 times smaller than crystals reported in
reference [5]. Similar result can be expected with adding
surfactant containing In, Zn, Sn, or Bi.
Example 12
[0095] In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the ammonothermal growth
steps with Mg addition as an oxygen getter. Unlike the Examples 1,
2, this high-pressure vessel has an open end on one side. All
necessary sources and internal components including 5 g of
polycrystalline GaN nutrient held in a Ni basket, 0.4 mm-thick
single crystalline GaN seeds, and six flow-restricting baffles were
loaded into a glove box together with the high-pressure vessel. The
glove box is filled with nitrogen and the oxygen and moisture
concentration is maintained to be less than 1 ppm. Since the
mineralizers are reactive with oxygen and moisture, the
mineralizers are stored in the glove box at all times. 2.4 g of Na
was used as a mineralizer. As explained in Example 6, Na was melted
in Ni crucible, capped with Ni foil and solidified. 0.1 g of Mg was
added in the Ni crucible on top of the Ni foil. After loading the
Ni crucible into the high-pressure vessel, six baffles together
with seeds and nutrient were loaded. After sealing the
high-pressure vessel, it was taken out of the glove box. The
high-pressure vessel was connected to a gas/vacuum system, which
can pump down the vessel as well as supply NH.sub.3 to the vessel.
The high-pressure vessel was evacuated with a turbo molecular pump
to achieve pressure less than 1.times.10.sup.-5 mbar. The actual
pressure before filling ammonia was 1.4.times.10.sup.-7 mbar. In
this way, residual oxygen and moisture on the inner wall of the
high-pressure vessel were removed. After this, the high-pressure
vessel was chilled with liquid nitrogen and NH.sub.3 was condensed
in the high-pressure vessel. Approximately 40 g of NH.sub.3 was
charged in the high-pressure vessel. After closing the
high-pressure valve of the high-pressure vessel, it was transferred
to a two zone furnace. The high-pressure vessel was heated up. For
the first 24 hours, only the furnace for the nutrient region was
turned on and maintained at 550.degree. C. while maintaining the
furnace for the crystallization region off in order to back etch
the surface of the seed. This reverse temperature setting was
discovered to be beneficial for improving crystal quality as shown
in Example 15. Then, the temperatures for the crystallization
region and the nutrient region were set to 590 and 575.degree. C.
for growth. After 9 days, ammonia was released and the
high-pressure vessel was opened. The grown crystal showed almost
transparent as shown in FIG. 3. Thus, reducing oxygen contamination
in the process is very effective to improve transparency of the
crystal.
Example 13
[0096] Seed crystals are usually prepared by slicing bulk GaN with
wire saw followed by polishing. There is a damage layer at the
surface of the seeds created by these slicing and polishing
process. To improve structural quality of GaN, it is necessary to
remove this damage layer. In addition, impurities physically or
chemically adsorbed on the seed surface can be removed with back
etching. In this example, a high-pressure vessel having an inner
diameter of 1 inch was used to demonstrate the back etching of the
seed in the ammonothermal high-pressure vessel. Unlike the Examples
1, 2, this high-pressure vessel has an open end on one side. A GaN
seed crystal was back etched by setting the temperature of the
crystallization region lower than that of the nutrient region. All
necessary sources and internal components including 10 g of
polycrystalline GaN nutrient held in a Ni basket, one 0.4 mm-thick
single crystalline GaN seed, and six flow-restricting baffles were
loaded into a glove box together with the high-pressure vessel. The
glove box is filled with nitrogen and the oxygen and moisture
concentration is maintained to be less than 1 ppm. Since the
mineralizers are reactive with oxygen and moisture, the
mineralizers are stored in the glove box at all times. 4 g of
NaNH.sub.2 was used as a mineralizer. After loading the mineralizer
into the high-pressure vessel, six baffles together with the seed
and nutrient were loaded. After sealing the high-pressure vessel,
it was taken out of the glove box. Then, the high-pressure vessel
was connected to a gas/vacuum system, which can pump down the
vessel as well as supply NH.sub.3 to the vessel. The high-pressure
vessel was heated at 125.degree. C. and evacuated with a turbo
molecular pump to achieve pressure less than 1.times.10.sup.-5
mbar. The actual pressure before filling ammonia was
1.8.times.10.sup.-6 mbar. In this way, residual oxygen and moisture
on the inner wall of the high-pressure vessel were removed. After
this, the high-pressure vessel was chilled with liquid nitrogen and
NH.sub.3 was condensed in the high-pressure vessel. Approximately
40 g of NH.sub.3 was charged in the high-pressure vessel. After
closing the high-pressure valve of the high-pressure vessel, it was
transferred to a two zone furnace. The high-pressure vessel was
heated up. The furnace for the crystallization region was
maintained off and the nutrient region was maintained at
550.degree. C. The actual temperature measured in the
crystallization region was 366.degree. C. It is very important to
maintain the temperature difference more than 50.degree. C. to
avoid GaN deposition on the seed due to concentration gradient.
After 24 hours, ammonia was released and the high-pressure vessel
was opened. The seed crystal was back etched by 36 microns, which
is sufficient to remove damage layer created by slicing and
polishing.
Example 13
[0097] Although GaN has retrograde solubility at temperatures
higher than approximately 400.degree. C., the solubility has normal
(i.e. positive) dependence on temperature below approximately
400.degree. C. Therefore it is possible to back etch the seed
crystals by setting the crystallization zone lower than 400.degree.
C. and maintaining the temperature of crystallization zone higher
than that of other zones. It is important to maintain the
temperature of the nutrient region at least 50.degree. C. lower
than the crystallization region to avoid GaN deposition on the seed
due to concentration gradient.
Example 14
[0098] Seed crystals are usually prepared by slicing bulk GaN with
wire saw followed by polishing. There is a damage layer at the
surface of the seeds created by these slicing and polishing
process. To improve structural quality of GaN, it is necessary to
remove this damage layer. In addition, impurities physically or
chemically adsorbed on the seed surface can be removed by back
etching. In this example, seed crystals were back etched in a
separate reactor prior to the ammonothermal growth. Seed crystals
with thickness of approximately 0.4 mm were loaded in a furnace
reactor which can flow ammonia, hydrogen chloride, hydrogen and
nitrogen. In this example, seed crystals were etched in mixture of
hydrogen chloride, hydrogen and nitrogen. The flow rate of hydrogen
chloride, hydrogen and nitrogen was 25 sccm, 60 sccm, and 1.44 slm,
respectively. The etching rate was 7, 21, 35, and 104 microns/h for
800, 985, 1000, 1050.degree. C. Therefore, back etching above
800.degree. C. gives sufficient removal of the surface layer.
Example 15
[0099] We discovered that structural quality of grown crystals was
improved by utilizing reverse temperature condition during initial
temperature ramp. In this example, a high-pressure vessel having an
inner diameter of 1 inch was used to demonstrate the effect of
reverse temperature setting in the initial stage of growth. Unlike
the Examples 1, 2, this high-pressure vessel has an open end on one
side. All necessary sources and internal components including 10 g
of polycrystalline GaN nutrient held in a Ni basket, 0.4 mm-thick
single crystalline GaN seeds, and three flow-restricting baffles
were loaded into a glove box together with the high-pressure
vessel. The glove box is filled with nitrogen and the oxygen and
moisture concentration is maintained to be less than 1 ppm. Since
the mineralizers are reactive with oxygen and moisture, the
mineralizers are stored in the glove box at all times. 4 g of
NaNH.sub.2 was used as a mineralizer. After loading the mineralizer
into the high-pressure vessel, three baffles together with seeds
and nutrient were loaded. After sealing the high-pressure vessel,
it was taken out of the glove box. Then, the high-pressure vessel
was connected to a gas/vacuum system, which can pump down the
vessel as well as supply NH.sub.3 to the vessel. The high-pressure
vessel was heated at 120.degree. C. and evacuated with a turbo
molecular pump to achieve pressure less than 1.times.10.sup.-5
mbar. The actual pressure before filling ammonia was
1.5.times.10.sup.-6 mbar. In this way, residual oxygen and moisture
on the inner wall of the high-pressure vessel were removed. After
this, the high-pressure vessel was chilled with liquid nitrogen and
NH.sub.3 was condensed in the high-pressure vessel. Approximately
40 g of NH.sub.3 was charged in the high-pressure vessel. After
closing the high-pressure valve of the high-pressure vessel, it was
transferred to a two zone furnace and heated up. The furnace for
the crystallization region was maintained at 510.degree. C. and the
nutrient region was maintained at 550.degree. C. for 6 hours. Then,
the temperatures for the crystallization region and the nutrient
region were set to 575 and 510.degree. C. for growth. After 7 days,
ammonia was released and the high-pressure vessel was opened. The
grown crystals had thickness of approximately 1 mm. The structural
quality was improved compared to one with conventional
ammonothermal growth [5]. The full width half maximum (FWHM) of
X-ray rocking curve from (002) and (201) planes were 169 and 86
arcsec, respectively.
Example 16
[0100] We discovered that setting the crystallization temperature
slightly lower than the nutrient temperature during growth
significantly reduced parasitic deposition of polycrystalline GaN
on the inner surface of reactor. Although the GaN has retrograde
solubility in supercritical ammonobasic solution as disclosed in
the Ref. [6, 13-16], we discovered that GaN crystal growth occurred
even with reverse temperature condition. Moreover, structural
quality of grown crystal was improved without sacrificing growth
rate.
[0101] A high-pressure vessel having an inner diameter of 1 inch
was used to demonstrate the effect of reverse temperature setting
during growth. Unlike the Examples 1, 2, this high-pressure vessel
has an open end on one side. All necessary sources and internal
components including 10 g of polycrystalline GaN nutrient held in a
Ni basket, 0.47 mm-thick single crystalline GaN seeds, and six
flow-restricting baffles were loaded into a glove box together with
the high-pressure vessel. The glove box is filled with nitrogen and
the oxygen and moisture concentration is maintained to be less than
1 ppm. Since the mineralizers are reactive with oxygen and
moisture, the mineralizers are stored in the glove box at all
times. 2.6 g of Na solidified in Ni crucible and capped with Ni
foil as explained in Example 6 was used as a mineralizer. After
loading the Ni crucible into the high-pressure vessel, six baffles
together with seeds and nutrient basket were loaded. After sealing
the high-pressure vessel, it was taken out of the glove box. Then,
the high-pressure vessel was connected to a gas/vacuum system,
which can pump down the vessel as well as supply NH.sub.3 to the
vessel. The high-pressure vessel was and evacuated with a turbo
molecular pump to achieve pressure less than 1.times.10.sup.-5
mbar. The actual pressure before filling ammonia was
9.5.times.10.sup.-8 mbar. In this way, residual oxygen and moisture
on the inner wall of the high-pressure vessel were removed. After
this, the high-pressure vessel was chilled with liquid nitrogen and
NH.sub.3 was condensed in the high-pressure vessel. Approximately
44 g of NH.sub.3 was charged in the high-pressure vessel. After
closing the high-pressure valve of the high-pressure vessel, it was
transferred to a two zone furnace and heated up. For the first 24
hours, the furnace for the crystallization zone and nutrient zone
were ramped and maintained at 360.degree. C. and 570.degree. C.,
respectively. As explained in Example 15, this reverse temperature
setting ensures back etching of the seed. Then, the crystallization
zone was heated and maintained to 535.degree. C. in about 10
minutes and the nutrient region was cooled from 570.degree. C. down
to 540.degree. C. in 24 hours. The slow cooling of the nutrient
region is expected to reduce sudden precipitation of GaN in the
initial stage of growth. After the temperature of the nutrient
region became 540.degree. C., this reverse temperature setting
(i.e. 535.degree. C. for crystallization region and 540.degree. C.
for nutrient region) was maintained for 3 more days. Then, the
ammonia was released, reactor was unsealed and cooled. The grown
crystals had thickness of approximately 1.2 mm. The average growth
rate was 0.183 mm/day. The structural quality was improved compared
to one with conventional ammonothermal growth [5]. The full width
half maximum (FWHM) of X-ray rocking curve from (002) and (201)
planes were 56 and 153 arcsec, respectively. Also, parasitic
deposition was about 0.3 g, which was more than 50% reduced
compared to the conventional temperature setting.
Advantages and Improvements
[0102] The disclosed improvements enable mass production of
high-quality GaN wafers via ammonothermal growth. By utilizing the
new high-pressure vessel design, one can maximize the high-pressure
vessel with size limitation of available Ni--Cr based precipitation
hardenable superalloy. Procedures to reduce oxygen contamination in
the growth system ensures high-purity GaN wafers. Small amount of
additives such as Ce, Ca, Mg, Al, Mn, Fe, B, In, Zn, Sn, Bi helps
to improve crystal quality. In-situ or ex-situ back etching of seed
crystals is effective way to remove damaged layer and any adsorbed
impurities from GaN seeds. In addition, reverse temperature setting
even for growth scheme is beneficial for reducing parasitic
deposition of polycrystalline GaN on the inner surface of the
reactor and improving structural quality. These techniques help to
improve quality of group III nitride crystals and wafers.
Possible Modifications
[0103] Although the preferred embodiment describes a two-zone
heater, the heating zone can be divided into more than two in order
to attain favorable temperature profile.
[0104] Although the preferred embodiment describes the growth of
GaN as an example, other group III-nitride crystals may be used in
the present invention. The group III-nitride materials may include
at least one of the group III elements B, Al, Ga, and In.
[0105] Although the preferred embodiment describes the use of
polycrystalline GaN nutrient, other form of source such as metallic
Ga, amorphous GaN, gallium amide, gallium imide may be used in the
present invention.
[0106] In the preferred embodiment specific growth apparatuses are
presented. However, other constructions or designs that fulfill the
conditions described herein will have the same benefit as these
examples.
[0107] Although the example in the preferred embodiment explains
the process step in which NH.sub.3 is released at elevated
temperature, NH.sub.3 can also be released after the high-pressure
vessel is cooled as long as seizing of screws does not occur.
[0108] The foregoing description of the preferred embodiment of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto.
REFERENCES
[0109] The following references are incorporated by reference
herein: [0110] [1]. S. Porowski, MRS Internet Journal of Nitride
Semiconductor, Res. 4S1, (1999) G1.3. [0111] [2] T. Inoue, Y. Seki,
O. Oda, S. Kurai, Y. Yamada, and T. Taguchi, Phys. Stat. Sol. (b),
223 (2001) p. 15. [0112] [3] M. Aoki, H. Yamane, M. Shimada, S.
Sarayama, and F. J. DiSalvo, J. Cryst. Growth 242 (2002) p. 70.
[0113] [4] T. Iwahashi, F. Kawamura, M. Morishita, Y. Kai, M.
Yoshimura, Y. Mori, and T. Sasaki, J. Cryst Growth 253 (2003) p. 1.
[0114] [5] T. Hashimoto, F. Wu, J. S. Speck, S, Nakamura, Jpn. J.
Appl. Phys. 46 (2007) L889. [0115] [6] R. Dwili ski, R. Doradzi
ski, J. Garczy ski, L. Sierzputowski, Y. Kanbara, U.S. Pat. No.
6,656,615. [0116] [7] R. Dwili ski, R. Doradzi ski, J. Garczy ski,
L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,132,730. [0117] [8]
R. Dwili ski, R. Doradzi ski, J. Garczy ski, L. Sierzputowski, Y.
Kanbara, U.S. Pat. No. 7,160,388. [0118] [9] K. Fujito, T.
Hashimoto, S, Nakamura, International Patent Application No.
PCT/US2005/024239, WO07008198. [0119] [10] T. Hashimoto, M. Saito,
S, Nakamura, International Patent Application No.
PCT/US2007/008743, WO07117689. See also US20070234946, U.S.
application Ser. No. 11/784,339 filed Apr. 6, 2007. [0120] [11] S.
Kawabata, A. Yoshikawa, JP 2007-290921. [0121] [12] M. P. D'Evelyn,
K. J. Narang, U.S. Pat. No. 6,398,867 B1. [0122] [13] D. Peters, J.
Cryst. Crowth, 104 (1990) 411. [0123] [14] T. Hashimoto, K. Fujito,
B. A. Haskell, P. T. Fini, J. S. Speck, and S, Nakamura, J. Cryst.
Growth 275 (2005) e525. [0124] [15] M. Callahan, B. G. Wang, K.
Rakes, D. Bliss, L. Bouthillette, M. Suscavage, S. Q. Wang, J.
Mater. Sci. 41 (2006) 1399. [0125] [16] T. Hashimoto, M. Saito, K.
Fujito, F. Wu, J. S. Speck, S, Nakamura, J. Cryst. Growth 305
(2007) 311.
[0126] Each of the references above is incorporated by reference in
its entirety as if put forth in full herein, and particularly with
respect to description of methods of making using ammonothermal
methods and using these gallium nitride substrates.
* * * * *