U.S. patent application number 15/472125 was filed with the patent office on 2017-07-13 for methods for producing improved crystallinity group iii-nitride crystals from initial group iii-nitride seed by ammonothermal growth.
The applicant listed for this patent is SIXPOINT MATERIALS, INC.. Invention is credited to Tadao Hashimoto, Masanori Ikari, Edward Letts.
Application Number | 20170198407 15/472125 |
Document ID | / |
Family ID | 40897460 |
Filed Date | 2017-07-13 |
United States Patent
Application |
20170198407 |
Kind Code |
A1 |
Letts; Edward ; et
al. |
July 13, 2017 |
METHODS FOR PRODUCING IMPROVED CRYSTALLINITY GROUP III-NITRIDE
CRYSTALS FROM INITIAL GROUP III-NITRIDE SEED BY AMMONOTHERMAL
GROWTH
Abstract
The present invention discloses methods to create higher quality
group III-nitride wafers that then generate improvements in the
crystalline properties of ingots produced by ammonothermal growth
from an initial defective seed. By obtaining future seeds from
carefully chosen regions of an ingot produced on a bowed seed
crystal, future ingot crystalline properties can be improved.
Specifically, the future seeds are optimized if chosen from an area
of relieved stress on a cracked ingot or from a carefully chosen
N-polar compressed area. When the seeds are sliced out, miscut of
3-10.degree. helps to improve structural quality of successive
growth. Additionally a method is proposed to improve crystal
quality by using the ammonothermal method to produce a series of
ingots, each using a specifically oriented seed from the previous
ingot. When employed, these methods enhance the quality of Group
III nitride wafers and thus improve the efficiency of any
subsequent device.
Inventors: |
Letts; Edward; (Buellton,
CA) ; Hashimoto; Tadao; (Santa Barbara, CA) ;
Ikari; Masanori; (Santa Barbara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIXPOINT MATERIALS, INC. |
Buellton |
CA |
US |
|
|
Family ID: |
40897460 |
Appl. No.: |
15/472125 |
Filed: |
March 28, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14192715 |
Feb 27, 2014 |
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15472125 |
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12455760 |
Jun 4, 2009 |
8728234 |
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14192715 |
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61058900 |
Jun 4, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/0254 20130101;
H01L 29/2003 20130101; C30B 29/403 20130101; C30B 29/406 20130101;
C30B 33/00 20130101; C30B 7/10 20130101; C30B 7/105 20130101 |
International
Class: |
C30B 7/10 20060101
C30B007/10; C30B 33/00 20060101 C30B033/00; C30B 29/40 20060101
C30B029/40 |
Claims
1. A method of fabricating a group III nitride wafer comprising: a.
growing a first ingot of group III nitride on a first seed crystal
of group III nitride by an ammonothermal method until i. a radius
of curvature of a c-plane lattice of the first ingot becomes
larger, and ii. the radius of curvature of the c-plane lattice of
the first ingot inverts from a bowing direction of the first seed
crystal along the c-plane lattice of the first seed crystal; and b.
slicing a wafer out of the first ingot and from a nitrogen polar
side of the first ingot.
2. The method of claim 1, wherein the wafer has a lattice curvature
that is convex toward the +c direction.
3. The method of claim 2, wherein the act of slicing the wafer
comprising slicing a c-plane oriented wafer with miscut angle
within 10 degrees.
4. The method of claim 1, wherein the first ingot has a N-polar
face during growth, and the N-polar face of the first ingot grows
under compression.
5. The method of claim 4, wherein the compression provides
continuous oriented growth.
6. The method of claim 4, wherein the compression prevents cracking
of the first ingot at the N-polar face during growth of the first
ingot.
7. The method of claim 6, wherein the group III nitride of the
first ingot is GaN, and the group III nitride of the first seed
crystal is GaN.
8. The method of claim 7, wherein the wafer has a lattice curvature
convex toward the +c direction.
9. The method of claim 8, wherein the act of slicing the wafer
comprising slicing a c-plane oriented wafer with miscut angle
within 10 degrees.
10. The method of claim 1, wherein the wafer is formed as a second
seed crystal for growth of a second ingot of group III nitride.
11. The method of claim 10, wherein the wafer has a lattice
curvature that is convex toward the +c direction.
12. The method of claim 11, wherein the act of slicing the wafer
comprising slicing a c-plane oriented wafer with miscut angle
within 10 degrees.
13. The method of claim 1, wherein strain in the wafer is reduced
from strain in the first seed crystal.
14. The method of claim 13, wherein the wafer has a lattice
curvature that is convex toward the +c direction.
15. The method of claim 14, wherein the act of slicing the wafer
comprising slicing a c-plane oriented wafer with miscut angle
within 10 degrees.
16. The method of claim 1, wherein the wafer has a thickness
between about 0.1 mm and about 2 mm.
17. The method of claim 1, wherein the act of slicing the wafer
comprises slicing multiple wafers out of the first ingot.
18. The method of claim 17, wherein at least one of said wafers has
a lattice curvature that is convex toward the +c direction.
19. The method of claim 18, wherein said multiple wafers are sliced
from the ingot with miscut angle within 10 degrees.
20. The method of claim 1, wherein the act of slicing the wafer
comprising slicing a c-plane oriented wafer with miscut angle
within 10 degrees.
21. A wafer formed by the method of claim 2.
22. A wafer formed by the method of claim 3.
23. A wafer formed by the method of claim 8.
24. A wafer formed by the method of claim 9.
25. A wafer formed by the method of claim 11.
26. A wafer formed by the method of claim 12.
27. A wafer formed by the method of claim 14.
28. A wafer formed by the method of claim 15.
29. A plurality of wafers formed by the method of claim 18.
30. A plurality of wafers formed by the method of claim 19.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. patent application
Ser. No. 14/192,715, filed Feb. 27, 2014, and entitled "Methods For
Producing Improved Crystallinity Group III-Nitride Crystals From
Initial Group III-Nitride Seed By Ammonothermal Growth", inventors
Edward Letts, Tadao Hashimoto, and Masanori Ikari, which is a
Divisional of U.S. patent application Ser. No. 12/455,760, filed
Jun. 4, 2009, and entitled "Methods For Producing Improved
Crystallinity Group III-Nitride Crystals From Initial Group
III-Nitride Seed By Ammonothermal Growth", with same inventors,
which claims the benefit of priority to U.S. Provisional Patent
Application No. 61/058,900, filed Jun. 4, 2008, having same
inventors, and entitled "Methods For Producing Improved
Crystallinity Group III-Nitride Crystals From Initial Group
III-Nitride Seed By Ammonothermal Growth". The entire contents of
each of the foregoing applications is incorporated by reference
herein as if put forth in full below. This application is also
related to PCT Application No. PCT/US2009/046316, filed Jun. 4,
2009, and entitled "Methods For Producing Improved Crystallinity
Group III-Nitride Crystals From Initial Group III-Nitride Seed By
Ammonothermal Growth", same inventors, the entire contents of which
are incorporated by reference herein as if put forth in full
below.
[0002] This application is 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);
[0004] 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);
[0005] 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);
[0006] 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);
[0007] 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;
[0008] which applications are incorporated by reference herein.
BACKGROUND
[0009] 1. Field of the Invention
[0010] The invention is related to the production method of group
III-nitride wafers using the ammonothermal method combined with
cutting and processing of an ingot to improve the crystal quality
from an initial group III-nitride seed.
[0011] 2. Description of the Existing Technology
[0012] (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.")
[0013] Gallium nitride (GaN) and its related group III alloys are
the key material 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 disc drives. However, the majority of these
devices are grown epitaxially on heterogeneous substrates, such as
sapphire and silicon carbide. 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.
[0014] Most of the problems inherent in heteroepitaxial growth
could be avoided by instead using homoepitaxial growth with single
crystalline group III-nitride wafers sliced from bulk group
III-nitride crystal ingots for homoepitaxy. For the majority of
devices, single crystalline GaN wafers are favored because it is
relatively easy to control the conductivity of the wafer and GaN
wafers will provide the smallest lattice/thermal mismatch with
device layers. Currently, however, the GaN wafers needed for
homogeneous growth are extremely expensive compared to
heteroepitaxial substrates. This is because it has been difficult
to grow group III-nitride crystal ingots due to their high melting
point and high nitrogen vapor pressure at high temperature. 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 using molten Ga
is a thin platelet because molten Ga has low solubility of nitrogen
and a low diffusion coefficient of nitrogen.
[0015] An ammonothermal method, which is a solution growth method
using high-pressure ammonia as a solvent, has been used to achieve
successful growth of real bulk GaN ingots [5]. Ammonothermal growth
has the potential for growing large GaN crystal ingots because
high-pressure ammonia has advantages as a fluid medium including
high solubility of source materials, such as GaN polycrystals or
metallic Ga, and high transport speed of dissolved precursors.
[0016] Currently, state-of-the-art ammonothermal method [6-8]
relies on seed crystals to produce large ingots. A lack of large
seed crystals free of strains and defects limits the growth of high
quality bulk GaN ingots with a diameter of 3'' or greater. Several
potential seeds produced by different methods exist; however the
seeds tend to be either small or defective. For instance, 2'' free
standing GaN wafers have been produced by the Hydride Vapor Phase
Epitaxy (HYPE) on sapphire or SiC substrates. Due to the large
lattice mismatch between GaN and the sapphire or SiC substrates,
the resulting GaN growth is bowed, strained and has a large defect
density. Continued growth on a free standing seed produced by HVPE
typically produces defective growth. In contrast, GaN crystals
produced by the high pressure synthesis or sodium flux method tend
to have high quality but limited size and availability. A method to
improve defective seed crystals would improve the feasibility of
producing large ingots suitable for use as substrates for
devices.
SUMMARY OF THE INVENTION
[0017] To address the problems inherent with growth on the
available defective seed, the present invention discloses a new
growth scheme including 3 different methods to improve the crystal
quality of group III-nitride crystals grown by the ammonothermal
method. Due to the lattice mismatch of GaN and typical
heteroepitaxial substrates, seed crystals produced by
heteroepitaxial methods show concave bowing of c-plane lattice
along +c direction with typical curvature radius of 1 m. However,
we discovered that subsequent growth of GaN by the ammonothermal
method on such seed crystals results in flipping over the bowing
direction. Therefore, GaN on the Ga-polar (0001) surface grows
under tensile stress while GaN on the N-polar (000-1) surface grows
under compression. The compression on the N-polar surface prevents
cracking and allows continuous oriented growth. Moreover, one can
obtain very flat crystal by choosing appropriate growth thickness
before the bowing direction flips. After group III-nitride ingots
are grown by the ammonothermal method, the ingots are sliced into
wafers whose thickness is between about 0.1 mm and about 2 mm. By
cutting from N-polar growth at the optimized position, orientation
and miscut so that the cut surface is not along the crystal face
but at an angle to the crystal face, the resulting wafer can be
used as an improved seed for subsequent growths that will then have
limited bowing and reduced stress.
[0018] By comparison, growth on the Ga-polar surface tends to
crack. Another method to obtain a seed with lower strain and bowing
is to harvest a small crack free region on the Ga-polar (0001) face
of an initial ingot in which cracking occurred. Cracking relieves
the stress in the surrounding region of growth. By harvesting one
of these localized regions of relieved stress as a seed crystal,
subsequent ingot growth would produce an improved crystal quality
compared to the initial seed crystal.
[0019] Lastly, a method is disclosed to produce seed crystal(s)
with improved crystal quality from an initial seed crystal and can
be achieved by the growth of a series of ingots each produced on a
wafer with a specific crystal orientation harvested from the
previous ingot.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0021] FIG. 1 Primary crystallographic planes of the III:nitride
wurtzite crystal lattice. On the left is the historically used
c-plane, on the right are the non-polar a-plane and m-plane.
[0022] FIG. 2 Photomicrograph of N-polar facet of GaN grown by the
ammonothermal method. No cracking was observed after 400 .mu.m of
growth on the N-polar facet. The scale bar is equal to 100
.mu.m.
[0023] FIG. 3 An exaggerated illustration of the bowing profile on
a potential seed crystal and the expected bowing profile of the
resulting growth.
[0024] FIG. 4 Photomicroscope of Ga-polar facet of GaN grown by the
ammonothermal method. Cracking was observed after 400 .mu.m of
growth on the Ga-polar facet. The scale bar is equal to 100
.mu.m.
[0025] FIG. 5 An illustration of the seed's c-plane growth
orientation before (left) and after (right) ammonothermal growth
for the first ingot in the series. The lines indicate the direction
of the wires to slice orientated wafers out of the ingot.
[0026] FIG. 6 An illustration of the seed's a-plane growth
orientation before (left) and after (right) ammonothermal growth
for the second ingot in the series. The lines indicate the
direction of the wires to slice orientated wafers out of the
ingot.
[0027] FIG. 7 An illustration of the seed's a-plane growth
orientation before (left) and after (right) ammonothermal growth
for the third ingot in the series. The lines indicate the direction
of the wires to slice orientated wafers out of the ingot.
DETAILED DESCRIPTION OF THE INVENTION
[0028] In the following description of the preferred embodiment,
reference is made to the accompanying drawings which form a part
hereof, and in which is shown by way of illustration a specific
embodiment in which the invention may be practiced. It is to be
understood that other embodiments may be utilized and structural
changes may be made without departing from the scope of the present
invention.
Technical Description of the Invention
[0029] The present invention provides a method of producing group
III-nitride wafers, primarily group III-nitride single crystalline
wafers that include at least one of the group III elements B, Al,
Ga and In, such as GaN, AlN and InN. The group III-nitride ingots
are grown by the ammonothermal method which utilizes high-pressure
NH.sub.3 as a fluid medium, nutrient containing group III elements,
and seed crystals that are group III-nitride single crystals. The
high-pressure NH.sub.3 provides high solubility of the nutrient and
high transport speed of dissolved precursors. After the group
III-nitride ingots are grown, the ingots are sliced into wafers of
thickness between about 0.1 mm and about 2 mm using conventional
means such as by mechanically sawing with a wire saw, a dicing saw,
or by laser cutting. The III-nitride crystal structure of interest
has a wurtzite crystal structure with the important facets c, m,
and a-planes shown in FIG. 1.
[0030] In one instance, a method for growing group III nitride
crystals includes:
[0031] (a) growing group III nitride ingots on original seed
crystals by the ammonothermal method;
[0032] (b) slicing wafers out of the ingots;
[0033] (c) using wafers taken from the nitrogen-polar side of the
original seed crystals as new seed crystals for subsequent growth
of ingots by the ammonothermal method.
[0034] The group III nitride may be GaN, for instance.
[0035] The original seed crystals may be formed using a
heteroepitaxial deposition process if desired.
[0036] The method may also include slicing new wafers from the
subsequently-grown ingots and using these new wafers as seeds in
subsequent ammonothermal growth of new ingots.
[0037] The method may therefore be practiced under conditions that
provide:
[0038] improvement in the bowing of crystallographic lattice along
the slicing direction as compared to the initial seed crystals;
[0039] strain in the new seed is reduced from the initial seed
crystals;
[0040] crystallinity is improved over crystallinity of the initial
seed crystals;
[0041] bowing of the crystallographic lattice along the slicing
direction is inverted from the initial seed crystal;
[0042] bowing of crystallographic lattice along the slicing
direction is improved over the initial seed crystals;
[0043] strain in the new seed is reduced from the initial seed
crystals; and/or crystallinity is improved from the initial seed
crystals.
[0044] In any of the instances above, wafers may be sliced from the
ingot in a plane misoriented from c plane of the grown crystals by
3 to 15 degrees.
[0045] The slice may be formed to provide:
[0046] bowing of crystallographic lattice along the slicing
direction is improved from the initial seed crystals;
[0047] strain in the new seed is reduced from the initial seed
crystals; and/or
[0048] crystallinity is improved from the initial seed
crystals.
[0049] An additional method for growing group III nitride crystals
involves:
[0050] (a) growing group III-nitride ingots on original seed
crystals by the ammonothermal method until some cracking
occurs;
[0051] (b) separating a crack free region out of the ingots;
and
[0052] (c) using the separated region as a new seed for subsequent
growth of an ingot.
[0053] The group III nitride may be e.g. GaN.
[0054] The original seed crystals may optionally be formed using a
heteroepitaxial deposition process for forming group III-nitride
crystals such as GaN.
[0055] The method may additionally include slicing new wafers from
the subsequently-grown ingots and using these new wafers as seeds
in a subsequent ammonothermal growth of new ingots.
[0056] Any of these methods may be performed under conditions
wherein:
[0057] the bowing of crystallographic lattice along the slicing
direction is improved from the initial seed crystals;
[0058] strain in the new seed is reduced from strain in the initial
seed crystals; and/or
[0059] crystallinity is improved over the crystallinity of the
initial seed crystals.
[0060] Wafers may be sliced from the ingot along a plane
misoriented from c plane by 3 to 15 degrees, and optionally the
wafers may be used as new seed material in ammonothermal growth of
new ingots.
[0061] A third method of growing group III-nitride crystals may
include:
[0062] (a) growing ingots (e.g. a first ingot) on c-facets of seed
crystals (e.g. a first seed crystal) by the ammonothermal method to
a thickness greater than 5 mm;
[0063] (b) slicing the ingots (e.g. the first ingot) along the
a-plane or a semi-polar plane to form seeds (e.g. a second seed
crystal);
[0064] (c) using the a-plane or semi-polar plane seeds (e.g. the
second seed crystal) to grow new ingots (e.g. a second ingot);
[0065] (d) slicing the new ingots (e.g. the second ingot) along the
a-plane or the semi-polar plane; and
[0066] (e) using a-plane or semi-polar plane wafers not containing
any original material of the initial seed crystal (e.g. third seed
crystal) to grow additional new ingots (e.g. a third ingot).
[0067] The method may be practiced using only a-plane slices or
only semi-polar plane slices, or the method may be performed by
using one slicing direction for one ingot and another slicing
direction for a subsequent ingot.
[0068] The group III-nitride may be e.g. GaN.
[0069] The method in any of these instances may further include
slicing an ingot obtained in step (e) above to produce c-plane
wafers.
[0070] The method may be performed under conditions where:
[0071] bowing of crystallographic lattice along the slicing
direction is improved from the initial seed crystals;
[0072] strain in the new seed is reduced from the initial seed
crystals; and/or
[0073] crystallinity is improved from the initial seed
crystals.
[0074] GaN wafers may be produced in which the c-plane lattice bows
convexly in the +c direction.
[0075] These GaN wafers may have a basal plane that is c-plane and
miscut within 10 degrees.
[0076] The GaN wafers may have a basal plane that is m-plane and
miscut within 10 degrees.
[0077] The GaN wafers may have a basal plane that is a-plane and
miscut within 10 degrees.
[0078] The following additional detailed explanation describes
detailed procedures to aid in further understanding of the
invention.
[0079] Method 1
[0080] A reaction vessel with an inner diameter of 1 inch was used
for the ammonothermal growth. All necessary sources and internal
components were loaded together with the reaction vessel into a
glove box. In one growth occasion, these components included 10 g
of polycrystalline GaN nutrient held in a Ni mesh basket, 0.34
mm-thick single crystalline c-plane GaN seeds, and six baffles to
restrict flow. The initial GaN seed was produced by HVPE on
sapphire which caused the seed crystal to be bowed and strained.
The glove box is filled with nitrogen, and the oxygen and moisture
concentration was maintained at less than 1 ppm. Since the
mineralizers are reactive with oxygen and moisture, the
mineralizers were stored in the glove box all the time. 4 g of
as-received NaNH.sub.2 was used as a mineralizer. After loading
mineralizer into the reaction vessel, six baffles together with
seeds and nutrient were loaded. After closing the lid of the
reaction vessel, the reaction vessel was taken out of the glove
box. Then, the reaction vessel was connected to a gas/vacuum
system, which can pump down the vessel as well as can supply
NH.sub.3 to the vessel. First, the reaction vessel was evacuated
with a turbo molecular pump to achieve a pressure of less than
1.times.10.sup.-5 mbar. The actual pressure achieved for this
example was 1.2.times.10.sup.-6 mbar. In this way, residual oxygen
and moisture on the inner wall of the reaction vessel were
partially removed. After this, the reaction vessel was chilled with
liquid nitrogen and NH.sub.3 was condensed in the reaction vessel.
About 40 g of NH.sub.3 was charged in the reaction vessel. After
closing the high-pressure valve of the reaction vessel, the
reaction vessel was transferred to a two zone furnace. The reaction
vessel was heated to 510.degree. C. in the crystallization zone and
550.degree. C. in the dissolution zone for the first 24 hrs before
being to adjusted to 575.degree. C. in the crystallization zone and
510.degree. C. in the dissolution zone. After 8 days, ammonia was
released and the reaction vessel was opened. The total thickness of
the grown GaN ingot was 1.30 mm.
[0081] Microscope images of the growth on the Ga-polar surface
showed cracking while the N-polar surface showed no cracking and a
relatively flat surface, see FIG. 2. Crystal structure measured on
the N-polar surface showed a single peak from 002 reflection. The
Full Width Half Max (FWHM) of the peak was 209 arcsecs. On the
other hand, the Ga-polar surface showed multiple sharp peaks from
002 reflections with FWHM of 2740 arcsec. The multiple sharp peaks
from Ga-polar side represent a gathering of high-quality grains.
This difference in growth on the different polarities is caused by
the bowing of the seed crystal, as diagrammed in FIG. 3. Bowing of
the seed crystal causes the growth on the Ga-polar surface to be
under tensile strain and prone to cracking while the growth on the
N-polar surface is under compressive strain which prevents cracking
of the growth.
[0082] The bowing profile was improved in the N-polar growth
compared to the initial seed bowing profile, as shown in FIG. 3. In
one growth occasion, the radius of lattice bowing on N-polar side
was improved to 130 m (convex) from 1.15 m (convex), which was the
original radius of lattice bowing of the seed.
[0083] By harvesting the N-polar growth as a seed for future
ingots, problems associated with bowing may be minimized allowing
subsequent crack free growth on the Ga-polar surface as well. In
addition, optimization of the growth thickness should yield
improved crystallinity for future ingots.
[0084] It was also confirmed that using miscut substrates as seed
crystals helps to improve crystal quality. In one growth occasion,
ammonothermal growth was conducted with two kinds of miscut seeds,
one with 7.degree. off from the c-plane and the other with
3.degree. off from the c-plane. The FWHM of X-ray rocking curve
from 002 reflection of the original seeds were 605 arcsec and 405
arcsec for 7.degree. off and 3.degree. off, respectively. After
growth, the FWHM of X-ray rocking curve became 410 arcsec and 588
arcsec for 7.degree. off and 3.degree. off, respectively. From this
result, it was confirmed that miscut as much as approximately
7.degree. helps improve structural quality. Miscut could be up to
10.degree. or 15.degree. off axis rather than up to 3.degree. or up
to 7.degree. off axis.
[0085] Method 2
[0086] With similar growth condition as indicated for Method 1, a
GaN ingot as thick as 1.3 mm was obtained after 8 day-growth.
Microscope images of the growth on the Ga Polar surface showed
cracking as shown in FIG. 4. while the N-polar surface showed no
cracking and a relatively flat surface. As explained for Method 1,
the crystal on Ga-polar side consists of many high-quality grains.
Therefore, it is expected that after cracking occurs, harvesting a
relaxed region of the growth on the Ga-polar surface as a seed
crystal would enable future ingots to exhibit improved
crystallinity from the initial seed crystal. Harvested regions are
expected to have a Ga-polar surface area between about 0.1 mm.sup.2
and about 5.0 mm.sup.2.
[0087] Method 3
[0088] The ammonothermal growth technique discussed above can be
used to produce a series of ingots and by selecting specific
regions with a crystallographic orientations for subsequent seeds,
the crystallinity of III-nitride material can be improved. Starting
with an imperfect c-plane seed crystal, the first ingot primary
growth direction is along the c-axis, as shown in FIG. 5. Due to
cracking problems the growth on the Ga-polar surface may not be
suitable for continued growth. The first ingot is then sliced using
a wire saw to produce a-plane wafers. Using an a-plane wafer as a
seed, a new ingot is then produced by the ammonothermal growth
techniques as shown in FIG. 6. The second ingot is then sliced
using a wire saw to produce a-plane wafers. By choosing a wafer
which contains no initial seed crystal as the new seed, a third
ingot can be produced which contains none of the initial seed
crystal, as shown in FIG. 7. This third ingot can then be sliced
with a wire saw in any given orientation to produce seed crystals
of improved crystallinity.
[0089] This method promotes growth by limiting the size and effect
of the dislocations, bowing, and strain of the seed. This method
realizes bulk crystal growth with very low threading dislocations
densities and an improved bowing profile. This method can be
modified to use a semipolar or m-plane growth instead of the
a-plane orientation.
Advantages and Improvements
[0090] The present invention disclosed new production methods of
group III-nitride wafers with improved crystal structure. Using
several possible strategies, specific regions of a grown ingot may
be harvested as a future seed to drastically improve the quality of
future ingots compared to the initial seed. Additionally, a method
is proposed to produce a series of ingots that could produce an
drastic improvement of crystalline quality. These improvement would
improve efficiencies for any optical devices fabricated on the
wafers.
REFERENCES
[0091] The following references are incorporated by reference
herein: [0092] [1]. S. Porowski, MRS Internet Journal of Nitride
Semiconductor, Res. 4S1, (1999) G1.3. [0093] [2] T. Inoue, Y. Seki,
O. Oda, S. Kurai, Y. Yamada, and T. Taguchi, Phys. Stat. Sol. (b),
223 (2001) p. 15. [0094] [3] M. Aoki, H. Yamane, M. Shimada, S.
Sarayama, and F. J. DiSalvo, J. Cryst. Growth 242 (2002) p. 70.
[0095] [4] T. Iwahashi, F. Kawamura, M. Morishita, Y. Kai, M.
Yoshimura, Y. Mori, and T. Sasaki, J. Cryst Growth 253 (2003) p. 1.
[0096] [5] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, Jpn. J.
Appl. Phys. 46 (2007) L889. [0097] [6] R. Dwili ski, R. Doradzi
ski, J. Garczy ski, L. Sierzputowski, Y. Kanbara, U.S. Pat. No.
6,656,615. [0098] [7] K. Fujito, T. Hashimoto, S. Nakamura,
International Patent Application No. PCT/US2005/024239, WO07008198.
[0099] [8] T. Hashimoto, M. Saito, S. Nakamura, International
Patent Application No. PCT/US2007/008743, WO07117689. See
US20070234946, U.S. application Ser. No. 11/784,339 filed Apr. 6,
2007.
[0100] 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 growth using ammonothermal
methods and using gallium nitride substrates.
CONCLUSION
[0101] This concludes the description of the preferred embodiment
of the invention. The following describes some alternative
embodiments for accomplishing the present invention.
[0102] 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.
[0103] In the preferred embodiment specific growth apparatuses and
slicing apparatus are presented. However, other constructions or
designs that fulfill the conditions described herein will have the
same benefit as these examples.
[0104] The present invention does not have any limitations on the
size of the wafer, so long as the same benefits can be
obtained.
[0105] 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.
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