U.S. patent application number 15/452700 was filed with the patent office on 2017-06-22 for method for producing group iii nitride wafers and group iii nitride wafers.
The applicant listed for this patent is SIXPOINT MATERIALS, INC.. Invention is credited to Tadao Hashimoto, Masanori Ikari, Edward Letts.
Application Number | 20170175295 15/452700 |
Document ID | / |
Family ID | 40602704 |
Filed Date | 2017-06-22 |
United States Patent
Application |
20170175295 |
Kind Code |
A1 |
Hashimoto; Tadao ; et
al. |
June 22, 2017 |
METHOD FOR PRODUCING GROUP III NITRIDE WAFERS AND GROUP III NITRIDE
WAFERS
Abstract
The present invention discloses a method of removing contaminant
from group III nitride single-crystal wafers. The method involves
annealing a wafer to concentrate a contaminant in a region of the
crystal near the surface of the crystal and removing some of the
crystal near the surface that contains at least a portion of the
region containing concentrated contaminant. The resultant thinner
wafer therefore has less contaminant in it.
Inventors: |
Hashimoto; Tadao; (Santa
Barbara, CA) ; Letts; Edward; (Buellton, CA) ;
Ikari; Masanori; (Santa Barbara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIXPOINT MATERIALS, INC. |
Buellton |
CA |
US |
|
|
Family ID: |
40602704 |
Appl. No.: |
15/452700 |
Filed: |
March 7, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12392960 |
Feb 25, 2009 |
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15452700 |
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61067117 |
Feb 25, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 29/403 20130101;
C30B 7/10 20130101; C30B 7/105 20130101; C30B 29/406 20130101; C30B
33/02 20130101 |
International
Class: |
C30B 33/02 20060101
C30B033/02; C30B 7/10 20060101 C30B007/10; C30B 29/40 20060101
C30B029/40 |
Claims
1. A method of removing a contaminant from a group III nitride
single-crystalline wafer comprising: a. annealing the wafer, which
has a N-face and a group III-face, in an ambient containing ammonia
vapor for a time at a temperature and pressure that concentrate a
contaminant toward the group III-polar face, thereby forming a
region of concentrated contaminant in the group III-polar face; and
b. removing a sufficient amount of the crystalline wafer at the
group III-polar face to reduce a thickness of the crystalline wafer
and to remove at least a portion of the region of concentrated
contaminant; c. wherein the contaminant is selected from an alkali
metal and an alkaline earth metal.
2. The method of claim 1, wherein the contaminant comprises at
least one of sodium and lithium.
3. The method of claim 1, wherein the contaminant comprises
potassium.
4. The method of claim 1, wherein the contaminant comprises
magnesium.
5. The method of claim 1, wherein sufficient ammonia vapor is
present to suppress decomposition of the wafer.
6. The method of claim 1, wherein the contaminant is positively
charged.
7. The method of claim 1, wherein the annealing conditions also
reduce a concentration of a heavy metal at both the N face and the
group III face.
8. The method of claim 7, wherein the heavy metal is selected from
the group consisting of Ti, Cr, Fe, Ni, and Co.
9. The method of claim 1, wherein the annealing is performed at a
temperature greater than or equal to 1100.degree. C.
10. The method of claim 1, wherein the annealing is performed at a
temperature greater than or equal to 1200.degree. C.
11. The method of claim 1, wherein the annealing is performed at a
sufficiently high temperature to reduce a number of point defects
in the wafer.
12. The method of claim 1, wherein the group III nitride
single-crystalline wafer comprises gallium.
13. The method of claim 1, wherein the group III nitride
single-crystalline wafer was formed using an ammonothermal
process.
14. A single-crystalline wafer formed by an ammonothermal process
having, at an N-face of the wafer, a concentration of lithium about
equal to or less than 4.times.10.sup.13 or a concentration of
potassium about equal to or less than 3.times.10.sup.14
cm.sup.-3.
15. A single-crystalline wafer formed by an ammonothermal
processing having, at an N-face of the wafer, a concentration of
chromium about equal to or less than 6.times.10.sup.14 cm.sup.-3 or
a concentration of nickel of about equal to or less than
1.times.10.sup.16 cm.sup.-3.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 12/392,960, filed Feb. 25, 2009, and entitled
"Method for Producing Group III Nitride Wafers and Group III
Nitride Wafers," inventors Tadao Hashimoto, Edward Letts, and
Masanori Ikari, which claims the benefit of priority to U.S.
Application Ser. No. 61/067,117, filed Feb. 25, 2008, and entitled
"Method for Producing Group III Nitride Wafers and Group III
Nitride Wafers," inventors Tadao Hashimoto, Edward Letts, and
Masanori Ikari, the entire contents of each of which are
incorporated by reference herein as if put forth in full below.
This application also incorporates by reference the International
Application WO2009/108700 filed Feb. 25, 2009, entitled "Method for
Producing Group III Nitride Wafers and Group III Nitride Wafers,"
inventors Tadao Hashimoto, Edward Letts, and Masanori Ikari.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The invention is related to the production of group III
nitride wafers using the ammonothermal method.
[0004] 2. Further Information on Group III-Nitride Materials and
Manner of Making
[0005] 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 discs. However, a 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.
[0006] 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 a 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 smallest lattice/thermal mismatch with
device layers. However, due to high melting point and high nitrogen
vapor pressure at high temperature, it has been difficult to grow
group III nitride crystal ingots. Growth methods using molten Ga,
such as high-pressure high-temperature synthesis ([1] S. Porowski,
MRS Internet Journal of Nitride Semiconductor, Res. 4S1, (1999)
G1.3; [2] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, and T.
Taguchi, Phys. Stat. Sol. (b), 223 (2001) p. 15) and sodium flux
([3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, and F. J.
DiSalvo, J. Cryst. Growth 242 (2002) p. 70; [4] T. Iwahashi, F.
Kawamura, M. Morishita, Y. Kai, M. Yoshimura, Y. Mori, and T.
Sasaki, J. Cryst Growth 253 (2003) p. 1), have been proposed to
grow GaN crystals, nevertheless the crystal shape grown in molten
Ga favors thin platelet formation because molten Ga has low
solubility of nitrogen and a low diffusion coefficient of
nitrogen.
[0007] An ammonothermal method, which is a solution growth method
using high-pressure ammonia as a solvent, has demonstrated
successful growth of bulk GaN ingots ([5] T. Hashimoto, F. Wu, J.
S. Speck, S. Nakamura, Jpn. J. Appl. Phys. 46 (2007) L889). This
newer technique called ammonothermal growth has a potential for
growing large GaN crystal ingots, because high-pressure ammonia
used as a fluid medium has high solubility of source materials,
such as GaN polycrystals or metallic Ga, and has high transport
speed of dissolved precursors. However, state-of-the-art
ammonothermal method ([6] R. Dwili ski, R. Doradzi ski, J. Garczy
ski, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615; [7] K.
Fujito, T. Hashimoto, S. Nakamura, International Patent Application
No. PCT/US2005/024239, WO07008198; [8] 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), can only produce brownish crystals.
This coloration is mainly attributed to impurities. In particular,
oxygen, carbon and alkali metal concentration of the sliced wafers
from GaN ingots is extremely high. The brownish wafer shows large
optical absorption, which deteriorates the efficiency of light
emitting devices grown on such wafers.
[0008] 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.
SUMMARY OF THE INVENTION
[0009] The present invention provides a new production method for
group III nitride wafers. In one embodiment of the invention, after
group III nitride ingots are grown by the ammonothermal method, the
ingots are sliced into pieces such as wafers having a thickness
between about 0.1 mm and about 1 mm, for instance. Then, the pieces
are annealed in a manner that improves transparency of the pieces
and avoids dissociation and/or decomposition of the pieces. A
surface portion of the pieces may then be removed if desired.
[0010] Resultant pieces such as wafers may differ from other
ingot-derived or individually-grown pieces or wafers in their (1)
transparency and (2) in their amount and/or distribution of
impurities or their surface morphology resulting from having a
surface layer removed, or both.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0012] FIG. 1 is a flow chart for the production of group III
nitride wafers;
[0013] FIG. 2 is an example of the ammonothermal growth apparatus.
1 reaction vessel, 2 lid, 3 gasket, 4 heater for the dissolution
region, 5 heater for the crystallization region, 6
convection-restricting device, 7 group III-containing nutrient, 8
nutrient basket, 9 group III nitride seed crystals;
[0014] FIG. 3 is the concentration of heavy metal impurities before
and after the annealing in Example 1 measured by secondary ion mass
spectroscopy (SIMS). The unit of the concentration is
atoms/cm3.
[0015] FIG. 4 is the concentration of light metal impurities before
and after the annealing in Example 1 measured by secondary ion mass
spectroscopy (SIMS). The unit of the concentration is
atoms/cm.sup.3.
DETAILED DESCRIPTION OF THE INVENTION
[0016] 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 Aspects of the Invention
[0017] The present invention provides a method of producing group
III (group 13) 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 process
flow for one embodiment of the invention is indicated in FIG. 1. A
group III nitride ingot is grown by the ammonothermal method, the
ingot is cut into pieces, and the pieces are annealed preferably in
a manner which limits or avoids dissociation and decomposition of
piece surfaces. The pieces may be wafers as are typically used to
form various semiconductor or optoelectronic devices, such as LEDs,
laser diodes, solar cells, and photodetectors.
[0018] Ammonothermal Growth of Group III-Nitride Ingot
[0019] Ammonothermal ingot growth utilizes high-pressure NH.sub.3
as a fluid medium, a nutrient containing at least one of the group
III elements, and one or more 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. FIG. 2 shows an example of one ammonothermal reaction
vessel in which the method may be carried out.
[0020] Methods of ammonothermal growth are discussed in WO08/51589
and in U.S. application Ser. No. 11/977,661, the contents of which
are incorporated by reference in their entirety herein as if put
forth in full below. The growth medium may therefore optionally
contain a mineralizer. Basic mineralizers include Li, Na, K,
LiNH.sub.2, NaNH.sub.2, and/or KNH.sub.2. Acidic mineralizers
include NH.sub.4F, NH.sub.4Cl, NH.sub.4Br, and/or NH.sub.4I.
[0021] Other ammonothermal growth methods may be used, such as
those discussed in WO07/08198 and WO07/117689, which are
incorporated by reference in their entirety herein as if put forth
in full below.
[0022] The group III-nitride ingot (such as GaN) may be in the
wurtzite crystal configuration or in the zincblende crystal
configuration, for example.
[0023] Ingot Slicing
[0024] After a group III nitride ingot is grown, the ingot is
sliced into wafers or crystalline pieces of other shape(s). An
ingot may be sliced by any suitable equipment or method. A cutter
such as a mechanical saw (e.g. a wire saw), a dicing saw, or a
laser may be used. Wafers cut from the ingot may have a thickness
between about 0.1 mm and about 1 mm, for instance. Wafers or other
ingot pieces may be cut from an ingot along a Group III element
face of the crystal (e.g. Ga face of the crystal, (0001) face,
(000-1) face, {11-20} face, {10-10} face or other low index
planes.
[0025] Annealing Wafer or Other Piece
[0026] Wafers or other ingot pieces are annealed to improve
transparency and reduce impurities, preferably in a manner that
limits or avoids substantial dissociation or decomposition of the
pieces. Once pieces are cut from an ingot, those pieces may be
individually or collectively annealed in an annealing reactor.
[0027] An annealing reactor may be configured to expose all
surfaces of the piece (e.g. wafer) to an annealing gas in an
annealing environment if desired. The annealing gas may be ammonia,
hydrogen, a mixture of hydrogen and ammonia, or other gas that may
create a reducing environment. While not being bound by theory, it
is postulated that a reducing gas either maintains the pieces
intact, without substantial degradation or decomposition or reacts
with contaminants in the crystalline pieces, making the
contaminants volatile and thereby removing the contaminants from
the pieces, or both. The annealing gas may alternatively be an
inert gas such as nitrogen, or the annealing gas may be a mixture
of nitrogen, ammonia, and/or hydrogen and/or other gas that may
create a reducing environment.
[0028] The annealing temperature may be selected to remove the
amount of contaminant desired for removal. The temperature is
sufficiently high to cause contaminants to migrate within the piece
or pieces being annealed. For GaN, the temperature is often between
about 500 and 1300.degree. C. Typically, the temperature is at or
above 500, 700, 800, 900, 1000, 1100, or 1200.degree. C. in ambient
gas comprising NH.sub.3 and H.sub.2. At about 1300.degree. C., the
pieces being annealed may decompose or etch somewhat. Consequently,
it may be desirable to anneal at a temperature of no more than
about 1300.degree. C. in NH.sub.3 and H.sub.2 ambient. An annealing
temperature of 1200.degree. C. works well.
[0029] The pieces are annealed for a sufficient length of time to
remove contaminants to a desired concentration in the pieces.
Pieces may be annealed for at least about 15, 30, 45, or 60
minutes, for instance. If a lower temperature is used, often pieces
may be annealed for a longer period of time to reduce contaminant
concentration to the desired level. Although the annealing time
depends on annealing temperature, the length of time that the
pieces or ingot are annealed is sufficiently long to remove
contaminants but not too long in order to avoid substantial
degradation of crystal quality.
[0030] It has been observed that certain contaminants such as
alkali metals concentrate at a Ga face of the crystal. Likewise,
alkaline earth metals may concentrate at a Ga face. The annealing
gas may be preferentially directed at the Ga face in order to
reduce the concentration of these contaminants in the crystalline
pieces or ingot. The length of time and annealing temperature may
be selected based on the high concentration of these contaminants
at a Ga face, and therefore the annealing conditions such as
temperature and time may be different than for the case where these
contaminants are dispersed throughout the crystalline pieces.
[0031] Annealing is typically carried out at atmospheric pressure
(i.e. 1 bar). If the annealing temperature is close to the
dissociation temperature, annealing can be carried out under
pressure, for instance at or above 10 bar, 100 bar, or 1000 bar. On
the other hand, if major contaminants are less volatile material,
annealing can be carried out at subatmospheric or low pressure, for
instance at or below 100 mbar, 10 mbar, 1 mbar, or less.
[0032] Ingot Annealing
[0033] A similar method can be utilized on the ingot itself. The
ingot may be annealed in an annealing environment that limits or
avoids substantial ingot dissociation or decomposition. The ingot
may be annealed in addition to annealing pieces such as wafers cut
from the ingot, or the ingot may be annealed instead of annealing
its pieces.
[0034] Optional Surface Removal
[0035] Impurities may be further reduced by removing a surface
layer of the wafers to which impurities have migrated. In one
instance, at least a portion of a Group III element surface layer
is removed (e.g. a Ga surface layer). Subsequent to ingot
annealing, an outer surface or layer of the ingot may optionally be
removed to reduce the concentration of the impurities in the
ingot.
[0036] In some instances, any of the methods as discussed above
reduce the concentration of heavy metals such as Ti, Cr, Fe, Ni,
and Co (each metal alone or any combination of these heavy
metals).
[0037] In some instances, any of the methods as discussed above
reduce the concentration of alkali or alkaline earth metals such as
Li, Na, Mg, and K (each metal alone or any combination of these
metals). A portion of a surface layer may be removed that contains
these metals, especially a Ga surface layer of the crystal of the
ingot or wafer. The amount of surface of wafers or ingot that may
be removed can vary depending upon how much impurity can be
tolerated in the wafer or ingot during use.
[0038] Further Considerations
[0039] The amount of impurity may be reduced by any method above to
no more than about 60%, 50%, 40%, 30%, 20%, 15%, 10%, or 5% of the
concentration of that impurity at a face (e.g. Ga or other group
III element or nitride face) prior to annealing. Annealing may
reduce the level of the contaminant to below a detectable level.
See in particular the tables in FIGS. 3 and 4, which indicate how
much the concentration was reduced for various contaminants at the
Group III element face and the nitride face. The concentration of
contaminants was measured with secondary ion mass spectroscopy
(SIMS).
[0040] The contaminant removed may be a metal. Alkali and alkaline
earth metals may be removed in certain embodiments. Likewise, heavy
metals selected from transition metals (e.g. Ti, Cr, Fe, Ni, and
Co), metalloids such as Ge or heavier, rare earth metals, and other
metals having similar atomic weight may be removed. The
concentration of heavy metal impurities such as Ti, Cr, Fe, Ni or
Co may be less than about 1.times.10.sup.17 cm.sup.-3 after an
ingot or pieces are treated according to a method herein. The
concentration of light metals (metals such as Li, Na, K, and Mg)
may likewise be less than about 1.times.10.sup.17 cm.sup.-3 after
an ingot or pieces are treated according to a method herein.
[0041] The annealing above differs from annealing after e.g.
implanting a crystalline material with a dopant. Typically a
substrate is annealed after implanting dopant atoms in order to
diffuse the atoms to a certain depth and therefore decrease the
concentration of the dopant in the substrate at the point of
implantation. In the method of the invention, impurities may be
concentrated locally (such as at a group III element face of the
crystalline structure) rather than diffused, and/or impurities may
be removed from the substrate by annealing it.
[0042] The annealed pieces or ingot may be used to form various
electronic or optoelectronic devices. Electronic and optoelectronic
devices include those disclosed in U.S. application Ser. No.
11/765,629, filed Jun. 20, 2007 and entitled "Opto-Electronic and
Electronic Devices Using N-Face or M-Plane GaN Substrate Prepared
With Ammonothermal Growth", the contents of which are incorporated
by reference herein in their entirety as if put forth in full
below.
[0043] The following examples describe a detailed procedure within
the scope of the current invention to help illustrate the invention
further.
Example 1
[0044] In this example, a reaction vessel having an inner diameter
of 1 inch was used for the ammonothermal growth. All necessary
sources and internal components including 10 g of polycrystalline
GaN nutrient held in Ni mesh basket, 0.3 mm-thick single
crystalline GaN seeds, and three baffles, which acts as a flow
restriction device were loaded into a glove box together with the
reaction 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 all the time. 4 g of
as-received NaNH.sub.2 was used as a mineralizer. After loading
mineralizer into the reaction vessel, three baffles together with
seeds and nutrient are loaded. After closing the lid of the
reaction vessel, it was taken out of the glove box. Then, the
reaction vessel is 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 pressure less than 1.times.10.sup.-5 mbar. The
actual pressure in 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 are 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, it was transferred to a two zone furnace. The
reaction vessel was heated to 575.degree. C. of the crystallization
zone and 510.degree. C. for the dissolution zone. After 7 days,
ammonia was released and the reaction vessel was opened. The total
thickness of the grown GaN ingot was 0.99 mm.
[0045] Since the thickness of the ingot was less than 1 mm, the
ingot shape was already wafer-like without slicing. The wafer-like
ingot had Ga-polar (0001) surface and N-polar (000-1) surface as
basal planes. The wafer-shaped ingot was then loaded into an
annealing reactor. The wafer-shaped ingot stood on its edge so that
the both sides of the basal planes were exposed to the gas stream.
After evacuating the air in the reactor, forming gas (4%
H.sub.2/96% N.sub.2) was introduced to the reactor. Then, the
reactor was heated. At 485.degree. C., ammonia was introduced to
the reactor to suppress dissociation or decomposition. The flow
rate of ammonia and the forming gas was 1 slm and 1.1 slm,
respectively. The wafer-shaped ingot was annealed at 1100.degree.
C. for about 1 hour. Then, the reactor was cooled. At about
400.degree. C., ammonia was shut off.
[0046] The coloration in a wafer-like ingot prepared as described
above was observably reduced when its coloration was compared to
the coloration of a wafer-like ingot that was not annealed. This
reduction in coloration in the annealed ingot indicates the
reduction of impurities. The impurity quantification by secondary
ion mass spectroscopy (SIMS) confirmed reduction of heavy metals
such as Ti, Cr, Fe, Ni, and Co as shown in FIG. 3. On the other
hand, light metals such as alkali metals and alkali earth metals
moved toward Ga-polar surface. As shown in FIG. 4, the
concentration of Li, Na, Mg, and K increased after annealing on the
Ga-polar side whereas it decreased on the N-polar side. This
suggests that alkali metals and alkali earth metals are positively
charged and they are attracted by the surface charge on the
Ga-polar surface, resulting in accumulation of these impurities on
the top Ga-polar surface. Therefore, we can efficiently remove
alkali metals and alkali earth metals from the wafer with annealing
followed by removing a portion of the Ga-polar surface by e.g.
grinding, lapping, polishing, or etching the Ga-polar surface.
Example 2
[0047] In this example a GaN ingot was formed by the same method as
described in Example 1. The GaN ingot was sliced into 0.4 mm-thick
wafers with a wire saw. Then 6 wafers were annealed at different
temperatures (500, 700, 900, 1100, 1200, and 1300.degree. C. in
NH.sub.3 ambient for 1 hour) by the following process.
[0048] A wafer was placed into a reactor. After evacuating air in
the reactor, a forming gas (4% H.sub.2/96% N.sub.2) was introduced
into the reactor, and subsequently the reactor was heated. At
485.degree. C., ammonia was introduced to the reactor to suppress
dissociation or decomposition of the GaN. The flow rate of ammonia
and the forming gas was 1 slm and 1.1 slm, respectively. During
annealing both the Ga-face and the N-face of each wafer was exposed
to the ambient gas. When the GaN wafer was annealed at 1300.degree.
C., the surface of the wafer was etched away. Therefore, if ammonia
is used to suppress dissociation or decomposition, the temperature
is typically less than 1300.degree. C. to avoid surface etching
during annealing.
[0049] Properties of wafers annealed by the method above were
compared to an unannealed wafer. Each wafer has three regions: a
Ga-face region; a seed region; and a N-face region from the left in
each wafer. The unannealed wafer had a dark N-face region and a
slightly tinted Ga-face region together with a clear seed region.
The coloration on the Ga-face was reduced with annealing even at
500.degree. C. Slight reduction of coloration was observed for
wafers annealed at 500, 700, 900, and 1100.degree. C. Annealing at
1200.degree. C. made a drastic change: the N-face region showed
much brighter color although the seed region and the Ga-face region
showed slight coloration. Therefore, annealing at 1200.degree. C.
is effective for N-face region.
[0050] The following theory is of course not limiting on the scope
of the invention. The difference in the coloration in the seed
region and the Ga-face region implies that coloration is not only
governed by an impurity, but also by native defects such as point
defects. From the color change in the N-face region, it is believed
that some impurities diffused out from the N-face region, thus the
N-face region acted as an impurity source. The seed region, which
was closer to the N-face region must have higher impurity
concentration than Ga-face region which is farther from the N-face
region. Therefore, if coloration is only due to impurity
concentration, one might expect the seed region to be darker than
the Ga-face region. However, the seed region was brighter than the
Ga-face region. This implies that the Ga-face region originally had
higher defects which, when combined with an impurity, will act as a
color center. Therefore, it appears to be desirable to reduce
native defects such as point defects in the ammonothermally grown
group III-nitride crystals.
[0051] From this example, we found that annealing in ammonia is
preferably performed at a temperature less than 1300.degree. C.
when surface etching of GaN is not desired, preferably between 500
and 1300.degree. C., or more preferably between 1100 and
1300.degree. C. The pressure may be about 1 bar, or the pressure
may be sub-atmospheric or above atmospheric pressure as discussed
above.
Advantages and Improvements
[0052] The present invention provides a new production method for
group III nitride wafers with improved transparency and purity.
Annealing the wafers after slicing is an effective way to reduce
impurities in the crystal since the necessary time for the
impurities to diffuse out of the crystal can be much smaller than
the situation of annealing the ingot before slicing. The purified
wafer showed improved transparency which improves efficiencies of
optical devices fabricated on the wafers.
CONCLUSION
[0053] This concludes the description of the preferred embodiment
of the invention. The following describes some alternative
embodiments for accomplishing the present invention.
[0054] 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.
[0055] Although the preferred embodiment describes the annealing of
ingots or wafers in ammonia ambient, other method to avoid
dissociation or decomposition can be used. For example, covering
the wafer surface with a silicon oxide layer, a silicon nitride
layer, a metal layer or other protective layer is expected to be
effective way to avoid dissociation and decomposition of the wafer.
One or more of these layers may be deposited on a wafer using e.g.
chemical vapor deposition or sputtering. If desired, the protective
layer or layers may be removed using conventional etching
techniques immediately before using a wafer to form a device.
[0056] Although the preferred embodiment describes the annealing at
1100-1200.degree. C. for 1 hour or other time sufficient to improve
wafer clarity, other temperatures and/or times can be utilized so
long as the same or similar benefit can be obtained.
[0057] In the preferred embodiment specific growth apparatuses and
annealing apparatus are presented. However, other constructions or
designs that fulfill the conditions described herein will have the
same benefit as these examples.
[0058] The present invention does not have any limitations on the
size of the wafer, so long as the same benefits can be
obtained.
[0059] 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.
* * * * *