U.S. patent application number 11/134200 was filed with the patent office on 2009-02-26 for method for achieving low defect density algan single crystal boules.
This patent application is currently assigned to Freiberger Compound Materials GmbH. Invention is credited to Vladimir DMITRIEV, Vladimir IVANTSOV, Yuri MELNIK, Vitali SOUKHOVEEV, Katie TSVETKOV.
Application Number | 20090050913 11/134200 |
Document ID | / |
Family ID | 25416850 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090050913 |
Kind Code |
A2 |
MELNIK; Yuri ; et
al. |
February 26, 2009 |
METHOD FOR ACHIEVING LOW DEFECT DENSITY ALGAN SINGLE CRYSTAL
BOULES
Abstract
A method for growing bulk GaN and AlGaN single crystal boules,
preferably using a modified HVPE process, is provided. The single
crystal boules typically have a volume in excess of 4 cubic
centimeters with a minimum dimension of approximately 1 centimeter.
If desired, the bulk material can be doped during growth to achieve
n-, i-, or p-type conductivity. In order to have growth cycles of
sufficient duration, preferably an extended Ga source is used in
which a portion of the Ga source is maintained at a relatively high
temperature while most of the Ga source is maintained at a
temperature close to, and just above, the melting temperature of
Ga. To grow large boules of AlGaN, preferably multiple Al sources
are used, the Al sources being sequentially activated to avoid Al
source depletion and excessive degradation. In order to achieve
high growth rates, preferably a dual growth zone reactor is used in
which a first, high temperature zone is used for crystal nucleation
and a second, low temperature zone is used for rapid crystal
growth. Although the process can be used to grow crystals in which
the as-grown material and the seed crystal are of different
composition, preferably the two crystalline structures have the
same composition, thus yielding improved crystal quality.
Inventors: |
MELNIK; Yuri; (Rockville,
MD) ; SOUKHOVEEV; Vitali; (Gaithersberg, MD) ;
IVANTSOV; Vladimir; (Gaithersberg, MD) ; TSVETKOV;
Katie; (North Potomac, MD) ; DMITRIEV; Vladimir;
(Gaithersberg, MD) |
Correspondence
Address: |
EMPK & Shiloh, LLP;c/o Landon IP, Inc.
1700 Diagonal Road
Suite 450
Alexandria
VA
22314
UNITED STATES
212-608-4141
212-608-4144
PUSDKT@EM-LG.COM
|
Assignee: |
Freiberger Compound Materials
GmbH
Am Junger Loewe Schacht 5
Freiberg/ Sachsen
DE
D-09599
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050212001 A1 |
September 29, 2005 |
|
|
Family ID: |
25416850 |
Appl. No.: |
11/134200 |
Filed: |
May 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09/903,047 |
Jul 9, 2001 |
|
|
|
11134200 |
May 20, 2005 |
|
|
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09/900,833 |
Sep 2, 2003 |
6613143 |
|
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09/903,047 |
Jul 9, 2001 |
|
|
|
Current U.S.
Class: |
257/94 |
Current CPC
Class: |
C30B 25/00 20130101;
C30B 29/40 20130101; H01L 21/02378 20130101; H01L 21/0254 20130101;
C30B 29/403 20130101; H01L 21/0237 20130101; C30B 11/14 20130101;
C30B 11/00 20130101; H01L 21/0262 20130101; C30B 29/406 20130101;
H01L 21/02389 20130101 |
Class at
Publication: |
257/094 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Claims
1. A method of growing an AlGaN single crystal boule, the method
comprising the steps of: growing an AlGaN single crystal layer on a
substrate; removing said substrate from said AlGaN single crystal
layer; growing the AlGaN single crystal boule on a surface of said
AlGaN single crystal layer; and continuing said step of growing the
AlGaN single crystal boule until the AlGaN single crystal boule has
a length of greater than 1 centimeter.
2. The method of claim 1, wherein said step of growing said AlGaN
single crystal layer on said substrate further comprises the steps
of: locating an extended Ga source within a first source zone of a
reactor; locating said substrate within a growth zone of said
reactor; locating an Al source within a second source zone of said
reactor; heating said substrate to a first temperature, wherein
said first temperature is greater than 1,000.degree. C.; heating a
first portion of said extended Ga source to a second temperature,
wherein said second temperature is greater than 450.degree. C.;
maintaining a second portion of said extended Ga source at a third
temperature, wherein said third temperature is greater than
30.degree. C., and wherein said third temperature is less than
100.degree. C.; heating said Al source to a fourth temperature,
wherein said fourth temperature is greater than 700.degree. C.;
introducing a halide reaction gas into said first source zone to
form a first halide metal compound; introducing said halide
reaction gas into said second source zone to form a second halide
metal compound; transporting said first halide metal compound to
said growth zone; transporting said second halide metal compound to
said growth zone; introducing a reaction gas into said growth zone,
said reaction gas containing nitrogen; and growing said AlGaN
single crystal layer on said substrate, said AlGaN single crystal
layer formed by said reaction gas reacting with said first halide
metal compound and said second halide metal compound.
3. The method of claim 2, further comprising the step of selecting
a HCl gas as said halide reaction gas, wherein said first halide
metal compound is comprised of gallium chloride, and wherein said
second halide metal compound is comprised of aluminum
trichloride.
4. The method of claim 2, further comprising the step of selecting
an ammonia gas as said reaction gas.
5. The method of claim 2, further comprising the step of selecting
said second temperature as approximately 650.degree. C.
6. The method of claim 2, wherein said step of transporting said
first halide metal compound to said growth zone is further
comprised of the step of flowing an inert gas through said first
source zone, and wherein said step of transporting said second
halide metal compound to said growth zone is further comprised of
the step of flowing said inert gas through said second source
zone.
7. The method of claim 2, further comprising the steps of: locating
at least one acceptor impurity metal in a third source zone of said
reactor; heating said at least one acceptor impurity metal to a
fifth temperature; and transporting said at least one acceptor
impurity metal to said growth zone, wherein said AlGaN single
crystal layer contains said at least one acceptor impurity
metal.
8. The method of claim 2, further comprising the steps of: locating
at least one donor in a third source zone of said reactor; heating
said at least one donor to a fifth temperature; and transporting
said at least one donor to said growth zone, wherein said AlGaN
single crystal layer contains said at least one donor.
9. The method of claim 2, further comprising the steps of: locating
a second Al source within a third source zone of said reactor;
heating said second Al source to a fifth temperature, wherein said
fifth temperature is greater than 700.degree. C.; introducing said
halide reaction gas into said third source zone to form said second
halide metal compound; transporting said second halide metal
compound from said third source zone to said growth zone;
discontinuing said step of transporting said second halide metal
compound from said second source zone to said growth zone; and
discontinuing said step of introducing said halide reaction gas
into said second source zone.
10. The method of claim 1, wherein said step of removing said at
least one substrate from said AlGaN single crystal layer further
comprises the steps of slicing a wafer from said AlGaN single
crystal layer; and polishing said surface of said wafer.
11. The method of claim 10, further comprising the step of etching
said polished surface.
12. The method of claim 1, wherein said step of removing said
substrate from said AlGaN single crystal layer further comprises
the step of etching said substrate from said AlGaN single crystal
layer to expose said surface of said AlGaN single crystal
layer.
13. The method of claim 12, wherein said etching step further
comprises the step of placing said substrate with said AlGaN single
crystal layer into a crucible containing molten KOH.
14. The method of claim 13, further comprising the step of reactive
ion etching said exposed surface, said reactive ion etching step
proceeding after the step of removing said substrate from said
crucible of molten KOH.
15. The method of claim 12, further comprising the step of
polishing said exposed surface.
16. The method of claim 15, further comprising the step of reactive
ion etching said polished, exposed surface.
17. The method of claim 15, further comprising the step of
chemically etching said polished, exposed surface.
18. The method of claim 1, wherein said step of removing said at
least one substrate from said AlGaN single crystal layer further
comprises the steps of: polishing said substrate, wherein a first
portion of said substrate is removed from said AlGaN single crystal
layer through said polishing step; and reactive ion etching said
substrate, wherein a second portion of said substrate is removed
from said AlGaN single crystal layer through said reactive ion
etching step.
19. The method of claim 18, wherein said reactive ion etching step
uses an Si.sub.3F/Ar mixture.
20. The method of claim 1, wherein said step of growing the AlGaN
single crystal boule on said surface of said AlGaN single crystal
layer further comprises the steps of: locating an extended Ga
source within a first source zone of a reactor; locating said AlGaN
single crystal layer within a growth zone of said reactor; locating
an Al source within a second source zone of said reactor; heating
said AlGaN single crystal layer to a first temperature, wherein
said first temperature is greater than 1,000.degree. C.; heating a
first portion of said extended Ga source to a second temperature,
wherein said second temperature is greater than 450.degree. C.;
maintaining a second portion of said extended Ga source at a third
temperature, wherein said third temperature is greater than
30.degree. C., and wherein said third temperature is less than
100.degree. C.; heating said Al source to a fourth temperature,
wherein said fourth temperature is greater than 700.degree. C.;
introducing a halide reaction gas into said first source zone to
form a halide metal compound; introducing said halide reaction gas
into said second source zone to form a second halide metal
compound; transporting said first halide metal compound to said
growth zone; transporting said second halide metal compound to said
growth zone; introducing a reaction gas into said growth zone, said
reaction gas containing nitrogen; growing a first portion of the
AlGaN single crystal boule on said AlGaN single crystal layer, said
first portion of the AlGaN single crystal boule formed by said
reaction gas reacting with said first halide metal compound and
said second halide metal compound; continuing said growing step for
at least 10 minutes; heating said AlGaN single crystal layer to a
fifth temperature, wherein said fifth temperature is greater than
850.degree. C. and less than 1,000.degree. C.; growing a second
portion of the AlGaN single crystal boule, said second portion of
the AlGaN single crystal boule formed by said reaction gas reacting
with said first halide metal compound and said second halide metal
compound; and continuing said step of growing said second portion
of the AlGaN single crystal boule for at least 12 hours.
21. The method of claim 20, wherein said step of transporting said
first halide metal compound to said growth zone is further
comprised of the step of flowing an inert gas through said first
source zone, and wherein said step of transporting said second
halide metal compound to said growth zone is further comprised of
the step of flowing said inert gas through said second source
zone.
22. The method of claim 20, further comprising the step of
selecting a HCl gas as said halide reaction gas, wherein said first
halide metal compound is comprised of gallium chloride, and wherein
said second halide metal compound is comprised of aluminum
trichloride.
23. The method of claim 20, further comprising the step of
selecting an ammonia gas as said reaction gas.
24. The method of claim 20, further comprising the step of
selecting said second temperature as approximately 650.degree.
C.
25. The method of claim 20, further comprising the steps of:
locating at least one acceptor impurity metal in a third source
zone of said reactor; heating said at least one acceptor impurity
metal to a sixth temperature; and transporting said at least one
acceptor impurity metal to said growth zone, wherein said AlGaN
single crystal boule contains said at least one acceptor impurity
metal.
26. The method of claim 20, further comprising the steps of:
locating at least one donor in a third source zone of said reactor;
heating said at least one donor to a sixth temperature; and
transporting said at least one donor to said growth zone, wherein
said AlGaN single crystal boule contains said at least one
donor.
27. The method of claim 20, further comprising the steps of:
locating a second Al source within a third source zone of said
reactor; heating said second Al source to a sixth temperature,
wherein said sixth temperature is greater than 700.degree. C.;
introducing said halide reaction gas into said third source zone to
form said second halide metal compound; transporting said second
halide metal compound from said third source zone to said growth
zone; discontinuing said step of transporting said second halide
metal compound from said second source zone to said growth zone;
and discontinuing said step of introducing said halide reaction gas
into said second source zone.
28. A method of growing an AlGaN single crystal boule, the method
comprising the steps of: growing an AlGaN single crystal layer on a
substrate; removing said substrate from said AlGaN single crystal
layer; growing the AlGaN single crystal boule on a surface of said
AlGaN single crystal layer utilizing a modified HVPE process and an
extended, multi-temperature zone Ga source; and continuing said
step of growing the AlGaN single crystal boule until the AlGaN
single crystal boule has a volume in excess of 4 cubic centimeters,
and wherein an x, a y, and a z dimension of said AlGaN single
crystal boule each exceed 1 centimeter.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 09/903,047 filed Jul. 9, 2001, priority of
which is claimed under 35 U.S.C. .sctn. 120, which is a
continuation of U.S. application Ser. No. 09/900,833, filed Jul. 6,
2001, priority of which is also claimed under 35 U.S.C.
.sctn.120.
FIELD OF THE INVENTION
[0002] The present invention relates generally to semiconductor
materials and, more particularly, to a method for growing bulk
single crystals.
BACKGROUND OF THE INVENTION
[0003] Recent results in the development of GaN-based
light-emitting diodes (LEDs) and laser diodes LDs) operating in the
green, blue, and ultraviolet spectrum have demonstrated the
tremendous scientific and commercial potential of group III nitride
semiconductors (e.g., GaN, AlN, InN, and their alloys). These
applications require electrically conducting substrates (e.g., GaN
or AlGaN) so that a vertical device geometry can be utilized in
which the electrodes are located on top and bottom surfaces of the
device structure. In addition to opto-electronic devices, group III
nitride semiconductors can be used in a host of other applications
such as communication electronics (e.g., high power microwave
devices). These devices require electrically insulating
substrates.
[0004] In order to achieve the desired device performance (e.g.,
high efficiency, low leakage current, high device yield, long
lifetime, etc.) for devices fabricated from group III nitride
semiconductor materials, it is expected that such devices will have
to be fabricated on native GaN or AlGaN substrates. Native
substrates as used herein refers to substrates that have been
obtained from bulk material, the bulk material preferably grown
from seeds of the same composition, thus allowing the substrates to
achieve extremely low defect densities as well as low residual
stress levels. Unfortunately, due to the lack of bulk GaN and AlGaN
substrates, device developers have been forced to attempt various
work-around solutions.
[0005] Initially GaN-based structures were developed on foreign
substrates. Some of the substrates that have been used in these
attempts are sapphire, silicon carbide, ZnO, LiGaO.sub.2,
LiAlO.sub.2, and ScMgAlO.sub.4. These devices suffer from a high
defect density including a high density of dislocations, domains,
and grain boundaries. Additionally, these devices suffer from
cracking and bending of the epitaxial structures. Most of these
problems arise from the lattice and thermal mismatch between the
foreign substrates and the GaN-based device structures. As a
result, these devices exhibit greatly reduced performance.
[0006] In order to attempt to overcome the problems in growing a
GaN-based device on a foreign (i.e., non-GaN) substrate, a number
of developers have attempted to grow GaN single crystals suitable
for use with microelectronic device structures. For example, U.S.
Pat. No. 5,679,152 discloses a technique for growing a GaN layer on
a sacrificial substrate and then etching away the substrate. As
disclosed, the substrate is etched away in situ while the
substrate/GaN layer is at or near the growth temperature. The GaN
layer may be deposited directly on the sacrificial substrate or
deposited on an intermediate layer that has been deposited on the
substrate.
[0007] Alternately, U.S. Pat. No. 5,770,887 discloses a repetitive
growth technique utilizing alternating epitaxially grown layers of
a buffer material and GaN single crystal. Removing the buffer
layers, for example by etching, produces a series of GaN single
crystal wafers. The patent discloses using sapphire as the initial
seed crystal and a material such as BeO, MgO, CaO, ZnO, SrO, CdO,
BaO or HgO as the buffer layer material.
[0008] Another technique for growing GaN substrates is disclosed in
U.S. Pat. No. 6,146,457. As disclosed, a thick layer of GaN is
epitaxially deposited on a thin, disposable substrate using a CVD
process. In the preferred embodiment, the substrate (e.g.,
sapphire) has a thickness of 20-100 microns while the GaN epitaxial
layer has a thickness of 50-300 microns. Accordingly, upon material
cooling, the strain arising from the thermal mismatch of the
material is relieved by cracking the disposable substrate rather
than the newly deposited epitaxial layer. As noted by the
inventors, however, the disclosed process solves the problems
associated with thermal mismatch, not lattice mismatch.
[0009] U.S. Pat. No. 6,177,292 discloses a method for growing a GaN
group material on an oxide substrate without cracking. After the
growth of the GaN group material, a portion of the oxide substrate
is removed by mechanical polishing. Another GaN layer is then grown
on the initial GaN layer to provide further support prior to the
complete removal of the remaining oxide substrate. As a result of
this process, a stand-alone GaN substrate is formed that can be
used as a substrate for the growth of a micro-electronic
device.
[0010] U.S. Pat. No. 6,218,280 discloses a method and apparatus for
depositing III-V compounds that can alternate between MOVPE and
HVPE processes, thus not requiring that the substrate be cooled and
transported between the reactor apparatus during device growth. As
disclosed, the MOVPE process is used to grow a thin III-V nitride
compound semiconductor layer (e.g., a GaN layer) on an oxide
substrate (e.g., LiGaO.sub.2, LiAlO.sub.2, MgAlScO.sub.4,
Al.sub.2MgO.sub.4, and LiNdO.sub.2). The HVPE process is then used
to grow the device structure on the GaN layer.
[0011] The main problems associated with growing true bulk GaN or
AlGaN crystals relate to fundamental properties of the materials.
For example, sublimation growth of GaN is limited by the
incongruent decomposition of GaN that becomes noticeable at
temperatures from 800.degree. C. to 1100.degree. C. In U.S. Pat.
No. 6,136,093, a technique is disclosed for growing GaN on a GaN
seed. As disclosed, Ga is heated to or above the evaporation
temperature of Ga and the seed crystal is heated to a temperature
higher than that of the Ga. The Ga vapor then reacts with a
nitrogen containing gas to form GaN which is deposited on the seed
crystal.
[0012] Another method to grow bulk GaN crystals is to grow them
from the liquid phase. The main problem associated with liquid
phase growth of GaN is the extremely low solubility of nitrogen in
melts in general, and in Ga melts in particular. The low solubility
limits the GaN growth rate and results in small volume GaN
crystals, generally less than 0.2 cubic centimeters.
[0013] In order to overcome the low solubility of nitrogen in the
Ga melts, typically growth temperatures between 1500.degree. C. and
1600.degree. C. are used with a nitrogen gas pressure in the range
of 10 to 20 kilobars. Even at these high pressures and
temperatures, nitrogen solubility in the Ga melt is very low. As a
result, only growth rates of approximately 0.01 to 0.05 millimeters
per hour can be obtained. Lateral growth rate, i.e., the growth
rate perpendicular to the [0001] crystallographic direction, is
typically about 1 millimeter per day. In addition to the low growth
rates, due to the high temperatures and pressures it is difficult
to develop production techniques using this process.
[0014] Accordingly, although III-V nitride compound semiconductor
materials offer tremendous potential for a variety of
micro-electronic devices ranging from opto-electronic devices to
high-power, high-frequency devices, the performance of such devices
is limited by the lack of suitable substrates. The present
invention provides a method for fabricating suitable
substrates.
SUMMARY OF THE INVENTION
[0015] The present invention provides a method and apparatus for
growing bulk GaN and AlGaN single crystal boules, preferably using
a modified HVPE process. The single crystal boules fabricated in
accordance with the present invention typically have a volume in
excess of 4 cubic centimeters with a minimum dimension (i.e., x, y,
or z dimension) of approximately 1 centimeter.
[0016] According to one aspect of the invention, an extended Ga
source is located within a reactor such that a portion of the Ga
source is maintained at a relatively high temperature while most of
the Ga source is maintained at a temperature close to, and just
above, the melting temperature of Ga. In at least one embodiment of
the invention, in order to obtain the desired temperature spread in
the Ga source, a portion of the source tube extends outside of the
reactor. Assuming the use of a modified HVPE process for the growth
of the single crystal boule, preferably the high temperature is in
the range of 450.degree. C. to 850.degree. C., and more preferably
at a temperature of approximately 650.degree. C., while the low
temperature is less than 100.degree. C. and preferably in the range
of 30.degree. C. to 40.degree. C. As a result of this source
configuration, the amount of Ga undergoing a reaction is
controllable and limited, thus allowing extended growth cycles to
be realized without experiencing degradation in the as-grown
material.
[0017] In another aspect of the invention, an extended Al source,
preferably comprised of one or more individual Al sources, is
included in the reactor, thereby allowing the growth of AlGaN
boules. Although a single Al source can be used, in order to
accomplish the desired extended growth cycles, thereby allowing the
growth of large single crystal boules, multiple Al sources are
used. If multiple Al sources are used, they are sequentially
activated. Accordingly, as one Al source begins to degrade and/or
become depleted, another Al source is activated and the first
source is deactivated. The cycling of Al sources continues as long
as necessary in order to complete the growth process.
[0018] In at least one embodiment of the invention, the reactor is
at least partially surrounded by a multi-temperature zone furnace.
The reactor includes at least one, and preferably two, growth
zones. One of the growth zones is maintained at a high temperature,
preferably within the range of 1,000.degree. C. to 1,1000.degree.
C. This growth zone is preferably used to initiate crystalline
growth. Although the high temperature of this zone allows high
quality crystal growth, the growth rate at this temperature is
relatively low. Accordingly a second growth zone, preferably held
to a temperature within the range of 850.degree. C. to
1,000.degree. C., is used after crystal growth is initiated. The
crystal growth rate at this temperature is relatively high.
[0019] In at least one embodiment of the invention, the reactor
includes an extended Ga source within one source tube, the Ga
source tube connected to a source of a halide gas (e.g., HCl) and
to a source of an inert gas (e.g., Ar). During the growth cycle,
the halide gas is introduced into the Ga source tube where it
primarily reacts with the Ga held at a high temperature. As a
result of the reaction, a halide metal compound is formed which is
delivered to the growth zone by the inert gas. Simultaneously,
ammonia gas is delivered to the growth zone. As a result of the
reaction of the halide metal compound and the ammonia gases, GaN is
deposited on one or more seed substrates within the growth zone. By
simultaneously supplying aluminum trichloride to the growth zone,
for example by reacting an Al source with HCl, AlGaN is grown on
the seed crystals within the growth zone. Additionally, by
supplying a suitable dopant to the growth zone during the growth
cycle, GaN or AlGaN of n-, i-, or p-type conductivity can be
grown.
[0020] In at least one embodiment of the invention, in order to
achieve superior quality in a single crystal boule of GaN or AlGaN,
a repetitive growth cycle is used. During the first growth cycle, a
single crystal of the desired material (e.g., GaN or AlGaN) is
grown on a seed substrate of a different chemical composition.
Suitable seed crystals include sapphire, silicon carbide, and
gallium arsenide (GaAs). After completion of a relatively lengthy
growth cycle, preferably of sufficient duration to grow a single
crystal several millimeters thick, the substrate is removed. The
remaining crystal is then subjected to suitable surface preparatory
steps. This crystal is then used as the seed crystal for another
extended growth cycle, preferably of sufficient duration to grow a
single crystal boule of approximately 1 centimeter in
thickness.
[0021] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic illustration of a horizontal furnace
as used with the invention;
[0023] FIG. 2 is an illustration of one embodiment of a boat
suitable for use with the furnace shown in FIG. 1;
[0024] FIG. 3 is an illustration of an individual source tube and a
means of varying the source contained within the tube relative to
the reactor;
[0025] FIG. 4 is a block diagram outlining the preferred method of
fabricating bulk GaN according to the invention;
[0026] FIG. 5 is a schematic illustration of an alternate
embodiment for use in growing AlGaN;
[0027] FIG. 6 is a block diagram outlining the preferred method of
fabricating bulk AlGaN according to the invention;
[0028] FIG. 7 is a schematic illustration of an alternate
embodiment for use in growing doped material; and
[0029] FIG. 8 outlines a process used in at least one embodiment of
the invention to grow material with a matching seed crystal.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0030] The present invention provides a method and apparatus for
growing bulk gallium nitride (GaN) or aluminum gallium nitride
(AlGaN), preferably using a modified hydrid vapor phase epitaxial
(HVPE) approach. FIG. 1 is a schematic illustration of a horizontal
furnace as used with the invention. It is understood that the
invention is not limited to this particular furnace configuration
as other configurations (e.g., vertical furnaces) that offer the
required control over the temperature, temperature zone or zones,
gas flow, source and substrate location, source configuration,
etc., can also be used. The furnace configuration illustrated in
FIG. 1 is preferred for the growth of undoped GaN as it easily
accommodates the desired gallium source of the invention. Furnace
100 is comprised of multiple temperature zones, preferably obtained
through the use of multiple heaters 101, each of which at least
partially surrounds reactor tube 103. In the preferred embodiment,
a six zone configuration is used in which heaters 101 are resistive
heaters. It is understood that although reactor tube 103 preferably
has a cylindrical cross-section, other configurations can be used
such as a `tube` with a rectangular cross-section. Within reactor
tube 103 are one or more source tubes 105. As noted with respect to
reactor tube 103, although source tubes 105 preferably have a
cylindrical cross-section, the invention is not limited to
cylindrical source tubes.
[0031] In order to grow undoped bulk GaN, a single source tube 105
is required. Within source tube 105 is a source boat 107. As used
herein, the term "boat" simply refers to a means of holding the
source material. For example, boat 107 may be comprised of a
portion of a tube 201 with a pair of end portions 203 as
illustrated in FIG. 2. Alternately, the source material can be held
within source tube 105 without the use of a separate boat 107.
Alternate boat configurations are clearly envisioned by the
inventors.
[0032] As described in detail below, in the preferred embodiment of
the invention the desired growth temperature depends upon the stage
of crystal growth (e.g., crystal nucleation versus high growth
rate). The temperature of a source in general, and the temperature
of a specific portion of the gallium source in particular, are
preferably controlled by varying the heat applied by specific
heaters 101. Additionally, in the preferred embodiment of the
invention in which multiple source types are used, the location of
a particular source (e.g., an impurity source) relative to reactor
tube 103 can be controllably varied, typically by altering the
position of the source. For example, as illustrated in FIG. 3, a
source tube 301 typically includes a boat 303, a source 305 within
boat 303, and a gas inlet 307. A control rod 309 coupled to boat
303 can be used to alter the position of the boat, and thus the
source, within the reactor. Control rod 309 can be manually
manipulated, as provided for in the illustrated configuration, or
coupled to a robotic positioning system (not shown).
[0033] In the preferred embodiment, coupled to each source tube are
one or more sources of gas 109-111. The rate of gas flow through a
particular source tube is controlled via valves 113-115, either
manually or by an automatic processing system.
[0034] A substrate 117 is located on a pedestal 119 within the
growth zone of reactor 103. Although typically multiple substrates
117 are manually loaded into the reactor for co-processing, a
single substrate can be processed with the invention. Additionally,
substrates 117 can be automatically positioned within the furnace
for automated production runs. In order to vary the temperature of
the growth zone, and thus substrate or substrates 117, either the
position of the substrates relative to reactor 103 are changed or
the amount of heat applied by heaters 101 proximate to the growth
zone is varied.
[0035] FIGS. 1 and 4 illustrate a specific reactor 100 and the
steps used to grow bulk GaN, respectively. Although reactor 100 is
a hot-wall, horizontal reactor and the process is carried out in an
inert gas flow at atmospheric pressure, as previously noted other
reactor configurations can be used to perform the modified HVPE
process of the invention. Preferably source tube 105 and source
boat 107 are comprised of quartz. Other materials can be used for
boat 107, however, such as sapphire or silicon carbide. Within boat
107, or simply within tube 105 if no separate boat is used, is a Ga
metal source 121.
[0036] In order to achieve extended GaN growth, as required to grow
bulk GaN, the inventors have found that an extended source of Ga
must be used and that the extended source must be maintained at
more than one temperature. Specifically, Ga metal 121 is positioned
relative to reactor 103 such that a large quantity of source 121
(i.e., preferably greater than 90 percent of source 121) is
maintained at a relatively low temperature, preferably less than
100.degree. C. and more than the melting temperature of Ga (i.e.,
29.78.degree. C.), and more preferably within the temperature range
of 30.degree. C. to 40.degree. C. Due to the low temperature, this
portion of Ga source 121 has limited reaction with the halide
reactive gas coupled to and flowing through source tube 105. If
desired, as in the preferred embodiment, a portion of source tube
105 and Ga source 121 are maintained outside of the reactor volume
as illustrated in FIG. 1. Alternately, the lower temperature of
this portion of source 121 is achieved through control of heaters
101 adjacent to the lower temperature portion of the source.
[0037] At the high temperature end of source tube 105, the
temperature of Ga source 121 is held at a relatively high
temperature, typically between 459.degree. C. and 850.degree. C.
and preferably at a temperature of approximately 650.degree. C.
During crystal growth, a constant source of Ga is maintained due to
the flow of Ga from the low temperature portion of tube 105 to the
higher temperature portion of tube 105. Accordingly, by providing a
large Ga source, the present invention allows the growth of bulk
GaN while limiting the amount of the source that reacts with the
halide reactive gas. It is understood that although the preferred
embodiment of the invention utilizes a modified HVPE process in
conjunction with the large Ga source described above, the source
can be used with other bulk growth techniques (e.g., sublimation
techniques).
[0038] In order to grow bulk GaN according to the preferred
embodiment of the invention, a source of halide gas 109, preferably
HCl, is coupled to source tube 105 along with a source of inert gas
110, preferably Ar. A source of ammonia gas 111 is also coupled to
reactor 103. In order to grow bulk GaN, preferably seed crystals
117 are comprised of GaN, thus providing a lattice and coefficient
of thermal expansion match between the seed and the material to be
grown. As a result of using GaN seed crystals, improved quality in
the as-grown material is achieved. Alternately, seed crystals 117
can be of silicon carbide (SiC), sapphire, gallium arsenide (GaAs),
or other material. Seed crystal pedestal 119 is preferably
fabricated from quartz, although other materials such as silicon
carbide or graphite can also be used.
[0039] Initially reactor 103 is flushed and filled with an inert
gas, preferably Ar, from gas source 110 (step 401). The inert gas
can enter the reactor through source tube 105, thereby flushing the
source tube, through a separate entry line (not shown), or both.
The flow of inert gas is controlled by metering valve 114 and is
typically in the range of 1 to 25 liters per minute. Substrates (or
substrate) 117 are then heated to the desired growth temperature
(step 403). In one embodiment of the invention the growth zone, and
thus the substrates within the growth zone, are heated to a
temperature within the range of 1,000.degree. C. and 1,100.degree.
C. This temperature achieves a higher quality material in the
as-grown crystal, but yields relatively slow growth rate. In an
alternate embodiment, the growth zone is maintained at a
temperature within the range of 850.degree. C. and 1,000.degree. C.
Although this temperature is capable of fast crystal growth, the
resulting crystal is of lower quality. In the preferred embodiment
of the invention, the methodology of which is illustrated in FIG.
4, the growth zone and thus the substrates (or substrate) within
the growth zone are initially heated to a high temperature within
the range of 1,000.degree. C. and 1,100.degree. C., thus initiating
high quality crystal growth. Once crystal growth has been
initiated, the source temperature is lowered and maintained at a
temperature within the range of 850.degree. C. and 1,000.degree.
C., thus allowing rapid crystal growth to be achieved. Preferably
the period of high quality crystal growth is at least 10 minutes
and the period of rapid crystal growth is at least 12 hours. More
preferably the period of high quality crystal growth is at least 30
minutes and the period of rapid crystal growth is at least 24
hours.
[0040] Preferably prior to initiating crystal growth, the surfaces
of substrates 117 are etched to remove residual surface
contamination, for example by using gaseous HCl from supply 109.
The Ga source material 121 is initially heated to a temperature
sufficient to cause the entire source to melt (step 405). As
previously noted, the melting temperature of Ga is 29.78.degree. C.
and source 121 is preferably heated to a temperature within the
range of 30.degree. C. to 40.degree. C. A portion of source tube
105 closest to substrates 117, and thus the portion of source
material 121 closest to substrates 117, is heated to a relatively
high temperature (step 407), typically between 450.degree. C. and
850.degree. C. and preferably at a temperature of approximately
650.degree. C.
[0041] After the source material is heated a halide reactive gas,
preferably HCl, is introduced into source tube 105 (step 409). As a
result of the reaction between HCl and Ga, gallium chloride is
formed which is transported to the reactor's growth zone by the
flow of the inert (e.g., Ar) gas (step 411). Simultaneously,
ammonia gas (NH.sub.3) from source 111 is delivered to the growth
zone (step 413). The NH.sub.3 gas and the gallium chloride gas
react (step 415) to form GaN on the surface of seed substrates 117
(step 417). The initial growth rate of the GaN is in the range of
0.05 to 1 micron per minute. After a high quality GaN layer of
sufficient thickness has been grown, typically on the order of 20
microns and preferably on the order of 50 microns, the temperature
of the growth zone is lowered (step 419) to a temperature within
the range of 850.degree. C. and 1,000.degree. C., thereby allowing
GaN to be grown at an accelerated rate (i.e., in the range of 5 to
500 microns per hour). After the desired boule thickness has been
achieved, the flow of HCl and NH.sub.3 gas is stopped and
substrates 117 are cooled in the flowing inert gas (step 421).
[0042] FIGS. 5 and 6 illustrate another embodiment of the invention
that can be used to grow AlGaN boules. Reactor 500 is substantially
the same as reactor 100 except for the inclusion of an aluminum
(Al) source. Also in this embodiment Ga source tube 105 is shown to
be completely within the reactor. As the Al source tends to degrade
over time due to the reaction between the Al and the source
tube/boat materials, in the preferred embodiment of the invention,
reactor 500 includes multiple Al sources. As shown, reactor 500
includes three Al source tubes 501, although it is understood that
fewer or greater numbers of Al source tubes can be included,
depending upon the quantity of AlGaN to be grown. Within each Al
source tube 501 is a source boat 503 containing a quantity of Al
metal 505. Preferably each source boat 503 is fabricated from
sapphire or silicon carbide. Additionally, as discussed with
reference to FIG. 3, the position of each source boat 503 within
the reactor can be altered using either a manual or automatic
control rod 507
[0043] As previously noted, preferably the seed crystal is of the
same material as the crystal to be grown. Therefore in order to
grow bulk AlGaN, preferably seed crystal 609 is fabricated of
AlGaN. Alternately, seed crystal 609 can be of GaN, SiC, sapphire,
GaAs, or other material.
[0044] The methodology to grow AlGaN is very similar to that
outlined in FIG. 4 for GaN growth. In this embodiment, during
source heating one of the Al sources 505 is heated to a temperature
of preferably between 700.degree. C. and 850.degree. C. (step 601),
the selected Al source being appropriately positioned within the
reactor to achieve the desired temperature. Once all of the
materials have achieved the desired growth temperature, halide gas
(e.g., HCl) is introduced into Ga source tube 105 and the selected
Al source tube (step 603). As a result, gallium chloride and
aluminum trichloride are formed (step 605). Both the gallium
chloride and aluminum chloride are transported to the growth zone
using an inert gas (e.g., Ar) (step 607). NH.sub.3 gas 111 is
simultaneously introduced into the growth zone with the source
materials (step 609) resulting in a reaction by the three gases to
form AlGaN (step 611). As in the prior embodiment, preferably the
growth zone is initially held at a higher temperature in order to
initiate the growth of high quality material. Once a sufficiently
large layer is formed, preferably on the order of 50 microns thick,
the temperature of the growth zone is lowered (step 419) to a
temperature within the range of 850.degree. C. and 1,000.degree. C.
in order to achieve accelerated growth. Prior to exhaustion or
excessive degradation of the initially selected Al source, a second
Al source 503 is heated to a temperature within the preferred range
of 700.degree. C. and 850.degree. C. (step 613). Once the second Al
source is heated, halide gas (e.g., HCl) is introduced into the
second Al source tube (step 615) and the resultant aluminum
trichloride is transported to the growth zone (step 617). The flow
of halide and inert gas through the initially selected Al source
tube is then stopped and the first Al source is withdrawn from the
high temperature zone (step 619). The process of introducing new Al
sources continues as long as necessary to grow the desired AlGaN
boule. After the desired boule thickness has been achieved, the
flow of HCl and NH.sub.3 gas is stopped and substrates 117 are
cooled in the flowing inert gas (step 421).
[0045] The present invention can be used to grow GaN or AlGaN of
various conductivities, the conductivity dependent upon the dopants
added during crystal growth. FIG. 7 illustrates another embodiment
of the invention that allows the addition of dopants during crystal
growth. The embodiment shown includes Ga source tube 105, two Al
source tubes 501, and two dopant source tubes 701. It is understood
that the number of source tubes is based on the number of
constituents required for the desired crystal.
[0046] To grow p-type GaN or AlGaN, a suitable dopant (i.e.,
acceptor) is placed within one or more boats 703 within one or more
dopant source tubes 701, thus allowing the desired dopants to be
added to the crystal during growth. Preferably either magnesium
(Mg) or a combination of Mg and zinc (Zn) is used. If multiple
dopants are used, for example both Mg and Zn, the dopants may be in
the form of an alloy, and thus be located within a single boat, or
they may be in the form of individual materials, and thus
preferably located within separate boats. To grow insulating (i.e.,
i-type) GaN or AlGaN, preferably Zn is used as the dopant. Although
undoped GaN and AlGaN exhibit low n-type conductivity, controllable
n-type conductivity can be achieved by doping the growing crystal
with donors. Preferred donors include silicon (Si), germanium (Ge),
tin (Sn), and oxygen (O).
[0047] A detailed discussion of GaN and AlGaN doping is provided in
co-pending U.S. patent application Ser. No. 09/861,011, pages 7-14,
the teachings of which are hereby incorporated by reference for any
and all purposes. In the preferred embodiment of the invention,
dopant source boats 703 are formed of non-reactive materials (e.g.,
sapphire), extremely pure source materials are used (e.g., 99.999
to 99.9999 purity Mg), and the source materials are etched prior to
initiating the growth process to insure minimal surface
contamination. Although the temperature for a particular dopant
source depends upon the selected material, typically the
temperature is within the range of 250.degree. C. to 1050.degree.
C. If a Mg dopant is used, preferably the temperature is within the
range of 450.degree. C. to 700.degree. C., more preferably within
the range of 550.degree. C. to 650.degree. C., and still more
preferably at a temperature of approximately 615.degree. C. The
dopant source or sources are heated simultaneously with the
substrate and the Ga or the Ga and Al sources. The dopants are
delivered to the growth zone via inert gas (e.g., Ar) flow. The
flow rate depends upon the conductivity to be achieved in the
growing crystal. For example, for growth of p-type GaN or AlGaN,
the flow rate for a Mg dopant is typically between 1,000 and 4,000
standard cubic centimeters per minute, and preferably between 2,000
and 3,500 standard cubic centimeters per minute.
[0048] As previously described, the level of doping controls the
conductivity of the grown material. In order to achieve p-type
material, it is necessary for the acceptor concentration (N.sub.a)
to be greater than the donor concentration (N.sub.d). The inventors
have found that in order to achieve the desired N.sub.a/N.sub.d
ratio and grow p-type GaN or AlGaN, the concentration of the
acceptor impurity must be in the range of 10.sup.18 to 10.sup.21
atoms per cubic centimeter, and more preferably in the range of
10.sup.19 to 10.sup.20 atoms per cubic centimeter. For an i-type
layer, the doping level must be decreased, typically such that the
dopant concentration does not exceed 10.sup.19 atoms per cubic
centimeter.
[0049] As previously noted, improved crystal quality in the
as-grown material is achieved when the seed crystal and the
material to be grown are of the same chemical composition so that
there is no crystal lattice or coefficient of thermal expansion
mismatch. Accordingly, FIG. 8 outlines a process used in at least
one embodiment of the invention in which material is grown using a
matching seed crystal.
[0050] In the illustrated embodiment of the invention, initially
material (e.g., doped or undoped GaN or AlGaN) is grown from a seed
crystal of different chemical composition using the techniques
described in detail above (step 801). As previously noted, the seed
crystal can be of sapphire, silicon carbide, GaAs, or other
material. After the bulk material is formed, a portion of the grown
crystal is removed from the bulk for use as a new seed crystal
(step 803). For example, new seed crystals can be obtained by
cutting off a portion of the as-grown bulk (step 805) and
subjecting the surfaces of the cut-off portion to suitable surface
preparatory steps (step 807). Alternately, prior to cutting up the
as-grown bulk material, the initial seed crystal can be removed
(step 809), for example using an etching technique. Once a new seed
crystal is prepared, the bulk growth process of the present
invention is used to grow a second crystal (step 811). However, as
a consequence of the ability to grow bulk materials according to
the invention, the second growth cycle is able to utilize a seed
crystal of the same composition as the material to be grown, thus
yielding a superior quality material.
SPECIFIC EMBODIMENTS
Embodiment 1
[0051] According to this embodiment of the invention, the modified
HVPE process described above was used to grow thick GaN layers on
SiC substrates. Suitable GaN substrates were then fabricated and
used in conjunction with the modified HVPE process of the invention
to grow a GaN single crystal boule. The second GaN boule was cut
into wafers suitable for device applications.
[0052] In this embodiment, multiple SiC substrates of a 6H polytype
were loaded into the growth zone of a reactor similar to that shown
in FIG. 1. The substrates were placed on a quartz sample holder
with the (0001) Si on-axis surface positioned for GaN deposition.
One kilogram of Ga metal was positioned in the source boat within
the Ga source tube. After purging the reactor with Ar gas to remove
air, the growth zone and the Ga source zone were heated to
1100.degree. C. and 650.degree. C., respectively. The majority of
the Ga source, however, was maintained at a temperature of less
than 100.degree. C., typically in the range of 30.degree. C. to
40.degree. C. To prepare the substrates for GaN deposition, HCl gas
was introduced into the growth zone to etch the SiC substrates. The
HCl gas was then introduced into the Ga source zone, thereby
forming gallium chloride that was transported into the growth zone
by the Ar carrier gas. Simultaneously, NH.sub.3 gas was introduced
into the growth zone, the NH.sub.3 gas providing a source of
nitrogen. As a result of the reaction between the gallium chloride
and the NH.sub.3 gases, a GaN layer was grown on the SiC surface.
The NH.sub.3 and gallium chloride gases were expelled from the
reactor by the flow of the Ar gas. After allowing the growth
process to continue for a period of 24 hours, the flow of HCl and
NH.sub.3 gases was stopped and the furnace was slowly cooled down
to room temperature with Ar gas flowing through all of the gas
channels. The reactor was then opened to the air and the sample
holder was removed. As a result of this growth process, GaN layers
ranging from 0.3 to 2 millimeters were grown on the SiC substrates.
The range of GaN thicknesses were the result of the distribution of
GaN growth rates within the growth zone.
[0053] To prepare GaN seed substrates, the SiC substrates were
removed from the grown GaN material by chemically etching the
material in molten KOH. The etching was carried out in a nickel
crucible at a temperature within the range of 450.degree. C. to
650.degree. C. Prior to beginning the etching process, the molten
KOH was maintained at the etching temperature for several hours to
remove the moisture from the melt and the crucible. Once the
substrates were placed within the molten KOH, only a few hours were
required to etch away most of the SiC substrates from the grown
GaN. This process for substrate removal is favored over either
mechanical or laser induced substrate removal. The remaining SiC
substrate was removed by reactive ion etching in a Si.sub.3F/Ar gas
mixture. For some of the as-grown material, polycrystalline
material was noted in the peripheral regions, this material being
subsequently removed by grinding. Additionally, in some instances
the surface of the as-grown material required mechanical polishing
to smooth the surface. In these instances, after the polishing was
completed, reactive ion etching or chemical etching was used to
remove the thin surface layer damaged during polishing. As a result
of this procedure, the desired GaN seeds were obtained. The high
quality of the resultant material was verified by the x-ray rocking
.omega.-scan curves (e.g., 300 arc sec for the full width at half
maximum (FWHM) for the (0002) GaN reflection). X-ray diffraction
measurements showed that the as-grown material was 21H--GaN.
[0054] The inventors have found that SiC substrates are preferable
over sapphire substrates during the initial growth process as the
resultant material has a defined polarity. Specifically, the
resultant material has a mixture of gallium (Ga) polarity and
nitrogen (N) polarity. The side of the as-grown material adjacent
to the SiC substrates has an N polarity while the opposite,
outermost layer of the material has a Ga polarity. Prior to growing
a GaN boule utilizing the process of the invention, in some
instances the inventors found that it was beneficial to grow a thin
GaN layer, e.g., typically in the range of 10 to 100 microns thick,
on one or both sides of the GaN substrates grown above. The
additional material improved the mechanical strength of the
substrates and, in general, prepared the GaN surface for bulk
growth. Prior to bulk growth, the GaN seed substrates were
approximately 1 millimeter thick and approximately 6 centimeters in
diameter.
[0055] The growth of the GaN boule used the same reactor as that
used to grow the GaN seed substrates. The substrates were
positioned within the reactor such that the new material would be
grown on the (0001) Ga on-axis face. The inventors have found that
the Ga face is preferred over the N face as the resulting boule has
better crystal properties and lower dislocation density. It should
be noted that the (0001) surface can be tilted to a specific
crystallographic direction (e.g., [11-20] and that the tilt angle
may be varied between 0.5 and 90.degree. degrees. In the present
embodiment, the tilt angle was zero.
[0056] In addition to loading the seed substrates into the growth
zone of the reactor, two kilograms of Ga was loaded into the source
boat within the Ga source tube. After purging the reactor with Ar
gas, the growth zone and the Ga source zone were heated to
1050.degree. C. and 650.degree. C., respectively. As previously
described, only a small portion of the Ga source was brought up to
the high source temperature noted above (i.e., 650.degree. C.).
Most of the Ga source was maintained at a temperature close to room
temperature, typically in the range of 30.degree. C. and 40.degree.
C., Prior to initiating GaN growth, a mixture of NH.sub.3 and HCl
gas was introduced in the growth zone to refresh the GaN seed
surface. As in the growth of the seed crystal previously described,
HCl was introduced into the Ga source zone to form gallium chloride
that was then transported to the growth zone with Ar gas. At the
same time, NH.sub.3 gas used as a source of nitrogen was introduced
into the growth zone. The GaN was formed by the reaction between
the gallium chloride and the NH.sub.3 gases.
[0057] After approximately 30 minutes of GaN growth, the GaN
substrate was moved into a second growth zone maintained at a
temperature of approximately 980.degree. C., thereby achieving
accelerated growth rates as previously disclosed. This process was
allowed to continue for approximately 80 hours. After that, HCl
flow through the Ga source tube and NH.sub.3 flow though the growth
zone were stopped. The furnace was slow cooled down to room
temperature with Ar flowing through all gas channels. The reactor
was then opened to the air and the sample holder was removed from
the reactor. The resultant boule had a diameter of approximately 6
centimeters and a thickness of approximately 1 centimeter. The
crystal had a single crystal 2H polytype structure as shown by
x-ray diffraction measurements.
[0058] After growth, the boule was machined to a perfect
cylindrical shape with a 5.08 centimeter diameter (i.e., 2 inch
diameter), thereby removing defective peripheral areas. One side of
the boule was ground to indicate the (11-20) face. Then the boule
was sliced into 19 wafers using a horizontal diamond wire saw with
an approximately 200 micron diamond wire. Before slicing, the boule
was oriented using an x-ray technique in order to slice the wafers
with the (0001) oriented surface. The slicing rate was about 1
millimeter per minute. The wire was rocked around the boule during
the slicing. Thickness of the wafers was varied from 150 microns to
500 microns. Wafer thickness uniformity was better than 5
percent.
[0059] After slicing, the wafers were polished using diamond
abrasive suspensions. Some wafers were polished only on the Ga
face, some wafers were polished only on the N face, and some wafers
were polished on both sides. The final surface treatment was
performed using a reactive ion etching technique and/or a chemical
etching technique to remove the surface layer damaged by the
mechanical treatment. The surface of the wafers had a single
crystal structure as shown by high energy electron diffraction
techniques. The surface of the finished GaN wafers had a mean
square roughness, RMS, of 2 nanometers or less as determined by
atomic force microscopy utilizing a viewing area of 5 by 5 microns.
The defect density was measured using wet chemical etching in hot
acid. For different wafers, etch pit density ranged from 10 to 1000
per square centimeter. Some GaN wafers were subjected to heat
treatment in an argon atmosphere in a temperature range from
450.degree. C. to 1020.degree. C. in order to reduce residual
stress. Raman scattering measurements showed that such heat
treatment reduced stress from 20 to 50 percent.
[0060] In order to compare the performance of devices fabricated
using the GaN substrates fabricated above to those fabricated on
SiC and sapphire, GaN homoepitaxial layers and pn diode multi-layer
structures were grown. Device structures included AlGaN/GaN
structures. Prior to device fabrication, surface contamination of
the growth surface of the GaN wafers was removed in a side growth
reactor with a NH.sub.3--HCl gas mixture. The thickness of
individual layers varied from 0.002 micron to 200 microns,
depending upon device structure. For example, high frequency device
structures (e.g., heterojunction field effect transistors) had
layers ranging from 0.002 to 5 microns. For high power rectifying
diodes, layers ranged from 1 to 200 microns. In order to obtain
p-type layers, a Mg impurity was used while n-type doping was
obtained using a Si impurity. The fabricated device structures were
fabricated employing contact metallization, photolithography and
mesa insulation.
[0061] The structures fabricated on the GaN wafers were studied
using optical and electron microscopy, secondary ion mass
spectrometry, capacitance-voltage and current-voltage methods. The
devices showed superior characteristics compared with devices
fabricated on SiC and sapphire substrates. Additionally, it was
shown that wafer surface cleaning procedure in the reactor reduced
defect density, including dislocation and crack density, in the
grown epitaxial layers.
Embodiment 2
[0062] In this embodiment, a GaN seed was first fabricated as
described in Embodiment 1. The 5.08 centimeter diameter (i.e., 2
inch diameter) prepared GaN seed substrates were then placed within
a stainless steel, resistively heated furnace and a GaN single
crystal boule was grown using a sublimation technique. GaN powder,
located within a graphite boat, was used as the Ga vapor source
while NH.sub.3 gas was used as the nitrogen source. The GaN seed
was kept at a temperature of 1100.degree. C. during the growth. The
GaN source was located below the seed at a temperature higher than
the seed temperature. The growth was performed at a reduced
pressure.
[0063] The growth rate using the above-described sublimation
technique was approximately 0.5 millimeters per hour. After a
growth cycle of 24 hours, a 12 millimeter thick boule was grown
with a maximum boule diameter of 54 millimeters. The boule was
divided into 30 wafers using a diamond wire saw and the slicing and
processing procedures described in Embodiment 1. X-ray
characterization was used to show that the GaN wafers were single
crystals.
Embodiment 3
[0064] In this embodiment, bulk GaN material was grown in an inert
gas flow at atmospheric pressure utilizing the hot-wall, horizontal
reactor described in Embodiment 1. Six 5.08 centimeter diameter
(i.e., 2 inch diameter) silicon carbide substrates of a 6H
polytype, were placed on a quartz pedestal and loaded into a growth
zone of the quartz reactor. The substrates were located such that
the (0001) Si on-axis surfaces were positioned for GaN deposition.
Approximately 0.9 kilograms of Ga (7N) was located within a quartz
boat in the Ga source zone of the reactor. This channel was used
for delivery of gallium chloride to the growth zone of the reactor.
A second quartz tube was used for ammonia (NH.sub.3) delivery to
the growth zone. A third separate quartz tube was used for HCl gas
delivery to the growth zone.
[0065] The reactor was filled with Ar gas, the Ar gas flow through
the reactor being in the range of 1 to 25 liters per minute. The
substrates were then heated in Ar flow to a temperature of
1050.degree. C. and the hot portion of the metal Ga source was
heated to a temperature in the range of 350.degree. C. to
800.degree. C. The lower temperature portion of the Ga source was
maintained at a temperature within the range of 30.degree. C. to
40.degree. C. HCl gas was introduced into the growth zone through
the HCl channel. As a result, the SiC seed substrates were etched
at Ar--HCl ambient before initiating the growth procedure.
[0066] To begin the growth process, HCl gas was introduced into the
Ga source zone, creating gallium chloride that was delivered to the
growth zone by Ar gas flow. Simultaneously, NH.sub.3 was introduced
into the growth zone. As a result of the reaction between the
gallium chloride gas and the ammonia gas, a single crystal
epitaxial GaN layer was grown on the substrates. The substrate
temperature during the growth process was held constant at
1020.degree. C. After a growth period of 20 hours, the flow of HCl
and NH.sub.3 were stopped and the samples were cooled in flowing
Ar.
[0067] As a result of the growth process, six GaN/SiC samples were
obtained in which the GaN thickness was in the range of 1 to 3
millimeters. To remove the SiC substrates, the samples were first
glued to metal holders using mounting wax (e.g., QuickStick.TM.
135) at a temperature of 130.degree. C. with the GaN layer facing
the holder. The holders were placed on a polishing machine (e.g.,
SILT Model 920) and a thick portion of the SiC substrates were
ground away using a 30 micron diamond suspension at 100 rpm with a
pressure of 0.1 to 3 kilograms per square centimeter. This process
was continued for a period of between 8 and 24 hours. After removal
of between 200 and 250 microns of SiC, the samples were unglued
from the holders and cleaned in hot acetone for approximately 20
minutes.
[0068] The residual SiC material was removed from each sample using
a reactive ion etching (RIE) technique. Each sample was placed
inside a quartz etching chamber on the stainless steel holder. The
RIE was performed using Si.sub.3F/Ar for a period of between 5 and
12 hours, depending upon the thickness of the residual SiC. The
etching rate of SiC in this process is about 10 microns per hour.
After the RIE process was completed, the samples were cleaned to
remove possible surface contamination. As a result of the above
processes, freestanding GaN plates completely free of any trace of
SiC were obtained.
[0069] After completion of a conventional cleaning procedure, the
GaN plates were placed in the HVPE reactor. A GaN homoepitaxial
growth was started on the as-grown (0001) Ga surface of the GaN
plates. The growth temperature was approximately 1060.degree. C.
After a period of growth of 10 minutes, the samples were cooled and
unloaded from the reactor. The GaN layer grown on the GaN plates
was intended to cover defects existing in the GaN plates.
Accordingly, the samples at the completion of this step were
comprised of 5.08 centimeter diameter (i.e., 2 inch diameter) GaN
plates with approximately 10 microns of newly grown GaN. Note that
for some samples a GaN layer was grown not only on the (0001) Ga
face of the GaN plates, but also on the (0001) N face of the
plates. Peripheral highly defective regions of the GaN plates were
removed by grinding.
[0070] Three of the GaN plates from the previous process were
loaded into the reactor in order to grow thick GaN boules. Gallium
chloride and ammonia gas served as source materials for growth as
previously disclosed. In addition, during the growth cycle the GaN
boules were doped with silicon supplied to the growth zone by
S.sub.2H.sub.4 gas. Growth temperatures ranged from 970.degree. C.
to 1020.degree. C. and the growth run lasted for 48 hours. Three
boules with thicknesses of 5 millimeters, 7 millimeters, and 9
millimeters, respectively, were grown.
[0071] The boules were sliced into GaN wafers. Prior to wafer
preparation, some of the boules were ground into a cylindrical
shape and peripheral polycrystalline GaN regions, usually between 1
and 2 millimeters thick, were removed. Depending upon wafer
thickness, which ranged from 150 to 500 microns, between 7 and 21
wafers were obtained per boule. The wafers were then polished on
either one side or both sides using an SBT Model 920 polishing
machine with a 15 micron diamond suspension at 100 rpm with a
pressure of between 0.5 and 3 kilograms per square centimeter for 9
minutes per side. After cleaning all parts and the holder for 5 to
10 minutes in water with soap, the polishing process was repeated
with a 5 micron diamond suspension for 10 minutes at the same
pressure. After subjecting the parts and the holder to another
cleaning, the wafers were polished using a new polishing cloth and
a 0.1 micron diamond suspension for an hour at 100 rpm with a
pressure of between 0.5 and 3 kilograms per square centimeter.
[0072] After cleaning, the GaN wafers were characterized in terms
of crystal structure, electrical and optical properties. X-ray
diffraction showed that the wafers were single crystal GaN with a
2H polytype structure. The FWHM of the x-ray rocking curve measured
in .omega.-scanning geometry ranged from 60 to 360 arc seconds for
different samples. After chemical etching, the etch pit density
measured between 1100 and 10,000 per square centimeter, depending
upon the sample. Wafers had n-type conductivity with a
concentration N.sub.d-N.sub.a of between 5 and 9.times.10.sup.18
per cubic centimeter. The wafers were used as substrates for device
fabrication, particularly for GaN/AlGaN multi-layer device
structures grown by the MOCVD process. Pn diodes were fabricated
using a vertical current flow geometry, which was possible due to
the good electrical conductivity of the GaN substrates.
[0073] As will be understood by those familiar with the art, the
present invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof.
Accordingly, the disclosures and descriptions herein are intended
to be illustrative, but not limiting, of the scope of the invention
which is set forth in the following claims.
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