U.S. patent application number 11/606624 was filed with the patent office on 2007-06-21 for low defect group iii nitride films useful for electronic and optoelectronic devices and methods for making the same.
This patent application is currently assigned to Kyma Technologies, Inc.. Invention is credited to Andrew D. Hanser, Lianghong Liu, Edward A. Preble, N. Mark Williams, Xueping Xu.
Application Number | 20070138505 11/606624 |
Document ID | / |
Family ID | 39314541 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070138505 |
Kind Code |
A1 |
Preble; Edward A. ; et
al. |
June 21, 2007 |
Low defect group III nitride films useful for electronic and
optoelectronic devices and methods for making the same
Abstract
In a method for making a low-defect single-crystal GaN film, an
epitaxial nitride layer is deposited on a substrate. A first GaN
layer is grown on the epitaxial nitride layer by HVPE under a
growth condition that promotes the formation of pits, wherein after
growing the first GaN layer the GaN film surface morphology is
rough and pitted. A second GaN layer is grown on the first GaN
layer to form a GaN film on the substrate. The second GaN layer is
grown by HVPE under a growth condition that promotes filling of the
pits, and after growing the second GaN layer the GaN film surface
morphology is essentially pit-free. A GaN film having a
characteristic dimension of about 2 inches or greater, and a
thickness normal ranging from approximately 10 to approximately 250
microns, includes a pit-free surface, the threading dislocation
density being less than 1.times.10.sup.8 cm.sup.-2.
Inventors: |
Preble; Edward A.; (Raleigh,
NC) ; Liu; Lianghong; (Cary, NC) ; Hanser;
Andrew D.; (Raleigh, NC) ; Williams; N. Mark;
(Raleigh, NC) ; Xu; Xueping; (Stamford,
CT) |
Correspondence
Address: |
THE ECLIPSE GROUP
10605 BALBOA BLVD., SUITE 300
GRANADA HILLS
CA
91344
US
|
Assignee: |
Kyma Technologies, Inc.
Raleigh
NC
|
Family ID: |
39314541 |
Appl. No.: |
11/606624 |
Filed: |
November 30, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60749728 |
Dec 12, 2005 |
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60750982 |
Dec 16, 2005 |
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60810537 |
Jun 2, 2006 |
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60843036 |
Sep 8, 2006 |
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60847855 |
Sep 28, 2006 |
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Current U.S.
Class: |
257/190 ;
257/E21.121; 257/E29.091; 438/60 |
Current CPC
Class: |
H01L 21/0242 20130101;
H01L 21/0262 20130101; H01L 33/007 20130101; C30B 25/183 20130101;
H01L 21/02389 20130101; H01L 21/02502 20130101; H01L 29/2003
20130101; H01L 21/02581 20130101; H01L 21/02458 20130101; H01L
21/02631 20130101; H01L 33/30 20130101; H01L 21/02433 20130101;
H01L 21/0237 20130101; H01L 21/0257 20130101; H01L 21/02595
20130101; C30B 29/406 20130101; H01L 21/02694 20130101; H01L
29/0688 20130101; H01L 29/04 20130101; H01L 33/32 20130101; C30B
25/02 20130101; C30B 25/18 20130101; C30B 29/403 20130101; H01L
21/0254 20130101; H01L 21/02505 20130101 |
Class at
Publication: |
257/190 ;
438/060; 257/E29.091 |
International
Class: |
H01L 31/00 20060101
H01L031/00; H01L 21/00 20060101 H01L021/00 |
Claims
1. A method for making a low-defect single-crystal gallium nitride
(GaN) film, comprising: depositing an epitaxial aluminum nitride
(AlN) layer on a substrate; growing a first epitaxial GaN layer on
the AlN layer by HVPE under a growth condition that promotes the
formation of pits, wherein after growing the first GaN layer the
GaN film surface morphology is rough and pitted; and growing a
second epitaxial GaN layer on the first GaN layer to form a GaN
film on the substrate, wherein the second GaN layer is grown by
HVPE under a growth condition that promotes filling of the pits,
and after growing the second GaN layer the GaN film surface
morphology is essentially pit-free, wherein the GaN growth
condition for growing the first GaN layer is selected from the
group consisting of a higher growth rate than during growth of the
second GaN layer, a lower growth temperature than during growth of
the second GaN layer, a higher ammonia flow than during growth of
the second GaN layer, and two or more of the foregoing.
2. The method of claim 1, wherein the substrate is sapphire.
3. The method of claim 1, wherein the substrate is selected from
the group consisting of sapphire, silicon, silicon carbide,
diamond, lithium gallate, lithium aluminate, zinc oxide, spinel,
magnesium oxide, and gallium arsenide.
4. The method of claim 1, wherein the substrate has surface
orientation ranging from about 0.degree. and about 5.degree. with
respect to a (0001) crystal orientation.
5. The method of claim 1, wherein the epitaxial AlN layer is
deposited by high temperature reactive sputtering.
6. The method of claim 1, wherein the epitaxial AlN layer is
deposited by a technique selected from a group consisting of
sputtering, molecular beam epitaxy, metal-organic vapor phase
epitaxy, hydride vapor phase epitaxy, and annealing in ammonia.
7. The method of claim 1, wherein the thickness of the deposited
AlN layer is between approximately 0.05 and approximately 2
microns.
8. The method of claim 7, wherein the thickness of the grown first
GaN layer ranges from approximately 2 to approximately 50 microns,
and the thickness of the grown second GaN layer is approximately 3
microns or greater.
9. The method of claim 1, wherein the GaN surface morphology after
growing the first GaN layer is not specular and has a pit coverage
greater than 50%.
10. The method of claim 1, wherein the growth condition for the
first GaN layer includes a first GaN layer growth temperature
ranging from about 900.degree. C. to about 1000.degree. C., a first
GaN layer V:III ratio ranging from about 10 to about 100, and a
first GaN layer growth rate ranging from about 50 .mu.m/hr to about
500 .mu.m/hr.
11. The method of claim 10, wherein the growth condition for the
second GaN layer includes a second GaN layer growth temperature
ranging from about 920.degree. C. to about 1100.degree. C., a
second GaN layer V:III ratio ranging from about 8 to about 80, and
a second GaN layer growth rate ranging from about 5 .mu.m/hr to
about 50 .mu.m/hr.
12. The method of claim 1, wherein the thickness of the grown first
GaN layer ranges from approximately 2 to approximately 50
microns.
13. The method of claim 1, wherein the growth condition for the
second GaN layer includes a growth temperature ranging from about
920.degree. C. to about 1100.degree. C., a V:III ratio ranging from
about 8 to about 80, and a growth rate ranging from about 5
.mu.m/hr to about 500 .mu.m/hr.
14. The method of claim 1, wherein the thickness of the grown
second GaN layer is about 3 microns or greater.
15. The method of claim 1, wherein the thickness of the grown
second GaN layer ranges from approximately 3 to approximately 200
microns.
16. The method of claim 1, wherein the total thickness of the GaN
film after growing the second GaN layer ranges from approximately
10 to approximately 250 microns.
17. The method of claim 1, wherein the ratio of the thickness of
the grown first GaN layer to the thickness of the grown second GaN
layer ranges from approximately 2:1 to approximately 1:5.
18. The method of claim 1, wherein the GaN film has a
characteristic dimension of about 2 inches or greater.
19. The method of claim 1, wherein the threading dislocation
density on the GaN film surface after growing the second GaN layer
is less than 1.times.10.sup.8 cm.sup.-2.
20. The method of claim 1, wherein the threading dislocation
density on the GaN film surface after growing the second GaN layer
is less than 5.times.10.sup.7 cm.sup.-2.
21. The method of claim 1, wherein the threading dislocation
density on the GaN film surface after growing the second GaN layer
is less than 1.times.10.sup.7 cm.sup.-2.
22. The method of claim 1, wherein the threading dislocation
density on the GaN film surface after growing the second GaN layer
is less than 5.times.10.sup.6 cm.sup.-2.
23. The method of claim 1, further including polishing the GaN
film.
24. The method of claim 23, wherein the surface root-mean square
roughness of the GaN film after polishing is about 0.5 nm or
less.
25. The method of claim 1, further including, after growing the
second GaN layer, mechanically lapping a back side of the
substrate.
26. The method of claim 1, wherein the bow of the GaN film on the
substrate is less than 200 microns.
27. The method of claim 1, wherein the bow of the GaN film on the
substrate is less than 100 microns.
28. The method of claim 1, wherein the bow of the GaN film on the
substrate is less than 50 microns.
29. The method of claim 1, wherein the bow of the GaN film on the
substrate is less than 25 microns.
30. A low-defect single-crystal GaN film produced according to the
method of claim 1.
31. A low-defect single-crystal GaN film having a characteristic
dimension of about 2 inches or greater, and a thickness normal to
the characteristic dimension ranging from approximately 10 to
approximately 250 microns, the GaN film including a pit-free
surface, the threading dislocation density on the GaN film surface
being less than 1.times.10.sup.8 cm.sup.-2.
32. A low-defect single-crystal gallium nitride (GaN) on substrate
structure, comprising: a substrate; an epitaxial aluminum nitride
(AlN) layer on the substrate; and a GaN film on the substrate, the
GaN film including a first epitaxial GaN growth layer and a second
epitaxial GaN growth layer, wherein: the first epitaxial GaN layer
is grown on the AlN layer under a growth condition that promotes
the formation of pits, and after growing the first GaN layer the
GaN film surface morphology is rough and pitted; and the second
epitaxial GaN is grown on the first GaN layer by HVPE under a
growth condition that promotes filling of the pits formed, and
after growing the second GaN layer the GaN film surface morphology
is essentially pit-free.
33. The GaN on substrate structure of claim 32, wherein the
substrate is sapphire.
34. The GaN on substrate structure of claim 32, wherein the
substrate is selected from the group consisting of sapphire,
silicon, silicon carbide, diamond, lithium gallate, lithium
aluminate, zinc oxide, spinel, magnesium oxide, and gallium
arsenide.
35. The GaN on substrate structure of claim 32, wherein the
thickness of the deposited AlN layer is between approximately 0.05
and approximately 2 microns, the thickness of the grown first GaN
layer ranges from approximately 2 to approximately 50 microns, and
the thickness of the grown second GaN layer is approximately 3
microns or greater.
36. The GaN on substrate structure of claim 32, wherein the total
thickness of the GaN film ranges from approximately 10 to
approximately 250 microns.
37. The GaN on substrate structure of claim 32, wherein the ratio
of the thickness of the grown first GaN layer to the thickness of
the grown second GaN layer ranges from approximately 2:1 to
approximately 1:5.
38. The GaN on substrate structure of claim 32, wherein the GaN
film has a characteristic dimension of about 2 inches or
greater.
39. The GaN on substrate structure of claim 32, wherein the
threading dislocation density on the GaN film surface is less than
1.times.10.sup.8 cm.sup.-2.
40. The GaN on substrate structure of claim 32, wherein the surface
root-mean square roughness is less than 0.5 nm.
41. The GaN on substrate structure of claim 32, wherein the bow of
the GaN film on the substrate is less than 200 microns.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/749,728, filed Dec. 12, 2005, titled
"Bulk Gallium Nitride Crystals and Method of Making the Same;" U.S.
Provisional Patent Application Ser. No. 60/750,982, filed Dec. 16,
2005, titled "Method of Producing Freestanding Gallium Nitride by
Self-Separation;" U.S. Provisional Patent Application Ser. No.
60/810,537, filed Jun. 2, 2006, titled "Low Defect GaN Films Useful
for Electronic and Optoelectronic Devices and Method of Making the
Same;" U.S. Provisional Patent Application Ser. No. 60/843,036,
filed Sep. 8, 2006, titled "Methods for Making Inclusion-Free
Uniform Semi-Insulating Gallium Nitride Substrate;" and U.S.
Provisional Patent Application Ser. No. 60/847,855, filed Sep. 28,
2006, titled "Method of Producing Single Crystal Gallium Nitride
Substrates by HVPE Method Incorporating a Polycrystalline Layer for
Yield Enhancement," the contents of which are incorporated by
reference herein in their entireties.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates to low-defect density, device-quality
gallium nitride (Al, Ga, In)N films useful for producing electronic
and optoelectronic devices, such as high electron mobility
transistors (HEMTs), heterojunction bipolar transistors (HBTs),
light emitting diodes (blue, UV and white LEDs), and laser diodes
(LDs). The invention also relates to methods for producing such GaN
films.
[0004] 2. Description of the Related Art
[0005] Group III-V nitride compounds such as aluminum nitride
(AlN), gallium nitride (GaN), indium nitride (InN), and alloys such
as AlGaN, InGaN, and AlGaInN, are direct bandgap semiconductors
with bandgap energy ranging from about 0.6 eV for InN to about 6.2
eV for AlN. These materials may be employed to produce light
emitting devices such as LEDs and LDs in short wavelength in the
green, blue and ultraviolet (UV) spectra. Blue and violet laser
diodes may be used for reading data from and writing data to
high-density optical data storage discs, such as those used by
Blu-Ray and HD-DVD systems. By using proper color conversion with
phosphors, blue and UV light emitting diodes may be made to emit
white light, which may be used for energy efficient solid-state
light sources. Alloys with higher bandgaps can be used for UV
photodetectors that are insensitive to solar radiation. The
material properties of the III-V nitride compounds are also
suitable for fabrication of electronic devices that can be operated
at higher temperature, or higher power, and higher frequency than
conventional devices based on silicon (Si) or gallium arsenide
(GaAs).
[0006] Most of the III-V nitride devices are grown on foreign
substrates such as sapphire (Al.sub.2O.sub.3) and silicon carbide
(SiC) because of the lack of available low-cost, high-quality,
large-area native substrates such as GaN substrates. Blue LEDs are
mostly grown on insulating sapphire substrates or semi-conducting
silicon carbide substrates using a metal-organic chemical vapor
deposition (MOCVD) process.
[0007] The MOCVD process is a slow growth rate process with a
growth rate of a few microns per hour. In a typical GaN-based
device growth process, a low-temperature buffer layer of GaN or
Al.sub.xGa.sub.1-xN (x=0-1) is first grown on a foreign substrate
(e.g., sapphire or silicon carbide), followed by the growth of a
few microns of GaN. The active device layer, such as quantum well
structures for LEDs, is subsequently grown. For example, U.S. Pat.
No. 5,563,422 to S. Nakamura et al. describes a GaN-based device
grown by an MOCVD process. A thin GaN nucleation layer of about 10
nanometers is first deposited on a sapphire substrate at a low
temperature of 500-600.degree. C. The GaN nucleation layer is
annealed at high temperature to recrystallize the GaN, and
epitaxial GaN film is grown at higher temperature (approximately
1000-1200.degree. C.).
[0008] Because of the lattice mismatch between gallium nitride and
the non-native substrate, there is a large number of crystal
defects in the GaN film and active device layer. The defect density
in the GaN nucleation layer is thought to be on the order of
10.sup.11 cm.sup.-2 or greater, and in the subsequently grown GaN
layer and active device layer, the typical density of crystal
defects, in particular, the threading dislocation density, is on
the order of 10.sup.9-10.sup.10 cm.sup.-2 or greater in typical
GaN-based LEDs. Despite the high defect density of LEDs grown on
these substrates, commercial low-power blue/white LEDs have long
lifetimes suitable for some applications.
[0009] Group III-V nitride-based laser diodes, however, show a
remarkable dependence of lifetime on the crystal defect density.
The lifetime of these LDs dramatically decreases with the increase
of the dislocation density (see, for example, "Structural defects
related issues of GaN-based laser diodes," S. Tomiya et al., MRS
Symposium Proceedings, Vol. 831, p. 3-13, 2005). Low-defect density
single-crystal gallium nitride is needed for the long lifetime
(>10,000 hours) nitride laser diodes. For LEDs based on an AlGaN
active layer operating at the deeper UV range, it is also found
that dislocation density has a detrimental effect on the
performance and lifetime of the devices. For LEDs operating at
higher power levels, it is also desirable to have a lower defect
density GaN layer.
[0010] There are several growth methods that may possibly be
performed to reduce the defect density of the gallium nitride film.
One common approach in MOCVD growth of gallium nitride is epitaxial
lateral overgrowth (ELOG) and its variations. In an ELOG GaN growth
process, a GaN film is first grown by a MOCVD method with the
2-step process (low-temperature buffer and high-temperature
growth). A dielectric layer such as silicon oxide or silicon
nitride is deposited on the first GaN film. The dielectric layer is
patterned with a photolithographic method and etched so that
portions of GaN surface are exposed and portions of the GaN film
are still covered with the dielectric mask layer. The patterned GaN
film is reloaded into the MOCVD reactor and growth is re-commenced.
The growth condition is chosen such that the second GaN layer can
only be grown on the exposed GaN surface, but not directly on the
masked area. When the thickness of the second GaN layer is thicker
than the dielectric layer, GaN can grow not only along the original
c direction, but also along the sidewalls of the GaN growing out of
the exposed area and gradually covering the dielectric mask. At the
end of the growth, the dielectric mask will be completely covered
by the GaN film and the GaN film overall is quite smooth. However,
the distribution of the threading dislocation density is not
uniform. Since the dislocation density of the first GaN layer is
quite high, the defect density is also high in the area of the
second GaN layer grown directly on the exposed first GaN layer. In
comparison, the defect density is much reduced in the area above
the dielectric mask area where the second GaN layer was grown
laterally in the direction parallel to the surface. The defect
density is still high in the area where the second GaN layer was
grown directly on the first GaN layer and in the area where the
lateral grown GaN coalesced. Multiple ELOG processes can be used to
further reduce the defect density by patterning a second dielectric
mask covering the high defect density areas of the first ELOG GaN
film, and growing GaN film in the ELOG condition that yields a
coalesced second ELOG film.
[0011] The manufacturing cost of the prior-art low defect density
GaN film based on MOCVD is high due to multiple growth and
photolithographic steps. The high cost of the film also increases
the overall manufacturing cost of end products such as UV LEDs.
[0012] Therefore, there is still a compelling need in the art for a
low-cost method of producing high-quality, low defect density GaN
films that are suitable for electronic and optoelectronic devices
to be built on.
SUMMARY
[0013] The present invention generally relates to high-quality
gallium nitride (Al, Ga, In)N films (articles, substrates, layers,
etc.) and methods for making the same.
[0014] According to one implementation, a method for making a
low-defect single-crystal gallium nitride (GaN) film is provided.
An epitaxial aluminum nitride (AlN) layer is deposited on a
substrate. A first epitaxial GaN layer is grown on the AlN layer by
HVPE under a growth condition that promotes the formation of pits,
wherein after growing the first GaN layer the GaN film surface
morphology is rough and pitted. A second epitaxial GaN layer is
grown on the first GaN layer to form a GaN film on the substrate.
The second GaN layer is grown by HVPE under a growth condition that
promotes filling of the pits, and after growing the second GaN
layer the GaN film surface morphology is essentially pit-free.
[0015] According to another implementation, the GaN growth
condition for growing the first GaN layer is selected from the
group consisting of a higher growth rate than during growth of the
second GaN layer, a lower growth temperature than during growth of
the second GaN layer, a higher ammonia flow than during growth of
the second GaN layer, and two or more of the foregoing.
[0016] According to another implementation, a low-defect
single-crystal GaN film produced according to any of the above
methods is provided.
[0017] According to another implementation, a low-defect
single-crystal GaN film is provided. The GaN film has a
characteristic dimension of about 2 inches or greater, and a
thickness normal to the characteristic dimension ranging from
approximately 10 to approximately 250 microns. The GaN film
includes a pit-free surface. The threading dislocation density on
the GaN film surface being less than 1.times.10.sup.8
cm.sup.-2.
[0018] According to another implementation, low-defect
single-crystal gallium nitride (GaN) on substrate structure is
provided. The structure includes a substrate, an epitaxial aluminum
nitride (AlN) layer on the substrate, and a GaN film on the
substrate. The GaN film includes a first epitaxial GaN growth layer
and a second epitaxial GaN growth layer. The first epitaxial GaN
layer is grown on the AlN layer under a growth condition that
promotes the formation of pits, and after growing the first GaN
layer the GaN film surface morphology is rough and pitted. The
second epitaxial GaN is grown on the first GaN layer by HVPE under
a growth condition that promotes filling of the pits formed, and
after growing the second GaN layer the GaN film surface morphology
is essentially pit-free.
[0019] According to any of the above implementations, the threading
dislocation density on the GaN film surface is minimal. In one
example, the threading dislocation density on the surface of the
GaN film may be less than 1.times.10.sup.8 cm.sup.-2, in another
example less than 5.times.10.sup.7 cm.sup.-2, in another example
less than 1.times.10.sup.7 cm.sup.-2, and in another example less
than 5.times.10.sup.6 cm.sup.-2.
[0020] According to any of the above implementations, the amount of
bowing the GaN film on an underlying substrate is minimal. In one
example, the bow of the GaN film may be less than about 200
microns. In another example, the bow of the GaN film may be less
than about 100 microns. In another example, the bow of the GaN film
may be less than about 50 microns. In another example, the bow of
the GaN film may be less than about 25 microns.
[0021] According to any of the above implementations, the surface
of the GaN film may have a root-mean square (RMS) surface roughness
of about 0.5 nm or less.
[0022] Other aspects, features and embodiments of the invention
will be more fully apparent from the ensuing disclosure and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic view of a vertical HVPE reactor.
[0024] FIG. 2 is an optical micrograph at 50.times. magnification
of the surface of a GaN film grown on an AlN-coated sapphire
substrate under a typical HVPE GaN growth condition. The GaN film
thickness was approximately 1 micron.
[0025] FIG. 3 is an optical micrograph at 50.times. magnification
of a GaN film grown on an AlN-coated sapphire substrate under the
same condition as the film shown in FIG. 2, but to a thickness of
approximately 5 microns. Microcracking of the film is visible.
[0026] FIG. 4 is an optical micrograph at 50.times. magnification
of a pitted GaN film, approximately 110 microns thick, grown on
AlN-coated sapphire substrate under a moderate NH.sub.3 partial
pressure growth condition.
[0027] FIG. 5 is a schematic illustration of an example of a growth
process of the present invention.
[0028] FIG. 6 is an optical micrograph at 200.times. magnification
of the surface of a GaN layer at the end of a growth step in which
the surface is pitted, according to one implementation of the
present invention.
[0029] FIG. 7 is a cross-sectional view of a bowed wafer or layer
of material.
[0030] FIG. 8 is an optical micrograph at 50.times. magnification
of the surface of an as-grown 60-micron GaN film on sapphire
substrate, according to one implementation of the present
invention.
[0031] FIG. 9 is a 10.times.10-micron AFM scan of a 60-micron thick
GaN film on sapphire, grown according to one implementation of the
present invention.
DETAILED DESCRIPTION
[0032] Throughout the disclosure, unless otherwise specified,
certain terms are used as follows. "Single crystalline film" or
"single crystal" means a crystalline structure that can be
characterized with x-ray rocking curve measurement. The narrower
the peak of the rocking curve, the better the crystal quality.
"Single crystal" does not necessarily mean that the whole crystal
is a single grain; it may contain many crystalline grains with
orientation more or less aligned. "Polycrystalline film" or
"polycrystal" means that a crystal has many grains whose crystal
orientations are randomly distributed. An X-ray rocking curve
measurement of a polycrystalline film does not exhibit a peak.
"Microcracks" are a cluster of localized cracks with high density
of cracks. The distance between the parallel cracks in the
microcrack cluster is typically less than 100 microns. "Growth
cracks" are the cracks formed during crystal growth. "Cool down
cracks" or "thermal cracks" are the cracks formed after the crystal
growth and during the cooling of the crystal from the growth
temperature to ambient or room temperature. "Pits" are typically
inverse pyramidal pits on the crystal surface. "Pit-free surface"
is a surface essentially having no pits on its surface. "Pitted
surface morphology" means a surface having a substantial amount of
pits on its surface. "Faceted surface morphology" means that a
single crystal film surface is completely covered with pits so that
the sides of the pits become the surface itself and the surface
appears faceted. "Smooth surface morphology" means that a surface
is specular and has no visual defects (such as pits). "Nucleation
layer" or "template layer" in some implementations may be the layer
first grown on a substrate. "V:III ratio" in some implementations
is the ratio of the ammonia flow to the HCl flow used during a
hydride vapor phase epitaxy GaN growth process. "Ammonia partial
pressure" is calculated according to the ammonia flow, the total
gas flow into a reactor, and the reactor pressure. "Growth surface"
or "growing surface" or "growth front" is the surface of the GaN
crystal during the instance of the growth.
[0033] For purposes of the present disclosure, it will be
understood that when a layer (or film, region, substrate,
component, device, or the like) is referred to as being "on" or
"over" another layer, that layer may be directly or actually on (or
over) the other layer or, alternatively, intervening layers (e.g.,
buffer layers, transition layers, interlayers, sacrificial layers,
etch-stop layers, masks, electrodes, interconnects, contacts, or
the like) may also be present. A layer that is "directly on"
another layer means that no intervening layer is present, unless
otherwise indicated. It will also be understood that when a layer
is referred to as being "on" (or "over") another layer, that layer
may cover the entire surface of the other layer or only a portion
of the other layer. It will be further understood that terms such
as "formed on" or "disposed on" are not intended to introduce any
limitations relating to particular methods of material transport,
deposition, fabrication, surface treatment, or physical, chemical,
or ionic bonding or interaction.
[0034] Unless otherwise indicated, terms such as "gallium nitride"
and "GaN" are intended to describe binary, ternary, and quaternary
Group III nitride-based compounds such as, for example, gallium
nitride, indium nitride, aluminum nitride, aluminum gallium
nitride, indium gallium nitride, indium aluminum nitride, and
aluminum indium gallium nitride, and alloys, mixtures, or
combinations of the foregoing, with or without added dopants,
impurities or trace components, as well as all possible crystalline
structures and morphologies, and any derivatives or modified
compositions of the foregoing. Unless otherwise indicated, no
limitation is placed on the stoichiometries of these compounds.
[0035] Single-crystal GaN films can be grown on sapphire substrates
with various vapor phase growth techniques, such as molecular beam
epitaxy (MBE), metal-organic vapor phase epitaxy (MOVPE), and
hydride vapor phase epitaxy (HVPE). In the MBE and MOVPE growth of
GaN films on sapphire, a low-temperature buffer layer is typically
needed to grow high-quality GaN film. It is not clear whether a
buffer layer is needed for HVPE GaN growth on sapphire. Lee in U.S.
Pat. No. 6,528,394 discloses a specific method of pre-treatment for
growing GaN on sapphire using HVPE. The pre-treatment involves
etching sapphire with a gas mixture of hydrochloric acid (HCl) and
ammonia (NH.sub.3), as well as nitridation of the sapphire
substrate. Molnar in U.S. Pat. No. 6,086,673 discloses the use of a
zinc oxide (ZnO) pretreatment layer that was further reacted in the
gaseous environment of HCl and/or NH.sub.3. After this treatment of
sapphire substrate, single-crystal GaN film is then grown by HVPE.
On the other hand, Vaudo et al in U.S. Pat. No. 6,440,823 discloses
the growth of a low defect density GaN layer on sapphire by the
HVPE method, without using any buffer layers or nucleation
layers.
[0036] Since teachings in the prior art regarding sapphire
substrate treatment or initiation prior to HVPE GaN growth are in
conflict, we systematically investigated the growth of gallium
nitride film on sapphire using an HVPE process. Vertical HVPE
reactors were used for the investigation. FIG. 1 schematically
illustrates an example of a vertical HVPE reactor 100. The HVPE
reactor 100 includes a quartz reactor tube 104 that is heated by a
multi-zone furnace 108. The reactor tube 104 is connected to gas
inlets 112, 116, and 120 for introducing reactants, carrier gases,
and diluting gases. The reactor tube 104 is also connected to a
pump and exhaust system 124. In some implementations, inside the
reactor 100, gaseous hydrochloric acid (HCl) is flowed through a
vessel 128 containing gallium metal 132, which is at a temperature
of, for example, about 850.degree. C. The hydrochloric acid reacts
with the gallium metal 132, forming gaseous GaCl, which is
transported by a carrier gas, such as nitrogen, to the deposition
zone in the reactor tube 104. Ammonia (NH.sub.3) and an inert
diluent gas, such as nitrogen, are also flowed to the deposition
zone where GaN crystals are deposited. The reactor 100 is designed
such that the mixing of GaCl and NH.sub.3 does not occur near the
gas outlets, ensuring no deposition of GaN on the outlets of GaCl
and NH.sub.3 and enabling long-term stability of gas flow patterns.
Epi-ready c-plane sapphire substrates or other suitable substrates
136 may be used. The substrate 136 is placed on a rotating platter
140 and heated to a temperature of, for example, 900-1100.degree.
C.
[0037] A typical deposition run process is as follows: (1) a
substrate 136 is placed on the platter 140, (2) the reactor 100 is
sealed, (3) the reactor 100 is evacuated and purged with
high-purity nitrogen to remove any impurities from the system, (4)
the platter 140 with the substrate 136 is raised to the deposition
zone, (5) the platter temperature is controlled at the desired
deposition temperature, (6) ammonia is flowed into the reactor 100,
(7) HCl is flowed to the reactor 100 to start the GaN deposition,
(8) deposition proceeds according to a predetermined recipe for a
predetermined time, (9) the HCl and NH.sub.3 gas flows are stopped,
(10) the platter 140 is lowered and the grown crystal is gradually
cooled down, and (11) the grown crystal is removed for
characterization and further processing.
[0038] After systematically investigating the HVPE growth of GaN on
sapphire substrates, we uncovered several issues that were not
disclosed in the prior art, namely, irreproducible nucleation of
single crystal GaN films on untreated sapphire substrates, and
microcracking of single crystalline GaN films.
[0039] First, we grew various HVPE GaN films directly on sapphire
substrates without any buffer layer or pretreatment under the
conditions taught by the prior art, i.e., a growth temperature of
about 950-1050.degree. C., V:III ratio (i.e., NH.sub.3/HCl) of
about 10-50, and a growth rate of about 100 microns per hour. The
bare sapphire substrate was heated up to the growth temperature,
ammonia flow was turned on first to fill the reactor to a
pre-determined partial pressure and HCl flow was turned on to
initiate the growth. The GaN film grown directly on the bare
sapphire substrate was not smooth. After analyzing the grown GaN
films with an x-ray rocking curve and optical microscope, we
determined that the GaN films grown directly on bare sapphire
substrates were not single-crystalline films. In fact, they were
polycrystalline GaN. We wish not to be bound by any particular
theory regarding the various results of HVPE GaN crystal growth on
sapphire, but the discrepancy in the various prior-art work and our
own work may be related to particular reactor configurations or
surface treatments. The prior art did not teach a reproducible
method to grow single crystal GaN films on sapphire substrate by
HVPE.
[0040] There is a large lattice mismatch between sapphire and
gallium nitride. Furthermore, c-plane GaN is a polar crystal, i.e.,
one face is terminated with gallium and the opposite face of the
crystal is terminated with nitrogen. On the other hand, sapphire is
not a polar crystal; the c-plane of sapphire is terminated with
oxygen on both faces. In other GaN thin-film deposition techniques
such as molecular beam epitaxy (MBE) or metal-organic chemical
vapor deposition (MOCVD), a thin buffer layer is required for the
high-quality single-crystalline GaN growth. The buffer layer may be
an AlN layer (S. Yoshida et al., Appl. Phys. Lett., 42, 427 (1983);
H. Amano et al., Appl. Phys. Lett., 48, 353 (1986)) or a GaN layer
grown at low temperature (S. Nakamura, Jpn. J. Appl. Phys., 30.
L1705 (1991)). Lee in U.S. Pat. No. 6,528,394 postulated the
formation of a thin AlN layer on the sapphire surface by the
pre-treatment step prior to HVPE GaN growth.
[0041] U.S. Pat. No. 6,784,085, the entire contents of which are
incorporated into the present disclosure, discloses a
high-temperature reactive sputtering method for growing
high-quality AlN film on sapphire substrates. Using this method, we
coated sapphire substrates with AlN for use as substrates for HVPE
GaN growth.
[0042] High-quality GaN thin films were successfully and
reproducibly grown on the AlN-coated sapphire substrate. We first
grew a thin layer of AlN film on a sapphire substrate by sputtering
using the method disclosed in U.S. Pat. No. 6,784,085. The typical
thickness of the AlN layer was approximately 0.5-2 microns. X-ray
rocking curve measurement indicated the AlN film was an epitaxial
and single-crystalline film with (0002) rocking curve full width at
half maximum (FWHM) of 50 arcsec. The AlN-coated sapphire substrate
was loaded into the HVPE reactor and a GaN film was grown using the
aforementioned procedure. The growth rate was about 60 microns per
hour, the GaCl partial pressure was about 2.97 Torr, the NH.sub.3
partial pressure was about 44.6 Torr, the V:III ratio was about 15,
and the growth temperature was about 950.degree. C. as measured
with a thermocouple under the platter. The growth time was 1
minute. The GaN film grown was transparent with a smooth, specular
surface. FIG. 2 shows an optical micrograph of the surface of the
GaN film. FIG. 2 shows a typical smooth surface morphology for an
HVPE GaN film with some hillock features. X-ray rocking curve
measurements confirm the single-crystalline nature of the GaN film,
with a FWHM value of 297 arcsec.
[0043] After developing a nucleation process for GaN single
crystalline films on AlN sputter-coated sapphire substrates, we
investigated the growth of thicker GaN films. We discovered a
problem, namely, microcracking in the GaN films. The HVPE growth
conditions were chosen to produce a smooth GaN surface. FIG. 3
shows an optical micrograph of thin GaN film, approximately 5
microns thick on an AlN-coated sapphire substrate, grown under the
same conditions as the film shown in FIG. 2. The surface exhibits a
typical smooth HVPE GaN morphology with hillock features. However,
microcracks in the GaN film are apparent. The sapphire substrate
remains intact without any cracking in this case.
[0044] Because of the difference between the coefficients of
thermal expansion of the sapphire substrate and the GaN film,
thermal stress builds up when the film cools down from the typical
growth temperature of about 1000.degree. C. to ambient room
temperature. As discussed in open literature (for example, E. V.
Etzkom and D. R. Clarke, "Cracking of GaN Films," J. Appl. Phys.,
89 (2001) 1025), sapphire substrate shrinks faster than GaN film
during cool down, causing a compressive stress in the GaN film due
to this thermal expansion mismatch. The compressive thermal stress
in the GaN film should not cause microcracking in the GaN film
during cool down. Therefore, the microcracks must be already formed
during the GaN growth and prior to cool down.
[0045] The microcracking of the GaN film during the growth suggests
a tensile stress in the GaN film during the growth. We wish not be
bound by any particular theory regarding the origin of
microcracking during GaN growth. However, the tensile stress may be
related to the AlN layer employed in the study, or may be related
to the HVPE growth condition used, or may be universal to the vapor
phase GaN growth in general. While cracking is noted in some
instances, most prior-art literature in HVPE GaN growth does not
disclose the formation of microcracks in GaN film during growth.
The prior art also does not teach how to eliminate the microcracks
during the HVPE GaN growth.
[0046] In order to eliminate the microcracks formed during the HVPE
GaN growth, we systematically investigated GaN growth on the
AlN-coated sapphire under various growth conditions by varying
growth parameters, such as GaCl flow or partial pressure (which may
be determined by the flow of hydrochloric acid (HCl)), NH.sub.3
flow or partial pressure, growth temperature, and associated
variables such as growth rate and V:III ratio (e.g., NH.sub.3/HCl
ratio). In this example, the V:III ratio is the ratio of the
NH.sub.3/HCl flow. The growth rate is typically proportional to
GaCl partial pressure, which is directly related to the HCl flow.
We found that the surface morphology of the GaN film varies
substantially with the growth temperature, growth rate and ammonia
partial pressure (or V:III ratio). At a constant growth temperature
and GaCl partial pressure, increasing NH.sub.3 partial pressure
dramatically alters behavior of microcracking and surface
morphology. For a constant growth time (similar film thickness,
about 100 microns), the HVPE GaN surface morphology gradually
changes from a smooth, hillocked morphology with microcracks at low
NH.sub.3 partial pressure, to a surface covered with pits at
moderately high NH.sub.3 partial pressure, and eventually to
polycrystalline material at high NH.sub.3 partial pressure. When
the GaN film is covered with pits, the microcracks formed during
the growth are also eliminated.
[0047] FIG. 4 is a micrograph of a GaN surface grown under
moderately high NH.sub.3 partial pressure (moderate V:III ratio).
This particular GaN film was grown on an AlN-coated sapphire
substrate. The growth rate was about 320 microns per hour, the GaCl
partial pressure was around 1.8 Torr, the NH.sub.3 partial pressure
was around 112.8 Torr, the V:III ratio was around 58, and the
growth temperature was about 990.degree. C. The growth time was 20
minutes. Although the GaN film surface is covered with pits, the
film is still epitaxial single-crystalline film, as confirmed by
x-ray rocking curve measurement, with FWHM of 400 arcsec. The
larger FWHM value of the film is due in part to curvature of the
sample, which is known to broaden the X-ray diffraction peak.
[0048] Similar surface morphology trends are observed with growth
temperature at otherwise constant conditions, or with growth rate
at otherwise constant conditions. Under constant GaCl and NH.sub.3
partial pressures (constant growth rate and V:III ratio), reducing
the growth temperature alters the growth morphology from a smooth,
hillocked structure to a pitted surface morphology and eventually
to a polycrystalline morphology. Similarly, at a constant growth
temperature and V:III ratio, increasing the growth rate (by
increasing both GaCl and NH.sub.3 partial pressure) alters the
surface morphology from a smooth pit-free surface to a pitted
surface morphology and eventually to a polycrystalline
morphology.
[0049] The pitted surface morphology eliminates the microcracks in
the GaN film during the HVPE GaN growth. However, the surface is
not desirable as a foundation of further growth of GaN-based device
structures. The present invention discloses methods for growing
high-quality, low defect density, pit-free and crack-free GaN films
by hydride vapor phase epitaxy. The GaN films are suitable for the
further growth of electronic and optoelectronic devices based on
group III nitride alloys.
[0050] The GaN growth method of the present invention may include
several growth steps, including depositing an epitaxial nitride
template layer on a suitable substrate, growing a thin GaN layer on
the nitride-coated substrate under a condition that yields a
surface covered with pits, and growing a GaN layer on or from the
pitted GaN layer under a condition that fills the pits and yields a
pit-free surface.
[0051] According to this implementation, the first step of the
growth process is to deposit a thin epitaxial nitride (e.g., AlN)
layer on a suitable substrate such as, for example, sapphire. The
purpose of this epitaxial nitride layer is to provide a template
for epitaxial growth of GaN. Without the epitaxial nitride
template, the HVPE GaN film grown on a substrate such as sapphire
under typical conditions is polycrystalline. The epitaxial nitride
layer in one implementation is prepared by high-temperature
reactive sputtering in a sputtering chamber. An aluminum target and
an AC plasma of an inert gas or gas mixture (e.g., an Ar/N.sub.2
gas mixture) may be utilized to deposit the epitaxial nitride layer
on a heated substrate. The epitaxial nitride layer may
alternatively be formed by molecular beam epitaxy (MBE),
metal-organic vapor phase epitaxy (MOVPE or MOCVD), hydride vapor
phase epitaxy, or high-temperature annealing in ammonia. In one
example, the thickness of the epitaxial nitride layer is in the
range (ranges) from about 0.05 to about 2 microns. In another
example, the thickness of the epitaxial nitride layer ranges from
about 0.2 to about 2 microns. Other types of template layers may
alternatively be used, for example, GaN or AlGaN layers, grown by
MOVPE, MBE or HVPE.
[0052] The second step of the growth process is to grow a GaN layer
by hydride vapor phase epitaxy in a growth condition that yields
pitted surface morphology. The nitride-coated substrate is loaded
into a HVPE reactor, and the reactor may be purged with high purity
nitrogen to remove impurities. A layer of gallium nitride is then
grown on the epitaxial nitride layer. The growth condition for this
GaN layer is typically higher growth rate, and/or higher ammonia
flow (or V:III ratio), and/or lower growth temperature than the
"optimal" thin-film growth condition. The "optimal" thin film
growth condition is one that would produce smooth, substantially
pit-free, crack-free thin films (e.g., with a thickness equal to or
less than 3 microns), but would produce microcracked thick films
(e.g., with a thickness equal to or greater than 20 microns). As
one specific example of an optimized growth condition, a 1-micron
thick GaN film that is transparent and has a smooth specular
surface has been grown on an AlN-coated sapphire substrate by the
inventors. The growth rate was about 60 microns per hour, the GaCl
partial pressure was about 3 Torr, the NH.sub.3 partial pressure
was about 45 Torr, the V:III ratio was about 15, the growth
temperature was about 950.degree. C., and the growth time was one
minute. When growing a thin film (.ltoreq.3 .mu.m), this "optimal"
thin-film growth condition typically produces a crack-free film,
whereas when growing a thick film (.gtoreq.20 .mu.m), the "optimal"
growth condition typically produces a microcracked film.
[0053] The GaN film grown under the growth condition of this second
step is very rough and covered with pits. There are two purposes
for this pitted layer: first is to prevent microcracking of GaN
during the growth; and second is to promote annihilation of
dislocations. In one example, the thickness of this pitted layer
ranges from approximately 2 to approximately 50 microns. In another
example, the thickness of this pitted layer ranges from
approximately 5 to approximately 50 microns. If the GaN film is
grown under the pitted growth condition with higher thickness, the
GaN film quality is gradually changed from an epitaxial
single-crystalline film to a polycrystalline film.
[0054] In one implementation, the growth temperature during growth
of the first (pitted) epitaxial GaN layer ranges from about
900.degree. C. to about 1000.degree. C., the V:III ratio ranges
from about 10 to about 100, and the growth rate ranges from about
50 .mu.m/hr to about 500 .mu.m/hr.
[0055] The third step of the growth process is to grow an
additional GaN layer under conditions that cause the pits to be
filled and yield a pit-free and crack-free surface. The growth
condition for this layer is typically lower growth rate, and/or
lower ammonia partial pressure, and/or higher growth temperature
than the growth condition utilized for the pitted layer. The
thickness of this layer is in one example greater than about 3
microns, in another example greater than about 5 microns, and in
another example greater than about 10 microns. In another example,
the thickness of second epitaxial GaN layer ranges from about 3 to
about 200 microns. In another example, the thickness of second
epitaxial GaN layer ranges from about 8 to about 200 microns. The
optimal thickness of the pit-free layer depends on the thickness of
the pitted layer. A thicker layer grown under pitted growth
conditions correspondingly requires a thicker layer grown under
pit-free conditions to completely fill the pits. The ratio of the
thickness of the layer grown under pitted conditions to the
thickness of the layer grown under pit-free condition is in one
example between about 2:1 and about 1:5.
[0056] In one implementation, the growth temperature during growth
of the second epitaxial GaN layer ranges from about 920.degree. C.
to about 1100.degree. C., the V:III ratio ranges from about 8 to
about 80, and the growth rate ranges from about 5 .mu.m/hr to about
500 .mu.m/hr.
[0057] FIG. 5 is a schematic illustration of an exemplary growth
process 500 of the present invention. First, a substrate 504 is
provided. An epitaxial nitride (e.g., AlN) layer 508 is then
deposited on the substrate 504. The deposition of the epitaxial
nitride layer 508 may be done in the same reactor for the
subsequent GaN growth or in a different deposition chamber. GaN
material is subsequently deposited on the nitride-coated substrate
504/508 by hydride vapor phase epitaxy in two steps with different
growth conditions. A first GaN layer 512 is grown under a condition
that results in a pitted surface morphology, and such conditions
are characterized by relatively higher growth rate, and/or high
ammonia flow, and/or lower growth temperature than utilized during
the second GaN growth step. A second GaN layer 516 is then grown
under a condition that fills the pits on the surface 514 of the
first GaN layer 512 and yields pit-free smooth GaN layer, and such
growth conditions are characterized by relatively lower growth
rate, and/or lower ammonia flow, and/or higher growth temperature
than employed in the first, pitted-growth step. The combination of
the two GaN growth steps both eliminates the GaN microcracking
during the growth and provides a smooth, low-defect GaN surface 518
that is suitable for the further growth of devices based on Group
III nitrides. The growth process 500 yields a GaN film generally
depicted at 524 in FIG. 5.
[0058] The substrate 504 may be any substrate that has a surface
having a 3-fold symmetry or close to having a 3-fold symmetry. Some
examples of the present disclosure utilize c-plane sapphire as the
substrate 504. Other substrates 504 such as silicon, silicon
carbide, diamond, lithium gallate, lithium aluminate, zinc oxide,
spinel, magnesium oxide, and gallium arsenide may be utilized for
the growth of low-defect, crack-free GaN films. In one example, the
substrate 504 has a characteristic dimension (e.g., diameter) of
about 2 inches or greater. In other examples, the diameter of the
substrate 504 is about 2'' or greater, about 3'' or greater, about
4'' or greater, or any other suitable size.
[0059] The substrate surface 506 may be exactly c-plane or vicinal
surfaces of the c-plane. Vicinal surfaces may promote step-flow
during the HVPE GaN growth and may yield smoother surface
morphology. The offcut angle of the vicinal surface with respect to
the c-plane in one example ranges from about 0.degree. to about
10.degree., in another example from about 0.10 to about 100, and in
another example from about 0.50 to about 5.degree.. The direction
of offcut may be along the <1-100> direction or along the
<11-20> direction, or along a direction between <1-100>
and <11-20>.
[0060] In some implementations, the deposition of the epitaxial
nitride layer 508 may be needed to grow single-crystalline GaN
films on substrates 504 such as sapphire substrates using the HVPE
process. In one implementation, the epitaxial nitride layer 508 is
deposited by reactive sputtering on a heated substrate 504 in a
sputter deposition chamber. The nitride-coated substrate 504/508 is
subsequently removed from the sputter chamber and loaded into the
HVPE reactor for GaN growth. As alternatives to depositing AlN by
HVPE, other nitride layers, such as AlN grown by MOCVD, GaN grown
by MOCVD, AlGaN grown by MOCVD, and the like may also be used. A
reactive sputtering-deposited AlN layer has the advantage of lower
cost than MOCVD or MBE deposited nitride layers. AlN layers may
also be grown in the HVPE reactor by incorporating an Al source so
that hydrochloric acid reacts with Al to form aluminum chloride
that reacts with ammonia in the deposition zone to form AlN on the
substrate surface 506.
[0061] The growth of GaN film 524 according to this implementation
includes at least two growth steps with different growth
conditions. The growth temperature is typically between 900.degree.
C. and 1100.degree. C., the growth rate is typically between 5 and
500 microns per hour, and V:III ratio is typically between 5 and
100. The two-step GaN growth is characterized by the growth
conditions of the first step having lower growth temperature,
and/or higher ammonia flow, and/or higher growth rate than the
second step. In one example, the growth temperature is about
15.degree. C. hotter in the second step than in the first step, and
the growth rate of the second step is about one-fourth of the first
step. At the end of the first step, the GaN surface 514 is rough
and covered with the pits. If the growth is stopped at the end of
the first step and wafer is taken out of the reactor, the GaN
surface 514 is not specular, as shown in a microphotograph in FIG.
6. The pit coverage, defined as the percentage of a surface covered
with the pits on the surface, is in one example greater than about
50%, and in another example greater than about 75%, and in another
example greater than about 90% at the end of the first GaN growth
step. At the end of the second step, the GaN surface 518 is smooth,
specular and pit-free. The pit coverage in the final film is in one
example less than 1%, in another example less than 0.1%, and in
another example less than 0.01%.
[0062] The resulting GaN film 524 may have a characteristic
dimension (e.g., diameter) as large as the initial substrate 504.
As examples, when a 2'' substrate 504 is utilized, a 2'' GaN film
524 may be obtained. When a 3'' substrate 504 is utilized, a 3''
GaN film 524 may be obtained. When a 4'' substrate 504 is utilized,
a 4'' GaN film 524 may be obtained. The thicknesses of the
respective GaN layers 512 and 516 grown in the two steps is in one
example in a ratio between about 2:1 and about 1:5, and in another
example in a ratio between about 1:1 and about 1:3, and in another
example in a ratio between about 1:1 and about 1:2. The exact
conditions of the two steps may strongly depend on the reactor
configuration and method of temperature measurement, and may be
easily found by those skilled in the arts. The total thickness of
the GaN film 524 in one example ranges from approximately 10 to
approximately 250 microns, in another example from approximately 10
to approximately 200 microns, and in another example from
approximately 20 to approximately 100 microns, and in another
example from approximately 20 to approximately 50 microns.
[0063] The HVPE GaN layers 512 and 516 may be grown without
intentionally introduced impurities. However, because of the
crystal defects and residual impurities such as oxygen and silicon
from the reactor, an unintentionally doped GaN layer may still have
n-type conductivity. The GaN may also be grown with the presence of
intentionally introduced impurities such as silane or oxygen for
n-type doping, or magnesium for p-type doping. When transition
metal impurities are introduced, the GaN film 524 can be made
semi-insulating. Transitional metal impurities, such as iron, may
be introduced using, for example, volatile metal-organic compounds
such as ferrocene. It will be understood that the growth conditions
may be slightly different when the doping impurities are
introduced. In one example, the dopant concentration (e.g., n-type,
p-type, transition metal, etc.) is greater than about
1.times.10.sup.18 cm.sup.-2. In one example of a semi-insulating
GaN film 524 produced according to the present disclosure, the GaN
film 524 has a resistivity greater than about 1.times.10.sup.5
ohm-cm.
[0064] Because of the thermal mismatch between the substrate 504
and the GaN film 524, the wafer is bowed after cool-down from the
growth temperature to the ambient temperature. The bow of the wafer
complicates the device fabrication process and a large bow of the
wafer is not desirable. One aspect of the present invention is that
the GaN material during growth develops a tensile stress that will
compensate the thermal stress and reduce the wafer bow. The tensile
stress of the GaN material during the growth is associated with the
reduction of dislocations in the GaN material. In another
implementation of the present invention, a thicker substrate 504
may also be employed to reduce the GaN film bow. In another
implementation, the backside of the substrate 504 is mechanically
lapped to introduce damage on the backside of the substrate 504,
which reduces the bow of the GaN film 524 on the substrate 504. In
one example, the wafer bow is less than about 200 microns. In
another example, the wafer bow is less than about 100 microns. In
another example, the wafer bow is less than about 50 microns. In
another example, the wafer bow is less than about 25 microns. Wafer
bow may be defined as the deviation of the center point of the
median surface of the wafer from a median-surface reference plane
of the wafer.
[0065] As an example of wafer bowing, FIG. 7 illustrates a bowed
wafer 704 having a bowed median surface 708 with a center point
712. A median surface reference plane 716 with a center point 720
is established by three equally-spaced points on the median surface
at the wafer circumference. In this example, the wafer bow b,
projected to the right of the bowed wafer 704, is the distance
between the center point 712 in the median surface of a free
unclamped wafer and the center point 720 in the median surface
reference plane 716. It will be understood that the radius of
curvature of the bowed wafer 704 as depicted in FIG. 7 is
exaggerated for illustrative purposes.
[0066] The crystal defect density, specifically, threading
dislocation density, decreases with the thickness of the GaN film
grown. In implementations described in the present disclosure, the
lattice mismatch between the GaN material and substrate that
generates dislocation is first accommodated by the AlN layer. The
dislocations in the GaN material are further annihilated during the
two-step GaN growth. The reduction of dislocation density during
HVPE GaN growth according to implementations described in the
present disclosure is much faster than those disclosed in the prior
arts. For example, U.S. Pat. Nos. 6,533,874 and 6,156,581 disclose
a GaN base structure grown by an HVPE process. According to the
prior art, the dislocation density of a 10-micron thick GaN film
grown by HVPE on sapphire is approximately 10.sup.9 cm.sup.-2, and
the dislocation density is reduced to approximately 10.sup.8
cm.sup.-2 for a 23-micron GaN film, and to approximately 10.sup.7
cm.sup.-2 for a 300-micron GaN film. In implementations of the
present invention, improved GaN films have been grown by HVPE on
sapphire, as represented by the following examples: a dislocation
density on the surface less than 10.sup.8 cm.sup.-2 for a 10-micron
GaN film, less than 5.times.10.sup.7 cm.sup.-2 for a 20-micron GaN
film, and less than 2.times.10.sup.7 cm.sup.-2 for a 50-micron GaN
film. The surface dislocation density of GaN film grown according
to implementations of the present invention is approximately
several factors lower than GaN films of the prior art at similar
thickness. According to some examples of the invention, the
threading dislocation density on the surface of the GaN film may be
less than 1.times.10.sup.8 cm.sup.-2, in other examples less than
5.times.10.sup.7 cm.sup.-2, in other examples less than
1.times.10.sup.7 cm.sup.-2, and in other examples less than
5.times.10.sup.6 cm.sup.-2.
[0067] The wafer structure and method for making the structure of
the present invention differ substantially from the prior art of
U.S. Pat. Nos. 6,533,874 and 6,156,581. We were not able to grow
device-quality epitaxial single-crystal GaN films using the methods
taught by prior art such as in these patent references. By
contrast, in accordance with the present invention, including the
use of the epitaxial nitride template layer 508 (FIG. 5) described
above, we can reproducibly grow device-quality epitaxial
single-crystal GaN films by HVPE. Additionally, the present
invention discloses methods for eliminating GaN film microcracking
during HVPE GaN growth. Microcracking of GaN film during the HVPE
growth and methods for eliminating the growth microcracking have
not been disclosed in the prior art. Implementations of the present
invention employ a two-step HVPE GaN growth process to eliminate
the growth microcracking and to produce smooth surfaces on the GaN
films.
[0068] Low-defect single-crystal film of Group III nitride alloys,
Al.sub.xGa.sub.yIn.sub.zN (x+y+z=1, 0<=x<=1, 0<=y<=1,
0<=z<=1), may be similarly grown according to additional
embodiments of the present invention. An AlN nucleation layer is
first deposited on a substrate. Single-crystal
Al.sub.xGa.sub.yIn.sub.zN film is grown on the AlN-coated substrate
by HVPE using the two-step growth process described above. The
Al.sub.xGa.sub.yIn.sub.zN film is grown under a condition that
yields a pitted surface morphology in the first step and then under
a growth condition that promotes filling the pits to produce a
smooth surface morphology in the second step. Typically, the first
step has a lower growth temperature, and/or higher growth rate,
and/or higher ammonia flow than the second growth step. The exact
condition for the two-step Al.sub.xGa.sub.yIn.sub.zN growth depends
on the reactor configuration and film composition, and may be
easily determined by those skilled in the art. Thus, as previously
noted, the term "GaN" encompasses "Al.sub.xGa.sub.yIn.sub.zN."
[0069] The surface morphology of the low-defect GaN film 524 may be
further improved by using a chemical mechanical polish (CMP). The
as-grown HVPE GaN film may exhibit some hillock features as shown
in FIG. 8. In some applications, the macroscopic roughness of the
GaN film surface 518 (FIG. 5) is less desirable for further device
layer growth. The GaN film surface 518 may be improved by chemical
mechanical polish. The CMP process does not produce surface and
subsurface damage on the GaN film surface 518 because of the active
chemical etching during the polish.
[0070] The present invention can be further understood by following
illustrative, non-limiting examples.
EXAMPLE 1
Low-Defect GaN Film Growth
[0071] In this example, we illustrate the growth of a high-quality,
low-defect GaN film suitable for the further growth of electronic
and optoelectronic devices. A 2''-diameter, 430-micron thick
sapphire was used as the starting substrate. Using the sputtering
method disclosed in U.S. Pat. No. 6,784,085, an AlN layer
approximately 0.7 .mu.m thick was grown on the sapphire substrate
for use as a template layer for the HVPE GaN growth. X-ray
diffraction was used to verify the AlN film was single-crystal. The
AlN/sapphire structure was loaded into a vertical HVPE system and
the GaN growth was commenced.
[0072] The HVPE GaN film was grown by a two-step method. The GaN
film was first grown under conditions of growth rate of
approximately 260 microns per hour, growth temperature of
955.degree. C., HCl flow rate of 92 sccm, and NH.sub.3 flow rate of
2500 sccm. After growth of approximately 4 minutes under these
growth conditions, the growth rate was reduced to approximately 65
microns per hour by reducing HCl flow to 23 sccm, and growth
temperature was raised by 20.degree. C. After growth of
approximately 7 minutes under these conditions, the NH.sub.3 flow
was further reduced to 750 sccm for approximately 32 minutes. The
total grown GaN film thickness was approximately 60 microns. The
bow of the wafer was approximately 190 microns. The GaN film was
specular visually, and under optical microscope observation,
hillock features were present on the surface as shown in FIG.
8.
[0073] An atomic force microscope (AFM) was used to image the wafer
surface and to measure the threading dislocation density. A
threading dislocation terminates on the surface as a pit that can
be observed with AFM. FIG. 9 is a 10-micron by 10-micron AFM scan
of the wafer surface. The pit density, i.e., the threading
dislocation density on the surface, was approximately
1.9.times.10.sup.7 cm.sup.-2.
EXAMPLE 2
Low-defect GaN Film Growth
[0074] In this example, we illustrate the growth of another
high-quality, low-defect GaN film suitable for the further growth
of electronic and optoelectronic devices. A 2''-diameter 430-micron
thick sapphire was used as the starting substrate. Using the
sputtering method disclosed in U.S. Pat. No. 6,784,085, an AlN
layer approximately 0.7 .mu.m thick was grown on the sapphire
substrate for use as a template layer for the HVPE GaN growth. The
AlN/sapphire structure was loaded into a vertical HVPE system and
the GaN growth was commenced.
[0075] The HVPE GaN film was grown by a two-step method. The GaN
film was first grown under conditions of growth rate of
approximately 260 microns per hour, growth temperature of
955.degree. C., HCl flow rate of 92 sccm, and NH.sub.3 flow rate of
2500 sccm. After growth of approximately 3 minutes under these
growth conditions, the growth rate was reduced to approximately 30
microns per hour by reducing the HCl flow rate to 10 sccm. At the
same time, the growth temperature was raised by 20.degree. C. and
the NH.sub.3 flow rate was reduced to 400 sccm for an additional 25
minutes. The total grown GaN film thickness was approximately 25
microns. The bow of the wafer was approximately 95 microns. The GaN
film was specular visually, and under optical microscope
observation hillock features were present on the surface.
EXAMPLE 3
Low-defect GaN Film Growth with Lapping Treatment
[0076] The GaN film on sapphire obtained from Example 2 is mounted
on a stainless steel plate using wax with the GaN film facing the
plate. The backside of the sapphire substrate is lapped on a metal
lapping plate with 30-micron diamond slurry. After removing
approximately 10 microns from the backside of the sapphire
substrate, the wafer bow is reduced from approximately 95 microns
to approximately 40 microns.
EXAMPLE 4
Low-Defect GaN Film Growth with Polishing Treatment
[0077] The GaN film on sapphire obtained from Example 3 is mounted
on a stainless steel plate using wax with the GaN film facing up.
The surface of the GaN film is then chemical mechanically polished
to remove approximately one micron of surface material. The
root-mean square (RMS) surface roughness of the GaN film is reduced
from approximately 5 nm for the as-grown film to approximately 0.5
nm or less for the CMP polished surface.
[0078] The examples of the present invention utilized several
specific growth sequences. It should be understood that these
specific growth process are meant for purposes of illustration and
not to be limiting. It should also be noted that growth conditions
cited in the examples are specific to the HVPE growth reactor used
in the examples. Different reactor design or reactor geometry may
need a different condition to achieve similar results. However, the
general trends are still similar.
[0079] It will be apparent to those skilled in the art that various
modifications and variations can be made in the growth of low
defect density GaN film within the scope of the present invention.
Thus it is construed that the present invention covers the
variations and modifications of this invention provided they come
within the scope of the appended claims and their equivalent.
* * * * *