U.S. patent application number 10/413691 was filed with the patent office on 2003-10-23 for non-polar a-plane gallium nitride thin films grown by metalorganic chemical vapor deposition.
Invention is credited to Craven, Michael D., Speck, James Stephen.
Application Number | 20030198837 10/413691 |
Document ID | / |
Family ID | 29250928 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030198837 |
Kind Code |
A1 |
Craven, Michael D. ; et
al. |
October 23, 2003 |
Non-polar a-plane gallium nitride thin films grown by metalorganic
chemical vapor deposition
Abstract
Non-polar (11{overscore (2)}0) a-plane gallium nitride (GaN)
films with planar surfaces are grown on (1{overscore (1)}02)
r-plane sapphire substrates by employing a low temperature
nucleation layer as a buffer layer prior to a high temperature
growth of the non-polar (11{overscore (2)}0) a-plane GaN thin
films.
Inventors: |
Craven, Michael D.; (Goleta,
CA) ; Speck, James Stephen; (Goleta, CA) |
Correspondence
Address: |
GATES & COOPER LLP
HOWARD HUGHES CENTER
6701 CENTER DRIVE WEST, SUITE 1050
LOS ANGELES
CA
90045
US
|
Family ID: |
29250928 |
Appl. No.: |
10/413691 |
Filed: |
April 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60372909 |
Apr 15, 2002 |
|
|
|
Current U.S.
Class: |
428/698 ;
257/E21.113; 427/314; 427/398.1 |
Current CPC
Class: |
C30B 29/605 20130101;
H01L 21/0242 20130101; H01L 21/02639 20130101; H01L 21/0237
20130101; H01L 21/02609 20130101; C30B 25/18 20130101; C30B 29/406
20130101; C30B 25/04 20130101; C30B 25/02 20130101; H01L 21/02433
20130101; H01L 21/02458 20130101; H01L 21/02647 20130101; C30B
29/403 20130101; H01L 21/0254 20130101; C30B 25/105 20130101; H01L
21/0262 20130101 |
Class at
Publication: |
428/698 ;
427/314; 427/398.1 |
International
Class: |
B05D 003/02; B32B
009/00 |
Claims
What is claimed is:
1. A method of growing a non-polar a-plane gallium nitride thin
film on an r-plane substrate through metalorganic chemical vapor
deposition, comprising: (a) annealing the substrate; (b) depositing
a nitride-based nucleation layer on the substrate; (c) growing the
non-polar a-plane gallium nitride film on the nucleation layer; and
(d) cooling the non-polar a-plane gallium nitride film under a
nitrogen overpressure.
2. The method of claim 1, wherein the substrate is an r-plane
sapphire substrate.
3. The method of claim 2, wherein an in-plane orientation of the
gallium nitride film with respect to the r-plane substrate is
[0001.sub.]GaN.parallel.[{overscore (1)}101].sub.sapphire and
[{overscore (1)}100].sub.GaN.parallel.[11{overscore
(2)}0].sub.sapphire.
4. The method of claim 1, wherein the substrate is selected from a
group comprising silicon carbide, gallium nitride, silicon, zinc
oxide, boron nitride, lithium aluminate, lithium niobate,
germanium, aluminum nitride, and lithium gallate.
5. The method of claim 1, wherein the annealing step (a) comprises
a high temperature annealing of the substrate.
6. The method of claim 1, wherein the depositing step (b) comprises
a low temperature deposit of the nitride-based nucleation layer on
the substrate.
7. The method of claim 1, wherein the depositing step (b) comprises
a low pressure deposit of the nitride-based nucleation layer on the
substrate.
8. The method of claim 1, wherein the low temperature depositing
conditions comprise approximately 400-900.degree. C. and
atmospheric pressure.
9. The method of claim 1, wherein the depositing step (b) initiates
gallium nitride growth on the r-plane substrate.
10. The method of claim 1, wherein the nucleation layer comprises
1-100 nanometers of gallium nitride.
11. The method of claim 1, wherein the growing step (b) comprises a
high temperature growth of the non-polar a-plane gallium nitride
film on the nucleation layer.
12. The method of claim 11, wherein the high temperature layer is
deposited at 0.2 atmospheres or less.
13. The method of claim 11, wherein the high temperature growth
conditions comprise approximately 1100.degree. C. growth
temperature, approximately 0.2 atmosphere or less growth pressure,
30 .mu.mol per minute gallium flow, and 40,000 .mu.mol per minute
nitrogen flow.
14. The method of claim 1, wherein the growing step (b) produces
the planar gallium nitride film.
15. A device manufactured using the method of claim 1.
16. A non-polar a-plane gallium nitride thin film on an r-plane
substrate, wherein the thin film is created using a process
comprising: (a) annealing the substrate; (b) depositing a
nitride-based nucleation layer on the substrate; (c) growing the
non-polar a-plane gallium nitride film on the nucleation layer; and
(d) cooling the non-polar a-plane gallium nitride film under a
nitrogen overpressure.
17. The thin film of claim 16, wherein the substrate is an r-plane
sapphire substrate.
18. The thin film of claim 17, wherein an in-plane orientation of
the gallium nitride films with respect to the r-plane substrate is
[0001].sub.GaN.parallel.[{overscore (1)}101].sub.sapphire and
[{overscore (1)}100].sub.GaN.parallel.[11{overscore
(2)}0].sub.sapphire.
19. The thin film of claim 16, wherein the substrate is selected
from a group comprising silicon carbide, gallium nitride, silicon,
zinc oxide, boron nitride, lithium aluminate, lithium niobate,
germanium, aluminum nitride, and lithium gallate.
20. The thin film of claim 16, wherein the annealing step (a)
comprises a high temperature annealing of the substrate.
21. The thin film of claim 16, wherein the depositing step (b)
comprises a low temperature deposit of the nitride-based nucleation
layer on the substrate.
22. The thin film of claim 16, wherein the depositing step (b)
comprises a low pressure deposit of the nitride-based nucleation
layer on the substrate.
23. The thin film of claim 16, wherein the low temperature
depositing conditions comprise approximately 400-900.degree. C. and
atmospheric pressure.
24. The thin film of claim 16, wherein the depositing step (b)
initiates gallium nitride growth on the r-plane substrate.
25. The thin film of claim 16, wherein the nucleation layer
comprises 1-100 nanometers of gallium nitride.
26. The thin film of claim 16, wherein the growing step (b)
comprises a high temperature growth of the non-polar a-plane
gallium nitride films on the nucleation layer.
27. The thin film of claim 26, wherein the high temperature layer
is deposited at 0.2 atmospheres or less.
28. The thin film of claim 26, wherein the high temperature growth
conditions comprise approximately 1100.degree. C. growth
temperature, approximately 0.2 atmosphere or less growth pressure,
30 .mu.mol per minute gallium flow, and 40,000 .mu.mol per minute
nitrogen flow.
29. The thin film of claim 16, wherein the growing step (b)
produces a planar gallium nitride film.
30. A structure having a non-polar a-plane gallium nitride thin
film on an r-plane substrate, comprising: (a) an annealed
substrate; (b) a nitride-based nucleation layer deposited on the
substrate; and (c) a non-polar a-plane gallium nitride film grown
on the nucleation layer and cooled under a nitrogen
overpressure.
31. The structure of claim 30, wherein the substrate is an r-plane
sapphire substrate.
32. The structure of claim 31, wherein an in-plane orientation of
the gallium nitride film with respect to the r-plane substrate is
[0001].sub.GaN.parallel.[{overscore (1)}101].sub.sapphire and
[{overscore (1)}100].sub.GaN.parallel.[11{overscore
(2)}0].sub.sapphire.
33. The structure of claim 30, wherein the substrate is selected
from a group comprising silicon carbide, gallium nitride, silicon,
zinc oxide, boron nitride, lithium aluminate, lithium niobate,
germanium, aluminum nitride, and lithium gallate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of the following copending and commonly-assigned U.S.
Provisional Patent Application Serial No. 60/372,909, entitled
"NON-POLAR GALLIUM NITRIDE BASED THIN FILMS AND HETEROSTRUCTURE
MATERIALS," filed on Apr. 15, 2002, by Michael D. Craven, Stacia
Keller, Steven P. DenBaars, Tal Margalith, James S. Speck, Shuji
Nakamura, and Umesh K. Mishra, attorneys docket number
30794.95-U.S. Pat. No. 1, which application is incorporated by
reference herein.
[0002] This application is related to the following co-pending and
commonly-assigned United States Utility Patent Applications:
[0003] Ser. No. ______, entitled "NON-POLAR (Al,B,IN,GA)N QUANTUM
WELL AND HETEROSTRUCTURE MATERIALS AND DEVICES," filed on same date
herewith, by Michael D. Craven, Stacia Keller, Steven P. DenBaars,
Tal Margalith, James S. Speck, Shuji Nakamura, and Umesh K. Mishra,
attorneys docket number 30794.101-US-U1; and
[0004] Ser. No. ______, entitled "DISLOCATION REDUCTION IN
NON-POLAR GALLIUM NITRIDE THIN FILMS," filed on same date herewith,
by Michael D. Craven, Steven P. DenBaars and James S. Speck,
attorneys docket number 30794.102-US-U1;
[0005] both of which applications are incorporated by reference
herein.
1. FIELD OF THE INVENTION
[0006] The invention is related to semiconductor materials,
methods, and devices, and more particularly, to non-polar a-plane
gallium nitride (GaN) thin films grown by metalorganic chemical
vapor deposition (MOCVD).
2. DESCRIPTION OF THE RELATED ART
[0007] (Note: This application references a number of different
patents, applications and/or publications as indicated throughout
the specification by one or more reference numbers. A list of these
different publications ordered according to these reference numbers
can be found below in the section entitled "References." Each of
these publications is incorporated by reference herein.)
[0008] Polarization in wurtzite III-nitride compounds has attracted
increased attention due to the large effect polarization-induced
electric fields have on heterostructures commonly employed in
nitride-based optoelectronic and electronic devices. Nitride-based
optoelectronic and electronic devices are subject to
polarization-induced effects because they employ nitride films
grown in the polar c-direction [0001], the axis along which the
spontaneous and piezoelectric polarization of nitride films are
aligned. Since the total polarization of a nitride film depends on
the composition and strain state, discontinuities exist at
interfaces between adjacent device layers and are associated with
fixed sheet charges that give rise to internal electric fields.
[0009] Polarization-induced electric fields, although advantageous
for two-dimensional electron gas (2DEG) formation in nitride-based
transistor structures, spatially separate electrons and hole wave
functions in quantum well (QW) structures, thereby reducing carrier
recombination efficiencies in QW based devices, such as laser
diodes and light emitting diodes. See References 1. A corresponding
reduction in oscillator strength and red-shift of optical
transitions have been reported for AlGaN/GaN and GaN/InGaN quantum
wells grown along the GaN c-axis. See References 2-7.
[0010] A potential means of eliminating the effects of these
polarization-induced fields is through the growth of structures in
directions perpendicular to the GaN c-axis (non-polar) direction.
For example, m-plane AlGaN/GaN quantum wells have recently been
grown on lithium aluminate substrates via plasma-assisted molecular
beam epitaxy (MBE) without the presence of polarization-induced
electric fields along the growth direction. See Reference 8.
[0011] Growth of a-plane nitride semiconductors also provides a
means of eliminating polarization-induced electric field effects in
wurtzite nitride quantum structures. For example, in the prior art,
a-plane GaN growth had been achieved on r-plane sapphire via MOCVD
and molecular beam epitaxy (MBE). See References 9-15. However, the
film growth reported by these early efforts did not utilize a low
temperature buffer layer and did not possess smooth planar
surfaces, and therefore, these layers were poorly suited for
heterostructure growth and analysis. Consequently, there is a need
for improved methods of growing films that exhibit improved surface
and structural quality as compared to previously reported growth of
GaN on r-plane sapphire via MOCVD.
SUMMARY OF THE INVENTION
[0012] The present invention describes a method for growing
device-quality non-polar aplane GaN thin films via MOCVD on r-plane
sapphire substrates. The present invention provides a pathway to
nitride-based devices free from polarization-induced effects, since
the growth direction of non-polar a-plane GaN thin films is
perpendicular to the polar c-axis. Polarization-induced electric
fields will have minimal effects, if any, on (Al,B,In,Ga)N device
layers grown on non-polar a-plane GaN thin films.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0014] FIG. 1 is a flowchart that illustrates the steps of the
MOCVD process for the growth of non-polar (11{overscore (2)}0)
a-plane GaN thin films on (1{overscore (1)}20) r-plane sapphire,
according to the preferred embodiment of the present invention;
[0015] FIG. 2(a) shows a 2.theta.-.omega. diffraction scan that
identifies the growth direction of the GaN film as (1{overscore
(1)}20) a-plane GaN;
[0016] FIG. 2(b) is a compilation of off-axis .phi. scans used to
determine the in-plane epitaxial relationship between GaN and
r-sapphire, wherein the angle of inclination .psi. used to access
the off-axis reflections is noted for each scan;
[0017] FIG. 2(c) is a schematic illustration of the epitaxial
relationship between the GaN and r-plane sapphire;
[0018] FIGS. 3(a) and 3(b) are cross-sectional and plan-view
transmission electron microscopy (TEM) images, respectively, of the
defect structure of the a-plane GaN films on r-plane sapphire;
and
[0019] FIGS. 4(a) and 4(b) are atomic force microscopy (AFM)
amplitude and height images, respectively, of the surface of the
as-grown a-plane GaN films.
DETAILED DESCRIPTION OF THE INVENTION
[0020] In the following description of the preferred embodiment,
reference is made to the accompanying drawings which form a part
hereof, and in which is shown by way of illustration a specific
embodiment in which the invention may be practiced. It is to be
understood that other embodiments may be utilized and structural
changes may be made without departing from the scope of the present
invention.
[0021] Overview
[0022] The present invention describes a method for growing device
quality non-polar (11{overscore (2)}0) a-plane GaN thin films via
MOCVD on (1{overscore (1)}02) r-plane sapphire substrates. The
method employs a low-temperature buffer layer grown at atmospheric
pressure to initiate the GaN growth on r-plane sapphire.
Thereafter, a high temperature growth step is performed at low
pressures, e.g., .about.0.1 atmospheres (atm) in order to produce a
planar film.
[0023] Planar growth surfaces have been achieved using the present
invention. Specifically, the in-plane orientation of the GaN with
respect to the r-plane sapphire substrate has been confirmed to be
[0001].sub.GaN.parallel.[{overscore (1)}101].sub.sapphire and
.parallel.[{overscore (1)}100].sub.GaN.parallel.[11{overscore
(2)}0].sub.sapphire.
[0024] The resulting films possess surfaces that are suitable for
subsequent growth of (Al,B,In,Ga)N device layers. Specifically,
polarization-induced electric fields will have minimal effects, if
any, on (Al,B,In,Ga)N device layers grown on non-polar a-plane GaN
base layers.
[0025] Process Steps
[0026] FIG. 1 is a flowchart that illustrates the steps of the
MOCVD process for the growth of non-polar (11{overscore (2)}0)
a-plane GaN thin films on a (1{overscore (1)}20) r-plane sapphire
substrate, according to the preferred embodiment of the present
invention. The growth process was modeled after the two-step
process that has become the standard for the growth of c-GaN on
c-sapphire. See Reference 16.
[0027] Block 100 represents loading of a sapphire substrate into a
vertical, close-spaced, rotating disk, MOCVD reactor. For this
step, epi-ready sapphire substrates with surfaces
crystallographically oriented within +/-2.degree. of the sapphire
r-plane (1{overscore (1)}20) may be obtained from commercial
vendors. No ex-situ preparations need be performed prior to loading
the sapphire substrate into the MOCVD reactor, although ex-situ
cleaning of the sapphire substrate could be used as a precautionary
measure.
[0028] Block 102 represents annealing the sapphire substrate
in-situ at a high temperature (>1000.degree. C.), which improves
the quality of the substrate surface on the atomic scale. After
annealing, the substrate temperature is reduced for the subsequent
low temperature nucleation layer deposition.
[0029] Block 104 represents depositing a thin, low temperature, low
pressure, nitride-based nucleation layer as a buffer layer on the
sapphire substrate. In the preferred embodiment, the nucleation
layer is comprised of, but is not limited to, 1-100 nanometers (nm)
of GaN and is deposited at low temperature, low pressure depositing
conditions of approximately 400-900.degree. C. and 1 atm. Such
layers are commonly used in the heteroepitaxial growth of c-plane
(0001) nitride semiconductors. Specifically, this depositing step
initiates GaN growth on the r-plane sapphire substrate.
[0030] After depositing the nucleation layer, the reactor
temperature is raised to a high temperature, and Block 106
represents growing the non-polar (11{overscore (2)}0) a-plane GaN
thin films on the substrate. In the preferred embodiment, the high
temperature growth conditions comprise, but are not limited to,
approximately 1100.degree. C. growth temperature, approximately 0.2
atm or less growth pressure, 30 .mu.mol per minute Ga flow, and
40,000 .mu.mol per minute N flow, thereby providing a V/III ratio
of approximately 1300). In the preferred embodiment, the precursors
used as the group III and group V sources are trimethylgallium and
ammonia, respectively, although alternative precursors could be
used as well. In addition, growth conditions may be varied to
produce different growth rates, e.g., between 5 and 9 .ANG. per
second, without departing from the scope of the present invention.
Non-polar GaN approximately 1.5 .mu.m thick have been grown and
characterized.
[0031] Upon completion of the high temperature growth step, Block
108 represents cooling the non-polar (11{overscore (2)}0) a-plane
GaN thin films under a nitrogen overpressure.
[0032] Finally, Block 110 represents the end result of the
processing steps, which is a nonpolar (11{overscore (2)}0) a-plane
GaN film on an r-plane sapphire substrate. Potential device layers
to be manufactured using these process steps to form a non-polar
(11{overscore (2)}0) a-plane GaN base layer for subsequent device
growth include laser diodes (LDs), light emitting diodes (LEDs),
resonant cavity LEDs (RC-LEDs), vertical cavity surface emitting
lasers (VCSELs), high electron mobility transistors (HEMTs),
heterojunction bipolar transistors (HBTs), heterojunction field
effect transistors (HFETs), and UV and near-UV photodetectors.
[0033] Experimental Results
[0034] The crystallographic orientation and structural quality of
the as-grown GaN films and r-plane sapphire were determined using a
Philips.TM. four-circle, high-resolution, x-ray diffractometer
(HR-XRD) operating in receiving slit mode with four bounce
Ge(220)-monochromated Cu K.alpha. radiation and a 1.2 mm slit on
the detector arm. Convergent beam electron diffraction (CBED) was
used to determine the polarity of the a-GaN films with respect to
the sapphire substrate. Plan-view and cross-section transmission
electron microscopy (TEM) samples, prepared by wedge polishing and
ion milling, were analyzed to define the defect structure of a-GaN.
A Digital Instruments D3000 Atomic Force Microscope (AFM) in
tapping mode produced images of the surface morphology.
[0035] FIG. 2(a) shows a 2.theta.-.omega. diffraction scan that
identifies the growth direction of the GaN film as (11{overscore
(2)}0) a-plane GaN. The scan detected sapphire (1{overscore
(1)}02), (2{overscore (2)}04), and GaN (11{overscore (2)}0)
reflections. Within the sensitivity of these measurements, no GaN
(0002) reflections corresponding to 2.theta.=34.604.degree. were
detected, indicating that there is no c-plane (0002) content
present in these films, and thus instabilities in the GaN growth
orientation are not a concern.
[0036] FIG. 2(b) is a compilation of off-axis .phi. scans used to
determine the in-plane epitaxial relationship between GaN and
r-sapphire, wherein the angle of inclination .psi. used to access
the off-axis reflections is noted for each scan. Having confirmed
the a-plane growth surface, off-axis diffraction peaks were used to
determine the in-epitaxial relationship between the GaN and the
r-sapphire. Two sample rotations .phi. and .psi. were adjusted in
order to bring off-axis reflections into the scattering plane of
the diffractometer, wherein .phi. is the angle of rotation about
the sample surface normal and .psi. is the angle of sample tilt
about the axis formed by the intersection of the Bragg and
scattering planes. After tilting the sample to the correct .psi.
for a particular off-axis reflection, .phi. scans detected GaN
(10{overscore (1)}0), (10{overscore (1)}1), and sapphire (0006)
peaks, as shown in FIG. 2(b). The correlation between the .phi.
positions of these peaks determined the following epitaxial
relationship: [0001].sub.GaN.parallel.[{overscore
(1)}101].sub.sapphire and [{overscore
(1)}100].sub.GaN.parallel.[11{overscore (2)}0].sub.sapphire.
[0037] FIG. 2(c) is a schematic illustration of the epitaxial
relationship between the GaN and r-plane sapphire. To complement
the x-ray analysis of the crystallographic orientation, the a-GaN
polarity was determined using CBED. The polarity's sign is defined
by the direction of the polar Ga--N bonds aligned along the GaN
c-axis; the positive c-axis [0001] points from a gallium atom to a
nitrogen atom. Consequently, a gallium-face c-GaN film has a [0001]
growth direction, while a nitrogen-face c-GaN crystal has a
[000{overscore (1)}] growth direction. For a-GaN grown on
r-sapphire, [0001].sub.GaN is aligned with the sapphire c-axis
projection [{overscore (1)}101].sub.sapphire, and therefore, the
epitaxial relationships defined above are accurate in terms of
polarity. Consequently, the positive GaN c-axis points in same
direction as the sapphire c-axis projection on the growth surface
(as determined via CBED). This relationship concurs with the
epitaxial relationships previously reported by groups using a
variety of growth techniques. See References 9, 12 and 14.
Therefore, the epitaxial relationship is specifically defined for
the growth of GaN on an r-plane sapphire substrate.
[0038] FIGS. 3(a) and 3(b) are cross-sectional and plan-view TEM
images, respectively, of the defect structure of the a-plane GaN
films on an r-plane sapphire substrate. These images reveal the
presence of line and planar defects, respectively. The diffraction
conditions for FIGS. 3(a) and 3(b) are g=0002 and g=10{overscore
(1)}0, respectively.
[0039] The cross-sectional TEM image in FIG. 3(a) reveals a large
density of threading dislocations (TD's) originating at the
sapphire/GaN interface with line directions parallel to the growth
direction [11{overscore (2)}0]. The TD density, determined by plan
view TEM, was 2.6.times.10.sup.10 cm.sup.-2. With the TD line
direction parallel to the growth direction, pure screw dislocations
will have Burgers vectors aligned along the growth direction
b=.+-.[11{overscore (2)}0]) while pure edge dislocations will have
b=.+-.[0001]. The reduced symmetry of the a-GaN surface with
respect to c-GaN complicates the characterization of mixed
dislocations since the crystallographically equivalent
[11{overscore (2)}0] directions cannot be treated as the
family<11{overscore (2)}0>. Specifically, the possible
Burgers vectors of mixed dislocations can be divided into three
subdivisions: (1) b=.+-.[1{overscore (2)}10] b and (3)
b=.+-.[{overscore (2)}110], (2) b=.+-.[11{overscore
(2)}0].+-.[0001], and (3) b=[11{overscore (2)}0].+-.[1{overscore
(2)}10] and b=.+-.[11{overscore (2)}0].+-.[{overscore (2)}110].
[0040] In addition to line defects, the plan view TEM image in FIG.
3(b) reveals the planar defects observed in the a-GaN films.
Stacking faults aligned perpendicular to the c-axis with a density
of 3.8.times.10.sup.5 cm.sup.-1 were observed in the plan-view TEM
images. The stacking faults, commonly associated with epitaxial
growth of close-packed planes, most likely originate on the c-plane
sidewalls of three-dimensional (3D) islands that form during the
initial stages of the high temperature growth. Consequently, the
stacking faults are currently assumed to be intrinsic and
terminated by Shockley partial dislocations of opposite sign.
Stacking faults with similar characteristics were observed in
a-plane AlN films grown on r-plane sapphire substrates. See
Reference 17. The stacking faults have a common faulting plane
parallel to the close-packed (0001) and a density of
.about.3.8.times.10.sup.5 cm.sup.-1 .
[0041] Omega rocking curves were measured for both the GaN on-axis
(11{overscore (2)}0) and off-axis (10{overscore (1)}1) reflections
to characterize the a-plane GaN crystal quality. The full-width
half-maximum (FWHM) of the on-axis peak was 0.29.degree. (1037"),
while the off-axis peak exhibited a larger orientational spread
with a FWHM of 0.46.degree. (1659"). The large FWHM values are
expected since the microstructure contains a substantial
dislocation density. According to the analysis presented by Heying
et al. for c-GaN films on c-sapphire, on-axis peak widths are
broadened by screw and mixed dislocations, while off-axis widths
are broadened by edge-component TD's (assuming the TD line is
parallel to the film normal). See Reference 18. A relatively large
edge dislocation density is expected for a-GaN on r-sapphire due to
the broadening of the off-axis peak compared to the on-axis peak.
Additional microstructural analyses are required to correlate a-GaN
TD geometry to rocking curve measurements.
[0042] FIGS. 4(a) and 4(b) are AFM amplitude and height images,
respectively, of the surface of the as-grown a-plane GaN film. The
surface pits in the AFM amplitude image of FIG. 4(a) are uniformly
aligned parallel to the GaN c-axis, while the terraces visible in
the AFM height image of FIG. 4(b) are aligned perpendicular to the
c-axis.
[0043] Although optically specular with a surface RMS roughness of
2.6 nm, the a-GaN growth surface is pitted on a sub-micron scale,
as can be clearly observed in the AFM amplitude image shown in FIG.
4(a). It has been proposed that the surface pits are decorating
dislocation terminations with the surface; the dislocation density
determined by plan view TEM correlates with the surface pit density
within an order of magnitude.
[0044] In addition to small surface pits aligned along GaN c-axis
[0001], the AFM height image in FIG. 4(b) reveals faint terraces
perpendicular to the c-axis. Although the seams are not clearly
defined atomic steps, these crystallographic features could be the
early signs of the surface growth mode. At this early point in the
development of the a-plane growth process, neither the pits nor the
terraces have been correlated to particular defect structures.
REFERENCES
[0045] The following references are incorporated by reference
herein:
[0046] 1. I. P. Smorchkova, C. R. Elsass, J. P. Ibbetson, R.
Vetury, B. Heying, P. Fini, E. Haus, S. P. DenBaars, J. S. Speck,
and U. K. Mishra, J. Appl. Phys. 86, 4520 (1999).
[0047] 2. O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K.
Chu, M. Murphy, W. J. Schaff, L. F. Eastman, R. Dimitrov, L.
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[0064] Conclusion
[0065] This concludes the description of the preferred embodiment
of the present invention. The following describes some alternative
embodiments for accomplishing the present invention.
[0066] For example, as the inclusions in the description above
indicate, there are many modifications and variations of the MOCVD
technique and equipment that could be used to grow non-polar
(11{overscore (2)}0) a-plane GaN thin films on (1{overscore (1)}02)
r-plane sapphire substrates. Moreover, different growth conditions
may be optimal for different MOCVD reactor designs. Many variations
of this process are possible with the variety of reactor designs
currently being using in industry and academia. Despite these
differences, the growth parameters can most likely be optimized to
improve the quality of the films. The most important variables for
the MOCVD growth include growth temperature, V/III ratio, precursor
flows, and growth pressure.
[0067] In addition to the numerous modifications possible with the
MOCVD growth technique, other modifications are possible. For
example, the specific crystallographic orientation of the r-plane
sapphire substrate might be changed in order to optimize the
subsequent epitaxial GaN growth. Further, r-plane sapphire
substrates with a particular degree of miscut in a particular
crystallographic direction might be optimal for growth.
[0068] In addition, the nucleation layer deposition is crucial to
achieving epitaxial GaN films with smooth growth surfaces and
minimal crystalline defects. Other than optimizing the fundamental
MOCVD parameters, use of AlN or AlGaN nucleation layers in place of
GaN could prove useful in obtaining high quality a-plane GaN
films.
[0069] Further, although non-polar a-plan GaN thin films are
described herein, the same techniques are applicable to non-polar
m-plane GaN thin films. Moreover, non-polar InN, AlN, and AlInGaN
thin films could be created instead of GaN thin films.
[0070] Finally, substrates other than sapphire substrate could be
employed for non-polar GaN growth. These substrates include silicon
carbide, gallium nitride, silicon, zinc oxide, boron nitride,
lithium aluminate, lithium niobate, germanium, aluminum nitride,
and lithium gallate.
[0071] In summary, the present invention describes the growth of
non-polar (11{overscore (2)}0) a-plane GaN thin films on r-plane
(1{overscore (1)}02) sapphire substrates by employing a low
temperature nucleation layer as a buffer layer prior to a high
temperature growth of the epitaxial (11{overscore (2)}0) a-plane
GaN films. The epitaxial relationship is
[0001].sub.GaN.parallel.[{overscore (1)}101].sub.sapphire and
[{overscore (1)}100] .sub.GaN.parallel.[11{overscore
(2)}0].sub.sapphire with the positive GaN c-axis pointing in the
same direction as the sapphire c-axis projection on the growth
surface.
[0072] The foregoing description of one or more embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto.
* * * * *