Molecular beam epitaxy and structural anisotropy of m-plane InN grown on free-standing GaN

Article abstract of DOI:10.1063/1.3001806

This study reports on the growth of high-quality nonpolar m-plane [1 100] InN films on free-standing m -plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized growth conditions (In/N ratio ～1 and T=390-430 °C) yielded very smooth InN fi

Full text of DOI:10.1063/1.3001806

Molecular beam epitaxy and structural anisotropy of m -plane InN grown on free-
standing GaN
G. Koblmüller, A. Hirai, F. Wu, C. S. Gallinat, G. D. Metcalfe, H. Shen, M. Wraback, and J. S. Speck
Citation: Applied Physics Letters 93, 171902 (2008); doi: 10.1063/1.3001806
View online: http://dx.doi.org/10.1063/1.3001806
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APPLIED PHYSICS LETTERS 93, 171902 ͑2008͒
Molecular beam epitaxy and structural anisotropy of m-plane InN
grown on free-standing GaN
G. Koblmüller,1,a͒ A. Hirai,1 F. Wu,1 C. S. Gallinat,1 G. D. Metcalfe,2 H. Shen,2
M. Wraback,2 and J. S. Speck1
1Materials Department, University of California, Santa Barbara, California 93106-5050, USA
2U. S. Army Research Laboratory, Sensors and Electron Devices Directorate, 2800 Powder Mill Road,
Adelphi, Maryland 20783, USA
͑Received 9 September 2008; accepted 25 September 2008; published online 27 October 2008͒
¯
This study reports on the growth of high-quality nonpolar m-plane ͓1100͔ InN films on
free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized
growth conditions ͑In/N ratio ϳ1 and T=390–430 °C͒ yielded very smooth InN films with
¯
undulated features elongated along the ͓1120͔ orientation. This directionality is associated with the
underlying defect structure shown by the anisotropy of x-ray rocking curve widths parallel to the
¯
͓1120͔ ͑i.e., 0.24° –0.34°͒ and ͓0001͔ ͑i.e., 1.2° –2.7°͒ orientations. Williamson–Hall analysis and
transmission electron microscopy identified the mosaic tilt and lateral coherence length and their
associations with different densities of dislocations and basal-plane stacking faults. Ultimately, very
low band gap energies of ϳ0.67 eV were measured by optical absorption similar to the best c-plane
InN. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3001806͔
With the revision of the band gap energy of InN to ap-
proximately 0.7 eV1 and its small electron effective mass to
ϳ0.05m0,2 this material has become an attractive candidate
for optoelectronic, and high electron mobility applications.
Most works performed to fabricate good InN films concen-
trated on the polar wurtzite c-plane orientations, i.e., In-face
and N-face InN, since appropriate substrates such as c-plane
sapphire, SiC, or free-standing c-plane GaN were readily
available. Despite substantial advancements of many physi-
cal properties, i.e., electron mobilities higher than
3500 cm2/V s3 and band gap energies lower than 0.65 eV,4
two severe problems for c-oriented ͑i.e., ͓0001͔͒ InN have
set limits to their commercial applicability.
dealt with highly defective a-plane InN films grown on
r-plane sapphire by molecular beam epitaxy ͑MBE͒.14,15
In this letter, we report on the growth of high-quality
m-plane InN films on m-plane GaN substrates by plasma-
assisted ͑PA͒ MBE. By identifying optimum growth condi-
tions, good structural quality and band gap energies as low as
0.65 eV could be achieved. Special emphasis was also placed
on the inherent film mosaic anisotropy and its correlation
with planar and extended defects.
All m-plane InN films were grown in a Varian Gen-II
MBE system using standard elemental effusion cells for In
and Ga while active nitrogen was supplied by a Veeco Un-
ibulb radio-frequency plasma source with a N limited growth
rate of 7.2 nm/min. The m-plane GaN substrates used in this
study were prepared at Mitsubishi Chemical Co., Ltd. by
slicing a low-defect bulk GaN crystal along its c-direction.
The on-axis substrates were specified with threading disloca-
tion ͑TD͒ and n-type carrier densities of ϳ5ϫ106 cm−2 and
mid-1017 cm−3, respectively. Prior to the InN growth, a 20-
nm-thin Ga-rich GaN nucleation layer was grown at 680 °C
for good interface quality.
First, the presence of internal electric fields caused by
discontinuities in the polarization along the c axis resulted in
many drawbacks for In-containing nitride devices. Specifi-
cally, the large band bending and reduced overlap in
electron-hole wave functions in ͓0001͔-oriented quantum
wells yielded undesired shifts in the emission spectra and
increased nonradiative recombination.5 Second, the Fermi
level pinning at surface states at ϳ0.7 eV above the conduc-
6
tion band minimum ͑CBM͒ demonstrated that c-plane InN
We explored a set of growth temperatures between
390–450 °C at constant In/N flux ratio of ϳ1. By monitor-
ing the growth using reflection-high energy electron diffrac-
surfaces contain large electron accumulation.7,8 This strong
n-type surface conductivity has heavily obscured both fabri-
cation and analysis of pure p-type InN.9
¯
tion ͑RHEED͒ along the ͓1120͔ azimuth, the intensity and
To avoid these situations, recent work highlighted that
streak spacing, i.e., strain relaxation profiles, indicated a
Stranski–Krastanow growth mode transition at ϳ2 monolay-
ers ͑ML͒ of InN, as also observed for c-plane InN growth.16
The formed three-dimensional islands coalesced quickly into
a continuous film at around 5–10 nm of growth shown by a
transition to its original streaky RHEED pattern. While for
growth temperatures below ϳ430 °C, no further change in
RHEED intensity was observed throughout the consecutive
growth, temperatures higher than ϳ430 °C yielded a gradual
decrease in the RHEED intensity contrast. This was associ-
ated with the accumulation of In droplets17 as confirmed for
a thick m-plane InN film grown at ϳ445 °C. The propensity
for In droplets under conditions without excess In flux ͑i.e.,
growth of ͑In͒GaN-based heterostructures along nonpolar,
i.e., the a-plane ͓1120͔ and m-plane ͓1100͔11,12, orienta-
tions enhanced nitride devices substantially by eliminating
the polarization-induced electric fields. Theoretical
calculations13 predicted also an absence of Fermi level pin-
ning above the CBM for nonpolar orientations, especially for
m-plane InN. Despite these apparent promises of nonpolar
InN, their growth was only rarely reported, i.e., there are few,
if any, reports on m-plane InN films, while some reports
10
¯
¯
a͒
Author to whom correspondence should be addressed. Electronic mail:
gregor@engineering.ucsb.edu.
0003-6951/2008/93͑17͒/171902/3/$23.00
93, 171902-1
© 2008 American Institute of Physics
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150.135.239.97 On: Wed, 17 Dec 2014 18:31:37

171902-2
Koblmüller et al.
Appl. Phys. Lett. 93, 171902 ͑2008͒
FIG. 1. ͑Color online͒ AFMs of the surfaces of ϳ0.5-␮m-thick m-plane InN
layers grown on m-GaN at ͑a͒ T=395 °C and ͑b͒ T=415 °C. Note the very
low rms surface roughness and morphological anisotropies along a- and
c-directions.
FIG. 2. Optical absorption and PL spectra at 300 K for an ϳ2-␮m-thick
m-plane InN film with a band gap of ϳ0.67 eV and PL peak energy of
ϳ0.60 eV.
In/N Յ1͒ results usually from thermal decomposition of the
InN crystal.17 The temperature 430 °C thus represents an
onset for thermal decomposition, which is much lower than
that reported for c-plane InN, i.e., ϳ500 °C for In-face
InN18 and Ͼ560 °C for N-face InN.17,19
¯
¯
¯
XRD ͑1100͒, ͑2200͒, and ͑3300͒ rocking curves ͑␻
scans͒ were also taken along the two orthogonal in-plane
For the m-plane InN films grown below the thermal de-
composition limit, their surfaces showed atomically smooth
morphologies, as obvious from the atomic force micrographs
͑AFM͒ in Fig. 1. These images were taken from
ϳ0.5-␮m-thick InN films grown at ͑a͒ T=395 °C and ͑b͒
T=415 °C after they were etched by a short 20-s-long HCl
dip to selectively remove any excess metal In adlayer. Sur-
prisingly, low surface root-mean-square ͑rms͒ roughness of
ϳ0.3 nm were measured from the 3ϫ3 ␮m2 AFM scans,
which are much lower than commonly reported for PAMBE-
grown c-plane InN surfaces.18,19 Undulated features elon-
gated along the a-direction were observed with step heights
of ϳ0.6–1.3 nm ͑corresponding to ϳ2–4 ML of m-InN,
¯
¯
͓0001͔ and ͓1120͔ orientations. Representative ͑1100͒ scans
are shown in Fig. 3͑b͒ for the m-plane InN film grown at T
=415 °C, demonstrating a large anisotropy in the rocking
curve broadening. In general, the rocking curve broadening
ͱ3a/2Ϸ3.07 Å͒ and widths of
where 1 ML equals
ϳ50–90 nm. The directionality of these undulated features
evidences strong morphological anisotropy parallel to the a-
and c-directions similar to the typical slatelike morphology
20
observed for m-plane GaN on ␥-LiAlO2 or m-plane SiC.21
Moreover, the surface undulations were attributed to signifi-
cant densities of basal-plane stacking faults ͑BPSFs͒ ͑as fur-
ther discussed below͒.22
The band gap energy of these m-plane InN films exhib-
ited very low values similar to c-plane InN. Both room tem-
perature optical absorption and photoluminescence ͑PL͒
spectra of a ϳ2-␮m-thick m-plane InN film grown at T
=415 °C ͑Fig. 2͒ indicated a band gap energy of 0.67 eV
and PL peak energy of 0.605 eV. The redshift of the PL peak
with regard to the band gap implies that the emission may be
related largely to bandtail states. All films grown within the
investigated thickness range of 0.5–2 ␮m resulted in con-
sistently low band gap energies between 0.65–0.69 eV and
PL peak energies between 0.605–0.617 eV.
Investigating the structural quality by x-ray diffraction,
¯
Fig. 3͑a͒ shows ͑1100͒ XRD ␻-2␪ scans of the three nomi-
nally ϳ0.5-␮m-thick m-plane InN films described above.
Well-resolved diffraction peaks were observed from both the
¯
¯
m-plane InN ͑1010͒ films and GaN ͑1010͒ substrates with-
out any ͑0001͒ oriented domains. For the InN film grown in
the thermal decomposition regime ͑T=445 °C͒, an addi-
tional peak was observed at ␻ϳ16.5°. This peak was com-
monly associated with a tetragonal ͑101͒ phase of In metal23
FIG. 3. ͑a͒ ␻-2␪ XRD scans of ϳ0.5-␮m-thick m-plane InN films grown on
¯
m-plane GaN as a function of growth temperature. ͑b͒ ͑1100͒ rocking curves
͑␻ scans͒ of the m-plane InN peak taken with an open detector parallel to
¯
the a-direction ͓1120͔ and c-direction ͓0001͔ showing significant aniso-
¯
tropy. ͑c͒ FWHM values of the SF-insensitive ͑3300͒ rocking curves along
due to the In droplets on the surface.
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these two orthogonal directions as a function of growth temperature.
150.135.239.97 On: Wed, 17 Dec 2014 18:31:37

171902-3
Koblmüller et al.
Appl. Phys. Lett. 93, 171902 ͑2008͒
The density of SFs was determined to be ϳ2ϫ105 cm−1
TABLE I. Summarized results from XRD rocking curves and WH analysis
¯
for the a-͓1120͔ and c-͓0001͔ directions. Values are given for the rocking
consistent with the WH analysis. The TD density was 4–5
curve widths, the LCL, and corresponding SF density.
10
ϫ1010 cm−2 for g=1120 and 3–4ϫ10 cm−2 for g=0002
¯
͑describing dislocations only with c-component͒ as measured
by plan-view TEM. The TD density was also determined
from the film grown at T=405 °C, which had identical SF
density but quite different crystal mosaic tilt. With a lower
¯
¯
⌬␻͑1100͒
⌬␻͑3300͒
T͑Growth͒
͑°C͒
a,c
͑deg͒
a,c
͑deg͒
LCL
͑nm͒
␳͑SF͒
͑cm−1
͒
395
405
410
415
445
0.31, 1.86
0.24, 1.26
0.28, 1.58
0.29, 1.62
0.34, 2.77
0.41, 0.71
0.28, 0.44
0.32, 0.53
0.30, 0.45
0.42, 1.25
51
48
21
29
6
1.95ϫ105
2.1ϫ105
4.7ϫ105
3.4ϫ105
1.6ϫ106
TD density of 3–4ϫ1010 cm−2 for g=1120 and 2–3
¯
ϫ1010 cm−2 for g=0002 and smaller crystal mosaic tilt in
this sample, this indicates a possible correlation between TD
density and crystal mosaic tilt, in consistence with c-plane
group-III nitrides.26
tilt
The authors acknowledge the fruitful discussions with
Professor C. Van de Walle ͑UCSB͒ and the support of
AFOSR ͑D. J. Silversmith, program manager͒ and SSLEC
͑UCSB͒. The experimental work made use of the MRSEC
facilities at UCSB ͑supported by the NSF͒.
⌬␻ contains contributions from crystal mosaic tilt ͑⌬␻ ͒
and stacking-fault-related lateral coherence length ͑LCL͒
tilt
͑L͒. The mosaic tilt ⌬␻ was derived from the full width at
¯
half maximum ͑FWHM͒ of the ͑3300͒ reflection, which is
entirely insensitive to stacking-fault-related anisotropic
broadening.24 The FWHM values of ͑3300͒ rocking curves
¯
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¯
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¯
¯
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Figure 4 shows the cross-sectional transmission electron
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¯
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FIG. 4. Cross-sectional TEM images for the ͓1120͔ zone axis at g=0002
¯
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