APPLIED PHYSICS LETTERS 94, 091905 ͑2009͒
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G. Koblmüller,
G. D. Metcalfe, M. Wraback, F. Wu, C. S. Gallinat, and J. S. Speck
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Materials Department, University of California, Santa Barbara, California 93106, USA
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Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Maryland 20783, USA
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Received 4 February 2009; accepted 5 February 2009; published online 4 March 2009͒
The role of the In adlayer on the morphological and structural properties of nonpolar a-plane InN
films was elucidated during the plasma-assisted molecular beam epitaxy on freestanding GaN.
Reflection high energy electron diffraction during In adsorption experiments on a-plane InN
surfaces revealed a stable In adlayer coverage of ϳ2 ML. This In adlayer-mediated growth was
responsible for achieving atomically smooth surfaces ͑rms roughness of Ͻ1 nm͒, phase-pure
material with lower x-ray rocking curve widths ͑⌬Ͻ0.5°͒, lower crystal mosaic tilt/twist, and
decreased stacking fault densities, compared to N-rich conditions. The photoluminescence
Despite intense efforts to enhance many physical prop-
erties of indium nitride ͑InN͒, this underdeveloped group-III
nitride material has found relatively few applications in de-
vices. These limitations are in part caused by an electron
accumulation layer at the InN surface due to surface Fermi
As substrate material, on-axis a-plane freestanding GaN
͑Mitsubishi Chemical Co.͒ was used, which was sliced from
a bulk GaN crystal along its c-direction. The a-plane InN
films were grown in a Varian Gen-II MBE system, equipped
with standard effusion cells for In and Ga. Active nitrogen
was supplied by a Veeco Unibulb radio-frequency plasma
level pinning in the conduction band, restricting the fabrica-
source, using a N flow rate of 0.4 sccm and plasma power of
tion of p-type InN layers. To facilitate pure p-type InN and
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00 W ͑i.e., equivalent to N limited growth rate of
drive research toward device applications, first-principle
ϳ6 nm/min͒. To produce low-impurity InN/GaN interfaces,
the a-plane GaN substrates were overgrown with ϳ50 nm of
MBE-GaN under Ga-rich conditions.
calculations suggested that growth along nonpolar, i.e., the
¯
¯
a-plane ͓͑1120͔͒ and m-plane ͓͑1100͔͒ orientations, would
yield surfaces without an accumulation layer. However, this
was proposed only for reconstructed nonpolar InN surfaces
without metal adlayers or In–In bonding states in the con-
duction band. The absence of electron accumulation on non-
polar InN surfaces was recently demonstrated, indeed, on in
First, the onset for thermal dissociation was determined
by recording the reflection high-energy electron diffraction
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RHEED͒ intensity during the growth of ϳ10 min long InN
pulses under variable temperatures ͑T=380–500 °C͒ at con-
stant N-rich conditions ͑In/N=0.8͒. As typical for N-rich
situ cleaved a-plane InN, but not on as-grown surfaces, due
group-III nitride growth, these pulses produced spotty
RHEED patterns, however, only up to Tϳ430 °C. Higher
temperatures resulted in very low-contrast RHEED patterns
due to the large In droplets accumulating on the surface as a
In coverage, which contributed to n-type surface states in
both a- and m-plane InN.
So far, all a-plane InN films were grown on r-plane
result of thermal dissociation. Thus, ϳ430 °C was the
¯
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͓1102͔͒ sapphire, which suffered from cubic inclusions,
bilities at Ͻ250 cm /V s͒, pointing to apparent difficulties
in fabricating high-quality nonpolar InN. More recently
though, the first step toward phase-pure nonpolar InN films
with smooth surfaces was achieved for the m-plane, owing to
the availability of high-quality freestanding m-plane GaN
nearly identical to that for m-plane InN, but much lower
than the dissociation temperature reported for c-plane
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InN.
We selected therefore T=420 °C as the maximum
growth temperature and further analyzed the effect of the
In/N flux ratio on the surface morphology. The atomic force
micrographs in Fig. 1 show surfaces of three a-plane InN
films grown under different In/N flux ratios, i.e., ͑a͒=0.6
N-rich growth͒, ͑b͒=1 ͑flux stoichiometry͒, and ͑c͒=1.25
In-rich growth͒. The film thicknesses varied between ͑a͒
substrates and knowledge of thermal dissociation.
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To establish a-plane InN films with similar qualities and
understand the critical In adlayer issues, we report in this
letter on the influence of the In adlayer kinetics during the
plasma-assisted molecular beam epitaxy ͑PAMBE͒ on the
surface and structural properties of the a-plane InN. Specifi-
cally, under In-rich conditions, In-adlayer-mediated growth
was found to enhance the surface diffusion, the film mosaic,
and the basal-plane stacking fault densities, surpassing pre-
vious problems associated with a-plane InN films.
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FIG. 1. 3ϫ3 m AFM images of a-plane InN grown on freestanding GaN
at constant T=420 °C, but variable In/N flux ratio: ͑a͒ In/N=0.6, ͑b͒
In/N=1, and ͑c͒ In/N=1.25.
a͒Author to whom correspondence should be addressed. Electronic mail:
Gregor.Koblmueller@wsi.tum.de.
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003-6951/2009/94͑9͒/091905/3/$25.00
94, 091905-1
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