Effect of substrate temperature on spontaneous GaN nanowire growth and
optoelectronic properties
A.P. Vajpeyi
a,b,
Ã
, A. Georgakilas
a,b
, G. Tsiakatouras
a,b
, K. Tsagaraki
a,b
, M. Androulidaki
a,b
,
S.J. Chua
c
, S. Tripathy
c
a
Microelectronics Research Group, Department of Physics, University of Crete, P.O. Box 2208, 71003 Herakilon-Crete, Greece
b
Institute of Electronic Structure and Laser (IESL), FORTH, P.O. Box 2208, 71110, Herakilon-Crete, Greece
c
Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology, and Research), 3 Research Link, Singapore 117602, Singapore
article info
Article history:
Received 11 July 2008
Received in revised form
2 September 2008
Accepted 3 September 2008
Available online 10 October 2008

performance. Nanodimensional nitrides such as nanowires
(NWs)/nanopillars are attractive materials for the reduction of
the dislocation density. The NWs could be free of threading
dislocations as their small lateral length could allow initially
formed threading dislocations to move out of the crystal. The free
surface at the sidewalls also permits elastic relaxation of the
strain [1]. In addition to the elimination of threading dislocations,
the NW geometry also allows the device dimensions to be scaled
down. Different techniques such as metal-organic vapor phase
epitaxy [2,3], molecular beam epitaxy (MBE) [4,5], chemical beam
epitaxy [6] and laser ablation [7,8] have been used to grow a
variety of semiconductor NWs with high crystalline quality.
Recently, Calarco et al. [9] studied the nucleation density of MBE
grown GaN NWs and their evolution in terms of growth time for a
fixed set of growth parameters. It has also been reported that the
GaN NW morphology is affected by the N/Ga flux ratio and the
substrate temperature [10–12]. However, it is desirable to
independently control the rod-like morphology of NWs in order
to optimize their crystalline quality.
In this paper, we report on the effect of substrate temperature
on the surface morphology, density, growth rate and optoelec-
tronic properties of GaN NWs spontaneously grown on n-type
(111) silicon, by nitrogen radio-frequency plasma source MBE
(RFMBE). The results suggest that during growth, Ga adatoms are
transferred from uncovered substrate areas to the GaN NWs and
the amount of transferred Ga atoms is limited by the Ga adatom
residence time on the surface, which decreases as the substrate
temperature increases. Photoluminescence spectra at 20 K
showed that PL intensity ratio of donor-bound exciton peak
(D1X) with defect-related peaks increased in the smaller diameter

photoluminescence at 20 K (20 K PL), and Raman spectroscopy. PL
spectra were recorded using 325 nm line excitation wavelength
from a He–Cd laser. Micro-Raman measurements were carried out
using 514.5 nm line excitations, where the scattered light was
dispersed through the JY-T64000 triple monochromatic system
attached to a liquid nitrogen cooled charge coupled device
detector. The spatial and spectral resolution of the Raman setup
is about 1.0
m
m and 0.2 cm
À1
, respectively.
During the initial stage of the growth, the RHEED shows a rings
pattern, indicating the formation of polycrystalline GaN and
silicon nitride on the major part of the Si surface. The formation of
amorphous silicon nitride layer has been previously reported by
Kim et al. and Grandal et al. [5–15] After MBE growth of 5–15 min,
the ring-like RHEED pattern gradually changed into a spotty one,
indicative for the formation of (0 0 0 1)-oriented GaN NWs. The
transition from a ring-like RHEED pattern to a spotty one
depended on the growth temperature and N/Ga flux ratio. In
general, growth at higher temperature needed longer time for the
appearance of the spotty RHEED pattern, because of reduced
coverage of the substrate surface by the GaN NWs at higher
temperature.
Fig. 1 shows the plane and cross-sectional view of the SEM
images of the GaN NWs samples A, B and C grown on (111) Si,
with N/Ga flux ratio kept constant at 5. SEM images revealed that
nanowires density decreased from 9 Â 10
9

inaccuracies of the thickness measurements. A slight NW height
reduction cannot be explained by initiation of GaN decomposition
at 800 1C, since GaN could be regrown under the high-excess flux
of nitrogen species, as N/Ga flux ratio of 5 was used. We believe
that the reduction of the GaN NW growth rate at 800 1C is due
to a reduced residence time (
t
) of Ga adatoms on the substrate
surface. The diffusion length of Ga adatoms before desorption
is proportional to (D
t
)
1/2
, where D is the diffusion coefficient
and
t
the residence time on the surface. When growth
temperature is increased, diffusion coefficient increases but
residence time is decreased and the overall effect is a reduction
in the Ga adatom diffusion length before desorption. Hence less
ARTICLE IN PRESS
Fig. 1. Plane and cross-sectional view of SEM images of GaN NWs grown at a fixed V/III flux ratio of 5 at (a, b) 700 1C, (c, d) at 750 1C and (e, f) at 800 1C.
A.P. Vajpeyi et al. / Physica E 41 (2009) 427–430428
Ga is transferred from the uncovered substrate area to the NWs.
The limited supply of Ga adatoms to the apex of the NWs limits
the growth rate and also reduces the tapering effect at higher
growth temperature. The reduction of the Ga adatom residence
time on the surface with increasing growth temperature is also
consistent with the observed temperature dependence of the
density of the GaN NWs.

GaN film.
Fig. 3 presents the Raman spectra of the GaN NWs and the
reference GaN film grown on Si(111). The spectra were recorded
in the zðxxÞ
¯
z scattering geometry. The spectra are dominated by
the strong E
2
(high) optical phonon peaks besides the strong
silicon peaks at 520.5 cm
À1
. The Raman spectrum of the GaN film
shows strong E
2
(high) and A
1
(LO) modes at 566.4 and 734.0 cm
À1
,
respectively, which are in agreement with Raman selection
rules for wurtzite GaN. The inset in Fig. 3 is an expansion of the
Raman spectrum in the vicinity of the E
2
(high) phonon peak of
GaN NWs and the GaN film. The E
2
(high) phonon line of the GaN
film indicates that the film is under tensile stress of
0.2570.05 GPa when compared to strain-free, 400
m

measurements which confirmed that doping level in GaN NWs is
of the order of 10
19
cm
À3
.
In conclusion, we have studied the effect of growth tempera-
ture on GaN NW morphology. SEM results revealed that the
density, diameter and growth rate of GaN NWs decrease
with increase in growth temperature. This behavior is attributed
to the reduction of Ga adatom residence time on the surface
with increase in growth temperature which limits the lateral
supply of Ga atoms to the NWs. Low-temperature photolumines-
cence at 20 K showed that PL intensity ratio of D1X peak with
defect-related peaks increased indicative of improvement in
optical quality of GaN NWs with increase in growth temperature.
PL and Raman spectra revealed that all the GaN NWs are stress
free irrespective of the used growth temperature. Such GaN NWs
are promising for the fabrication of nanostructure devices as well
as for the overgrowth of low density and stress-free III-Nitrides
films since the NWs form an air-bridge-like structure with a high
aspect ratio. They may also be used as compliant buffer layer for
the growth of compact III-Nitride films and device hetrostruc-
tures.
ARTICLE IN PRESS
3.1 3.2 3.3 3.4 3.5 3.6 3.7
20K
PL Intensity (a.u)
Emission Energy (eV)
A

Fig. 3. Raman spectra of GaN NWs grown at (A) 70 0 1C, (B) 750 1C, (C) 800 1C, and a
reference GaN film grown on (111) Si. The inset shows an expansion of the
spectrum in the vicinity of the E
2
(high) phonon peak.
A.P. Vajpeyi et al. / Physica E 41 (2009) 427–430 429
This research was carried out in the framework of the ‘‘PARSEM’’
Marie Curie Project (MRTN-CT-2004-005583) funded by European
Union.
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