Synthesis and optical properties of InN nanowires and nanotubes

Article abstract of DOI:10.1063/1.2712801

InN nanowires and faceted hexagonal InN nanotubes are synthesized by catalyst-free chemical vapor deposition at different temperatures. Both have the single crystalline wurtzite structure and grow along the c axis. Different growth dynamics are suggested

Full text of DOI:10.1063/1.2712801

Synthesis and optical properties of InN nanowires and nanotubes
H. Y. Xu, Z. Liu, X. T. Zhang, and S. K. Hark
Citation: Appl. Phys. Lett. 90, 113105 (2007); doi: 10.1063/1.2712801
View online: http://dx.doi.org/10.1063/1.2712801
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 90, 113105 ͑2007͒
Synthesis and optical properties of InN nanowires and nanotubes
H. Y. Xu, Z. Liu, X. T. Zhang, and S. K. Harka͒
Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong
͑Received 17 January 2007; accepted 5 February 2007; published online 13 March 2007͒
InN nanowires and faceted hexagonal InN nanotubes are synthesized by catalyst-free chemical
vapor deposition at different temperatures. Both have the single crystalline wurtzite structure and
grow along the c axis. Different growth dynamics are suggested for the difference in morphology.
Observations of phonon-plasmon coupled modes in their Raman scattering suggest of high electron
concentrations. Absorption edges in their optical spectra have energies slightly higher than 1 eV,
showing blueshifts from the fundamental band gap of ϳ0.7 eV, recently observed in epitaxial films.
The shifts are argued to be the result of the Burstein-Moss effect. © 2007 American Institute of
Physics. ͓DOI: 10.1063/1.2712801͔
Indium nitride ͑InN͒ is currently receiving much atten-
tion, in large part due to its recently observed narrow band
gap of ϳ0.7 eV.1–3 This implies that light emitting diodes
based on group-III nitride alloys such as ͑Al, Ga, In͒N can
cover a wide spectral range from ultraviolet ͑ϳ6.2 eV͒ to
near infrared ͑ϳ0.7 eV͒. In addition, InN has superior elec-
tron transport properties, including high mobility and high
saturation velocity at room temperature, which make it suit-
able for many optoelectronic applications, such as high-
speed field-effect transistors, terahertz emitters, and high-
efficiency solar cells.
minute at STP͒, and the furnace was maintained at 800 or
850 °C for 200 min. Finally, the tube and its contents were
rapidly cooled down to room temperature, all the time under
the protective flow of ammonia, by removing them from the
furnace, which suppressed the decomposition of the synthe-
sized InN. Fluffy products were collected from the down-
stream side of the starting materials.
The morphologies, structures, and compositions of the
synthesized products were characterized by scanning elec-
tron microscopy ͑SEM͒ ͑LEO 1450VP͒, high-resolution
transmission electron microscopy ͑HRTEM͒ ͑Philips Tecnai
20͒, x-ray diffraction ͑XRD͒ ͑Rigaku RU-300 with Cu K␣
radiation͒, and energy-dispersive x-ray ͑EDX͒ microanalysis.
Optical transmission measurements were performed using a
spectrophotometer ͑Hitachi U3501͒ on InN nanostructures
dispersed in ethanol. Raman scattering was measured by a
micro-Raman spectrometer ͑Renishaw RM1000B͒ in a back-
scattering configuration, employing the 514.5 nm line of an
Ar+ laser as the excitation source.
In recent years, one-dimensional ͑1D͒ semiconductor na-
nomaterials have become a subject of intense research, be-
cause of their importance in the understanding of the funda-
mental properties of low dimensional systems, as well as in
potential nanodevice applications. Among group-III nitrides,
1D GaN and AlN nanostructures have been extensively
studied.4,5 In contrast, the synthesis of high quality InN is
still challenging, due to its low thermal decomposition tem-
perature and the high equilibrium vapor pressure of nitrogen.
Thus, reports on 1D InN nanomaterials,6–13 especially those
with a tubular structure,14–17 are very limited. Chemical va-
por deposition ͑CVD͒ has proven to be a feasible method to
prepare InN nanomaterials. However, it is observed, in most
available reports on CVD-grown InN, that In2O3 is usually
contained in the precursors,7,8,14–16 which may lead to oxy-
gen contamination and increased band gap of the resultant
InN.18,19 In this work, InN nanowires and faceted hexagonal
InN nanotubes are prepared from high purity In metal and
ammonia by controlled normal pressure CVD without cata-
lysts and templates. Their optical properties are investigated.
High purity indium wires ͑99.999%͒, serving as the
starting material, were placed in a quartz boat. Prior to each
experiment, the indium wires were cleaned using dilute hy-
drochloric acid and de-ionized water to remove any native
oxide layer present on their surface. The quartz boat was
loaded into the center of a 15 mm diameter quartz tube,
which was placed in a horizontal tube furnace. The quartz
tube was degassed and then purged with high purity ammo-
nia. During the growth process, the flow rate of ammonia
was set at 30 SCCM ͑SCCM denotes cubic centimeter per
XRD measurements on the samples synthesized at 800
and 850 °C show that all diffraction peaks can be indexed to
wurtzite InN, suggesting the formation of high purity InN
material.
Figure 1͑a͒ shows a typical SEM image of the sample
synthesized at 800 °C, which consists of a high density of
InN nanowires. The nanowires have an average diameter of
ϳ100 nm with a narrow size distribution, and lengths up to a
few tens of microns. When the temperature increases to
850 °C, the synthesized product contains a large amount of
InN nanotubes and a fraction of nanowires, as shown in Fig.
1͑b͒. The nanotubes are well faceted with a hexagonal cross
section, about 750–1050 nm in outer diameter, and several
microns in length. The wall thicknesses are in the range of
50–100 nm. Most of the nanotubes have both ends open.
The microstructures and morphologies of InN nanostructures
were further studied by HRTEM and selected area electron
diffraction ͑SAED͒. The inset in Fig. 2͑a͒ shows a SAED
pattern of a single nanowire along the ͓100͔ zone axis, which
confirms that the synthesized InN nanowires are single crys-
talline with a ͓001͔ growth direction. The HRTEM image of
the nanowires in Fig. 2͑b͒ shows clearly the lattice fringes
with interplanar spacings of 0.57 and 0.31 nm corresponding
to the ͑001͒ and ͑100͒ planes of wurtzite InN. Figure 2͑c͒
displays a typical TEM image of an InN nanotube. The tube
wall is smooth. The obvious contrast between the wall and
a͒
Author to whom correspondence should be addressed; electronic mail:
skhark@phy.cuhk.edu.hk
0003-6951/2007/90͑11͒/113105/3/$23.00
90, 113105-1
© 2007 American Institute of Physics
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113105-2
Xu et al.
Appl. Phys. Lett. 90, 113105 ͑2007͒
FIG. 3. Micro-Raman spectra of InN nanowires and nanotubes.
only In and N elements. The Cu signals come from the
TEM grid. Quantitative analyses determine the In/N atomic
ratios as 48.3:51.7 and 47.9:52.1 for the individual wires and
tubes, respectively, which are stoichiometric, within experi-
mental errors.
It has been observed that both InN nanowires and nano-
tubes grow along the ͓001͔ direction. As is known, there is
no center of inversion in the wurtzite structure, and therefore,
the presence of an inherent asymmetry along the c axis al-
lows the anisotropic crystal growth along the ͓001͔
direction.5,20 In the present work, neither a catalyst nor a
template is used, and no metallic particles are found at the
ends of 1D InN nanostructures. Thus, the growths of InN
wires and tubes are likely to follow a vapor-solid ͑VS͒ pro-
cess. It is interesting to note that the morphologies of the
synthesized InN 1D nanostructures depend on the growth
temperatures. This can be understood by considering the
kinetics-limited and diffusion-limited processes. Similar
mechanism has been proposed in Refs. 16, 20, and 21 to
understand the nanowire and nanotube growths of different
materials.
Figure 3 gives the micro-Raman spectra of InN nano-
wires and nanotubes. Both show analogous features. The
peak at ϳ443 cm−1 corresponds to the A1 transverse optical
mode of wurtzite InN. The E2͑high͒ modes are observed at
488 and 493 cm−1 for InN wires and tubes, respectively. It is
known that the E2͑high͒ mode is especially sensitive to lat-
tice strain. The strain-free Raman frequency of the E2͑high͒
mode has been reported to be 490 cm−1 for wurtzite InN.22
The E2͑high͒ mode frequencies observed in our experiment
are very close to this reported value, within an instrumental
error of 2.5 cm−1, which indicates that the strain of the syn-
thesized InN nanostructures is fully relaxed. The Raman
peak at ϳ570 cm−1, which shows a redshift relative to the A1
longitudinal optical ͑LO͒ mode reported at 585–590 cm−1,23
may be attributed to the lower branch of the large-wave vec-
tor A1͑LO͒-phonon-plasmon ͑A1-LPP−͒ coupled mode.24
This coupled mode has also been observed in GaAs and InN
films of high carrier concentrations.24,25 The presence of
A1-LPP− mode, along with the broad feature of the Raman
peaks, suggests that the synthesized InN nanomaterials have
high electron concentrations,23,24,26 mainly due to the contri-
bution of native donor defects such as N vacancies and/or
dislocations.
FIG. 1. SEM images of InN ͑a͒ nanowires and ͑b͒ nanotubes. A high mag-
nification SEM image in the inset clearly shows an InN nanotube of hex-
agonal cross section.
the core region again confirms the hollow nature of the tube.
The SAED pattern indicates that the faceted hexagonal InN
single crystalline tubes grow along the ͓001͔ direction, and
are enclosed by low-index ͕100͖ planes. This observation
differs from that reported by Yin et al.16 EDX spectrum in
Fig. 2͑d͒ illustrates that the synthesized products consist of
Figure 4 shows the optical transmission spectra of the
synthesized 1D InN nanostructures dispersed in ethanol. The
transmittances of both samples start to decrease at around
0.9 eV. The optical band gap can be estimated by plotting
FIG. 2. TEM images of a single InN ͑a͒ nanowire and ͑c͒ nanotube. The
insets in ͑a͒ and ͑c͒ show their SAED patterns along the ͓100͔ zone axis. ͑b͒
A HRTEM lattice image of a typical InN nanowire. ͑d͒ EDX spectra of a
single InN nanowire and nanotube.
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113105-3
Xu et al.
Appl. Phys. Lett. 90, 113105 ͑2007͒
direction. Optical transmission measurements determine the
optical band gaps to be 1.26 and 1.05 eV for different
samples. Burstein-Moss shift is attributed as the factor that
affects the energy gap of InN nanostructures.
The work described was partially supported by a grant
from the Research Grants Council of the Hong Kong Special
Administrative Region, China ͑Project No. 401003͒ and
CUHK direct grants ͑Project codes 2060293 and 2060305͒.
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