# Templated Fabrication of InSb Nanowires for Nanoelectronics

Authors: M. Ibrahim Khan; Xu Wang; Krassimir N. Bozhilov; Cengiz S. Ozkan
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Among various ways
to produce nanowires, anodic alumina membrane-
(AAM-) based synthesis has constantly received
much attention, because AAM has a uniform and parallel porous nanostructure which makes it an ideal template material for fabricating highly ordered nanostructures. In this paper, we report
fabrication of InSb nanowire arrays with
diameter of 200 nm and 30 nm by
direct current electrodeposition inside the
nanochannels of anodic alumina membranes without
subsequent annealing. The nanowires have four
major growth directions, (220) being the most
dominant with structure defects such as twins.
The transmission electron microscopy (TEM) and
scanning electron microscopy (SEM) results
demonstrate that these InSb nanowires are
uniform with diameters about 200 nm and
30 nm, corresponding to the pore diameter
of the AAMs. The I-V measurement of a single
nanowire is also reported with encouraging
preliminary results.


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## Body

## 1. Introduction

InSb is well known for its direct narrow band gap (0.18 eV  at 300 K) and various applications in
electronic and optoelectronic devices with a very high-electron mobility (8×104 cm2V−1s−1at 300 K), electron velocity, and
ballistic length (up to 0.7 μm at 300 K), and ideal candidates for detector
arrays operating in the infrared wavelength, high-speed electronic devices, and
magneto resistive sensors [1]. In the past few years, there has been increasing
interest in nanostructure III-V semiconducting materials due to their potential
applications. Different methods have been used to fabricate the nanowires, such
as vapor-liquid-solid (VLS), physical vapor deposition (PVD), and hydrothermal
methods [2–10]. Comparatively, the pulsed electrodeposition process, using
anodic alumina membranes (AAM) as template, is an effective and inexpensive
method to fabricate nanowires. However, although it is considered that the electrodeposition
of high-quality compound InSb with precise stoichiometry at ambient temperature
from aqueous solutions is a challenge, it is surprising that only a few studies
on this important issue have been reported [11–13]. Other than thin films on a
substrate [14–16], fabrication of InSb nanowires and nanocables by simple electrodeposition in anodic alumina membranes was reported [17]. Transport properties along with thermal properties of InSb
nanowires have been shown very recently [18–22]. But these properties were
reported only for the whole template or membrane embedded with nanowires and
single nanowire properties are not measured or reported so far. Also, no
attention has been given to the photoconduction properties of InSb nanowires
despite the exciting possibilities for using InSb nanowires in optoelectronic
circuits or as infrared detectors. We have synthesized InSb nanowires with
single-crystal structures, diameters as small as from 30 to 200 nanometers by
electrochemical deposition in the porous anodic alumina template and exploring
the possible applications for nanoelectronics devices such as field effect
transistors and Infrared photodetectors [23–28].


## 2. Experimental Procedures

### 2.1. Synthesis

The AAMs were purchased from Whatman Inc. Florham Park, NJUSA and Synkera Technologies, Inc. (Longmont, Colo, USA). The diameters
of the pores in the AAMs are about 30 nm and 200 nm, and the thickness of all
AAMs used here is about 60μm. A layer of Au about 200 nm in thickness was
sputtered onto the bottom side of the AAMs to serve as a conduction contact.
The electrodeposition was carried out in a typical three-electrode
electrochemical cell equipped with a platinum electrode as the counter
electrode, Ag/AgCl as reference electrode, and the AAMs with Au back layer as
the working electrode controlled by a potentiostat/galvanostat (Princeton
Applied Research Company (Oak Ridge, Tenn, USA), model:
Potentiostat 263A). All chemicals were analytical grade used without further
purification. We had performed a series of experiments and optimized the
deposition parameters as follows: 0.1 M SbCl3, 0.15 M InCl3,
0.36 M citric acid, and 0.17 M potassium citrate at pH 1.8. The citrate ions
are used as the complexion agents to bring the deposition potential of In and
Sb closer to maintain binary growth. The deposition was conducted for 40 minutes
under the deposition potential of −1.5 V versus Ag/AgCl reference electrode at
room temperature. After the deposition, the sample was washed several times in
distilled water and rinsed with acetone.


### 2.2. Characterization

The as-synthesized products were characterized by X-ray diffraction (XRD) machine (BRUKER AXS, Inc. Madison, WI USA, model:
D8 Advance), transmission electron microscope (FEI Company (Hillsboro, Ore,
USA), model: CM 300), and Scanning electron microscope (JEOL
Company (Tokyo, Japan), model: JSM-6300) equipped with energy dispersive X-ray spectroscopy. For TEM
analysis, the products were immersed in 3 M NaOH solution for several minutes
to centrifuge and were washed with distilled water to remove the dissolved AAMs
and the remaining NaOH solution; the nanowires were dispersed in deionized
water. Then, drops of the solution were dripped onto copper grids coated with
holey carbon film. For SEM observation, the products were treated with 3 M NaOH
for about 30 minutes and washed with distilled water then was placed on Si
substrate.


## 2.1. Synthesis

The AAMs were purchased from Whatman Inc. Florham Park, NJUSA and Synkera Technologies, Inc. (Longmont, Colo, USA). The diameters
of the pores in the AAMs are about 30 nm and 200 nm, and the thickness of all
AAMs used here is about 60μm. A layer of Au about 200 nm in thickness was
sputtered onto the bottom side of the AAMs to serve as a conduction contact.
The electrodeposition was carried out in a typical three-electrode
electrochemical cell equipped with a platinum electrode as the counter
electrode, Ag/AgCl as reference electrode, and the AAMs with Au back layer as
the working electrode controlled by a potentiostat/galvanostat (Princeton
Applied Research Company (Oak Ridge, Tenn, USA), model:
Potentiostat 263A). All chemicals were analytical grade used without further
purification. We had performed a series of experiments and optimized the
deposition parameters as follows: 0.1 M SbCl3, 0.15 M InCl3,
0.36 M citric acid, and 0.17 M potassium citrate at pH 1.8. The citrate ions
are used as the complexion agents to bring the deposition potential of In and
Sb closer to maintain binary growth. The deposition was conducted for 40 minutes
under the deposition potential of −1.5 V versus Ag/AgCl reference electrode at
room temperature. After the deposition, the sample was washed several times in
distilled water and rinsed with acetone.


## 2.2. Characterization

The as-synthesized products were characterized by X-ray diffraction (XRD) machine (BRUKER AXS, Inc. Madison, WI USA, model:
D8 Advance), transmission electron microscope (FEI Company (Hillsboro, Ore,
USA), model: CM 300), and Scanning electron microscope (JEOL
Company (Tokyo, Japan), model: JSM-6300) equipped with energy dispersive X-ray spectroscopy. For TEM
analysis, the products were immersed in 3 M NaOH solution for several minutes
to centrifuge and were washed with distilled water to remove the dissolved AAMs
and the remaining NaOH solution; the nanowires were dispersed in deionized
water. Then, drops of the solution were dripped onto copper grids coated with
holey carbon film. For SEM observation, the products were treated with 3 M NaOH
for about 30 minutes and washed with distilled water then was placed on Si
substrate.


## 3. Results and Discussion

SEM observations in Figure1(a) show that the InSb nanowires are uniformly distributed, highly ordered,
parallel to each other, and their diameters are all about 200 nm, corresponding
to the pore diameter of the AAM used. It can be seen in Figure 1(b) that the
nanowires are uniform in diameter and have a smooth surface. Figures 1(c) and 1(d)
show the bundles of nanowires after removing from a membrane with 30 nm pores. Depending
on the pore diameter of the AAM, InSb nanowire arrays with different diameters
can be obtained. The length of InSb nanowires varies between 5 μm to 20 μm,
which corresponds with the deposition time used and also the speed of the
centrifuge during cleaning. The length of the nanowires can be modulated by
changing the thickness of the AAM or the pulsed deposition time. Figures 2(a)
and 2(b) show a typical EDS spectrum of InSb nanowire and the XRD patterns,
respectively. The diffraction peaks can be indexed to a cubic zinc-blended
phase of InSb crystal from the American Mineralogist Crystal Structure Database
(AMCSD). One can see that the intensity of the diffraction peak at 2θ = 39.34° is
relatively stronger than the other diffraction peaks, demonstrating that the
InSb nanowires deposited in AAM grow preferentially along (220) crystal
direction.(a) SEM of a bundle of aligned nanowires released from AAM with 200 nm pores, (b)
SEM of a single 200 nm-diameter InSb nanowire, (c) SEM of a bundle of nanowires
after they are removed from the template with 30 nm pores, and (d) SEM of one
single isolated 30 nm-diameter InSb nanowire.
(a)(b)(c)(d)(a) Energy dispersive spectrum of a single InSb nanowire, (b) XRD pattern of InSb
nanowire arrays show four preferred directions of growth.
(a)(b)The transmission
electron microscope (TEM) images of one single InSb nanowire released from AAMs
is shown in Figure3(a). The corresponding selected-area electron diffraction
(SAED) pattern indicates that the InSb nanowires are single crystalline in
Figure 3(b). The SAED pattern was taken along the nanowires, but changes along the
length of the nanowires. This means that the nanowires are single crystal with
a slight structural deformation along the length which is seen in the high
resolution TEM (HRTEM) image in Figure 3(c). The HRTEM obtained from the
nanowire shown in Figure 3(a) is shown in Figure 3(c) depicting the twinning
pattern present in the crystal structure. The deformations may be due to the
release process of nanowires from the membrane, which involves etching with NaOH,
washing with acetone and isopropanol and finally centrifuging in deionized water.(a) Electron transparent edge of an InSb nanowire in a TEM bright field image, (b) selected area diffraction
pattern (SAED) shows hexagonal pattern on a (0-2-2) zone axis, and (c) high-resolution TEM image of the junction between an atomically abrupt
inter-twinning. The arrows are showing the junctions.
(a)(b)(c)Numbered grids were written onto a silicon wafer using electron beam lithography technique (EBL)
for fabricating test beds for electrical measurements. The synthesized InSb
nanowires were suspended in deionized water and transferred onto each grid. A
scanning electron microscope was used to locate and image the deposited wires.
With the images as reference, contacts to the nanowires were designed using an
SEM/E-beam Lithography system (Carl Zeiss Company, model: Leo SUPRA 55). The Si
chip was spin-coated with a polymer resist layer (PMMA) for using with electron
beam lithography, which involves exposing the resist layer to high-energy
electrons in a pattern defined by the drawing programmed in the lithography
system. Through developing, the resist is removed from the exposed areas. A
layer of Au is evaporated for making contact patterns to the measurement devices.
After metals are deposited, the remaining polymer is removed using acetone and
the EBL patterns of gold lines were found on top of the nanowires at the
desired sites (Figure4(a)). Electrical measurements were made using a semiconductor
parameter analyzer (Agilent Technologies, Inc. Santa Clara, CA USA, model: 4155C) coupled with a probe station (Signatone Corporation Gilroy, CA USA,
model: H 100) by applying a constant voltage across the nanowire and current
was measured. Figure 4(b) shows that the I-V curves exhibit a nonlinear
characteristic and might be referred to the Au/InSb/Au junction structure. It
is well known that contacts made by EBL shows rectifying behavior which can be explained considering the existence of two
back-to-back Schottky barriers connected in series to both extremes of the
nanowire, that is, Au-InSb-Au. The observed rectifying behavior at room
temperature tends to disappear at higher temperatures which is being studied at
present and will be reported in near future. Nevertheless, it could be
ascertained to the fact that conduction electrons possess enough mobility to
overcome the energy barrier existing in Schottky junctions.(a) Au-InSb-Au contacts (Schottky) made by electron beam lithography (EBL)
technique for a single NW, (b) I-V characteristics show the nonlinear
rectifying effect due to the metal-semiconductor-metal contacts.
(a)(b)


## 4. Conclusion

In summary, near-stoichiometric InSb nanowires have been synthesized by using dc electrodeposition
inside the nanochannels of AAMs without subsequent annealing. Achievement of
better control of stoichiometry than demonstrated here may be possible and will
be important for future device applications. The nanowires also conduct almost
no current in the dark, but when hit with light, they conduct 10000 times more
current which was also found recently. This photoconduction property could lead
to a variety of tiny optoelectronic devices potentially useful in future
generations of nanoelectronics, chemical sensors, and eventually provide clues
to the fabrication of tiny solar cells. The light-induced conductivity increase
and the temperature dependent behavior of the nanowires will be reported in
near future.



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*Source: 698759-2008-07-27.xml*