Growth of amorphous silicon nanowires
Z.Q. Liu, W.Y. Zhou, L.F. Sun, D.S. Tang, X.P. Zou, Y.B. Li,
C.Y. Wang, G. Wang, S.S. Xie
*
Group 412, Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
P.O. Box 603-32#, Beijing 100080, People's Republic of China
Received 16 February 2001; in ®nal form 4 April 2001
Abstract
We have grown vertically aligned amorphous silicon nanowires on Au±Pd co-deposition silicon oxide substrate by
thermal chemical vapor deposition using SiH
4
gas at 800°C. The diameter of silicon nanowires is in the range 10±50 nm
and the length is about 1 lm. Transmission electron microscopy (TEM) observations show that the grown silicon
nanowires are of an amorphous state and some of nanowires appear to bifurcate in the vertically growth process. The
eect of H
2
gas etchings on the catalytic size and the eect of catalytic size on the formation of the vertical growth
nanowires are discussed. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
Nano-materials are attracting much attention
because of the electronic, magnetic, optical, bio-
logical, and chemical characteristics they have that
are not obtained with conventional materials.
Among these low-dimensional materials, one-
dimensional materials, such as nanotubes [1±4],
semiconductor nanowires [5±10] and metal nano-
wires [11] have been of recent heightened interest
because these materials oer fundamental scien-
ti®c opportunities for investigating the in¯uence of
size and shape with respect to optical, electronic,
and mechanical properties. For silicon, it is

Corresponding author. Fax: +86-10-8264-9531.
E-mail address: (S.S. Xie).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 513-9
nucleation site as well as dierentiate between
growth and tail ends of the nanowires. In this
Letter, we use chemical vapor deposition of silane
to prepare silicon nanowires. Our research group
had previously reported the growth amorphous
silicon nanowires on AuPd/SiO
2
/Si substrates by
using thermal chemical vapor deposition [17]. We
extend the work of H
2
gas etchings to the nano-
wires growth. The eect of H
2
gas etchings on the
catalytic size and the eect of the catalytic particle
size on the diameter and alignment of amorphous
silicon nanowires are discussed. Bifurcate phe-
nomena morphology was found in vertical growth
process.
2. Experimental
The substrates used in our experiments were
8 X cm n-type Si(1 0 0) wafers with an oxide layer
about 60 nm in thickness. They were ultrasoni-
cally stirred for 30 min in acetone solution to
clean their surfaces. The cleaned substrates were

nanowires grown on a substrate, which was de-
posited with AuPd alloy as catalyst for 10 nm. Fig.
1a shows that large amounts of nanowires are
formed, which are of a uniform length up to
2 micrometers. The growth rate of the nanowires is
estimated to be ca. 15 nm/min. The diameter of the
nanowires is about 40 nm. A lot of nanowires are
assembling along the same direction and are not
parallel to the surface of the substrate (see Fig.
1a,b). All of the nanowires are terminated by the
nanoparticles with the diameter about 60±80 nm at
their tips (see Fig. 1c). Some of the nanowires
become curved near their tops. The EDX spectra
taken from these nanoparticles showed the pres-
ence of gold, palladium, oxygen and silicon (0.6,
0.3, 26.6 and 72.5 at%, respectively). Compared
with the nanoparticles, the nanowires are com-
posed of silicon and a small amount of oxygen,
indicating that no catalytic elements exist in the
nanowires. Top view of the nanowires is shown in
Fig. 1d. An interesting phenomenon is that bifur-
cation of the nanowires was found at the tail of the
nanowires (see arrowhead in Fig. 1b,d)
High-resolution TEM was employed to detect
the structure of the nanowires in detail. Using an
ultrasonic treatment in alcohol solution for 30 min,
Table 1
Pretreatment and depostion parameters
Pretreatment Depostion
Gas H

size on the diameter and alignment of amorphous
silicon nanowires, we also use the substrate, which
has been sputtered with Au±Pd alloy thickness of
about 5 nm. The growth process is the same as 10
nm Au±Pd deposited substrate. Fig. 3a±b shows
the SEM images of the nanowires grown on 5 nm
Au±Pd deposited substrate. Comparing with the
Fig. 1. SEM images of the silicon nanowires grown on a substrate deposited with AuPd alloy as catalyst for 1 min. (a) Low-mag-
ni®cation images of silicon nanowires. (b) and (c) A magni®ed view of (a). (d) Top view of the vertically aligned silicon nanowires.
Z.Q. Liu et al. / Chemical Physics Letters 341 (2001) 523±528 525
thicker Au±Pd alloy ®lms, we can see that the di-
ameter of the nanowires is 30 nm. The alignment
of the nanowires is improved.
Compared with our previously work [17], we
got amorphous aligned silicon nanowires instead
of the randomly distributed nanowires. The only
dierence in our experiment is that the substrates
were pretreated by H
2
etching before the nano-
wires growth. For comparison, the substrate which
only annealed in ¯uent He without H
2
etching was
also studied. Fig. 4a show the SEM image of the
unetching substrate with 5 nm Au±Pd ®lms. From
Fig. 4 we can see that AuPd ®lms on the substrate
have broken up to form a large amount of nano-
particles. The nanoparticles size are uniform and
are of about 30 nm diameter. However, when the

explain the bifurcation growth process under VLS
growth mechanism (see diagram in Fig. 5). During
the aligned nanowires growth process, some of the
nanowires will form kinks due to the weight of the
catalyst on their tip. It may be easy for the melting
catalytic nanoparticles on its top to meet together
and coalesce to become a larger catalyst. From
Fig. 1, we found that the size of catalyst on the tip
of the nanowires become larger than that of the
particles shown in Fig. 4b. In the end, the bifur-
cation growth is formed.
4. Conclusion
Aligned amorphous silicon nanowires on a
large scale of Au±Pd co-deposition silicon oxide
substrate by thermal chemical vapor deposition
were obtained. The catalytic particle size of Au±Pd
catalysts decreases, the diameter of the nanowires
decreases and the vertical alignment is enhanced.
H
2
pretreatment before growth can deduce the
catalytic nanoparticles sizes. There are bifurcation
Fig. 5. Schematic diagrams of the bifurcation growth model.
Fig. 4. SEM images of the annealing substrates with 5 nm
Au±Pd ®lms at 800°C for 180 min. (a) without H
2
gas etching,
(b) with H
2
gas etching.

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