A study in the growth mechanism of silicon nanowires with or without metal catalyst

A study in the growth mechanism of silicon nanowires
with or without metal catalyst
Jun-Jie Niu
⁎
, Jian-Nong Wang
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, PR China
Received 11 April 2007; accepted 22 June 2007
Available online 28 June 2007
Abstract
The growth mechanism of silicon nanowires synthesized with or without a metal catalyst via chemical-vapor-deposition (CVD) is discussed by
using a developed vapor–liquid–solid and novel sulfide-assisted growth models, respectively. The metal catalyst plays an important role on the
catalytic growth. However, the growth of silicon nanowires with sulfide is chiefly affected by the compound decomposition, gas stream, and
temperature difference. Silicon nanowires fabricated with metal can be self-organized while a large scale of samples can be achieved with metal-
free catalyst. The growth mechanism comparison between metal- and non-metal assisted methods for synthesizing silicon nanowires will supply a
beneficial help in deepening the understanding of crystal procedure and improving the sample quality.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Nanomaterials; Crystal growth
1. Introduction
Silicon nanowires (SiNWs), as a candidate material for nano-
electronic devices, are being intensively studied [1,2]. This is
because of the feasibility of integrating SiNWs as functional
building blocks into the existing CMOS technology [3].
Specifically, SiNWs can be applied in the fields including ultra
sensitive bio-sensor, field effect transistors (FETs), and single -
electron detector [4].
The fabrication of SiNWs involves metal-assisted or metal-
free growth. The metal-assisted growth mainly follow s the
vapor–liquid–solid (VLS) mechanism, which was first de-
scribed by Wagner and Ellis in 1964 and developed by
Givargizov in 1975 [5,6]. The metal catalyst used was usually
Au although other metals were also used [7,8]. The synthesis

methods mainly include laser ablation [9], chemical-vapor-
deposition (CVD) [10,11], thermal evaporation [12,13], and
growth from organic solution [14]. Amongst these methods,
laser ablation is widely adopted. However, this method does not
allow the local ized growth on a patterned substrate for further
processing [8]. Recently, a solid–liquid–solid (SLS) process
derived from the VLS mode was also reported for obtaining
SiNWs [15]. The growth based on an oxide, which was
proposed by Lee and coworkers [16], is a typical sample of
synthesis of SiNWs without a metal catalyst [17]. In this case,
the SiO
x
vapor decomposes into Si atoms, which act as the
nuclear of SiNWs covered by shells of silicon oxide.
Metal-assisted growth by CVD technique is a simple and
efficient route for synthesizing controllable and even self-
organized SiNWs at a low temperature [11,18]. However, the
metal-free growth based on sulfide is believed to have a
potential for large-scale production. The growth mechanisms,
however, are still poorly understood under both circumstances.
This study thus attempts to improve the understanding on the
basis of new experimental observations.
2. Experimental
A CVD system was used for the fabrication of SiNWs
(Fig. 1). For metal-assisted growth, the transition metal such as
Au or Ni was used as a catalyst. The substrate was put in a quartz
tube furnace which was then pumped down to ∼ 20 Pa and
heated to 600–900 °C. The mixed gases of argon, hydrogen, and
silane with a desired flow rati o were flowed. For sulfide-assisted
growth, silicon wafer was used both as the silicon source and

A
vailable online at www.sciencedirect.com
Materials Letters 62 (2008) 767 – 771
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⁎
Corresponding author. Tel.: +86 21 62932050.
E-mail address: (J J. Niu).
0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2007.06.056
sample collector. The ZnS or S powder was put in one side and
would be flowed to the wafer. The reaction temperature was set
at about 1100 °C. The samples were analyzed by scanning
electron microscopy (SEM, JSM-5610LV) and transmission
electron microscopy (TEM, JEM2100F).
3. Results and discussion
3.1. Metal-assisted growth of SiNWs
A basic aspect of the VLS mechanism is the metal particle acting as
a catalyst for the anisotropic growth of SiNW with a crystalline
structure. A catalyst particle provides a site for absorption of vapor-
phase silicon atoms. The continuous absorption induces the supersat-
uration of the formed liquid alloy with silicon atoms, which leads to
nucleation and growth of a SiNW [19]. Therefore, the diameter and
location of the SiNW are determined by the features of the catalyst
particle [20]. Thus, it is important to control the catalyst size
distribution and the delicate catalyst positioning. The metal catalyst
can be generated by thermal evaporation, sputtering, or electrochemical
methods. The particle size can be modified by varying the reaction
parameters. In particular, the catalyst distribution can be well-organized
by using a nano-channel-alumina (NCA) technique [11]. Otherwise,
the pressure of reaction channel and temperature are also important for

the growth of SiNWs. In the present CVD system (Fig. 1), the pressure
can be adjusted by a gas-controlled valve. Normally, a high pressure
can lead to a high yield of the sample. If a very low pressure was used,
plenty of Si atoms will be flowed away and have no time to contact
Fig. 1. Diagrammatic sketch of the CVD system.
Fig. 2. A) Binary phase diagram of Au and Si. B) A mode of VLS mechanism.
768 J J. Niu, J N. Wang / Materials Letters 62 (2008) 767–771
with the catalyst. This results in formation of only a few of SiNWs. The
reaction temperature is usually decided with the consideration of the
catalyst type. According to the binary phase diagram of a metal and Si,
the temperature for SiNW formation must be above the eutectic point
as liquid droplets can form under this condition. Here we give an
example of using Au as the catalyst. As seen from Fig. 2A, if the
reaction temperature is increased to be above the eutectic point of
363 °C (such as T1), a liquid alloy can form and will assist the growth.
Considering the production output, a temperature of 620 °C was used
for the case of Au and 900 °C for the case of Ni to fabricate SiNWs
[11,21].
In general, the chemical reaction equations can be written as the
following:
SiH
4
¼ Si þ 2H
2
ð1Þ
Si þ O
x
¼ SiO
x
ðxb2Þð2Þ

(3)
Si is proffered by the decomposition of the precursor of SiH
4
(Eq. (1)). When dropping Si atoms come to contact with the metal
particle located on the substrate (Fig. 2B a), a liquid metal–Si alloy will
be formed. With more and more Si atoms added, the Si content in the
droplet will reach a saturated value (in the Au–Si system, this value is
∼ 25% at a point in Fig. 2A). If the supply of Si atoms is continued, the
liquid alloy will be supersaturated with Si atoms and excessive Si
atoms will then precipitate. The precipitated atoms will grow freely
with a crystalline structure (Fig. 2B b). Since the orientation of b111N
in Si lattice has the lowest energy, this orientation dominates the growth
direction and the final extending direction of SiNW (Fig. 3A). In
particular, when the nanowire becomes longer, the droplet at the tip will
be pushed randomly and thus sometimes two or more will combine to
form a bigger one, leading to an intercrossing structure. Fig. 3B clearly
shows the original growth stage of SiNWs. As can be seen from the
figure, if Si atoms cannot be added continuously into the droplet, the
growth will be terminated and a short SiNW is obtained. On the con-
trary, if enough Si atoms are provided, a long SiNW with random
direction will be observed (Fig. 3C). In this case, an excellent crystalline
nature of Si (111) is observed as shown in Fig. 3A. In addition, the as-
formed SiNW is easily oxidized with an oxide shell of 1–3nm
according to the Eq. (2) (Fig. 3A).
3.2. Sulfide-assisted growth of SiNWs
If the growth with a metal catalyst can cause contamination, the
synthesis without metal will be useful for obtaining clean samples with
high quality. The oxide-assisted growth is an effective approach for
large-scale production of high-quality SiNWs without metal [16,17].
During the reaction, SiO decomposes into Si and SiO

2
. The pre-
cipitated Si atoms under the assistance of outer SiO
2
shell will form a
nuclear and thus grow up to a wire. It is clear that silicon oxide plays a
key role on the formation of SiNW. Similar with this mode, we have
Fig. 3. High resolution TEM image (A), SEM image of original growth stage (B), and final stage (C) of SiNWs synthesized via VLS mechanism.
769J J. Niu, J N. Wang / Materials Letters 62 (2008) 767–771
developed a novel sulfide-assisted mechanism to synthesize SiNWs in
large quantity [22]. Relative to SiO decomposition, compound SiS also
plays a significant role on the growth of SiNW. Furthermore, the gas
flow and temperature difference have simultaneously important effects.
We now analyze the detailed growth process in the following sections.
Firstly, the whole reaction equations can be summarized to:
2ZnS þ O
2
¼ 2ZnO þ 2S↑ðN∼200-CÞ; ð4Þ
S þ Si ¼ SiSðN∼900 -C Þ ; ð5Þ
2SiS↑ ¼ Si þ SiS
2
ð∼950-C–1080-CÞ; ð6Þ
and
SiS
2
þ 2H
2
O ¼ SiO
2
þ 2H

2
S↑: ð7Þ
The S source can be originated from ZnS or direct S powders [22].As
presented in Fig. 4A, the S vapor formed at low temperature zone
(b ∼ 900 °C) will be carried to a higher temperature zone (∼ 900 °C
b Tb∼ 950 °C). In this region, the S vapor will encounter silicon wafer
which is used as a substrate and form plenty of SiS particles relative to the
Eq. (5). When the temperature is continually increased to ∼ 950–1080 °C,
the new-formed SiS compound will sublimate and decompose into Si and
SiS
2
(see Eq. (6) and zone A in Fig. 4A). At the beginning, the Si/SiS
2
is
present as a phase of quasi-liquid droplet. Thus, a large quantity of Si and
SiS
2
atoms are quickly flowed away by the protected gas to an area with
lower temperature (B in Fig. 4A). When temperature decreases, the Si
atoms are easier to reach saturation in quasi-liquid Si/SiS
2
and precipitate
along the droplet to form a nuclear. As a result, the precipitated Si atoms
develop to be a SiNW with crystal directions along the energy lowest
theory (B in Fig. 4A). Amongst this method, the reactions Eqs. (5) and (6)
are very fast and thus induce a quick growth of numbers of SiNWs with
a badly crystalline structure (Fig. 4B). Otherwise, the Si is easy to be
oxidized in low-vacuum system and the generated SiS
2
is very feasible to

react with H
2
O (Eq. (7)). The above factors easily cause a low-crystal
quality of as-received SiNWs even amorphous SiO
2
nanowires.
3.3. Advantage and disadvantage of with and without metal growth
The mentioned metal-assisted and free-metal methods are both simple
and convenient to synthesize SiNWs. The VLS mechanism with metal
catalyst has disadvantages of c ontamination, low quantity, and complicated
process. Furthermore, if the Si source is used by silane, t he vacuum
requirement for equipment is high. However, this process can be reacted at
a r elatively l ow tempera ture. And growth rate can be controlled even can be
self-organized with the assis tance of template. Thus i t provides an operable
route to combine with present nano-techniques for real applications. The
fabrication with sulfide-assisted mechanism cannot be easily controlled
and the structure of SiNWs is badly crystalline even amorphous. Further-
more, t he growth temperature is relatively h igher. However, by using this
quick-growth technique, we can receive large scales of sample with a very
simple device. If we can control the growth velocity and improve the
crystal nature, the process may have more applications in the future.
4. Conclusions
The catalyst features take important actions on metal-assisted
growth. Otherwise, the compound decomposition, gas stream,
and temperature difference play key roles on sulfide-assisted
synthesis. The growth with metal catalyst can be controlled while
the sulfide-assisted growth can obtain large scales of samples.
Acknowledgements
This wor k was supported by the Shanghai-Applied Materials
Research and Development Fund (No. 06SA06) and Youth

Teacher Fund of Shanghai Jiaotong University (A2306B). We
would like to thank Instrumental Analysis Center of Shanghai
Jiaotong University, for their great helps in measurements.
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