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

A
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Materials Letters 62 (2008) 767 – 771
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⁎
Corresponding author. Tel.: +86 21 62932050.
E-mail address: [email protected] (J J. Niu).
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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

(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
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

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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Fig. 4. A) The growth sketch of sulfide-assisted mechanism. B) SEM image of
SiNWs synthesized by sulfide-assisted growth.
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