Oriented silicon nanowires on silicon substrates from
oxide-assisted growth and gold catalysts
Yuan Yao
a
, Fanghua Li
b
, Shuit-Tong Lee
a,
*
a
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong,
Tat Chee Avenue, Kowloon, Hong Kong SAR, China
b
Institute of Physics and Center for Condensed Matter Physics, CAS, Beijing, China
Received 17 February 2005; in final form 4 March 2005
Available online 25 March 2005
Abstract
High-density, oriented silicon nanowires (SiNWs) array were fabricated on (0 0 1) silicon substrates by the oxide-assisted growth
method assisted with Au catalyst in a hot filament chemical vapor deposition system. The yield of SiNWs was different with the
synthesis temperature. Au particles were present at the tips of the SiNWs and limited the wire diameter. High resolution transmis-
sion electron microscopy revealed the epitaxial SiNWs on the Si substrate.
Ó 2005 Elsevier B.V. All rights reserved.
One-dimensional (1D) nanomaterials have attracted
intense interest because of their many unique properties
not found in the bulk materials. Silicon nanowires
(SiNWs) are a particularly important 1D nanomaterial
because silicon is most widely used in electronic indus-
try. Besides excellent electronic property, nanoscale sili-
con materials also possess interesting optical [1] and
field-emission [2] properties, which may be exploited
for optoelectronic applications.

no carrier gas. The temperature of the SiO source was
kept at 1300 °C, while that of the substrate varied from
600 to 1000 °C for different experiments. Thermocouples
(mounted to the source and substrate) supplemented by
infrared (IR) pyrometer were employ ed to measure the
temperature. The pressure in the chamber increased to
$10
À2
Torr after 30 min of growth. The processing
details are shown in Fig. 1.
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.03.027
*
Corresponding author. Fax: +852 2784 4696.
E-mail address: [email protected] (S T. Lee).
www.elsevier.com/locate/cplett
Chemical Physics Letters 406 (2005) 381–385
After growth, the Si substrate surface showed differ-
ent colors dependent on the temperature to which it
was heated; varying from light yellow at 900 °C to gray
at 600 °C. Figs. 2a,b are the scanning electron micros-
copy (SEM) images of the silicon substrate surface after
growth at 800 and 700 °C, respectively. It is clear that
the surface was covered with high density, oriented
array of aligned nanowires. The average length of the
nanowires was less than 5 lm, while the density of the
wires varied with the substrate or deposition tempera-
ture. The largest density ($400/lm
2
) of the nanowires

consist of a crystalline core and an amorphous silicon
oxide sheath, similar to the SiNWs grown by the OAG
method without Au catalyst. As illustrated in Fig. 3c,
although some nanowires grew along Æ111æ direction,
the dominant growth direction of SiNWs was Æ112æ
with Æ110æ being the second dominant direction, the
same as the SiNWs grown by the OAG method. The
Æ112æ direction is different from the common Æ111æ
growth direction for SiNWs fabri cated by the Au-
catalytic VLS method. To investigate the initial wire
growth, the cross-section of the sample grown for
10 min was examined by HRTEM. Fig. 4b shows a
typical SiNW of 20 nm in diameter grew epitaxially on
the (0 0 1) silicon substrate with a 40 nm Au cap. Signif-
icantly, the interface between the wire and substrate is
essentially free of defects. The high-quality epitaxy
at the interface is expected to subsequently guide the
oriented growth of SiNWs on the Si substrate.
The growth process of SiNWs can be described as
follows [8]: First, when the substrate is heated to high
temperature (700–800 °C), the 1 nm gold film would
break up to form 20–40 nm Au particles. The Au parti-
cles will dissolve Si from the Si substrate to form the eu-
tectic Au–Si alloy. As the SiO
x
vapor arrives at the Au
particle, SiO
x
will disproportionate at the particle sur-
face into Si and SiO

decomposition temperature. Close TEM observation
shows there is a thin silicon oxide layer covering the
Au particle tips. During growth, the arriving silicon
atoms have to diffuse through the thin oxide layer to
reach the Au particle. Thus, the growth temperature
should be sufficiently high to allow silicon penetrating
through the oxide layer to form the eutectic alloy so
as to sustain the growth of SiNWs. It is likely that the
oxide layer stops silicon diffusion into the Au particle
at temperatures less than 700 °C, thus hindering the
growth of SiNWs at lower temperatures. Moreover, it
was found the Si substrate was covered by a thin
poly-crystal silicon and amorphous silica film if the
growth temperate was higher than 800 °C.
The growth direction of SiNWs from the MC-VLS
method with Au catalyst is predominantly along the
Æ111æ direction [3]. In the present method, the SiNWs
primarily grew along the Æ112æ and Æ110æ directions,
similar to SiNWs grown by the OAG method without
any catalysts. As shown in Fig. 4a, the side surfaces of
the SiNW are made of the {1 1 1} and {1 1 0} facets
for the nanowire grown along [1 1 2] direction. The pres-
ence of those crystal facets could minimize the total en-
ergy of the nanowire because the surface energy of the
{1 1 1} facets is the lowest and the energy of the side sur-
faces dictates the total surface energy of a SiNW [11].
Recently, it was reported that the small-diameter SiNWs
Fig. 3. (a) Variation of wire density and diameter as a function of
deposition temperature. (b) Histogram of diameter distribution of
SiNWs and Au particle tips. (c) Growth direction of SiNWs deposited

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Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 385