Physica E 9 (2001) 305–309
www.elsevier.nl/locate/physe
Controlled growth of oriented amorphous silicon nanowires
via a solid–liquid–solid (SLS) mechanism
D.P. Yu
a; ∗
, Y.J. Xing
a; b
, Q.L. Hang
a
, H.F. Yan
a
,J.Xu
a
, Z.H. Xi
b
, S.Q. Feng
a
a
Department of Physics, Electron Microscopy Laboratory and Mesoscopic Physics National Laboratory, Peking University,
Beijing 100871, China
b
Department of Electronics, Peking University, Beijing, China
Received 19 May 2000; received in revised form 19 July 2000; accepted 24 July 2000
Abstract
Highly oriented amorphous silicon nanowires (a-SiNWs ) were grown on Si (1 1 1). The length and diameter of oriented
SiNWs are almost uniform, which are 1 m and 25 nm, respectively. Dierent from the well-known vapor–liquid–solid (VLS)
for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid–liquid–solid mechanism
(SLS). This synthesis method is simple and controllable. It may be useful in large-scale synthesis of various nanowires.
? 2001 Elsevier Science B.V. All rights reserved.
PACS: 61.46.+w; 68.65.+g; 73.20.Dx; 78.55.−m

1386-9477/01/$ - see front matter ? 2001 Elsevier Science B.V. All rights reserved.
PII: S 1386-9477(00)00202-2
306 D.P. Yu et al. / Physica E 9 (2001) 305–309
2. Experimentals
Heavily doped (1:5 × 10
−2
=cm) n-type Si (1 1 1)
chips wafers were used as substrate. The silicon subs-
trate was cleaned ultrasonically in pure petroleum
ether and in ethanol in turns for 5 min, and leached
in distilled water, then dried. A thin layer of 40 nm
nickel was thermally deposited on the substrate. The
substrate was placed in a quartz tube which was heated
in a tube furnace at 950
◦
C. Ar (36 sccm) and H
2
(4
sccm) were introduced during growth at an ambient
pressure of about 200 Torr. After cooling down to
room temperature, a thin layer of gray-colored deposit
was found on the surface of the substrate. An Am-
ray FEG-1910 scanning electron microscope (SEM),
and a Hitachi-9000NAR high-resolution transmis-
sion electron microscope (HREM) equipped with
energy-dispersive spectrum (EDS) were employed
for analysis of the morphology and microstructure of
the product.
3. Results and discussions
Fig. 1(a) shows plan view of SEM image revealing

this step. The ambient pressure of the tube was
Fig. 1. (a) SEM micrograph showing the general morphology of
the SiNWs grown via a SLS growth mechanism. The inset shows
EDS spectrum with the peak corresponding to Si, (b) TEM image
revealing that the SiNWs have smooth morphology and average
diameter around 40 nm. The SAED pattern shown in inset reveals
characteristic diusive ring pattern, showing that the nanowires
are completely amorphous.
kept near 750 Torr by adjusting the exit valve,
then the tube was evacuated. This procedure was
repeated three times. Finally, the temperature was
held at 950
◦
C at the pressure of about 200 Torr for
1 hr. A mixture of H
2
(4 sccm) and Ar (36 sccm) was
introduced to the tube.
A low-magniÿed SEM image of the oriented
a-SiNWs is shown in Fig. 2(a), representing a gen-
eral planar view of the oriented a-SiNWs. It is visible
that the nanowires were grown on centimeter-sized
substrate. An interesting phenomenon is that the
nanowire ÿlm was found chapped in a network
of white-contrasted lines. The inclined view at a
crossover point of the white in Fig. 2(b) revealed the
white lines are in fact V-shaped chaps. From this im-
age it is visible that the ÿlm consists of pure SiNWs.
D.P. Yu et al. / Physica E 9 (2001) 305–309 307
Fig. 2. (a) SEM image of oriented SiNWs on the substrate (top

with the vapor phase, and forms the NiSi
2
eutectic liq-
uid droplets. The vapor phase is rich in Si atoms. With
the further absorption of Si atoms into the droplets
from the vapor phase, the droplets become supersat-
urated, resulting in the precipitation of SiNWs from
the droplets.
In the present circumstance, however, the Si con-
centration in the vapor phase is negligible at the
growth temperature, because the speciÿc surface=
volume ratio of bulk Si substrate is extremely low.
On the other hand, the Si substrate was covered by a
thin layer of Ni. Therefore, the only possible silicon
source comes from the bulk silicon substrate, because
no extra Si source was introduced in the vapor phase.
From the binary Ni–Si diagram, it is visible that the
eutectic point of Si
2
Ni is 993
◦
C. However, due to
the melting eect of small-size grains, the eutectic
compound NiSi
2
can begin to form at a temperature
lower than 993
◦
C. As we proved, the deposited Ni
ÿlm can react with the Si substrate at temperature

and are all perpendicular to the substrate. The length
of the SiNWs is about 1 m. It is visible that be-
tween the SiNWs and the substrate there is a layer of
nano-sized particles which proved to consist of Ni and
Si. Fig. 4(b) shows a low-magniÿed cross-sectional
SEM image. It is visible that a layer of oriented
a-SiNWs was grown on the substrate. EDS analysis
between the Si substrate and the a-SiNW layer further
conÿrmed that there is a thin layer of Si–Ni alloy,
which is indicated with a white arrow. We found that
the a-SiNWs grow from base, which manifests itself
by the fact that the solidiÿed Si–Ni nano particles
were visible between the surface of the substrate and
the a-SiNW ÿlm, instead of being attached at the free
tip of the SiNWs.
The SiNWs are interesting to evaluate the quantum
conÿnement eect related to materials of low dimen-
sionality [10,11]. The a-SiNWs grown on substrate
have remarkable surface= volume ratio, possibly show-
ing physical–chemical properties completely dierent
from the bulk. From this point of view, it is speculated
that the a-SiNWs may have potential applications such
as rechargeable battery of high capacity with portable
size, which is closely related to the surface eects. In
fact, it was recently revealed that the lithium battery
using SiNWs as electrode materials showed a capacity
as high as 8 times than that of the ordinary one [12].
Fig. 4. (a) Low and (b) magniÿed cross-sectional SEM images of
the SiNWs grown on Si (111) substrate, which is controlled by
a SLS mechanism. The length of the SiNWs is about 1 m. The

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