Silicon nanowires fabricated by thermal evaporation of silicon monoxide

Physica E 23 (2004) 131 – 134
www.elsevier.com/locate/physe
Silicon nanowires fabricated by thermal evaporation of
silicon monoxide
Junjie Niu
a
, Jian Sha
a; b
, Deren Yang
a;∗
a
State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
b
Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China
Received 9 January 2004; accepted 30 January 2004
Abstract
A large-scale crystalline silicon nanowires (SiNWs) with a diameter of ∼30 nm and length of tens of micrometers on
Al
2
O
3
templates and silicon wafers were synthesized by the thermal evaporation of silicon monoxide (SiO). The SiNWs were
measured by transmission electron microscopy, scanning electron microscopy, X-ray diraction and Raman spectroscopy,
respectively. It was pointed out that the SiNWs possessed the well crystalline structure. Therefore, it is considered that SiO
could be used as Si sources to produce larger-scale SiNWs and crystalline SiNWs may grow from amorphous nuclei.
? 2004 Elsevier B.V. All rights reserved.
PACS: 71.55.Cn; 81.05.Ys
Keywords: Nanowires; Silicon; Thermal evaporation
1. Introduction
Recently one-dimensional materials such as silicon
nanowires (SiNWs) have stimulated much interest be-

cause of their dierent electronic and optical charac-
teristics compared with bulk materials [1–4]. Many
favorable specialities of SiNWs have been reported
extensively, including p–n junction [5], chemical sen-
sors [6], electrical transport properties [7], and noise
characteristics [8], etc. Therefore, both the fabrica-
tion of large-scale uniform SiNWs and the under-
standing of the growth mechanism of SiNWs are very
important for their application. In fact, several dif-
ferent methods have been used for producing SiNWs
∗
Corresponding author. Tel.: +86-571-87951667;
fax: +86-571-87952322.
E-mail address: (D. Yang).
such as laser ablation [9], chemical-vapor-deposition
(CVD) [10–12], thermal evaporation Si and SiO
2
at
1200
◦
C[13,14], and electrochemistry [15]. The dif-
ferent growth models including vapor–liquid–solid
(VLS) [10], oxygen-assisted [13,16,17], and solid–
liquid–solid (SLS) [18] have been reported.
In this paper, we successfully synthesized large
quantity of uniform crystalline SiNWs on Al
2
O
3
templates and silicon wafers by thermal evaporation

of silicon monoxide (SiO) at 1100
◦
C, respectively.
The results of the scanning electron microscope
(SEM), transmission electron microscope (TEM) and
X-ray diraction (XRD) experiments indicate that the
SiNWs crystallized well.
2. Experimental
The samples were prepared in a CVD system as
we reported previously [10]. An Al
2
O
3
template and
1386-9477/$ - see front matter ? 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2004.01.013
132 J. Niu et al. / Physica E 23 (2004) 131 – 134
a p-type (1 1 1) silicon wafer with a resistivity of
about 0:001  cm as substrates were placed in a hor-
izontal quartz tube furnace, respectively. And SiO
particles (purity: 99.99%) were placed in an alumina
boat which was placed in the center of the furnace.
The furnace was evacuated for several hours to reach
a low vacuum of 20 Pa. The temperature of the fur-
nace was then raised to 1100
◦
C at a heating rate of
20
◦
C=s and held at a constant pressure of 1500 Pa

for 6 h. When the temperature reached to 750
◦
C, the
steady mixture gas of 200 sccm argon and 70 sccm
hydrogen were sent through the chamber, which
acted as the protective gas. The thermal evaporation
conditions for SiNWs synthesis were similar to the
previously reported work [16,19]. After the reac-
tion, the as-grown materials with weak-yellow and
brown color on the dierent zones of the substrates
were removed out from the furnace and measured
by XRD (XRD: Rigaku, D/MAX 2550 PC), SEM
(SEM: JEOL, JSM-5610LV), TEM (TEM: 160 kV,
JEM200CX) and Raman scattering spectroscopy
(Nicolet Almega), respectively, and the chemical
composition was detected by energy-dispersive X-ray
spectroscopy (EDS) attached to the SEM.
3. Results and discussion
Fig. 1 shows the TEM images of the symmetrical
SiNWs generated on a silicon wafer and an Al
2
O
3
template, respectively. It can be seen that the di-
ameter of those SiNWs is about 30 nm. The insets
of Fig. 1(I) and the upper right inset of Fig. 1(II)
are the selected area electric diraction (SAED) im-
ages, which show that the SiNWs were crystallized
well. The top view of the SiNWs on the dierent
substrates observed by SEM is shown in Fig. 2.

The plenty of the SiNWs with the length of tens
of micrometers were observed. The corresponding
EDS in the insets indicate that the SiNWs consists
of high-intensity Si and small quantity of oxygen.
Fig. 3 shows the XRD spectrum of the SiNWs grown
on the Al
2
O
3
template, which displays high-intensity
peaks of the Si (1 1 1); (220); (311); (400)
and (3 3 1), indicating that the SiNWs were the well
crystalline structure [20]. Some SiO
2
and -Al
2
O
3
came from the decomposition of SiO and the Al
2
O
3
substrate, respectively.
Fig. 1. TEM images of the SiNWs produced by thermal evaporation
of SiO on a silicon wafer (I, the upper right inset is the SAED
image taken from the corresponding SiNWs), and on an Al
2
O
3
template (II, the upper right and lower right insets are the SAED

images taken from the middle position (a) and end position (c)
of the SiNW).
Lee et al. contributed that silicon oxide played an
important role on the formation of SiNWs, so called
oxygen-assisted mechanism [13,16]. In our experi-
ments, SiO powders were used as Si sources. During
annealing at 1100
◦
C, SiO evaporated ÿrstly and was
transported by the gas to the lower-temperature region
(850–1000
◦
C) to decompose on substrates. The reac-
tion is as follows:
2SiO(↑)=Si+SiO
2
:
J. Niu et al. / Physica E 23 (2004) 131 – 134 133
Fig. 2. (a) SEM images of the SiNWs on a silicon wafer, and (b) on an Al
2
O
3
template. The insets of (a) and (b) are the EDS taken
from the corresponding SiNWs.
20 30 40 50 60 70 80
500
1000
1500
2000
2500

3000
SiO
2
Al
2
O
3
Si
¦Ã-Al
2
O
3
Al(220)
Si(331)
Si(400)
Si(311)
Si(220)
Si(111)
Intensity
2 Theta (degree)
Fig. 3. XRD spectrum of the SiNWs grown on an Al
2
O
3
template.
Therefore, in the XRD spectrum (Fig. 3), SiO
2
could
also be detected.
When Si atoms precipitate, the atoms will easily

centralize to form nuclei on low-energy places of sil-
icon wafers such as defects. With the increase of Si
atoms, the nuclei will grow up to wires. And because
of oxide reaction and growth energy, some growth di-
rections of SiNWs will be limited; therefore, the 111
and 112 orientations of the SiNWs may be the main
directions, as reported in the previous work [10,14].
The lower left inset of Fig. 1(II) is the SAED image
of the tip of the nanowires, indicating that is amor-
phous. The tip should be the nuclear of the SiNW.
134 J. Niu et al. / Physica E 23 (2004) 131 – 134
400 600 800 1000
0
500
1000
1500
2000
2500
3000
3500
4000
960cm
-1
960cm
-1
517.75cm
-1
517.67cm
-1
b

a
Intensity
Raman Shift(cm
-1
)
Fig. 4. (a) Raman spectra of the SiNWs on a silicon wafer and
(b) on an Al
2
O
3
template.
Furthermore, the upside of the wire was crystalline,
as illustrated in the upper right inset of Fig. 1(II). It
is considered that crystalline SiNWs grew from amor-
phous nuclei.
Fig. 4 shows the Raman scattering spectra of the
SiNWs grown on a silicon wafer (a) and an Al
2
O
3
template (b), which reveals that there are the same
peaks at 517 and 960 cm
−1
for both the SiNWs. It
can be seen from the spectra that the peaks with high
intensity are good symmetric and narrow, which could
be due to the uniform diameter. Usually, those peaks
are regarded to be the ÿrst-order transverse optical
photon mode (TO) which is caused by the diameter
decrease of the SiNWs [21].

4. Conclusion
A large-scale crystalline SiNWs on silicon wafers
and Al
2
O
3
templates were fabricated by the thermal
evaporation of silicon monoxide (SiO), respectively.
The SiNWs were about ∼30 nm in diameter and tens
of micrometers in length. It was also found that the
SiNWs crystallized well. Finally, SiO is considered to
be Si sources to produce SiNWs.
Acknowledgements
This work was supported by the National Natu-
ral Science Foundation of China (No.50272057 and
60225010). The authors would like to thank Prof.
Youwen Wang and Mr. Z.C. Chen, Zhejiang Univer-
sity, for their great helps in measurements.
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