Oriented silicon nanowires on silicon substrates from oxide assisted growth and gold catalysts

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.

There are various ways to synthesize SiNWs, includ-
ing the classic metal-catalyti c vapor–liquid–solid (MC -
VLS) [3] method, the simple thermal-evaporation
oxide-assisted growth (OAG) method [4], and the solu-
tion-grown method [5]. MC-VLS and OAG are widely
used to fabricate SiNWs and they have their respective
merits and shortcomings. The MC-VLS method offers
better control of diameter [6] and patterning [7] of
SiNWs but relatively low yield of production; whereas
the OAG method can produce SiNWs in large quantities
and without metal contamination, but with less control
in wire pattern and diameter. Recently, SiNWs have
been grown by combining the OAG method and metal
catalysts [8]. Here, we report the successful fabrication
of well-oriented array of SiNWs with controlled diame-
ter using a similar approach.
SiNWs were fabricated in a hot-filament chemical va-
por deposition (CVD) system. (0 0 1) silicon substrates
were etched in 5% hydrofluoric (HF) acid, sonicated in
acetone, and resistively coated (thermal evaporation)
with 1 nm Au film at a base pressure of 5 · 10
À6
mbar.
A molybdenum foil was used to heat the silicon sub-
strate, while another boat-like molybdenum foil con-
taining SiO powder worked as a source heater. The
base pressure of the CVD chamber was 10
À6
Torr. Dur-
ing growth, the system was closed with no pumping and

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: (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

was obtained at 700 °C, while the wire orientation ap-
pears to be insensitive to the deposition temperature.
No nanowires were observed if the deposition tempera-
ture was above 800 °C or below 700 °C. The yellow
color above 800 °C came from the roughened surface
of the substrate. To investigate the microstructure of
SiNWs, the as-grown nanowires were scratched from
the substrate and dispersed in alcohol. A few drops were
put on the holey carbon copper grids for transmission
electron microscopy (TEM) and high-resolution TEM
(HRTEM) examination. TEM image (Fig. 2c) shows a
dark particle capping the nanowire. Silicon element
mapping image (Fig. 2d) indicates that Si was absent
in the dark cap. Figs. 2e,f display, respectively, the
EDX spectrum of the nanowire and its particle cap,
which reveals the gold signal only from the cap. It is thus
established that oriented SiNWs with the gold particle
cap were formed on the silicon substrate.
The diameter variation of the nanowires deposited at
different temperature is illustrated in Fig. 3a. The chart
shows the average wire diameter increased, while the
wire density decreased with increasing deposition tem-
perature. Fig. 3b depicts the diameter distribution of
the SiNWs and Au particle tips grown at 700 °C. The
bars indicate that the diameters of the SiNWs range
mostly between 10 and 30 nm, whereas the diameters
of the Au particle tips primarily fall between 20 and
40 nm. Consequently, the diameter of the nanowires is
well limited by that of the Au caps.
HRTEM image (Fig. 4a) shows the as-grown SiNWs

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

2
. Silicon will dissolve in the Au par-
ticle, while the silicon oxide will remain at the particle
surface. When the Si concentration in the Au particle
reaches super-saturation, Si will separate out at the
interface and grow epitaxially on the Si substrate. Sili-
con oxide will flow over the Au particle surface and
form a layer sheathing the SiNW. The diameter of the
SiNWs is thus limited by the size of the Au particles.
Fig. 1. Flowchart of the fabrication process of oriented silicon nanowires.
382 Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385
It is well recognized that the formation of metal–Si
eutectic alloy is the key point of the MC-VLS growth
of SiNWs and the growth temperature is dependent on
the metal catalyst. As Au and Si can form an eutectic al-
loy as low as 363 °C [9], consequently SiNWs have been
synthesized under 500 °C using Au as the catalyst and
SiH
4
as the source [10]. On the other hand, we have
shown that SiNWs are produced by the catalyst-free
OAG method only at temperatures above 900 °C [4].
The present results show that the production of SiNWs
Fig. 2. (a) SEM image of oriented SiNWs arrays synthesized at 800 °C and (b) 700 °C. The scale bar is 2 lm. (c) TEM image of a SiNW with an Au
particle tip and (d) corresponding silicon elemental mapping. The scale bar is 50 nm. (e) EDX spectrum of the nanowire and (f) its Au particle tip.
Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 383
is limited to the temperature range of 700–800 °C using
the OAG and Au catalyst. The reduced growth temper-
ature relative to that of metal-free OAG may be attrib-
uted to the Au catalytic effect in lowering 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

at 700 °C.
Fig. 4. (a) HRTEM image of a SiNW. The growth direction is along
Æ112æ orientation. (b) The initial growth of SiNWs with an Au cap
showing the epitaxial relation to the Si substrate.
384 Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385
grown from the MC-VLS method also adopted the
Æ110æ and Æ112æ orientations [12]. In the present work,
the initial growth of the SiNWs is along Æ001æ direction
as a result of epitaxy to the substrate, as depicted in
Fig. 4b. Therefore, the present results suggest that the
initial Æ 001æ orientation of SiNWs may change to the
more popular Æ110æ and Æ112æ directions during subse-
quent growth of SiNWs as described elsewhere [13].
In summary, high-density, oriented SiNWs array were
grown on (0 0 1) silicon substrates by the OAG method
assisted by Au catalyst. The yield of SiNWs reached the
largest at 700–800 °C. Au particles were present at
the tips of the SiNWs and limited the wire diameter. The
present SiO–Au approach offers certain advantages over
the common metal-catalytic VLS method and the OAG
method, such as the absence of toxic and flammable gases
and the control of size and epitaxial growth of SiNWs.
Acknowledgment
This work is supported by a Central Allocation Pro-
ject (No. CityU 3/04C) of the Research Grants Council
of Hong Kong SAR.
References
[1] A.G. Gullis, L.T. Canham, Nature 353 (1991) 335.
[2] W.K. Wong, F.Y. Meng, Q. Li, F.C.K. Au, I. Bello, S.T. Lee,
Appl. Phys. Lett. 80 (2002) 877.

[3] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964)
89.
[4] (a) R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003)
645;
(b) S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull. 24
(1999) 36.
[5] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science
287 (2000) 1471.
[6] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, C.M. Lieber,
Appl. Phys. Lett. 78 (2001) 2214.
[7] Y. Wu, R. Fan, P.D. Yang, Nano Lett. 2 (2002) 83.
[8] F.M. Kolb, D.D. Ma, M. Zacharias, S.T. Lee, U. Goesele, J.
Electrochem. Soc. 151 (2004) G472.
[9] T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak
(Eds.), Binary Alloy Phase Diagrams, second edn., ASM Inter-
national, Materials Park, OH, 1990, p. 428.
[10] N. Ozaki, Y. Ohno, S. Takeda, Appl. Phys. Lett. 73 (1998)
3700.
[11] C.P. Li, C.S. Lee, X.L. Ma, N. Wang, R.Q. Zhang, S.T. Lee,
Adv. Mater. 15 (2003) 607.
[12] Y. Wu, Y. Cui, L. Huynh, C.J. Barrelet, D.C. Bell, C.M. Lieber,
Nano Lett. 4 (2004) 433.
[13] Y. Yao, S.T. Lee, J. Phys. Chem. B (submitted).
Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 385