Theoretical investigation of silicon nanowires: Methodology, geometry, surface modification,
and electrical conductivity using a multiscale approach
Man-Fai Ng,
1
Liping Zhou,
1,2
Shuo-Wang Yang,
1,
* Li Yun Sim,
1
Vincent B. C. Tan,
2
and Ping Wu
1
1
Institute of High Performance Computing, 1 Science Park Road, #01-01 The Capricorn, Singapore 117528, Singapore
2
Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore
Received 23 April 2007; revised manuscript received 26 June 2007; published 29 October 2007
The structural and electronic properties of hydrogenated silicon nanowires SiNWs oriented in 100, 110,
111, and 112 directions are investigated systematically using a multiscale approach: geometry optimization
is done by a semiempirical method and electronic band structure is calculated by density-functional theory with
Gaussian basis set. The calculated band gaps agree very well with the available experimental data. We propose
that the multiscale approach is an accurate and effective way for calculating the structural and electronic
properties of SiNWs with diameter up to 3 nm. Besides, we found that surface modification of the SiNWs
using the hydroxyl and fluoro groups can strongly reduce the band gap by as much as 1 eV, and more
interestingly, alter the gap nature. On the contrary, the 110 SiNWs exhibiting different patterns at the cross
section do not show significant difference in their band gaps 0.09 eV, indicating that the electronic struc-
tures of SiNWs are much more sensitive to surface modification than the change of cross section. Moreover,
SiNWs of different orientations exhibit different degrees of band gap reduction upon surface modification, in
which the 110 SiNW demonstrates the highest sensitivity. The result indicates that SiNWs oriented in 110
direction are the better candidate for sensor application. On the contrary, the 111 SiNW is found to be very
tight to structural change with diameter and the most reluctant to surface modification, showing that it is
structurally stable and rather inert. In addition, from the I-V curves of the SiNWs with surfaces modified with
hydroxyl groups, which are calculated by the nonequilibrium Green’s function theory, we found that the
electrical conductivity of the SiNWs is highly chemical sensitive. Besides, the phenomenon of negative dif-
ferential resistance is observed in the I-V curves.
DOI: 10.1103/PhysRevB.76.155435 PACS numbers: 73.22.f
I. INTRODUCTION
The intrinsic characteristics of silicon nanowire SiNW
such as one dimensionality, high surface-to-volume ratio,
biocompatibility, as well as the tunable band gap make it a
unique and special class of semiconductors. More impor-
tantly, because of its interface compatibility with the existing
silicon-based technology and the intensive pursuits of the
miniaturization of silicon electronic,
1
the development of
SiNW appears to be a natural choice. In fact, SiNWs have
been successfully synthesized and have already been utilized
in various applications such as field-effect transistors,
2–5
chemical and biological sensors,
6,7
complementary metal-
oxide semiconductor devices,
8
as well as for developing op-
tical and photonic devices.
9
Currently, most applications
make use of SiNWs with diameters of few tens nanometer.
However, SiNWs with diameters of few nanometer can be-
come more important in application not only due to the size
matter but also for the fact that small diameter SiNWs ex-
hibit unique properties because of the growth orientation and
the pronounced quantum confinement effect.
SiNWs below 10 nm in diameter have been synthesized
recently by various techniques.
10–14
These SiNWs exhibit
various diameter distributions and growth orientations. In
particular, the solution techniques employed by Holmes et
al.
11
produced SiNWs with 4 – 5 nm in diameter that oriented
in 100 and 110 directions. Cui et al.
12
reported that
SiNWs grown by a vapor-liquid-solid mechanism have a
range of diameters from 6 to 31 nm with 110 and 111
growth orientations. Ma et al.
13
adopted the oxide-assisted
catalyst-free method, by which the 110 and 112 SiNWs
are grown with diameters ranged from 1.3 to 7 nm. Wu et
al.
14
obtained SiNWs with 110, 111, and 112 growth
orientations down to 3 nm in diameter by employing the
chemical vapor deposition method using gold nanocluster as
catalyst. Normally the synthesized SiNWs are coated with an
oxide layer. Hydrogenated SiNWs can be obtained after re-
moving the oxide layer by hydrofluoric acid treatment. It is
noted from these experimental results that the 110 SiNW is
the most commonly produced by these “bottom-up” syn-
thetic strategies with diameter as small as 3 nm. The occur-
rence of the 111 and 112 SiNWs tends to be more tech-
nique dependent. The 100 SiNW is relatively less reported.
Nevertheless, these experiments show that 110, 111,
112, and 100 are the most important and the most com-
mon growth orientations for small diameter SiNWs.
Understanding the electronic properties of SiNW within
the quantum confined regime 3 nm diameter via theoret-
ical studies is important for two reasons: i it is still difficult
for the experimentalists to synthesize SiNWs with such small
diameters and thus their properties have only been partially
determined by experiments, and ii SiNWs within this re-
gime have larger band gaps that can extend their functional-
ity and they are also more unique. For instance, the elec-
tronic structures of SiNWs of different growth orientations
within this regime are more differentiated. The calculated
results can thus provide more guidelines for the band gap
engineering purposes. In fact, the electronic properties of
SiNWs have been extensively investigated theoretically with
respect to diameters, growth orientations, morphologies, sur-
face modifications, and doping effects.
15–26
Moreover, the
optical properties,
27–29
structural stability,
30–34
and electrical
properties
35
have also been reported.
PHYSICAL REVIEW B 76, 155435 2007
1098-0121/2007/7615/15543511 ©2007 The American Physical Society 155435-1