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Physica E 23 (2004) 221–225
Raman spectrum of array-ordered crystalline silicon nanowires
Jianxun Liu
a
, Junjie Niu
b
, Deren Yang
b
, Mi Yan
c
, Jian Sha
a,b,
*
a
Department of Physics, Zhejiang University, P.O. Box 1232, Hangzhou 310027, China
b
State Key Laboratory of Silicon Materials, Zhejiang University, China
c
Department of Materials Science and Engineering, Zhejiang University, 310027 Hangzhou, China
Received 27 February 2004; accepted 10 March 2004
Available online 11 May 2004
Abstract
Array-ordered single-crystal silicon nanowires were fabricated by the nanochannel-aluminal and CVD method. The
average length and diameter of the nanowires is about 10 mm and 60 nm, respectively. A study of the Raman spectrum
of the nanowires shows that the Raman shift to low frequency is due to the quantum confinement effect, which is
discussed by using the phonon confinement model. Also we determine the peaks of the Raman spectrum to be
corresponding to that of crystal silicon (c-Si).
r 2004 Published by Elsevier B.V.

PACS: 61.46.+w; 68.65.La; 78.30.Àj
Keywords: Nanowires; Raman spectra; Phonon confinement model
1. Introduction
Silicon, the most important semiconductor
material, plays an dominant role in the area of
microelectronics materials. Silicon-based photo-
electric integration is one of the highlights of
todays science research. Crystal silicon is an
indirect band material, and its light emission
efficiency at room temperature is very low.
Recently, one-dimensional silicon nanomaterials
had been prepared successfully by many methods
[1–8], stimulating intensive interest in it [9–11].
Silicon nanowires (SiNWs) are particularly inter-
esting because they are expected to exhibit unusual
confinement effects on electrical and optical
properties due to its low dimension and high
surface-to-volume ratio and fulfill the technology
requirements for integration.
As with micro-Raman technique, which offers
many advantages such as much less amount of
sample and lower-power laser. Raman spectra
play an increasingly important role in the study of
nano-structured materials. Especially that of
SiNWs reveals many interesting features: the
Raman peak of SiNWs changes compared to
the c-Si in position, FWHM, symmetry and the
Raman peak changes with the wavelength of
ARTICLE IN PRESS
*Corresponding author. Department of Physics, Zhejiang

University, P.O. Box 1232, Hangzhou 310027, China. Tel.:
+86-0571-879516593.
E-mail address: (J. Sha).
1386-9477/$ - see front matter r 2004 Published by Elsevier B.V.
doi:10.1016/j.physe.2004.03.016
exciting laser, etc. [12–15]. In this paper, we report
a Raman investigation of the SiNWs and give
some explanation for the discrepancy between the
SiNWs and the c-Si.
2. Experiment
The nanochannel-aluminal (NCA) and CVD
techniques were used for preparing the array-
ordered single crystal silicon nanowires [16]. The
NCA were fabricated by electrochemistry method
with chromium acid [17], of which the average
channel diameter is 60 nm. Then gold as a catalyst
was deposited by the magnetic sputtering method
on one side of the template, and nanowires were
grown in the channels using CVD method. After
the deposition, Raman spectra of samples (NCA
with SiNWs heading out of the channel) were
measured with Raman spectrometer (Nicolet
Thermo) at room temperature. Excitation was
done with 532-nm line of an Ar-ion laser. Raman
signals were measured with a spectral resolution of
1.0 cm
À1
. Then the morphology of the cross
section of the sample was taken by SEM (JSM-
T20, JEOL). Furthermore, the sample was dis-

solved in ethanol solution which was placed
dropwise on a copper grid covered with a very
thin carbon film, so that the deposited nanowires
could be analyzed by the TEM (Philips CM2000)
equipped with energy-dispersive X-ray spectrum
(EDX) facility.
3. Results and discussions
Fig. 1 is a SEM image of many SiNWs that
revealed the general array-ordered morphology of
the SiNWs. It can also be seen from Fig. 1 that the
average diameter and the length of the SiNWs are
60 nm and 10 mm, respectively. TEM image of one
SiNW indicates that the nanowire has a relatively
homogeneous and smooth structure, and is cov-
ered with a silicon oxide sheath, as shown in Fig. 2.
The selected area electrical diffraction (SAED)
analysis of nanowire (Fig. 2, inset) exhibits a
silicon single crystal structure.
Raman spectra of the two samples that were
prepared in the same condition are measured as
shown in Fig. 3, in which the five typical peaks of
the two samples almost coincide. In order to
investigate the Raman spectra of the SiNWs, we
first investigate the Raman spectra of the c-Si.
There is a peak at 519.4 cm
À1
in c-Si. It is the first-
order optical phonon mode with the full-width at
half-maximum (FWHM) at 4.5 cm
À1

. It is the
most intensive peak in the Raman spectra. There
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Fig. 1. SEM image of many SiNWs.
Fig. 2. The TEM image of single SiNWs (The inset is the image
of SAED that is taken from the same nanowire).
J. Liu et al. / Physica E 23 (2004) 221–225222
are two broad peaks at 301.2 and 966.4 cm
À1
,
corresponding to the second transverse optical
phonon model (2TO) and the second transverse
acoustic phonon model (2TA), respectively. Be-
cause of the similar Raman spectra for the two
samples, we only investigate Raman spectra of
sample a. In the Raman spectra of sample a, there
are five peaks at 505, 927, 296, 604 and 423 cm
À1
.
Since Si
x
O
y
has no contribution to Raman
spectrum, we neglect the influence of silicon oxide
sheath of SiNWs. It can be seen that the spectrum
of the sample is very similar to that of the c-Si,
except for the weak peaks at 604 and 423 cm
À1
.

These two peaks were found in nanoparticle Si [18]
and porous Si [19] and can be related to quantum
confinement effect of Si. The most intensive peak
in the SiNWs is found to shift to the lower
frequency and to be asymmetric. The Raman
spectra of microcrystalline silicon films were
combined with the spectra of c-Si and the spectra
of porous Si. So its Raman spectrum is asym-
metric. In our experiment, the asymmetry can also
be seen as the combination of the c-Si and the
porous Si. The FWHM of sample a is 20 cm
À1
which is increasingly larger than that of c-Si. The
explanation is that the FWHM increase is
evidently associated with the disorder at bound-
aries of crystallites which leads to a decrease of the
phonon time. We give a qualitative explanation of
shift of the first-order optical phonon of SiNWs in
phonon confinement model. In an ideal crystal, the
correlation length is infinite, and hence the phonon
eigenstates are plane waves. Therefore the usual
~
kk ¼ 0 momentum selection rule of the first-order
Raman spectrum can be satisfied. As the crystallite
is reduced to nanosize, the most important effect
on the Raman spectra is that the crystal momen-
tum conservation rules are relaxed. This allows
phonons with wave vector j
~
kkj¼j

~
kk
0
j¼72p=L to
participate in the first-order Raman scattering.
Here
~
kk
0
is the wave vector of the incident light and
L is the size of crystal. The phonon scattering is no
longer limited to the center of the Brillioun zone,
and the scattering near the zone center must be
considered. As a result, the symmetry-forbidden
modes must be observed, in addition to shift of the
first-order optical phonon.
The size-dependent Raman shift was investi-
gated, in which Raman shift increased with the
decrease of the diameter of SiNWs. In our
experiment, the Raman shift of SiNWs TO
compared to c-Si is 15 cm
À1
, which is larger than
than it should be [15]. If we only consider the
diameter of the crystal in the SiNWs, the shift of
15 cm can be explained qualitatively. This means
that the Raman shift actually depends on the size
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296
423

505
604
927
(a)
292
416
507
600
925
(b)
5000
1

0000
1

5000
2

0000
2

5000
3

0000
3

5000
4


0000
4

5000
5

0000
5

5000
6

0000
6

5000
7

0000
Int
200 400 600 800 1000 1200 1400 1600 1800
Raman shift (cm
-1
)
Fig. 3. The Raman spectrum of the SiNWs.
J. Liu et al. / Physica E 23 (2004) 221–225 223
of crystals in the nanowire, and other matters do
not contribute to the shift. The MCM based on
quantum confinement has been successfully ap-

plied to many nanoscale materials [20]. According
to MCM, the theoretical first Raman spectrum can
be obtained from the following equation [21]:
IðoÞ¼
Z
d
3
qjCð0;
~
qqÞj
½o À oð
~
qqÞ
2
þðG
0
=2Þ
2
; ð1Þ
where oð
~
kkÞ represents the phonon dispersion
curve. G
0
is the natural linewidth (inversely
proportional to the intrinsic phonon lifetime);
Cð0;
~
qqÞ is the coefficient describing the phonon
confinement at q

0
=0, which is appropriate for the
first-order Raman scattering. The integration must
be performed over the entire Brilloun zone. Cð0;
~
qqÞ
is the Gaussian one:
Cð0;
~
qqÞ¼exp À
q
2
L
2
4p
2

; ð2Þ
where L is the size of the crystal. From Eq. (2) we
can see that the crystal size determines the TO
peak the wavelength of the exciting light. Our
experiment uses an Ar-ion laser with wavelength
532.0 nm, the exciting light of Raman spectrum in
the experiment above discussed is 514.5 nm. It is
known that because of resonance Raman scatter-
ing, the Raman peak will shift with the wavelength
of exciting laser. Here the difference between the
two wavelengths is small. So the first explanation
is the most important.
However, there are also some discrepancies.

There should be amorphous Si covering the SiNW.
But there is no signal of amorphous Si in the
Raman spectra. The possible explanation is that
the Raman peak of the amorphous Si is about
490 cm
À1
and the FWHM of the sample is 20 cm
À1
,
so the peak may be covered by the TO peak.
4. Conclusion
Array-ordered silicon nanowires with single-
crystal structure were prepared by the NCA and
CVD method. Their Raman spectra have been
measured, in which the Raman shift could be
considered. This results from the confinement
effect of nanomaterials, some peaks could belong
to that of c-Si, expect for the breadth and
asymmetry of the peak. Also, we gave the
qualitative explanation of the changes of the peak.
We obtained the conclusion that the crystal size
determines the effect of the Raman shift. It will be
helpful for our further investigation.
Acknowledgements
This work was supported by the National
Natural Science Foundation of China (Project
No. 50272057 and No. 90201038) and Zhejiang
Provincial Natural Science Foundation (Project
No. 601092), and the Center for Nanoscience and
Nanotechnology of ZheJiang University.

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