APPLIED PHYSICS LETTERS 93, 203101 共2008兲

Raman characterization of Ge distribution in individual Si1−xGex
alloy nanowires
Chiharu Nishimura,1 Go Imamura,1 Minoru Fujii,1,a兲 Takahiro Kawashima,2 Tohru Saitoh,2and Shinji Hayashi1
1Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan
2
Image Devices Development Center, Panasonic Co., Ltd., 3-1-1 Yagumo-Nakamachi, Moriguchi,
Osaka 570-8501, Japan

共Received 30 July 2008; accepted 23 October 2008; published online 17 November 2008兲
The distribution of Ge composition in an individual Si1−xGex alloy nanowire 共SiGeNW兲 grown by
the vapor liquid solid 共VLS兲 process was studied by micro-Raman spectroscopy by changing themeasurement position from the catalyst side to the substrate side. The Si–Si mode in Raman spectra
were found to split into two peaks and the intensity ratio as well as the wavenumbers of the twopeaks depended on the measurement position. The Raman data revealed that SiGeNWs have a
core-shell structure with a low-Ge composition core grown by the VLS process covered with ahigh-Ge composition shell grown by the conformal deposition. © 2008 American Institute ofPhysics. 关DOI: 10.1063/1.3028027兴
Semiconductor nanowires have attracted considerable attention in recent years because of their unique electronic
properties and potential applications as transistors,1,2 photovoltaic cells,3–8 and chemical sensors.9,10 Among various
kinds of semiconductor nanowires, group IV semiconductor
nanowires such as Si,3,5–8,11 Ge,12–15 and Si1−xGex nanowires
共SiGeNWs兲 共Refs. 1, 2, 16, and 17兲 have most intensively
been studied due to their high compatibility with conventional complementary metal-oxide semiconductor technology. In particular, wide controllability of the electronic structure and possibility for the modulation of the band
gap within a nanowire make SiGeNWs very fascinating
materials.
SiGeNWs are usually grown by the vapor liquid solid
共VLS兲 process by using metal nanoparticles as
catalysts.1,2,16,17 Since the performance of SiGeNW-based
devices is strongly affected by the composition, size, shape,etc., detailed studies have been made to realize controlled
VLS-growth of SiGeNWs. In particular, the effects of growthtemperature and inlet gas composition were investigated indetail to clarify suitable conditions for the growth of SiGeNWs with uniform and controlled alloy composition inboth the radial and axial directions.17 However, because of
the complex growth process compared to Si or Ge nanowires, there still remain unknown factors that determine themorphology and composition of SiGeNWs. For example,composition of SiGeNWs is reported to depend not only on
growth conditions but also on the diameter when the diameter is below ⬃50 nm.16
Raman spectroscopy is a powerful tool to characterize
SiGeNWs and is widely used to discuss the growthmechanism.2,18,19 In a previous work,19 we studied SiGeNWs
by Raman spectroscopy combined with transmission electronmicroscopy 共TEM兲 and TEM energy-dispersive x-ray spectroscopy 共TEM-EDS兲. Our results suggest that axial VLS
growth of SiGeNWs and a conformal growth of Si1−xGexlayer on the sidewall of the SiGeNWs core occur simultaa兲

Electronic mail: fujii@eedept.kobe-u.ac.jp.

0003-6951/2008/93共20兲/203101/3/$23.00

neously in a wide growth condition range. We also showedthat the Ge composition of the conformally deposited layer
共shell兲 is always higher than that of the Si1−xGex core grownby the VLS process. SiGeNWs have core-shell structureswith a low-Ge composition core surrounded by a high-Gecomposition shell.
Raman studies of SiGeNWs have been performed for
ensembles of SiGeNWs or a whole region of a single
SiGeNW.2,18,19 Therefore, information on microstructures inside SiGeNWs was not obtained from Raman spectroscopy
and combination with other characterization techniques wasindispensable. In this work, we demonstrate that microRaman spectroscopy is a useful tool to characterize inner
structures of SiGeNWs and to discuss the growth mechanism
when the spectra of small regions are measured from one endof a SiGeNW to the other with a small step. By analyzing
Raman data, we show that there is Ge distribution both in theaxial and radial directions. The observed data suggests acore-shell structure. Moreover, we show that the compositions of core and shell are not uniform but change graduallyalong the nanowire axis. The change of the compositions
suggests diffusion of Ge from the shell to the core during thegrowth.
SiGeNWs were synthesized via gold 共Au兲 catalyzed
VLS growth using a cold-wall infrared lamp-heated chemicalvapor deposition 共CVD兲 apparatus. Details of the growth
procedure are found in our previous work.19 The flow rate of
Si2H6 and GeH4 precursors, and H2 were 200, 200, and 1080
SCCM 共SCCM denotes standard cubic centimeters per
minute at STP兲, respectively. The gas pressure and temperature for CVD growth were 0.3 Torr and 450 ° C, respectively.
The growth duration was 10 min. The diameter and length ofSiGeNWs were 80–100 nm and 7 – 12 ␮m, respectively.
For Raman measurements of individual SiGeNWs, they
were separated from the growth substrates by ultrasonicationin ethanol, and the diluted solution was dropped on Au
共⬃100 nm in thickness兲 coated Si共100兲 substrates. The purpose of Au coating was to avoid a Raman signal from a Sisubstrate. Raman spectra were measured by a confocal mi-93, 203101-1© 2008 American Institute of Physics

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203101-2Appl. Phys. Lett. 93, 203101 共2008兲

Nishimura et al.

1Pm
catalystBCDEF0246810 (m)

(b)

(a)

FIG. 1. Typical TEM image of a SiGeNW. The length is 7.5 ␮m.

(c)AIntensity (arb.units)

croscope 共50⫻ objective lens, numerical aperture= 0.75兲
equipped with a single monochromator and a charge-coupleddevice. Although the diameter of SiGeNWs was smaller thanthe diffraction limit of the resolution, they were observablefrom scatter of imaging light through a microscope monitor.
From the image on the monitor, an isolated SiGeNW wasselected and the catalyst side of the SiGeNW was brought tothe center. The catalyst side and the other side 共the substrateside兲 could be distinguished due to slightly conical shape ofSiGeNWs, i.e., the substrate side looked darker than the catalyst side. Raman signal from the region of 2 ␮m in diameter
was collected. After the Raman measurement, the stage wasmoved for 2 ␮m to bring the next measurement point to thecenter. The procedure was repeated until reaching the substrate side. The excitation source for Raman measurements
was the 632.8 nm light from a He–Ne laser. Rayleigh scattering was removed by a low pass filter 共Semrock: LP02–633RU-25兲.
Figure 1 shows a typical TEM image of a SiGeNW. Thelength is 7.5 ␮m. We can see that the SiGeNW is slightly
conical. The catalyst side is thinner than the substrate side.
The formation of conical SiGeNWs suggests that in addition
to axial VLS growth, conformal deposition of a Si1−xGexlayer proceeds. It should be noted here that in the case of Si
nanowires 共SiNWs兲 grown in the same condition, the diameter was almost the same from the catalyst side to the substrate side. Therefore, the addition of Ge precursors 共GeH4gas兲 is considered to initiate the conformal deposition.19
Figure 2共a兲 shows a typical Raman spectrum of a single
undoped SiGeNW. The signal from the whole region of aSiGeNW is collected. Three major peaks can be seen at 500–520, 400–410, and 280– 290 cm−1, which are assigned to
optical phonons caused by the motions of adjacent Si–Si,Si–Ge, and Ge–Ge pairs, respectively.20 The peak around420 cm−1 is a local vibration mode of the Si–Si pairs modulated by the adjacent Ge atoms.21 Figure 2共b兲 shows an expansion of the Si–Si mode. We can see a small shoulder at
the high-wavenumber side of the peak. In a previous work,19
we showed that the composition of as-grown SiGeNWs canbe estimated with a reasonable accuracy from the shift of theSi–Si peak. The Ge composition estimated from Fig. 2共b兲 isabout x = 0.28.
Figure 2共c兲 shows the micro-Raman spectra of a SiGeNW measured at the positions A–F indicated in the illustration. Signals from the region of 2 ␮m in diameter aredetected in each measurement. Note that the Raman intensitychanges depending on the measurement position. The intensity is smaller at the catalyst side than at the substrate side.
This is mainly due to different volumes caused by slightly
conical shape of SiGeNWs and to different compositions discussed later. To make the comparison of the spectral shapeeasier, the spectra are normalized at the position of the Si–Si
mode. We see that the relative intensity of the Ge–Ge andSi–Ge modes increases only slightly by changing the measurement position from A to F. On the other hand, the shapeA(d)AB

BCCDDEEF
200

400600-1

Raman Shift (cm )F490

500510-1520

Raman Shift (cm )FIG. 2. 共Color online兲 共a兲 Typical Raman spectrum obtained from a whole
region of a single SiGeNW. 共b兲 Expansion of the region of the Si–Si vibration mode of 共a兲. 共c兲 Typical Raman spectra of a SiGeNW measured at thepoint indicated by bars in the illustration. Distance between the measurement points is 2 ␮m. Signals from the region of 2 ␮m in diameter aredetected in each measurement. 共d兲 Expansion of the region of the Si–Sivibration mode. The dotted curves in F are the results of the fitting by twoLorentzians.

of the Si–Si mode changes drastically. Figure 2共d兲 shows theexpansion of the Si–Si mode. We can clearly see that theSi-Si mode splits into two peaks. This implies that SiGeNWs
are not uniform. The intensity ratio of the two peaks depends
on the measurement position. To discuss the change of theintensity and the wavenumbers of the two peaks more indetail, we deconvolute the peak by two Lorentzian functions.
Dotted curves in F are the results of the fitting. All the data
could be well-fitted by two Lorentzian functions.
Figure 3共a兲 shows the intensity ratio of the low- to highwavenumber peaks of the Si–Si mode as a function of thedistance from the catalyst. The low-wavenumber peak arises
from a high-Ge concentration region, while the highwavenumber one from a low-Ge concentration region because the Si–Si mode shifts to lower wavenumber with increasing Ge concentration.20 The relative intensity increases
with increasing the distance from the catalyst. The Ramanintensity is roughly proportional to the volume contributing
to the mode. Therefore, Fig. 3共a兲 suggests that the volume ofthe high-Ge concentration region with respect to that of thelow-Ge concentration region increases from the catalyst side
to the substrate side. This result combined with slightly conical shape of SiGeNWs strongly suggests that the low-Ge
concentration region is a SiGeNW core grown by the VLS
process and the high-Ge concentration region is the shellmade by the conformal deposition. This conclusion is consistent with our previous work by TEM-EDS.19 Note that we
performed the same measurements for more than ten single
SiGeNWs and obtained almost the same results for all ofthem.

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203101-3

Appl. Phys. Lett. 93, 203101 共2008兲

Nishimura et al.
5

Intensity ratio(a)
4I SiSi _ low

3I SiSi _ high2100.15

Ge composition (x)-1Raman Shift (cm )

(b)
510

SiSi_high

0.20

5050.25

SiSi_low

0.30

500

0.35
0246810Distance from the catalyst (m)FIG. 3. 共Color online兲 共a兲 Intensity ratio of low- to high-wavenumber Si–Si
modes as a function of the distance from the catalyst. The dashed line is a
guide to the eyes. 共b兲 Raman shift of high- and low-wavenumber Si–Si
modes as a function of distance from the catalyst. The dashed curves are
guides to the eyes. The horizontal dotted lines represent the wavenumber ofthe low-wavenumber mode at the catalyst side and that of the highwavenumber mode at 10 ␮m.

The analysis of Raman data provides more detailed information on the core-shell structure. In Fig. 3共b兲, Raman
shifts of the two Si–Si peaks are plotted as a function of thedistance from the catalyst 共left axis兲. Corresponding Ge compositions obtained from the relation between the Si–Si mode
wavenumber and the Ge composition at a stress-free condition, i.e., ␻SS = 520.2− 62x, where ␻SS is the frequency of theSi–Si mode and x is the Ge composition, are shown on theright axis.20 We note that the peak wavenumbers are notconstant but both peaks shift to lower wavenumber from thecatalyst side to the substrate side. At distance zero, i.e., theposition at the catalyst, where the conformal deposition isvery small, the Ge composition is about 0.19. The value isconsidered to be a Ge composition of the core grown by theVLS process. The Ge composition of the core increases withincreasing the distance and saturates around 0.23. The increase of the Ge composition suggests diffusion of Ge fromthe higher Ge concentration shell to the core during thegrowth. Ge diffusion from the shell to the core decreases Ge
composition of the shell. This effect is significant in the region where the shell is thin, i.e., when the distance from thecatalyst is small. In fact, in Fig. 3共b兲, at 2 ␮m the Ge composition is about 0.29. It increases with the distance andsaturates around 0.31 at about 8 ␮m. This saturated value isthought to be the Ge composition of the shell grown by theconformal deposition. The present results suggest that the Gecomposition distribution depends not only on growth parameters such as inlet gas composition, temperature, etc., butalso on the length. Therefore, growth duration is also an
important parameter to control the energy band structure ofSiGeNWs.
In conclusion, distribution of Ge composition in single
SiGeNWs were studied by micro-Raman spectroscopy. We
demonstrated that despite poor spatial resolution, microRaman spectroscopy is a powerful tool to characterize theinner structure of SiGeNWs when small regions of a single
SiGeNW were analyzed systematically. The Raman results
demonstrated that SiGeNWs have a core-shell structure witha low-Ge composition core grown by the VLS process covered with a high-Ge composition shell grown by the conformal deposition. Furthermore, detailed analysis of the Raman
data revealed that Ge compositions in the core and the shell
are not uniform but gradually change along the axis, i.e.,there is Ge composition distribution both in the radial andaxial directions. This implies that VLS-grown SiGeNWs
have complicated energy band structures and they should betaken into account to design SiGeNW-based devices.
This work is supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,Sports, Science and Technology, Japan.
1

S. J. Whang, S. J. Lee, W. F. Yang, B. J. Cho, and D. L. Kwong, Appl.
Phys. Lett. 91, 072105 共2007兲.
2C.-J. Kim, J.-E. Yang, H.-S. Lee, M. Jang, M.-H. Jo, W.-H. Park, Z. H.
Kim, and S. Maeng, Appl. Phys. Lett. 91, 033104 共2007兲.
3
K. Peng, Y. Xu, Y. Wu, Y. Yan, S.-T. Lee, and J. Zhu, Small 1, 1062
共2005兲.
4
H.-S. Shim, S.-I. Na, S. H. Nam, H.-J. Ahn, H. J. Kim, D.-Y. Kim, and W.
B. Kim, Appl. Phys. Lett. 92, 183107 共2008兲.
5
Y. Cheng, G. Fang, C. Li, L. Yuan, L. Ai, B. Chen, X. Zhao, Z. Chen, W.
Bai, and C. Zhan, J. Appl. Phys. 102, 083516 共2007兲.
6K. Peng, X. Wang, and S.-T. Lee, Appl. Phys. Lett. 92, 163103 共2008兲.
7
B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, and C.
M. Lieber, Nature 共London兲 449, 885 共2007兲.
8L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J.
Rand, Appl. Phys. Lett. 91, 233117 共2007兲.
9Y. Cui, Q. Wei, H. Park, and C. M. Lieber, Science 293, 1289 共2001兲.
10J. Hahm and C. M. Lieber, Nano Lett. 4, 51 共2004兲.
11
K. Mori, K. Shoda, and H. Kohno, Appl. Phys. Lett. 87, 083111 共2005兲.
12
E. Tutuc, S. Guha, and J. O. Chu, Appl. Phys. Lett. 88, 043113 共2006兲.
13
A. Morales and C. Lieber, Science 279, 208 共1998兲.
14
E. Sutter and P. Sutter, Nano Lett. 8, 411 共2008兲.
15
S. Kodambaka, J. Tersoff, M. C. Reuter, and F. M. Ross, Science 316, 729
共2007兲.
16
X. Zhang, K.-K. Lew, P. Nimmatoori, J. M. Redwing, and E. C. Dickey,Nano Lett. 7, 3241 共2007兲.
17
K.-K. Lew, L. Pan, E. C. Dickey, and J. M. Redwing, Adv. Mater. 共Weinheim, Ger.兲 15, 2073 共2003兲.
18
Q. Lu, K. W. Adu, H. R. Gutierrez, G. Chen, K.-K. Lew, P. Nimmatoori,X. Zhang, E. C. Dickey, J. M. Redwing, and P. C. Eklund, J. Phys. Chem.
C 112, 3209 共2008兲.
19
T. Kawashima, G. Imamura, T. Saitoh, K. Komori, M. Fujii, and S.
Hayashi, J. Appl. Phys. 102, 124307 共2007兲.
20
J. C. Tsang, P. M. Mooney, F. Dacol, and J. O. Chu, J. Appl. Phys. 75,8098 共1994兲.
21
M. I. Alonso and K. Winter, Phys. Rev. B 39, 10056 共1989兲.

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