JOURNAL OF APPLIED PHYSICS 102, 124307 共2007兲

Raman and electron microscopic studies of Si1−xGex alloy nanowires
grown by chemical vapor deposition
Takahiro Kawashimaa兲
Advanced Devices Development Center, Matsushita Electric Industrial Co., Ltd.,
3–1–1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, JapanGoh Imamura, Minoru Fujii, and Shinji Hayashi
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
1-1 Rokkodai, Nada, Kobe 657-8501, Japan

Tohru Saitoh and Kazunori Komori
Image Devices Development Center, Matsushita Electric Industrial Co., Ltd., 3-1-1 Yagumo-Nakamachi,
Moriguchi, Osaka 570-8501, Japan

共Received 31 July 2007; accepted 1 October 2007; published online 26 December 2007兲
Si1−xGex alloy nanowires 共SiGeNWs兲 were grown by Au-catalyzed chemical vapor deposition andstudied by Raman spectroscopy, transmission electron microscopy 共TEM兲, and energy-dispersive
x-ray spectroscopy 共EDS兲 in TEM 共TEM-EDS兲. The relationship between the growth parameters
and the structure of the SiGeNWs was clarified by systematically changing the growth conditions
over a wide range. Raman and TEM-EDS results demonstrated that the SiGeNWs consist of a lower
Ge composition core and a higher Ge composition shell epitaxially grown on the surface of the core.
The effects of oxidation on the structure of the SiGeNWs were studied. It was found that oxidation
leads to segregation of the Ge atoms at the interface between the SiGeNWs and SiO2, which in turnresults in a large inhomogeneity in Ge composition. Oxidation at a very low rate in a diluted oxygengas atmosphere is required to avoid the formation of Ge particles and minimize the inhomogeneity.
© 2007 American Institute of Physics. 关DOI: 10.1063/1.2817619兴
I. INTRODUCTION

In recent years, semiconductor nanowires 共NWs兲 haveattracted significant attention due to their unique electronic
properties and possible device applications as transistors,1,2chemical sensors,3,4 and light-emitting devices.5,6 Among thedifferent kinds of semiconductor NWs, group IV semiconductor NWs such as Si and Ge NWs 共SiNWs and GeNWs兲
have been the most widely studied because of their highcompatibility with conventional complementary metal-oxide
semiconductor 共CMOS兲 technology. Nanowires of Si1−xGex
alloy 共SiGeNWs兲 are also potentially very useful materials,
since their energy band gap can be modulated within the wire
and their electronic properties can be tailored over an extended range. Recently, several groups have reported the synthesis of SiGeNWs by a vapor-liquid-solid 共VLS兲 process.7,8
Wu et al. synthesized Si/ Si1−xGex superlattice nanowires using a hybrid pulse laser ablation/chemical vapor deposition
共PLA-CVD兲 process and demonstrated single crystalline
nanowires with a longitudinal Si/ Si1−xGex superlattice
structure.7 Core-shell type SiGeNWs with different Ge compositions between the core and the shell have also been
fabricated.8
Despite the possible applications of SiGeNWs in future
electronic devices, research on the growth mechanism is stillrather limited, and thus the growth parameters and strategies
for the fabrication of high-quality SiGeNWs are not fully
elucidated. The mechanism of the VLS growth of SiGeNWsa兲Electronic mail: kawashima.takahiro@jp.panasonic.com.

0021-8979/2007/102共12兲/124307/6/$23.00

is considered to be more complex than those of SiNWs and
GeNWs, since different kinds of source gases are mixed during growth, and the solubility of Si and Ge in Au catalysts isdifferent. Furthermore, because of these different solubilities,
the growth mechanism is believed to depend on the composition.
In this study, we grew SiGeNWs by systematicallychanging the growth parameters and studied them by Raman
spectroscopy and transmission electron microscopy 共TEM兲
combined with energy-dispersive x-ray spectroscopy 共EDS兲.
The purpose of this work is to clarify the parameters that
control the shape, crystallinity, and homogeneity of the Gecomposition, etc., of SiGeNWs and also clarify their growthmechanism. We show that Ge composition and the shapes of
SiGeNWs depend strongly on the growth temperature aswell as the partial pressure of source gases. Furthermore, ourresults demonstrate that SiGeNWs consist of a lower-Ge
composition core and a higher-Ge composition shell epitaxially grown on the core. We also studied how the structure of
SiGeNWs is modified by oxidation, and demonstrated thatthe oxidation of SiGeNWs leads to the segregation of Ge
atoms at the interface between SiGeNWs and SiO2: this segregation results in a large inhomogeneity in the Ge composition.
II. EXPERIMENTS

SiGeNWs were synthesized via VLS growth using a
cold-wall infrared 共IR兲 lamp-heated chemical vapor deposition 共CVD兲 apparatus. To attach the catalysts to the Si substrates, thermally oxidized Si 共100兲 wafers were first treated102, 124307-1© 2007 American Institute of Physics

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Kawashima et al.

with oxygen plasma and dipped into 1 wt % solution of 3,5diaminopentyltrimethoxysilane. The substrates were thendipped into a solution of gold 共Au兲-colloid 共Tanaka Kikinzoku Co.兲 for 5–30 min. After drying, the substrates were
loaded into the CVD chamber. The flow rate of the Si2H6
precursor, GeH4 precursor and H2 were 100–450, 100–450,
and 0–1100 sccm 共cubic centimeters per minute at STP兲,
respectively, and the total pressure was about 0.2–1 Torr. The
partial pressures of the Si2H6 and GeH4 precursors were
0.002–0.008 and 0.003–0.016 Torr, respectively. The totalpressure was controlled by changing the H2 flow rate. The
growth temperature was varied between 350 and 450 ° C,
and was monitored using a pyrometer. The growth duration
was 10 min. To study the effects of oxidation on the structureof SiGeNWs, they were annealed in an oxygen atmosphere
共about 1 atm兲 at 900– 1100 ° C for 2–6 min.
Raman measurements were carried out in a conventional
90° scattering configuration by using a triple monochromator
共Spex 1877E兲 equipped with a charge-coupled device 共CCD兲
detector. The excitation source was the 514.5 nm line of anargon 共Ar兲 ion laser. The laser power applied to the sample
was about 3 mW. At this power level, no significant shift or
broadening of the Raman spectra by laser heating was observed. The signal from the Si substrate was not observed
because of the high density of the SiGeNWs grown on it.
The morphology of the SiGeNWs was studied by a scanning electron microscope 共SEM兲 共Hitachi, S-4000兲 and atransmission electron microscope 共TEM兲 共JEOL JEM4000DX兲 operated at 400 kV. For the TEM observations, the
SiGeNWs were separated from the growth substrates by ultrasonication in ethanol, and the solvent containing the SiGeNWs was then dropped onto a microgrid. The crystallinityof the SiGeNWs was also studied by TEM. The chemicalcompositions of the SiGeNWs, especially the Ge distribution
within the SiGeNWs, were analyzed using energy-dispersive
x-ray spectroscopy 共EDS兲 in TEM 共TEM-EDS兲. The electron
beam diameter for the TEM-EDS measurements was about
1–2 nm. To quantitatively determine the chemical compositions by TEM-EDS, we used Si1−xGex thin films 共100 nm inthickness兲 with three different x 共19.6, 24.9, and 30.1 at %兲
values, epitaxially grown on Si substrates using the same
apparatus as the reference samples. The chemical composition of the reference Si1−xGex thin films was determined by
x-ray diffraction 共XRD兲.
III. RESULTS AND DISCUSSION
A. Raman scattering studies of as-grown SiGeNWs

Figure 1 shows plan-view SEM images of the SiGeNWs
prepared at different GeH4 partial pressures 共flow rates兲, i.e.,
0.003 共100 sccm兲, 0.007 共200 sccm兲, and 0.01 共300 sccm兲
Torr in Figs. 1共a兲–1共c兲, respectively. The other growth parameters are fixed 关Si2H6 partial pressure 共flow rate兲: 0.002
Torr 共100 sccm兲, H2 flow rate: 1100 sccm, growth temperature: 450 ° C, growth duration: 10 min兴. In all the samples,the diameter and the length of the SiGeNWs are almost thesame and are ⬃50– 100 nm and ⬃7 ␮m, respectively,
while the shape is significantly different. As can be seen in
Fig. 1, the shape becomes more conelike when the SiGeNWs

J. Appl. Phys. 102, 124307 共2007兲

FIG. 1. Plan-view SEM images of SiGeNWs grown with the GeH4 gas
partial pressures 共flow rates兲 of 共a兲 0.003 Torr 共100 sccm兲; 共b兲 0.007 Torr
共200 sccm兲; and 共c兲 0.01 Torr 共300 sccm兲. Other growth parameters are
fixed. The ratio of GeH4 partial pressure with respect to GeH4 and Si2H6
partial pressures 关GeH4 / 共GeH4 + Si2H6兲兴 in 共a兲, 共b兲, and 共c兲 is 0.6, 0.78, and0.83, respectively. The growth temperature is 450 ° C.

are grown at a higher GeH4 partial pressure. To find out
whether the change in shape is chiefly caused by the different
ratios of the Si2H6 and GeH4 partial pressures or different
total gas pressure, which changes from 0.2 to 0.5 Torr due tothe change in the GeH4 flow rate, we studied a large numberof samples prepared under different conditions. Our results
show that, within the present growth condition range,
whether the wires become cylinderlike or conelike depends
most strongly on the ratio of the Si2H6 and GeH4 partialpressures but depends little on the total pressure. Therefore,
in the following, we fixed the partial pressure of Si2H6
共0.002 Torr兲 and changed only that of GeH4 共0.003–0.01
Torr兲 to control the shape. The ratio of the GeH4 partialpressure with respect to the Si2H6 and GeH4 partial pressures
关GeH4 / 共GeH4 + Si2H6兲兴 is changed from 0.6 to 0.83. Hereafter, we will use the value of GeH4 / 共GeH4 + Si2H6兲 to represent the samples.
Figure 2共a兲 shows the Raman spectra of the SiGeNWs.
GeH4 / 共GeH4 + Si2H6兲 is changed from 0.6 to 0.83. Three major peaks at 500– 520, 400– 410, and 280– 290 cm−1 correspond to optical phonons caused by the motions of adjacent
Si-Si, Si-Ge, and Ge-Ge pairs, respectively.9 The peakaround 420 cm−1 is assigned to a local vibration mode of the
Si-Si pairs modulated by the adjacent Ge atoms.10 We seethat the Ge-Ge and Si-Ge modes grow with increasing
GeH4 / 共GeH4 + Si2H6兲. This indicates that, similar to the epitaxial growth of Si1−xGex thin films, Ge composition can becontrolled by GeH4 / 共GeH4 + Si2H6兲 in SiGeNWs. The Gecomposition can also be controlled by the growth temperature. In Fig. 2共b兲, the growth temperature is changed from
350 to 450 ° C and other growth parameters are fixed. The
intensity of the Ge-Ge mode with respect to the Si-Si modeis greater when the growth temperature is higher.
In Si1−xGex thin films epitaxially grown on Si substrates,
the Ge composition dependence on the frequencies of Si-Si,

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Kawashima et al.

FIG. 2. 共Color online兲 Raman spectra of SiGeNWs. 共a兲 GeH4 gas partialpressure is changed 共0.003, 0.007, and 0.01 Torr, GeH4 / 共GeH4 + Si2H6兲
= 0.6, 0.78, and 0.83兲, while other growth parameters are fixed. Growthtemperature is 450 ° C. 共b兲 Growth temperature is changed 共350, 400, and
450 ° C兲, while other parameters are fixed. The partial pressures of GeH4
and Si2H6 are 0.003 and 0.002 Torr, respectively, and thus GeH4 / 共GeH4+ Si2H6兲 is 0.6.

Si-Ge, and Ge-Ge modes was empirically obtained by taking
into account the strain exerted from the Si substrates.9 In the
SiGeNWs in the first approximation, the strain is relieved.
We thus try to estimate the Ge composition from the Raman
peak frequencies by using the relations under strain-free
conditions,9

␻SS = 520.2 − 62␹ ,共1兲␻SG = 400.5 + 14.2␹ ,共2兲

␻GG = 282.5 + 16␹ ,共3兲

where ␻SS, ␻SG, and ␻GG are the frequencies of the Si-Si,
Si-Ge, and Ge-Ge modes, respectively, and x is the Ge composition. Figure 3 shows the intensity ratio of the Ge-Ge and
Si-Si modes as a function of the Ge composition 共x兲 estimated from Eqs. 共1兲–共3兲 for 共a兲 as-grown and 共b兲 oxidized
SiGeNWs. In ideal cases, x obtained from the three Raman
modes agrees perfectly. However, in the present SiGeNWs,
the values scatter over a rather wide range for some samples.
In Eqs. 共1兲–共3兲, the frequency of the Si-Si mode depends
most strongly on Ge composition, and thus is most reliable inobtaining the composition, especially in the range where theRaman intensities of Ge-Ge and Si-Ge modes are very weak
due to the small Ge composition. Therefore, in Fig. 3, x
obtained from Eq. 共1兲 is represented by symbols 共䊐兲 andthose from Eqs. 共2兲 and 共3兲 by error bars.
In Fig. 3共a兲, the data obtained for the SiGeNWs grown at
different growth temperatures 共350– 450 ° C兲 and different
GeH4 / 共GeH4 + Si2H6兲 共0.6–0.83兲 are summarized. We cansee that the error bars are rather large in some samples. The
large error bars suggest that the Ge composition is not uniform within the SiGeNWs and it cannot be expressed as asingle value. This will be confirmed later by TEM-EDS. It
should be noted here that, despite the composition distribution, the x values estimated from Eq. 共1兲 关䊐 in Fig. 3共a兲兴FIG. 3. 共Color online兲 共a兲 Relative intensities of Si-Si and Ge-Ge Raman
modes as a function of Ge composition determined from the shifts of Ramanpeaks. The data obtained for SiGeNWs grown at different temperatures
共350– 450 ° C兲 and different ratios of GeH4 and Si2H6 partial pressures
关GeH4 / 共GeH4 + Si2H6兲 = 0.6, 0.78, and 0.83兴 are summarized. The solid
curve is the result of fitting by Eq. 共4兲. Ge composition is estimated from
Eqs. 共1兲–共3兲. Ge composition obtained from Eq. 共1兲 is represented by symbols 共䊐兲 and those from Eqs. 共2兲 and 共3兲 by error bars. 共b兲 The same data for
SiGeNWs after oxidation. The data of the samples grown at different temperatures: 350 ° C 共open symbol兲 and 450 ° C 共filled symbol兲, are shown.
Other growth parameters are fixed 关GeH4 / 共GeH4 + Si2H6兲 = 0.6兴. The oxidation temperatures are 900 °C 共䊊쎲兲, 1000 °C 共䉭䉱兲 to 1100 ° C 共䊐䊏兲.

agree in most cases with those obtained from EDS and Rutherford backscattering 共RBS兲 spectroscopy within an accuracy of 5 at %. Therefore, Raman scattering is still usefulfor roughly estimating the Ge composition of as-grown SiGeNWs, and x obtained from the Si-Si mode frequency
probably represents the average Ge composition. This conclusion is not applicable to oxidized SiGeNWs, as discussed
later.
If we assume a random mixing of Si and Ge in theSi1−xGex alloy, the fractions of nearest-neighbor Ge atoms,
those of Si atoms, and those of nearest-neighbor Si-Ge pairs
are x2, 共1 − x兲2, and 2x共1 − x兲, respectively.11 Since the scattering intensity of each of the three Raman modes should
depend linearly on the fraction of pairs of atoms of each
type, the intensity ratio of the modes is expected to vary with
the Ge composition as11
IGG/ISS = A

␹2,
共1 − ␹兲2

共4兲

where ISS and IGG are the scattering intensities of the Si-Si
and Ge-Ge modes, respectively, and A is a fitting parameterthat must be determined experimentally for each wavelengthand may vary weakly with the alloy composition. The solid
curve in Fig. 3共a兲 is the result of the fitting of the data by Eq.
共4兲. For the fitting, the Ge compositions are assumed to be
those obtained from Eq. 共1兲, i.e., 䊐 in Fig. 3共a兲. We can seethat the data obey nearly perfectly the relation in Eq. 共4兲.
This confirms that the SiGeNWs are indeed random alloys
on the scale of Raman measurements and not a mixture of Si
and Ge crystals. Furthermore, the nearly perfect fitting supports the validity of Eq. 共1兲 in estimating the Ge compositionof as-grown SiGeNWs.

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124307-4

Kawashima et al.

FIG. 4. 共Color online兲 共a兲 Low-magnification TEM image of a SiGeNW.
The partial pressures 共flow rates兲 of GeH4 and Si2H6 are 0.007 共200 sccm兲
and 0.002 Torr 共100 sccm兲, respectively 关GeH4 / 共GeH4 + Si2H6兲 = 0.78兴, andthe growth temperature is 450 ° C. The numbers represent positions where
Ge composition is measured by TEM-EDS. 共b兲 High-magnification TEM
image of a SiGeNW around the line 3 in 共a兲. 䊏 and 䉱 represent positions
where TEM-EDS observations are made. 共c兲 Ge composition measured at
the center 共䊏兲 and the peripheral 共䉱兲 regions of a SiGeNW as a function of
nanowire diameter. The numbers correspond to those in 共a兲 and the symbols
correspond to those in 共b兲.

B. TEM studies of as-grown SiGeNWs

In CVD growth of Si1−xGex thin films, the addition of a
Ge precursor, i.e., GeH4 gas, is known to enhance the growthrate, i.e., to decrease the growth temperature due to complex
interactions between source gases and the low pyrolysis
character of GeH4 gas.12 The low pyrolysis temperature of
GeH4 results in a substantially lower growth temperature of
GeNW 共⬃260 ° C兲 than that of SiNWs.13 In the VLS growthof SiGeNWs, the low pyrolysis temperature of GeH4 causes
conformal deposition of the Si1−xGex layer on the sidewall,
which is considered to cause the growth of conelike-shaped
NWs.14 The formation of conelike-shaped SiGeNWs in thepresent work suggests a similar growth process. To study themechanism of the formation of cone-shaped SiGeNWs in
more detail, the Ge composition distribution in SiGeNWs
was studied by TEM and TEM-EDS. Figure 4 shows theresults of TEM-EDS analyses of an as-grown SiGeNW
关GeH4 / 共GeH4 + Si2H6兲 : 0.78兴. Figure 4共a兲 shows a lowmagnification TEM image in which we can clearly see a
conelike SiGeNW. The lines with numbers in Fig. 4共a兲 represent the positions where the EDS measurements are made.
Figure 4共b兲 shows a high-magnification TEM image aroundposition 3 in Fig. 4共a兲. TEM-EDS measurements are performed at the center 共䊏兲 and the peripheral 共䉱兲 regions. The
Ge compositions of the center and the peripheral regions are
39.4 and 50.7 at %, respectively. The same measurements
are performed at 6 positions, labeled from 1 to 6 in Fig. 4共a兲.
The results are summarized in Fig. 4共c兲 as a function of NW
diameter. In all the positions, Ge composition is higher in the

J. Appl. Phys. 102, 124307 共2007兲

FIG. 5. High-resolution TEM images of 共a兲 top 共catalyst side兲 and 共b兲 bottom 共substrate side兲 of an as-grown SiGeNW. The partial pressures 共flow
rates兲 of GeH4 and Si2H6 are 0.007 共200 sccm兲 and 0.002 Torr 共100 sccm兲,respectively 关GeH4 / 共GeH4 + Si2H6兲 = 0.78兴, and the growth temperature is
450 ° C.

peripheral regions than in the central regions. This meansthat the SiGeNW has a core-shell structure with a low-Ge
composition Si1−xGex core covered by a high-Ge composition Si1−xGex shell. It is highly plausible that the core isformed via a VLS process and the shell is formed by a conformal growth process.
In Fig. 4共c兲, the Ge composition of the peripheral regionis almost constant from the bottom to the top except for
position 1, where the shell is very thin. This result suggeststhat the Ge composition of the shell is close to being uniform. The thickness of the shell can easily be estimated fromthe shape of the SiGeNWs under the assumption that thediameter of the core is the same as that of the Au catalyst.
The rate of conformal growth can be obtained from the shell
thickness. In the case of the SiGeNW in Fig. 4, the conformal growth rate of the shell is about 2% of the VLS growthof the core. The conformal growth rate increases as
GeH4 / 共GeH4 + Si2H6兲 increases, resulting in a more conelike
shape, as can be seen in Fig. 1.
In position 6 in Fig. 4共a兲, the core diameter is about 76
nm, while the shell thickness is about 110 nm. If we average
the Ge composition of the shell 共⬃50 at %兲 and the core
共⬃35 at %兲 by taking into account these core and shellsizes, the result is about 46 at %. This value is very close tothat obtained at the central region of position 6. The same
estimations made from positions 2 to 6 were able to reproduce the diameter dependence of Ge composition in the center region 关Fig. 4共c兲兴 within an accuracy of 10%. The agreement indicates that the Ge composition within the core, as
well as that within the shell, tends to be uniform.
Figure 5 shows HRTEM images of an as-grown SiGeNW grown under the same conditions as those in Fig. 4.
Figures 5共a兲 and 5共b兲 correspond to the top 共catalyst side兲and the bottom 共substrate side兲 of the SiGeNW. Lattice

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124307-5

Kawashima et al.

fringes correspond to the 共111兲 planes of the Si1−xGex alloy.
We can see that the SiGeNW is defect-free single crystal
from the bottom to the top, i.e., both the core grown by a
VLS process and the shell by conformal growth are single
crystals.
The growth of conelike core-shell type SiNWs withhigher B concentration in the shell was reported in previous
work on B-doped SiNWs.15 The formation of these conelike
shapes could also be explained by the enhancement of conformal growth under high B2H6 concentration growth conditions. However, in that case, the high B-concentration shell
was not a single crystal but a highly defective polycrystal.16
The formation of a polycrystal shell is also reported in VLS
growth SiGeNWs.8 Therefore, the shell of core-shell typeSi-based NWs is not necessarily a single crystal, but the
crystallinity depends on the growth condition.
As shown in Fig. 5, in the present SiGeNWs, a single
crystal shell is epitaxially grown on the core. The growth ofa single crystal shell in the present growth condition is understandable by considering the fact that uncatalyzed epitaxial growth of Si1−xGex alloy thin films is possible under thepresent growth parameters. Epitaxial growth at fairly low
temperatures 共⬃450 ° C兲 is partly due to the choice of Si
source gas, Si2H6, the thermal stability of which is lower
than that of the more commonly used SiH4. The lower thermal stability decreases the lowest possible growth temperature and increases the growth rate. In fact, at a growth temperature of 500 ° C, the growth rate of a Si film from Si2H6 ismore than 20 times faster than that from SiH4. The additionof GeH4, the thermal stability of which is even lower thanthat of Si2H6, further decreases the lowest possible growthtemperature and increases the growth rate. At a growth temperature of 500 ° C, the growth rate of a Si1−xGex film isabout 7 nm/min when x = 0.25, while it increases to 13 nm/
min when x = 0.3. The high growth rate of a Si1−xGex film ata low growth temperature without catalysts results in theformation of a single crystal Si1−xGex shell on the surface of
the SiGeNWs.
C. Effects of oxidation

For some device applications of SiGeNWs, the formation of a SiO2 layer on the surface is required. We thus studied the effects of oxidation on the structure and Ge composition of SiGeNWs. Figure 6 shows Raman spectra of
SiGeNWs before and after oxidation at different temperatures for 2 min. The Ge composition of the as-grown SiGeNWs, estimated from Eq. 共1兲, is 0.21. The small Ge composition results in small intensities of Si-Ge and Ge-Ge
modes. These modes grow significantly by oxidation at
900 ° C. By further increasing the oxidation temperature,they grow further, and all the modes are broadened. Whenthe oxidation temperature is 1100 ° C, the Ge-Ge mode splits
into two peaks. The existence of two different frequency
Ge-Ge modes indicates that two different Ge composition
regions are formed in the SiGeNWs.
We attempted to estimate the Ge composition of oxidized SiGeNWs using the same procedure as employed forthe as-grown ones. However, the values estimated from Eqs.

J. Appl. Phys. 102, 124307 共2007兲

FIG. 6. Raman spectra of as-grown and oxidized SiGeNWs. 共a兲 as-grown;
共b兲, 共c兲, and 共d兲 oxidized at 900, 1000, and 1100 ° C, respectively, for 2 min.
The ratio of partial pressures of GeH4 and Si2H6 关GeH4 / 共GeH4 + Si2H6兲兴 is
0.6 and that of H2 is 1100 sccm. Growth temperature is 450 ° C and growthduration is 10 min.

共1兲–共3兲 are very much scattered, preventing this method frombeing applied to oxidized SiGeNWs. To demonstrate howmuch the estimated values were scattered, we plotted theintensity ratio of the Ge-Ge and Si-Si modes as a function of
Ge composition in Fig. 3共b兲. As before, the symbols are theGe composition estimated from Eq. 共1兲; and the error bars
correspond to those estimated from Eqs. 共2兲 and 共3兲. The
solid curve is the same as that shown in Fig. 3共a兲. The data
for the samples with GeH4 / 共GeH4 + Si2H6兲 of 0.6 and thegrowth temperatures of 350 ° C 共open symbols兲 and 450 ° C
共filled symbols兲 are shown. The oxidation temperatures are
900 °C 共䊊쎲兲, 1000 °C 共䉭䉱兲 to 1100 ° C 共䊐䊏兲. Since there
are two Ge-Ge peaks in the sample oxidized at 1100 ° C, twodata are simultaneously shown for the sample. It is clear thatoxidation significantly increases the intensity ratio and theerror bars in the abscissa become extremely large. These results suggest that segregation of Ge occurs and SiGeNWs
become very inhomogeneous. It is noted that Eqs. 共1兲–共3兲 arevalid in the x range of 0–0.5, and for x ⬎ 0.5 different relations are used.17 For some oxidized samples with high intensity ratios of the Ge-Ge and Si-Si modes, we try to estimate
x by the relations applicable to the higher Ge composition
ranges. However, the x values do not change significantly
from those estimated from Eqs. 共1兲–共3兲.
To confirm the above discussion, we performed TEM
observations of oxidized SiGeNW 共Fig. 7兲. The oxidation
temperature and duration are 1100 ° C and 4 min, respectively. We can see the growth of a SiO2 layer about 25 nm inthickness by oxidation. At the interface between the SiO2
layer and the SiGeNW, 5–15 nm particles are seen. TEMEDS analyses revealed that these particles are composed
mainly of Ge atoms. We also found that the Ge compositionin the SiGeNW increases after oxidation.
In the oxidation of Si1−xGex thin films, Si is preferentially oxidized and Ge atoms are piled up at SiO2 / Si1−xGex
interfaces. This causes the formation of Ge particles at theinterface, and inhomogeneity in the composition of the remaining Si1−xGex layer.18 Similarly, in the oxidation of
SiGeNWs, segregation of the Ge atoms at the interface be-

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FIG. 7. TEM image of a SiGeNW after oxidation at 1100 ° C for 4 min. The
ratio of partial pressures of GeH4 and Si2H6 关GeH4 / 共GeH4 + Si2H6兲兴 is 0.78
and the growth temperature is 450 ° C.

tween the SiGeNW core and SiO2 shell leads to inhomogeneity of the Ge composition, as can be seen in Fig. 3共b兲. The
present results indicate that great care should be taken inoxidizing SiGeNWs. Under the present oxidation conditions,oxidation is faster than the diffusion of piled-up Ge atoms
toward the NW core. To avoid the formation of Ge particles
and minimize the inhomogeneity, SiGeNWs should be oxidized at a lower rate by keeping the Ge diffusion rate high.
This can be achieved by oxidizing SiGeNWs in a diluted
oxygen gas atmosphere. To confirm this, we oxidized SiGeNWs in atmospheres of 1% O2 in N2 and 10% of O2 in
N2. In these atmospheres, no splitting of the Ge-Ge Raman
mode was observed, even when fairly high Ge composition
samples 共x = 0.2– 0.3兲 were oxidized at 1100 ° C.
In discussing the Raman spectra, we did not take into
account the effect of strain at the core/shell interfaces,
SiO2 / SiGeNWs interfaces, Ge particle/SiGeNWs interfaces,
etc. In as-grown SiGeNWs, the effect of strain seems to be
not very large because the Raman data can be fully explainedby the stress-free empirical relations in Eqs. 共1兲–共3兲. On theother hand, in oxidized SiGeNWs, strain and its distribution
is considered to be one of the reasons for the large scatteringof Raman data in Fig. 3共b兲. However, unfortunately, at thepresent stage of research we cannot distinguish between the
distributions of Ge composition and that of strain.
IV. CONCLUSIONSSiGeNWs grown by UHV-CVD were studied in detailby Raman spectroscopy, TEM, and TEM-EDS. By systematically changing the growth conditions over a wide range,
we clarified the relationship between the growth parameters

and the structure of SiGeNWs. Our Raman and TEM-EDS
results demonstrated that SiGeNWs consist of a lower Ge
composition single crystal core grown via Au catalysts and ahigher Ge composition single crystal shell epitaxially grownon the core. The TEM-EDS results revealed that the Ge composition within the core and the shell is almost constant. We
also studied the effects of oxidation on the structure of
SiGeNWs and showed that oxidation leads to segregation of
Ge atoms at the interface between the SiGeNWs and SiO2,
which in turn results in a large inhomogeneity in Ge composition. We believe oxidation at a very low rate in a diluted
oxygen gas atmosphere to be effective in preventing the formation of Ge particles and minimizing any inhomogeneity.
ACKNOWLEDGMENTS

The authors thank Mr. Tatsunori Mizutani of Matsushita
Technoresearch for the TEM measurements. The authors
thank Mr. Hiroyuki Masuda, Dr. Tohru Nakagawa, Dr. Hideo
Torii, and Mr. Sadayoshi Hotta for the experimental supportand discussions on this work. This work was supported by aGrant-in-Aid for Science Research from the Ministry of Education, Science, Sports, and Culture of Japan.
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