Facile fabrication of ordered Si1-xGex nanostructures via hybrid process of selective epitaxial growth (SEG) and self-assembled nanotemplates

Article abstract of DOI:10.1016/j.jallcom.2012.04.107

In this study, a facile fabrication method of the ordered Si _1-xGe_x nanodots (NDs) and nanowires (NWs) was successfully developed via hybrid process of selective epitaxial growth (SEG) of Si_1-xGe_x and self-assembled nanotemplates, i.e., anodic aluminum oxide (AAO) and diblock copolymer (DBC) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA). Si_{1- x}Ge_x films were selectively grown on the Si windows against the oxide area at a growth temperature of 550 °C by repeating the unit cycle consisting of two consecutive steps; Si_{1- x}Ge_x deposition step using Si₂H₆ and GeH₄ and Cl₂ exposure step for removing the nuclei or deposits formed on the oxide area during the preceding Si_{1- x}Ge_x deposition step. The chemical composition of the Si_1-xGe_x films was readily controlled by adjusting the flow rate of GeH₄ from 20 sccm to 45 sccm while that of Si₂H₆ gas was fixed at 10 sccm, giving rise to the variation of Ge composition in Si_1-xGe_x from 22.2% to 34.0%. In order to fabricate well-ordered Si_{1- x}Ge_x nanostructures, Si windows with hexagonal arrangement were fabricated using AAO and PS-b-PMMA. AAO was prepared through multi-step anodization of the Al films of Al/Si(0 0 1) substrate under suitable anodizing conditions. Subsequently, ordered Si windows were fabricated by removing the barrier layer at the bottom of the AAO membrane by reactive ion etching (RIE). In case of PS-b-PMMA, SiO₂ templates with ordered Si windows were fabricated through replication of nano-cylindrical pattern of PS-b-PMMA to the 20-nm thick SiO₂ layers of SiO₂/Si. By utilizing the ordered Si windows obtained from both AAO and PS-b-PMMA, Si_{1- x}Ge_x was selectively grown on Si windows against the oxide area, viz., aluminum oxide in AAO and SiO₂ templates. Hexagonally ordered NDs and freestanding NWs were facilely fabricated on the Si substrates after removing the AAO and SiO₂ templates.

Full text of DOI:10.1016/j.jallcom.2012.04.107

Journal of Alloys and Compounds 536 (2012) 166–172
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Facile fabrication of ordered Si1 Ge nanostructures via hybrid process
Àx
x
of selective epitaxial growth (SEG) and self-assembled nanotemplates
Sang-Joon Park , Inchan Hwang, Heung Soon Lee, Sunggi Baik, Hyungjun Kim 1
⇑
Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Àx
In this study, a facile fabrication method of the ordered Si1 Gex nanodots (NDs) and nanowires (NWs)
Received 22 February 2012
Received in revised form 17 April 2012
Accepted 21 April 2012
was successfully developed via hybrid process of selective epitaxial growth (SEG) of Si1ÀxGe
assembled nanotemplates, i.e., anodic aluminum oxide (AAO) and diblock copolymer (DBC) of polysty-
rene-b-polymethylmethacrylate (PS-b-PMMA). Si1ÀxGe films were selectively grown on the Si windows
against the oxide area at a growth temperature of 550 °C by repeating the unit cycle consisting of two
consecutive steps; Si1ÀxGe deposition step using Si and GeH and Cl exposure step for removing
the nuclei or deposits formed on the oxide area during the preceding Si1ÀxGe deposition step. The chem-
ical composition of the Si1ÀxGe films was readily controlled by adjusting the flow rate of GeH from
0 sccm to 45 sccm while that of Si gas was fixed at 10 sccm, giving rise to the variation of Ge com-
from 22.2% to 34.0%.
x
and self-
x
Available online 9 May 2012
x
H
2 6
4
2
Keywords:
x
Ordered SiGe nanostructures
Selective epitaxial growth
Anodic aluminum oxide
Diblock copolymer
x
4
2
2 6
H
position in Si1ÀxGe
x
x
In order to fabricate well-ordered Si1ÀxGe nanostructures, Si windows with hexagonal arrangement
were fabricated using AAO and PS-b-PMMA. AAO was prepared through multi-step anodization of the
Al films of Al/Si(001) substrate under suitable anodizing conditions. Subsequently, ordered Si windows
were fabricated by removing the barrier layer at the bottom of the AAO membrane by reactive ion etching
(
RIE). In case of PS-b-PMMA, SiO
of nano-cylindrical pattern of PS-b-PMMA to the 20-nm thick SiO
ordered Si windows obtained from both AAO and PS-b-PMMA, Si1ÀxGe
dows against the oxide area, viz., aluminum oxide in AAO and SiO templates. Hexagonally ordered NDs
2
templates with ordered Si windows were fabricated through replication
layers of SiO /Si. By utilizing the
was selectively grown on Si win-
2
2
x
2
and freestanding NWs were facilely fabricated on the Si substrates after removing the AAO and SiO
templates.
2
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
phorus and boron can be facilely altered by adjusting the gas flow
rates of each Si and Ge source and those of dopant sources during
Various nanostructures such as nanodots (NDs) and nanowires
NWs) have been widely studied due to their unique properties
the VLS [16]. Si1ÀxGe
neous self-organized growth of Si1ÀxGe
x
NDs are generally fabricated through sponta-
on Si substrate, which is
(
x
originated from their low dimension [1–6]. They have been
expected to play a key role in emerging electronics such as high-
sensitivity sensor, energy storage/generation applications, and
photonic and optoelectronic devices [7–10]. Among a variety of
originated from their lattice mismatch [17–19]. Precise control of
position, size and shape is one of the prerequisite for actual and
reliable application of the Si1ÀxGe
tronics. Aforementioned fabrication methods of Si1ÀxGe
structures, however, have shown lack of such controllability.
Moreover, incorporation of the metal impurity into Si1ÀxGe is
x
nanostructures to practical elec-
x
nano-
nanostructure materials, Si1ÀxGe
most representative materials for nanoelectronic applications
10–12]. The most common method for fabrication of Si1ÀxGe
NWs is vapor–liquid–solid (VLS) growth [13–16]. The chemical
composition and doping concentration of Si1ÀxGe such as phos-
x
nanostructures are one of the
x
[
x
unavoidable in VLS method because of the use of metal catalyst
[20], resulting in the degradation of device performance [21].
To resolve these issues of existing fabrication techniques of
x
x
Si1ÀxGe nanostructures, we employed the selective epitaxial
growth (SEG). SEG has recently drawn huge attention as one of
the promising catalyst-free fabrication methods of nanostructured
⇑
Corresponding author. Address: Engineering Building #1, Pohang University of
Science and Technology (POSTECH), San31, Hyoja-dong, Nam-gu, Pohang, Gyeong-
buk 790-784, Republic of Korea. Tel.: +82 54 279 2822; fax: +82 54 279 5090.
E-mail address: narcciss@postech.ac.kr (S.-J. Park).
semiconductors including Si1ÀxGe
an attempt to deliberately control the position and composition
of Si1ÀxGe nanostructures, i.e., NWs and NDs, we employed the
hybrid process of Si1ÀxGe SEG and self-assembled nanotemplates.
x
[22], Ge [23], and GaN [24]. In
1
x
Present address: School of Electrical and Electronic Engineering, Yonsei Univer-
x
sity, Seoul 120-749, Republic of Korea.
0
925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.04.107

S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
167
In this study, we adopted two different kinds of nanotemplates;
one is anodic aluminum oxide (AAO) and the other is diblock
copolymer (DBC). Both of nanotemplates exhibit the regularly ar-
ranged nanopatterns [25–27]. They have been widely used for
the purpose of fabricating the nanostructured materials [24,28–
3
1]. For the case of AAO, vertically penetrated and hexagonally ar-
ranged cylindrical nanoholes are readily formed by anodizing alu-
minum under the proper anodizing conditions; electrolytes,
temperature, applied voltage or current [25,27]. The hole dimen-
sion including the diameter and length can be easily controlled
by simply varying the anodizing voltages, times, and additional
pore-widening. On the other hand, for the case of DBC, various
nanopatterns such as sphere, lamella, and cylinder can be fabri-
cated through the microphase separation of DBC, in which the
shape of the nanopattern can be altered by simply changing the
molecular weight of each polymer block [26,29,30]. Among a diver-
sity of the nanopatterns which can be obtained from DBC, cylindri-
cal nanohole pattern is advantageous in fabricating the
nanostructures which directly contact on the substrates by virtue
of direct formation of nanoholes normal to the surface of
substrates.
x
In SEG, Si1ÀxGe is preferentially well grown on the Si surfaces,
whereas it is rarely deposited on the oxide or nitride ones [32–34].
Enabled by this selective growth characteristic of SEG, we devel-
oped the facile fabrication method for ordered and composition-
controlled Si1ÀxGe
SEG and self-assembled nanotemplates. Si1ÀxGe
x
nanostructures, i.e., hybrid process of Si1ÀxGe
SEG with good
x
x
selectivity between Si windows and oxide layer was established
by repeating the unit cycle of SEG composed of two consecutive
steps; one is Si1ÀxGe
deposition (UHV-CVD) using pure disilane (Si
GeH ) gases and the other is etching step of small Si1ÀxGe
or deposits, which might have been formed on oxide layer during
the preceding Si1ÀxGe growth step, by introducing pure chloride
Cl ) gas to reaction chamber. Two different types of nano-sized
x
growth step by ultra-high vacuum chemical
) and germane
nuclei
2 6
H
(
4
x
x
Fig. 1. Schematic diagrams for two different ways of Si1ÀxGe
fabrication using (a) AAO templates and (b) PS-b-PMMA.
x
nanostructures
(
2
Si windows were fabricated by employing the nanotemplates;
AAO and DBC nanotemplates polystyrene-block-polymethylmeth-
acrylate (PS-b-PMMA). One is Si windows surrounded with alumi-
num oxide of AAO template, which was prepared by anodizing Al
layer directly deposited on Si substrate and subsequently removing
the barrier layer of the holes of AAO. The other ones were fabri-
cated using the hexagonal nanohole arrays of PS templates ob-
tained from PS-b-PMMA. Ordered Si windows were prepared via
ordering of AAO [35]. A carbon counter electrode and Si substrate with Al film were
dipped into the electrolyte of 0.3 M oxalic acid. Anodization was performed under
the applied voltage of 40 V at 15 °C. The AAO membrane formed during the 1st
anodization step was removed by dipping the sample into the mixture solution of
chromic and phosphoric acid, which was maintained at 65 °C. After repeating these
two steps several times alternately and carrying out the final anodization step, the
hexagonally ordered hole arrays were obtained at last. The thickness of AAO mem-
brane could be controlled by adjusting the duration of the final anodization step in
consideration of the pursued AAO thickness. Fig. 2a is the SEM image for the result-
ing AAO template, prepared by anodizing the Al film on Si. The hole diameter was
enlarged to 70 nm by dipping AAO template into the 0.1 M phosphoric acid solution
which was maintained at 30 °C. In order to fabricate the exposed Si window at each
of nanoholes, the barrier layer of AAO was etched away by reactive ion etching (RIE)
using the mixture gas of 20-sccm BCl3 and 10-sccm Ar under the same power of
150 W for source and bias of plasma. The etching duration was altered depending
on the AAO thickness; the longer the AAO thickness was, the longer etching dura-
tion to remove the barrier layer was.
transfer of nanohole pattern of PS templates to SiO
reactive ion etching (RIE) by utilizing PS as an etching mask. Hex-
agonally well ordered Si1ÀxGe NDs were fabricated on Si(001)
substrate through Si1ÀxGe SEG on the resulting Si windows and
subsequent removal of the oxide layers of aluminum oxide and
SiO . Moreover, vertically aligned free standing Si1ÀxGe NWs with
2 4
layer by CF
x
x
2
x
uniform diameter were readily fabricated on Si(001) substrate
using the Si windows with thick AAO templates by increasing
the number of Si1ÀxGe
Si1ÀxGe NDs and NWs were found to exhibit the good agreement
with the hole size of AAO and PS-b-PMMA.
x
SEG cycle. The lateral size of the resulting
2.1.2. Fabrication of Si windows using PS-b-PMMA
The Si windows were also fabricated by using DBC of PS-b-PMMA in order to
fabricate the Si windows with smaller size of 20-nm diameter in comparison those
x
prepared by utilizing the AAO. The Si windows with the nanohole-patterned SiO
template was obtained through pattern transfer of hexagonally ordered nanoholes
of PS-b-PMMA to 20-nm thick SiO layer on Si(001) substrate. Self-assembled
monolayer (SAM) of 3-(p-methoxy-phenyl)propyltrichloro-silane [MPTS, CH OPh
CH SiCl ] was coated on SiO /Si substrate prior to PS-b-PMMA coating, leading
2
2
. Experimental details
2
3
2.1. Fabrication of ordered Si windows
(
2
)
3
3
2
to the hydrophobically neutral surface of the substrate. During the vacuum anneal-
ing of PS-b-PMMA-coated Si substrate at 180 °C for 24 h, microphase separation of
PMMA and PS took place, resulting in PMMA nanocylinder and PS matrix. The
ordering of PMMA cylinders was facilitated by virtue of the hydrophobically neutral
substrate surface. Finally, vertically aligned cylindrical nanoholes were formed on
As shown in the schematic diagrams of Fig. 1, hexagonally ordered Si windows
with sub-100 nm in lateral dimension were fabricated via two different processes
using AAO (Fig. 1a) and PS-b-PMMA (Fig. 1b).
2.1.1. Fabrication of Si windows using AAO
2
the SiO /Si substrates as a consequence of removal of the PMMA cylinders by per-
The first type of Si window was prepared using AAO. The AAO was prepared by
forming ultra-violet (UV) exposure to the sample for 90 min, giving rise to degrada-
tion of the PMMA, and subsequent dipping of sample into the glacial acetic acid for
1 min. Fig. 2b shows the resulting 20-nm diameter hole array fabricated by utilizing
anodizing the 1.4-lm thick Al film, which had been thermally evaporated on the Si
(
001) substrate. Multi-step anodization was employed in order to improve hole

1
68
S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
Fig. 2. Top-view SEM images: (a) AAO fabricated through multi-step anodization of the Al films evaporated on the Si substrate, (b) cylindrical nanoholes formed on the Si
with native oxide using PS-b-PMMA.
PS-b-PMMA. By using this nanohole pattern, SiO
was fabricated through the nanocylindrical pattern transfer of PS-b-PMMA to
0-nm thick SiO layer of SiO /Si by CF RIE. In the end, ordered Si windows of
2
template with ordered nanoholes
2
2
2
4
diameter of 20 nm were obtained. More details for these were reported by our
previous study [36].
x x
2.2. Si1ÀxGe nanostructure fabrication by the Si1ÀxGe SEG
Si1ÀxGe
Si1ÀxGe on the Si windows was achieved by repeating the unit cycle of SEG, which
was composed of two separate steps, namely, the Si1ÀxGe deposition step and the
Cl exposure step to etch away the Si1ÀxGe nuclei or deposits which could be
formed on oxide layer during the preceding Si1ÀxGe deposition step. During the
Si1ÀxGe deposition step of 20 s, pure GeH and Si gases were introduced to
the reaction chamber at a flow rate of 20 and 10 sccm, respectively. On the other
hand, during the Si1ÀxGe etching step, samples were exposed to Cl gas of flow rate
of 1 sccm in an attempt to remove the Si1ÀxGe nuclei or deposits which could be
formed on the oxide area, viz., aluminum oxide of AAO templates or SiO templates
obtained using PS-b-PMMA in this study. The Si1ÀxGe thicknesses were controlled
by altering the number of cycles of the Si1ÀxGe SEG.
Si1ÀxGe nanostructures including NDs and NWs were fabricated through
Si1ÀxGe SEG on Si windows. The numbers of cycles of Si1ÀxGe SEG were adjusted
depending on the pursuing dimensions of Si1ÀxGe nanostructures, i.e., the height of
NDs or the length of NWs. The morphology and the shapes of Si1ÀxGe nanostruc-
x
was grown by UHV-CVD at a growth temperature of 550 °C. SEG of
x
x
2
x
x
x
4
2 6
H
x
2
x
2
x
x
x
x
x
x
x
tures were observed using atomic force microscopy (AFM), scanning electron
microscopy (SEM), high-resolution transmission electron microscopy (HRTEM),
and scanning tunneling electron microscopy (STEM). The quantitative chemical
composition as well as epitaxial growth of Si1ÀxGe
X-ray diffraction (HR-XRD). The qualitative composition analysis on Si1ÀxGe
was performed by the energy-dispersive spectroscopy (EDS) mapping. The crystal-
line structure of the Si1ÀxGe NDs was investigated by HR-XRD at Pohang Acceler-
x
ator Laboratory (PAL).
x
was analyzed by high-resolution
Fig. 3. HR-XRD
rates of GeH and a fixed flow rate of 10 sccm of Si
measured normal to Si(004) plane: (a) 45-sccm GeH
GeH , Si0.715Ge0.285; (c) 20-sccm GeH , Si0.778Ge0.222. (S: Diffraction peak from Si(0 0
), L: diffraction peak from Si1–xGe layer, and F: Fringe peaks originated from
interference between Si and Si1–xGe
x
À 2h scan spectra of Si1ÀxGe
x
films grown with different flow
. The each of spectra were
, Si0.660Ge0.340; (b) 31-sccm
x
NWs
4
2 6
H
4
4
4
4
x
x
)
3
. Results and discussion
tions, viz., the flow rates of Si
sitions of each Si and Ge in the resulting Si1ÀxGe
determined from the difference of diffraction angle between
Si(004) and Si1ÀxGe (004) in À 2h scan spectra, which is origi-
nated from the larger lattice constant of Si1ÀxGe than that of Si.
Fig. 3 shows the films.
À 2h scan spectra of the resulting Si1ÀxGe
As manifested by Fig. 3, the distinct diffraction peaks of Si1ÀxGe
(004) as well as fringe peaks observed in
À 2h spectra supply
a evidence on the high quality Si1ÀxGe film deposition by our
deposition process. More details for chemical composition control
2
H
6
and GeH
4
. The chemical compo-
3
.1. Si1ÀxGe film deposition on Si(001) substrates & control of its
x
x
films were
chemical composition
x
x
The chemical composition of the Si1ÀxGe
x
films was modulated
x
by adjusting the flow rate of the precursor gas of Ge (i.e., GeH
while that of Si precursor gas (i.e., Si ) was fixed at 10 sccm. Ta-
ble 1 presents the effect of the flow rate of each precursor gas of
Si and GeH , and the resulting chemical composition of Si1ÀxGe
prepared at different conditions of precursor gases. As described in
Table 1, the chemical composition of Ge in the present Si1ÀxGe
4
)
x
x
H
2 6
x
x
2
H
6
4
x
x
x
and growth behaviors of our Si1ÀxGe
x
films will be reported
films was adjusted from 22.2% to 34.0% depending growth condi-
elsewhere.
Table 1
x 2 6 4
Chemical composition of Si1ÀxGe films grown under different gas flow rates of Si H and GeH .
Condition I
Condition II
Condition III
Precursor gas flow rate (sccm)
Si
2
H
6
10
10
10
GeH
4
20
31
45
Ge composition (%)
Chemical formula
22.2
Si0.788Ge0.222
28.5
Si0.715Ge0.285
34.0
Si0.660Ge0.340

S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
169
3
.2. Fabrication of Si1ÀxGe
x
NDs and NWs using AAO
quent 450 cycles of Si1ÀxGe
growth of Si1ÀxGe
via holes as shown in Fig. 4. The faceting of the Si1ÀxGe1Àx observed
in the SEM image is commonly observed in Si1ÀxGe SEG on the fi-
nite areas [37]. On the basis of the results of two preliminary tests,
we can conclude that our Si1ÀxGe SEG has a great feasibility of
selective growth on the Si windows of small lateral size down to
x
SEG exhibited the good selective
on the Si windows at the bottom of each of
x
Prior to applying Si1ÀxGe
reliable selective growth of Si1ÀxGe
square-shaped Si windows with various sizes, which were formed
in 200-nm thick SiO layers on Si substrates. The Si1ÀxGe SEG was
achieved by repeating the unit cycle of two sequential steps of
x
SEG to the nano-sized Si windows, the
x
was checked out using the
x
2
x
x
Si1ÀxGe
x
film deposition and Cl
nuclei or deposits which could be formed on oxide layer.
and GeH were introduced
to reaction chamber at flow rates of 20 and 10 sccm, respectively.
On the other hand, during the Cl exposure step of 10 s, 1 sccm
of Cl gas was delivered to the reaction chamber.
From Si1ÀxGe SEG on the Si windows of which minimum size is
00 Â 500 nm , it turned out that Si1ÀxGe films were selectively
grown on the Si windows against on the SiO area. In other word,
Si1ÀxGe SEG by our approach was successfully developed. The
good selectivity of our Si1ÀxGe SEG was also confirmed when it
was carried out on the substrate of an array of via holes in SiO
2
exposure step to eliminate the
x
100 nm and high aspect ratio, and fabrication of Si1ÀxGe nano-
structures using nano-sized Si windows.
Si1ÀxGe
x
During the former step of 20 s, Si
H
2 6
4
Fig. 5 shows the Si1ÀxGe
strate through Si1ÀxGe SEG on the 70-nm-diameter Si windows,
obtained from 150-nm thick AAO template. After 20 cycles of
SEG, Si1ÀxGe was selectively grown on the Si windows at the bot-
tom of each hole of AAO (Fig. 5a), and ordered Si1ÀxGe NDs were
x
nanodots (NDs) fabricated on Si sub-
x
2
2
x
x
x
2
5
x
successfully obtained after removing the AAO using the mixture
solution of chromic and phosphoric acid (Fig. 5b) at a temperature
of 60 °C. The average diameter and inter-dot distance was 70 and
2
x
x
x
100 nm, respectively. Si1ÀxGe ND arrays showed the good replica-
2
.
tion of the hole size and inter-hole distance as well as hexagonal
ordering of AAO. During the anodization of Al film deposited on
the Si substrate, silicon oxide could be formed beneath the barrier
layer of AAO [38]. Complying with this, it can be thought that sil-
icon oxide was remained even after removing the barrier layer of
The aspect ratio of a via hole was 1:5; the diameter of the top
and the bottom of a via hole was 95 and 82 nm, respectively and
the depth of them was 470 nm. The bottom of as-received via holes
was covered by thin SiN
to no Si1ÀxGe growth on the nitride layer at the bottom of via holes
even after 450 cycles of Si1ÀxGe SEG. In order to obtain the ex-
posed Si surface at the bottom of each via hole, prior to carrying
out Si1ÀxGe SEG, SiN layer was selectively etched by dipping sam-
ples into 85% phosphoric acid heated at 160 °C for 5 min. Subse-
x
etch-stop layer on Si substrates, leading
x
the AAO by RIE, resulting in non-uniform formation of Si1ÀxGe
NDs and different height of each Si1ÀxGe dot. In other words, re-
mained silicon oxide hindered or retarded the nucleation of the
Si1ÀxGe on the Si windows of the barrier-layer-etched holes of
AAO, causing no growth of Si1ÀxGe or different height of each
Si1ÀxGe NDs.
HRTEM and HR-STEM images of Fig. 6 show the epitaxial
growth of Si1ÀxGe NDs on the Si windows. It is noteworthy that
Si1ÀxGe was epitaxially grown on the Si windows. Epitaxial
growth of Si1ÀxGe also confirmed in the result of HR-XRD. Fig. 7
is the curve for
À 2h scan normal to Si(004) plane for Si1ÀxGe
ND arrays, presenting the epitaxial growth of SiGe dots, which is
in consistent with the STEM results. The curve of
À 2h scan is
the typical one for Si1ÀxGe film epitaxially grown on the Si(001)
substrate [39]. Discernible diffraction peak originated from
Si1ÀxGe was appeared at the 2h of 68.4° although the different
peak of Si1ÀxGe was relatively broad. The smaller diffraction angle
of Si1ÀxGe (004) than that of Si (004) is due to larger lattice
constant of the resulting Si1ÀxGe than that of Si.
Si1ÀxGe NWs were fabricated using 700-nm thick AAO tem-
plate, which is thicker than that used in Si1ÀxGe NDs fabrication,
as shown in the SEM images of Fig. 8. Si1ÀxGe SEG of 450 cycles
was carried out on the Si windows and Si1ÀxGe NWs were ob-
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
tained after subsequent removal of AAO template. Non-uniform
length of each NW may be ascribed to the residual oxide layer
on the Si windows, which is the same reason as non-uniform
Fig. 4. Si1ÀxGe
at a growth temperature of 550 °C.
x x
selectively grown on via holes during the 450 cycles of Si1ÀxGe SEG
Fig. 5. SEM images of Si1ÀxGe
x x
NDs fabricated on the Si windows, which were fabricated using 200-nm thick AAO template, through 20 cycles of Si1ÀxGe SEG: (a) cross
sectional image before removing AAO and (b) top-view image after removing AAO.

1
70
S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
x x
Fig. 6. (a) Cross-sectional view HRTEM images of Si1ÀxGe NDs and (b) HR-STEM image for the interface between Si1ÀxGe layer and Si(001) substrate.
formed in the hole of AAO template. It should be noted that
Si1ÀxGe was grown only on the Si but not on AAO, i.e., alumina,
presenting Si1ÀxGe NWs fabrication without using metal catalysts.
In addition, Al contamination, probably come from AAO, was also
not detected (data not shown). It is obvious that pure Si1ÀxGe
was selectively well grown on the Si windows against aluminum
x
x
x
oxide of AAO even after Si1ÀxGe
The hybrid process of Si1ÀxGe
be promising method for facile Si1ÀxGe
x
SEG of 450 cycles.
SEG and AAO nanotemplates will
x
x
nanostructures fabrication
because Si1ÀxGe nanostructures, viz., NDs and NWs, can be fabri-
x
cated without metal catalysts and incorporation of metal impuri-
ties via our method. This also can be applied to fabrication of Si
and Ge nanostructures. As mentioned earlier, dimension of AAO,
namely, hole dimension, inter-hole distance, and depth of hole, is
readily modulated by simply adjusting the anodization conditions
and additional pore-widening. Based on this, dimensions of
Si1ÀxGe NDs and NWs in our approach can be facilely controlled
x
by employing AAO templates with the variety of hole dimensions.
Additionally, the axial length scale of the NDs and NWs can be
varied depending on the number of cycles of Si1ÀxGe SEG.
x
x
3.3. Fabrication of Si1ÀxGe NDs using PS-b-PMMA
Fig. 7. HR-XRD curve of
x
À 2h scan normal to Si(004) plane for Si1ÀxGe
x
NDs.
To gain more insight into applicability of our approach for fabri-
cation of Si1ÀxGe nanostructure with smaller size, Si1ÀxGe SEG was
applied to the 20-nm thick SiO2 template with ordered circular-
shaped Si windows of which diameter was 20 nm. Twenty-cycles
Àx
Si1 Gex SEG was performed on SiO2 template. Good selective
x
x
formation of Si1ÀxGe
process of residual oxide can improve the uniformity of fabrication
of Si1ÀxGe NDs and NWs by SEG.
Energy-dispersive spectroscopy (EDS) mapping result of Fig. 9
revealed that Si1ÀxGe was selectively grown on the Si windows
x
NDs on the Si windows. Clearer elimination
x
growth of Si1 Ge was observed as shown in Fig. 10. Fig. 10a and
Àx
x
x
1Àx
b are the SEM images of SiO2 template after 20-cycles Si Gex
x x
Fig. 8. SEM images for Si1ÀxGe NWs fabricated on the Si windows through 450 cycles of Si1ÀxGe SEG. Si windows were fabricated using 450-nm thick AAO.

S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
171
x
Fig. 9. (a) Bright field TEM image and (b) EDS mapping result for Si1ÀxGe NW fabricated in a hole of AAO.
Fig. 10. SEM top-view images of Si1ÀxGe
with 20-nm thick SiO template, fabricated using PS-b-PMMA. (c) and (d) AFM images of Si1ÀxGe
m  1 m, Z-scale: 50 nm).
x
NDs (a) before and (b) after removing SiO
2
template. Si1ÀxGe
x
were selectively grown during the 20 cycles of SEG on the Si windows
2
x
NDs of Fig. 10(b): (c) oblique view and (d) plan-view image (scan size:
1
l
l
SEG and ordered Si1ÀxGe
plate, respectively. As shown in Fig. 10a, hexagonally arranged
nanohole pattern was well replicated to the SiO layers through
transfer of hexagonal nanohole pattern of PS-b-PMMA. Si1ÀxGe
was selectively grown on the Si windows against SiO area and final-
ly Si1ÀxGe NDs were fabricated on the Si substrate after removing
the SiO layer. Hexagonal arrangement of the resulting Si1ÀxGe
x
NDs obtained after taking away SiO
2
tem-
(NWs) was successfully developed via hybrid process of selective
epitaxial growth (SEG) of Si1ÀxGe and self-assembled nanotem-
plates, i.e., anodic aluminum oxide (AAO) and diblock copolymer
x
2
x
(DBC) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA).
2
Si1ÀxGe
the oxide area at a growth temperature of 550 °C by repeating
the unit cycles consisting of two consecutive steps; Si1ÀxGe depo-
sition step and Cl exposure step. During the former step, in order
for the Si1ÀxGe to be grown, Si and GeH were injected to the
reaction chamber as a Si and Ge gas source, respectively. Mean-
while, during the latter step, Cl was introduced to the reaction
x
films were selectively grown on the Si windows against
x
2
x
x
NDs was observed using AFM measurements as shown in Fig. 10c
and d. As confirmed from the SEM and AFM measurements, ordered
2
x
H
2 6
4
Si1ÀxGe
Si1ÀxGe
x
NDs were successfully fabricated on the Si substrates via
SEG on an array of 20-nm-diameter Si windows. This result
x
2
presents that the small size of Si1ÀxGe
of 20 nm or even smaller scale can be fabricated by our hybrid pro-
cess which employ the Si1ÀxGe SEG and DBCs.
x
nanostructures down to size
chamber to achieve the SEG on the Si windows, resulting from
etching of the nuclei or deposits formed on oxide area during the
x
preceding Si1ÀxGe
the Si1ÀxGe films was controlled by adjusting the flow rate of
GeH from 20 sccm to 45 sccm while that of Si gas source was fixed
at 10 sccm, giving rise to the variation of Ge composition in
Si1ÀxGe from 22.2% to 34.0%.
x
deposition step. The chemical composition of
x
4
4
. Conclusions
x
In this study, a facile fabrication method for the ordered
Si1ÀxGe nanostructures such as nanodots (NDs) and nanowires
The position, direction, and density as well as dimension of
nanostructures should be deliberately controlled for their applica-
x

1
72
S.-J. Park et al. / Journal of Alloys and Compounds 536 (2012) 166–172
tion to practical applications. To this end, in this study, hexagonally
ordered Si windows were utilized in order to fabricate the Si1ÀxGe
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Acknowledgment
This work was supported by the Technology Innovation Pro-
gram funded by the Ministry of Knowledge Economy (MKE, Korea).