Physica E 37 (2007) 153–157
Dimensional evolutionofsiliconnanowiressynthesized by
Au–Si island-catalyzed chemicalvapor deposition
D.W. Kwak, H.Y. Cho, W C. Yang
Ã
Department of Physics and Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 100-715, South Korea
Available online 11 September 2006
Abstract
This study explores the nucleation and morphological evolutionofsiliconnanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates
synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemicalvapor deposition. The Au–Si islands are formed by Au thin
film (1.2–3.0 nm) deposition at room temperature followed by annealing at 700 1C, which are employed as a liquid-droplet catalysis
during the growth of the Si-NWs. The Si-NWs are grown by exposing the substrates with Au–Si islands to a mixture of gasses SiH
4
and
H
2
. The growth temperatures and the pressures are 500–600 1C and 0.1–1.0 Torr, respectively. We found a critical thickness of the Au
film for Si-NWs nucleation at a given growth condition. Also, we observed that the dimensionalevolutionof the NWs significantly
depends on the growth pressure and temperature. The resulting NWs are 30–100 nm in diameter and 0.4–12.0 mm in length. For Si
(0 0 1) substrates 80% of the NWs are aligned along the / 111S direction which are 301 and 601 with respect to the substrate surface
while for Si (1 1 1) most of the NWs are aligned vertically along the /111S direction. In particular, we observed that there appears to be
two types of NWs; one with a straight and another with a tapered shape. The morphological and dimensionalevolutionof the Si-NWs is
significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes.
r 2006 Elsevier B.V. All rights reserved.
PACS: 66.30.h; 68.70.w; 81.15.Gh
Keywords: Si nanowires (Si-NWs); Au–Si alloy droplets; VLS; Chemicalvapor deposition; Diffusion kinetics; Morphological evolution
1. Introduction
The ongoing reduction of electronic device size has led to
a transition of technological approach from top–down to
bottom–up due to current lithographic limitations. The
controlled fabrication of the self-organized nanostructures
as a building blo ck in the bottom–up approach has been
significant for advanced technological applications. In
particular, one dimensionalsiliconnanowires (Si-NWs)
have recently become of interest for potential applications
in various technologies such as optics, electronics,
and chemical sensors. This is the Si-NW s can offer the
possibility of integration with conventional Si integrated-
circuit technology [1–3]. The quantum effects in the
electronic and optical properties of the nanodevices
are strongly related to nanostructure’s dimensions [2].
Therefore, good control of the dimensions and alignments
of the NWs is required to employ them as elements of
nanodevices.
Vapor–liquid–solid (VLS) growth method has been
widely employed for the NW growth of various materials
[3]. In metal island-catalyzed growth of Si-NWs via the
VLS processes, the diameter and alignment of the NWs can
be readily controlled by the size of the catalytic metal
islands and the substrate orientation [4,5]. However, the
diameters of the reported NWs did not correspond to that
of the metal islands but were extensively modified with
variation in growth conditions [4,6]. Thus, detai led studies
on the correlation of the growth parameters with morpho-
logical evolutionof Si-NWs in the VLS processes are still
required for the well-controlled growth. In the VLS growth
mechanism, the evolutionof the NWs proceeds with three
well-known stages: metal alloying process, crystal nuclea-
tion, and axial growth [7]. These processes involve mass
transport through metal alloying and energetics of the
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doi:10.1016/j.physe.2006.07.017
Ã
Corresponding author.
E-mail address: wyang@dongguk.edu (W C. Yang).
system. Thus, these factors will determine the dimension
and orientation of the evolving Si-NWs.
In this study, we investigate the morphological evolution
of Si-NW s on Si substrates grown by nanoscale Au–Si
island-catalyzed rapid thermal chemicalvapor deposition
(RTCVD). The initial nucleation of the NWs from the
Au–Si islands is examined while varying island sizes. Also,
we investigate the variation in the morphology and
dimension of the NWs depending on the growth pressures,
temperatures and times. In particular, preferential growth
directions of the NWs are identified for Si (0 0 1) and (1 1 1)
substrates. The results are discussed in terms of atomic
mass transport and energetics at interfaces of vapor/liquid
and liquid/solid in the NW growth via the VLS mechanism.
2. Experimental procedure
P-type Si (0 0 1) and (1 1 1) wafers were employed as
substrates. The wafers were cleaned ultrasonically with
acetone and methanol for 10 min, and then rinsed under
running de-ionized wat er. In order to remove native oxide,
the wafers were dipped into 2% HF (HF:H
2
O ¼ 1:50) for
3 min and then flushed by dry nitrogen. The cleaned wafers
were transferred into an e-beam evaporator chamber to
deposit 1.2–3.0 nm thick Au films with a growth rate of
0.01 nm/s at room tempe rature. Then, the Au-deposited
substrates were transferred to the RTCVD chamber, where
they were annealed in hydrogen ambient at 0.5 Torr and
700 1C for 10 min to form Au–Si islands. After the island
formation, the substrate temperature was reduced to
Si-NW growth temperatures and the substrates were
exposed to a mixture of SiH
4
(1–4 sccm) and H
2
(50 sccm)
for 30–120 min. The growth temperatures and total
chamber pressures are 500–600 1C and 0.1–1.0 Torr,
respectively. The morpho logy of the grown Au–Si islands
and the Si-NWs were characterized by using a field
emission scanning electron microscopy (FESEM) for a
301 tilted view and cross-sectional geometries. The dimen-
sions of the Au–Si isl ands and the Si-NWs were measured
from the obtained SEM images.
3. Results and discussion
To explore initial nucleation of Si-NWs depending on
catalysis island size, we initiated the NW growth with
varying Au film thicknesses (1.2, 2.0 and 3.0 nm). Fig. 1
shows the substrate morphology before and after exposing
the annealed Au films to the silane (SiH
4
) mixture gases.
Upon annealing at above the Au–Si eutectic temperature
(360 1C), the Au film reacts with Si substrate and
dissolves Si to form Au–Si alloy liquid [8]. Further
annealing leads to a transformation of the liquid into
Au–Si alloy droplet structures, whose shape is determined
by minimization of the surface and interface energy of the
liquid/substrate. Also, the composition of the Au–Si liquid
alloy droplets will follow the liquid at annealing tempera-
tures [8]. Thus, the islands in Figs. 1(a) and (b) were formed
from the Au–Si alloy droplets after cooling down to room
temperature. For annealing temperature of 700 1C, the
composition of the Au–Si alloy islands might be 9% Si,
which can be estimated from Au–Si binary phase diagram
[8]. The surface shape of the islands is smooth and circular
even though the edge of the islands are irregular in
Figs. 1(a) and (b), indicating that the islands were formed
from the liquid droplets.
As the Au film thickness was increa sed, the average size
of the islands became larger while the size distribution was
broader and the number density of the islands was reduced.
For a 1.2 nm thick Au film, the average diameter of the
islands was 8 nm and all islands were smaller than 20 nm
(Fig. 1(a)) while for 2.0 nm Au film, the average diameter
was 13 nm and the fraction of the islands smaller than
20 nm was about 85% (not shown in Fig. 1). For further
increase in thickness of 3.0 nm, the average diameter was
increased to 15 nm and the fraction of the islands smaller
than 20 nm was decreased to 77% (Fig. 1(b)). These results
indicate that for thicker films the initially nucleated Au–Si
alloy droplets would tend to grow larger through droplet
coarsening with neighboring droplets [9]. Thus, the
diameter of the islands will be more uniform for Au films
thinner than 1.2 nm at a given annealing temperature.
After silane exposure of the Au–Si droplets on Si in
Figs. 1(a) and (b), we observed the formation of
nanostructures on the surface. Figs. 1 (c) and (d) display
the resulting SEM images obtained with 301 tilt with
respect to the horizon. For the 1.2 nm Au film, the
nanostructures are observed to be pillar shaped over all
surfaces (Fig. 1(c)). The nanopillars (NP) seem to be grown
prior to the nucleation of the NWs [10]. For the 2.0 nm
film, similar NP structures were observed. However, for the
3.0 nm film we observed the nucleation of the Si-NWs with
average diameter of 60 nm and length of 350 nm
(Fig. 1(d)). It may indicate that a critical thickness of Au
films exists for the Si-NW nucleation in the given growth
conditions. In other words, there exists a minimum size of
Au–Si droplets to initiate Si-NW nucleation from the
droplet catalysis. The bright tips of the NWs seem to be
Au–Si droplets in Fig. 1(d). The diameters of the NWs are
similar to those of the Au–Si droplets. The existence of a
critical Au film thickness for initiation of NW nucleation
may be related to competition of Si homogeneous nuclei
formation in the Au–Si droplets with the adatom attach-
ment at the Au–Si droplet-substrate interface [11].
To examine the effects of growth pressure on the Si-NW
morphology and dimension, a series of samples of 3.0 nm
of Au films were exposed to the sil ane mixture gases at
total pressures ranging from 0.1 to 1.0 Torr. For 0.1 Torr,
the surface morphology displays an early stage of the
nucleation of the Si-NWs (Fig. 2(a)). Most of the
nanostructures seem to be NP shaped and some are of
NW shape. As the growth pressure increased to 0.5 Torr,
more NWs nucleated and the NWs coexisted with the NPs
(Fig. 2(b)). For 1.0 Torr, most of the NPs transformed to
the NWs and the resulting surface shows randomly
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D.W. Kwak et al. / Physica E 37 (2007) 153–157154
oriented and dense Si-NWs distribution (Fig. 2(c)). The
diameter of the NWs is essentially constant (60 nm) while
the number density of the NWs increases rapidly with
increasing pressure (Fig. 2(d)). Also, the length increases
linearly from 0.8 to 4.0 m m (not shown in Fig. 2(d)). In
the VLS processes, the nucleation and growth of the Si-
NWs are strongly related to the degree of Si super-
saturation in the Au–Si droplets because the difference of
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Fig. 1. SEM images ofAu–Si islands grown on Si (0 0 1) substrates for (a) 1.2 nm and (b) 3.0 nm of Au deposited at room temperature and followed by
annealing at 700 1C. (c) and (d) Tilted SEM images of the (a) and (b) substrates after exposure to a mixture of SiH
4
(2 sccm) and H
2
(50 sccm) for 60 min at
0.1 Torr and 550 1C, respectively.
Fig. 2. Tilted SEM images of Si-NWs grown on Si (0 0 1) for 60 min at 550 1C and a total pressure of (a) 0.1, (b) 0.5, and (c) 1.0 Torr. The Au films (3 nm)
annealed at 700 1C were exposed to a gas mixture of SiH
4
(4 sccm) and H
2
(50 sccm). The diagram (d) is number density and average diameter of the NWs
as a function of growth pressure.
D.W. Kwak et al. / Physica E 37 (2007) 153–157 155
Si concentration between vapor–liquid interface and
liquid–solid interface will determine the growth rate of
the NWs [10]. As the growth pressure increases, dissolution
of more Si into Au–Si droplets will lead to increasing Si
chemical potential in the liquid droplets with a relationship
of Dm
Si
¼ kT ln p, where Dm
Si
is Si chemical potential in
the liquid droplets and p is a molecular Si overpressure [4].
Note that the growth rate of the NWs, R, is determined by
the chemical potential difference at the interface of liquid/
solid by the relation ship, R expðm
Si
=kTÞ [4]. Correspond-
ingly, the growth rate of the NWs is proportional to
pressure. This enhanced growth rate with increasing
pressure would result in more transition from the NPs to
NWs and the corresponding rapid increase in the NW
density and length, as shown in Fig. 2 .
Also we explored the variation in the Si-NW morphol-
ogy and dimension depending on growth temperature. In
our growth temperature ranges, linear and randomly
oriented Si-NWs were formed on the surfaces, which have
similar morphologies as shown in Fig. 2(c). However, the
dimensional variation was distinct. As the temperature
increases from 500 to 600 1C, the average diameter of the
NWs increases from 55 to 130 nm while the density of the
NWs decreases rapidly (Fig. 3). In particular, the number
density for 500 1C is lower than for 550 1C, which would
result from the existence of more NPs at lower temperature
due to lesser transition of the NWs from the NPs. As the
temperature increases, more amount of Si can dissolve into
the Au–Si droplets following the liquid of the Au–Si binary
phase diagra m. The increased chemical potential can
enhance the Si-NW growth rate at the interface between
Au–Si droplet and Si-NW seeds as well as the transition of
the NWs from the existing NPs. In contrast, the rapid
increase in diameter at higher temperature can be explained
by the NW coalescence with neighboring NWs during NW
vertical growth, or possibly coalescence of the Au–Si
droplet or the NPs at the initial nucleation stage before the
NW nucleation. This explanation is consistent with
decreasing number density due to the coalescence.
To explore the axial growth direction of the NWs, we
grew the NWs on Si (0 0 1) and (1 1 1) with optimized
growth conditions obtained from the above studies. Fig. 4
displays cross-sectional SEM images of the Si (0 0 1) and
(1 1 1) sample surfaces. The dimensions of the NW on both
surfaces are similar. However, the growth direction was
distinct. For Si (0 0 1), the axial directions of the NWs
varied in the range 30–901 with respect to the substrate
surface and approximately 80% of the NWs were aligned
along the angles of 301 and 601 (Fig. 4(a)), which is a
preferential /111S growth direction of the NWs [12].In
contrast, for Si (1 1 1), most of the NWs are aligned along
the vertical direction /111S (Fig. 4(b)). Our results are
consistent with previous reports [12]. This preferential
growth direction might be explained by energetics at the
interface between Au–Si droplets and initial Si-NW seeds
on the substrate. After the Si-NWs seeds are nucleated
below the droplets, the free energy of the liquid–solid
system is determined by the surface energy of the Si-NW
seeds and the interface energy of the liquid droplet–solid
NW seed. Note that the surface energy of crystal structures
depends on the crystal orientation and that the interface
energy of the NWs is proportional to the diameter of the
NWs [12]. Thus, for the NWs with diameter larger than
60 nm grown with our growth conditions, the axial
growth along the /111S direction will give rise to having
a minimum surface and interface energies at the interface
of Au–Si droplets and Si seeds.
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Fig. 3. Number density and average diameter of the NWs as a function of
growth temperature for the samples grown on Si (0 0 1) for 60 min at
1.0 Torr and various temperatures. The Au films (3 nm in thickness) were
exposed to a mixture of SiH
4
(4 sccm) and H
2
(50 sccm).
Fig. 4. Cross-sectional SEM images of Si-NWs grown on (a) Si (0 0 1) and (b) Si (1 1 1) substrates for 60 min.at 550 1C and 1.0 Torr with SiH
4
(4 sccm) and
H
2
(50 sccm), respectively.
D.W. Kwak et al. / Physica E 37 (2007) 153–157156
In addition, we investigated the dimensional variation of
the NWs with growth time from 150 to 120 min. For
growth time shorter than 60 min, the average diameter of
the NWs is 60 nm and the diameter of each NW is
essentially constant along the length direction independent
of the growth time while the length is proportional to the
growth time. The average length growth rate was
0.08 mm/min. In contrast, longer growth time gave rise
to distinct morphology of the NWs. For 120 min growth,
the length of all the NWs increased to 12 mm while the
diameters varied (Fig. 5). The wider NWs would be formed
by NW coalescence. In particular, we found the existence
of two types of NWs from the NWs with different
diameters (inset of Fig. 5). Some of the narrower NWs
have slightly tapered shape, the diameter decreasing with
increasing distance from the Si substrate while most of the
NWs have uniform diameter along their length. The
formation of the tapered NWs might result from a slight
loss of Au during NW growth in length [10]. The
morphology of both types of the NWs indicates that the
vertical growth rate by catalyzing Au–Si droplet is more
dominant than lateral growth rate.
4. Conclusion
We studied the morphological and dimensional evolu-
tion of the Si-NWs on Si (0 0 1) and (1 1 1) surfaces grown
by using Au–Si nanoisland-catalyzed RTCVD. For a given
growth condition, a critical thickness of the Au film exists
for the nucleation of the Si-NWs. The growth rate,
dimension, and orientation of the NWs can be controlled
by the growth pressure, temperature, time, and substrate
orientation. The resulting NWs are 30–100 nm in
diameter and 0.4–12 mm in length depending on the
growth conditions. For both Si (0 0 1) and (1 1 1) sub-
strates, most of the NWs were aligned along the /111S
direction. We fabricated vertically well-aligned and rela-
tively uniform sized NWs on Si (1 1 1) surfaces with
optimized growth parameters. In addition, we observed
two types of NWs, with straight and tapered shapes for the
NWs grown with a longer growth time. In the Si-NW
growth via the VLS processes, the morphological and
dimensional evolutionof the Si-NWs are considerably
related to mass transport through the Au–Si liquid droplets
and surface and interface energies at the interface of
liquid–solid.
Acknowledgments
This work was supported by the Quantum-Functional
Semiconductor Research Center in the Dongguk Univer-
sity and by the National Program for Tera Level Nano
Devices through MOST.
References
[1] A.P. Alivisatos, Science 271 (1996) 933.
[2] Y. Cui, C.M. Lieber, Science 291 (2001) 851.
[3] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin,
F. Kim, H. Yan, Adv. Mater. 15 (2003) 353.
[4] E.I. Givargizov, J. Cryst. Growth 31 (1975) 20.
[5] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, Appl. Phys. Lett. 78
(2001) 2214.
[6] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci.
Technol. B 15 (1997) 554.
[7] Yiying Wu, Peidong Yang, J. Am. Chem. Soc. 123 (2001) 3166.
[8] B. Ressel, K.C. Prince, S. Heun, Y. Homma, J. Appl. Phys. 93 (2003)
3886.
[9] W C. Yang, M. Zeman, H. Ade, R.J. Nemanich, Phys. Rev. Lett. 90
(2003) 136102.
[10] J.W. Dailey, J. Taraci, T. Clement, D.J. Smith, J. Drucker,
S.T. Picraux, J. Appl. Phys. 96 (2004) 7556.
[11] S. Sharma, T.I. Kamins, R.S. Williams, Appl. Phys. A 80 (2005)
1225.
[12] V. Schmidt, S. Senz, U. Gosele, Nano Lett. 5 (2005) 931.
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Fig. 5. Cross-section SEM images of Si-NWs grown on Si (1 1 1)
substrates for 120 min. The NWs were grown at the same condition as
in Fig. 4 except for the growth time. Inset is a magnified SEM image of the
end regions of the NWs.
D.W. Kwak et al. / Physica E 37 (2007) 153–157 157
. Physica E 37 (2007) 153–157
Dimensional evolution of silicon nanowires synthesized by
Au–Si island-catalyzed chemical vapor deposition
D.W. Kwak, H.Y. Cho,. morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates
synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical