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
Trang 1Physica E 37 (2007) 153–157
Dimensional evolution of silicon nanowires synthesized by
Au–Si island-catalyzed chemical vapor deposition
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 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 vapor 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 SiH4and
H2 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 dimensional evolution of 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 /1 1 1S 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 /1 1 1S 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 dimensional evolution of the Si-NWs is significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes
r2006 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; Chemical vapor 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 block in the bottom–up approach has been
significant for advanced technological applications In
particular, one dimensional silicon nanowires (Si-NWs)
have recently become of interest for potential applications
in various technologies such as optics, electronics,
and chemical sensors This is the Si-NWs 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, detailed studies
on the correlation of the growth parameters with morpho-logical evolution of Si-NWs in the VLS processes are still required for the well-controlled growth In the VLS growth mechanism, the evolution of 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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1386-9477/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.physe.2006.07.017
Corresponding author.
E-mail address: wyang@dongguk.edu (W.-C Yang).
Trang 2system Thus, these factors will determine the dimension
and orientation of the evolving Si-NWs
In this study, we investigate the morphological evolution
of Si-NWs on Si substrates grown by nanoscale Au–Si
island-catalyzed rapid thermal chemical vapor 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 water In order to remove native oxide,
the wafers were dipped into 2% HF (HF:H2O 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 temperature 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 SiH4(1–4 sccm) and H2(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 morphology 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 islands 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 (SiH4) 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 inFigs 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 increased, 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 inFig 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 inFig 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 silane 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
Trang 3oriented 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 mm (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
Fig 1 SEM images of Au–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.
Trang 4Si 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 DmSi¼kT ln p, where DmSi 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 relationship, R expðmSi=kT Þ[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 inFig 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 inFig 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 diagram 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 /1 1 1S growth direction of the NWs [12] In contrast, for Si (1 1 1), most of the NWs are aligned along the vertical direction /1 1 1S (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 /1 1 1S direction will give rise to having
a minimum surface and interface energies at the interface
of Au–Si droplets and Si seeds
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 (50 sccm), respectively.
Trang 5In 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 /1 1 1S 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 evolution of 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
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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.