Fabrication of Cobalt Silicide Nanowire Contacts to Silicon Nanowires

Article abstract of DOI:10.1149/1.1598966

A novel method to fabricate cobalt suicide nanowire contacts to silicon nanowires is described. Nanoporous anodized alumina membranes were used to define the wires' dimensions. Cobalt was deposited electrochemically within the pores of the membranes. On top of the cobalt, gold was deposited electrochemically to catalyze the growth of the silicon nanowires. The well-known vapor-liquid-solid mechanism was used to grow the silicon nanowires. During the growth of silicon, cobalt reacts with the growing silicon nanowire, forming cobalt silicide, and under certain conditions, all of the Au used to catalyze the growth of the silicon nanowire travels along the tip of the growing wire, away from the cobalt silicide/Si interface. We also discuss the influence of the amount of Au catalyst on the structural quality of the wires.

Full text of DOI:10.1149/1.1598966

Journal of The Electrochemical Society, 150 ͑9͒ G577-G580 ͑2003͒
G577
0
013-4651/2003/150͑9͒/G577/4/$7.00 © The Electrochemical Society, Inc.
Fabrication of Cobalt Silicide Nanowire Contacts to Silicon
Nanowires
Ahmad M. Mohammad,* Soham Dey, K.-K. Lew, J. M. Redwing,
and S. E. Mohney**,
z
Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State
University, University Park, Pennsylvania 16802, USA
A novel method to fabricate cobalt silicide nanowire contacts to silicon nanowires is described. Nanoporous anodized alumina
membranes were used to define the wires’ dimensions. Cobalt was deposited electrochemically within the pores of the membranes.
On top of the cobalt, gold was deposited electrochemically to catalyze the growth of the silicon nanowires. The well-known
vapor-liquid-solid mechanism was used to grow the silicon nanowires. During the growth of silicon, cobalt reacts with the growing
silicon nanowire, forming cobalt silicide, and under certain conditions, all of the Au used to catalyze the growth of the silicon
nanowire travels along the tip of the growing wire, away from the cobalt silicide/Si interface. We also discuss the influence of the
amount of Au catalyst on the structural quality of the wires.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1598966͔ All rights reserved.
Manuscript submitted December 26, 2002; revised manuscript received March 26, 2003. Available electronically July 25, 2003.
Semiconductor nanowires have generated tremendous interest in
recent years due to their potential applications in nanoelectronic and
nanoelectromechanical devices.1 Considerable interest has been
devoted to the fabrication and characterization of silicon nanowires
For nanoelectronic devices to reach their potential, ohmic con-
tacts to the SiNWs are required. In microelectronics, the silicide
-3
CoSi has emerged as a leading choice as a contact material due to
2
its low resistivity, stability on Si, and small lattice mismatch
2
7
͑
SiNWs͒, and researchers have synthesized SiNWs using various
͑Ϫ1.2%͒ with silicon. Furthermore, the low mutual solubility of
4
-6
28
methods including electron beam lithography,
reactive ion
Co and Au protects cobalt from mixing extensively with the Au
etching,1
,4,6-8
laser ablation
9-13
͑which may be oxide
9,10
or metal
11,13
catalyst during the VLS growth. Therefore, we were motivated to
incorporate Co in the SiNW fabrication scheme reported by Lew
assisted͒, and the classical metal catalyzed vapor-liquid-solid ͑VLS͒
1
4-16
26
29
growth mechanism introduced by Wagner and Ellis
and de-
et al. Interestingly, Li et al. have attempted another method to
scribed in detail by Givargizov and co-workers.1
7,18
In this method,
fabricate cobalt silicide contacts to SiNWs. They used a metal vapor
ϩ
a catalyst such as Au, Zn, Ga, or Pt is usually deposited on a silicon
wafer. Subsequently, the wafer is placed in a furnace at a tempera-
ture high enough to form a liquid alloy droplet between the catalyst
and silicon. When a silicon-containing gas such as SiCl or SiH is
vacuum arc ion source to implant Co ions in SiNWs presynthe-
sized by thermal decomposition of SiO powder. Since the incident
ϩ
Co beam was directional, only the side of the nanowire that was
ϩ
facing the Co beam was implanted. Therefore, the contacts
4
4
introduced, the liquid droplet catalyzes the decomposition of the
silicon-containing gas, and silicon is released. The released silicon
subsequently diffuses into the liquid droplet until it becomes satu-
rated with silicon. After saturation, excess silicon precipitates from
the liquid droplet. Using the VLS growth mechanism, large numbers
spanned the length of the nanowires. In this paper, we describe the
fabrication of cobalt silicide nanowire contacts to silicon nanowires
using the VLS technique in nanoporous alumina templates, where an
interface between the cobalt silicide and Si is formed along the cross
section of the wire.
1
9-21
of SiNWs with high aspect ratios have been synthesized.
How-
ever, not all of these efforts have produced long, thin, straight
nanowires.
Experimental
Figure 1 shows the fabrication scheme of the cobalt silicide
nanocontacts to the SiNWs. We used nanoporous anodized alumina
membranes with a nominal 200 nm diam as templates for nanowire
growth. A thin film of approximately 150 nm Ag was sputtered on
one side of the membrane to provide a conductive layer for elec-
trodeposition within the pores. To fully cover all the pores with Ag,
the membrane was subsequently placed in an electrochemical cell,
A new approach in which the SiNWs were grown on gold-
coated, mesa-shaped silicon projections was reported by Okajima
2
2
et al. in an effort to position the wires without fusing or kinking on
a Si wafer. This technique yielded wires over 500 ␮m long; how-
ever, the wire diameter was still large ͑ϳ20 ␮m͒. Westwater et al.
conducted a similar study aimed at controlling the size and position
of the SiNWs grown on a silicon wafer.2
3,24
They patterned an
2
and Ag was electrodeposited at 22.27 mA/cm for 1 min on the
oxide-coated silicon wafer using photolithography to create an array
of circular holes. Gold was then deposited on top of the silicon
within the holes, and VLS growth was conducted. The diameter and
the position of the wires were controlled by the diameter and the
position of the holes, which may be useful for studying individual
wires, but the yield of SiNWs is low.
conductive side of the membrane. To electrodeposit within the
pores, the membrane was flipped upside down and the plating solu-
tions were added from the open side of the membrane. Ag, Co, and
Au were then sequentially electroplated within the pores. The Ag
͑1.125 tr. oz/qt͒ and Au ͑0.25 tr. oz/qt͒ plating solutions were pur-
chased ready from Technic, Inc., while the Co bath was made by
mixing 500 g/L hydrated cobalt sulfate, 17 g/L sodium chloride, and
45 g/L boric acid at 25°C. The pH of the Co bath was then adjusted
to 5. Ag was electroplated inside the membrane at 7.32 mA/cm2
for 2 h to control the position of the Co and Au layers. Co was
Recently, a novel method combining VLS growth with an anod-
ized alumina nanoporous membrane as an initial template was
developed.2
5,26
In this method, the pores of the alumina membrane
30
are used to define the size of the wires. Consequently, precise con-
trol of the wire diameter can be achieved by growing within nan-
oporous templates. Using this method, a high yield of uniform
SiNWs can be synthesized. This approach also provides a controlled
method for contact formation.
2
electroplated at 89.09 mA/cm for 5 min. The Au was electroplated
2
at 2.22 mA/cm for various lengths of time to participate in the VLS
growth of the Si nanowires. The template was next placed in a
chemical vapor deposition ͑CVD͒ chamber, which was purged with
hydrogen at room temperature. The temperature of the CVD furnace
was then raised to 500°C and left at this temperature for about 20
min to reach thermal equilibrium. After that, a mixture of 5% silane
in hydrogen was allowed to flow over the template at 500°C, and Si
nanowires were grown by the VLS mechanism. At this temperature,
*
Electrochemical Society Student Member.
*
* Electrochemical Society Active Member.
z
E-mail: mohney@ems.psu.edu
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G578
Journal of The Electrochemical Society, 150 ͑9͒ G577-G580 ͑2003͒
Figure 1. Fabrication scheme of the cobalt silicide contacts to the SiNWs.
the Si portion also reacted with the Co portion of the nanowire to
form one or more of the cobalt silicide phases. After growth, the
nanowires were released from the membranes using 8.0 M nitric
acid to dissolve the Ag and the remaining metallic Co, and further
treatment with 1.0 M sodium hydroxide was used to remove the
alumina membrane. After several centrifugation and rinsing steps,
the nanowires were then stored in deionized water.
time of the growth of the SiNWs. Hence, only a short cobalt silicide
segment ͑ϳ0.5 ␮m͒ was obtained. We also found Au nanoparticles
near the silicide segment, which appear as bright nanoparticles in
Fig. 2. These Au particles along the SiNWs will likely affect the
The morphology and the chemical analysis of the different layers
within the nanowires were studied using a FEI-Philips XL20 scan-
ning electron microscope ͑SEM͒ and a Philips 420 transmission
electron microscope ͑TEM͒. Both microscopes have an energy dis-
persive X-ray spectrometer ͑EDXS͒, which was used to identify the
elements in segments of the nanowires.
Results and Discussion
Figure 2 shows a typical nanowire from our early attempts to
grow multilayered nanowires, with the segments of the wires labeled
according to EDXS analysis. Before Si growth, the lengths of the
Ag, Co, and Au portions within the membrane were 20 ␮m, 3.5 ␮m,
and 230 nm, respectively. Silane at a partial pressure of 0.65 Torr
was introduced along with the carrier gas H at a total pressure of
2
12.8 Torr at 500°C for 12 min. Gold catalyzed the decomposition of
SiH into silicon and hydrogen, and a ball of Au-Si liquid alloy was
4
formed. After the liquid alloy became saturated with silicon, silicon
precipitated at the interface between the liquid alloy and the remain-
ing electrodeposited metal. These are essentially the steps for the
VLS mechanism whether or not the SiNWs are grown within a
membrane. However, in the present study, when the silicon comes in
contact with cobalt at 500°C, another reaction takes place between
the silicon and cobalt to produce one or more cobalt silicides, as has
been observed in thin-film contacts to Si wafers.31
After cooling the CVD chamber to room temperature, the mem-
brane was dipped in 8.0 M HNO for 3 min to etch away silver and
3
the remaining elemental cobalt. Silicon, Au, and the cobalt silicide
phase are resistant to HNO3 and are not attacked. In this study,
consistent with a report on the rate of reaction in Co/Si diffusion
couples by Jan et al.,32 the kinetics of the silicidation reaction was
relatively slow, so as not to consume all the cobalt within the short
Figure 2. SEM micrograph of one of the released nanowires.
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Journal of The Electrochemical Society, 150 ͑9͒ G577-G580 ͑2003͒
G579
Figure 3. SEM micrographs of a single nanowire grown with different Au
thicknesses of ͑a͒ 80, ͑b͒ 160, ͑c͒ 200, and ͑d͒ 780 nm.
electrical performance of the wire; consequently, effort was focused
on this problem.
We addressed this problem by searching for an optimal Au thick-
ness for the nanowires to be grown without leaving any Au particles
along the wires. To determine the effect of Au thickness, membranes
with various Au thicknesses of 80, 160, 200, and 780 nm above the
Co segment were prepared and investigated after growing the
nanowires for the same period.
Figure 4. TEM micrograph of one of the wires that was synthesized with a
¯¯
Au thickness of 160 nm. The inset is a diffraction pattern near the ͓145͔
zone axis of the Si segment, acquired transverse to the axis of the wire.
With the Au thickness of 80 nm, the Au particles disappeared, as
shown in Fig. 3a for a nanowire with diam 152 nm. The silicon
nanowire diameter was smaller than the diameters of both the cobalt
plug and the alumina membrane pore, and as a result the wires did
not take the exact shape of the pores during growth. They were
curved and bent rather than straight. Thus, the shape of the SiNWs
was not desirable. A gold thickness of 160 nm was optimal in this
study. No Au particles appeared within the wires, as shown in Fig.
1
66 nm that we predict using the formula for a spherical bead for
our nanowires of average diam 249 nm. However, the prediction we
make assuming a hemispherical cap ͑83 nm͒ provides a Au thick-
ness very close to the 80 nm thickness that was insufficient in our
study.
However, as we see from Fig. 5, the Au bead at the end of the Si
nanowire is neither a hemisphere nor a sphere. More careful exami-
nation in the TEM of 13 nanowires was next performed. The diam-
eter of each nanowire was measured, and the volume of the Au bead
at the end of each nanowire was approximated from the TEM im-
ages. From these experiments, the starting thickness of the Au plug
was estimated to vary from 0.5-0.9 D. Note that the value 2/3 D falls
within this range.
3b. Furthermore, the structure of the wires was good. With Au thick-
nesses of 200 and 780 nm, some Au was retained within the SiNWs.
Not surprisingly, the amount of retained Au became greater as the
thickness of the electrodeposited Au increased, as shown in Fig. 3c
and d. However, the structure of the wires was good, and the silicon
nanowires, the cobalt plugs, and the pore diameters are nearly the
same.
Using the TEM, we found that the nanowires that were released
from the alumina membranes containing the optimal Au thickness
had silicon segments that were single crystals or crystalline with a
small number of defects. Figure 4 shows a TEM image for one of
the nanowires, and the inset is a diffraction pattern from the Si
segment. From examination in the TEM of 13 nanowires, an average
diam of 249 nm with a standard deviation of 32 nm was measured.
The wires have a greater diameter than the nominal pore diameter,
along with a spread in diameters, because the pores are not always
the exact same size as the nominal value stated by the manufacturer,
nor are they all the same size as each other. The SEM and TEM
studies also show a little roughness along some of the wires that
may be due to the use of NaOH to release the wires from the alu-
mina membranes. The silicon segment of the nanowires is expected
to be attacked by NaOH but with a much slower rate than alumina.
Several approaches were next taken to provide a formula to de-
scribe the optimal thickness ͑or length͒ of Au plug required for VLS
growth. We assumed that the liquid Au-Si alloy present during VLS
growth forms either a hemispherical or a spherical bead of the same
diameter as the growing Si nanowire, and that the volume of the
Au-Si alloy is close to that of the solid Au plug. Therefore, we
calculated that we would need Au plugs that are either 1/3 D or 2/3
D thick for hemispherical or spherical beads, respectively, where D
is the diameter of the nanowire. The thickness of the Au plug that
worked well in our experiments, 160 nm, is very near the value of
Figure 5. The Au tip at the end of a nanowire synthesized with a Au thick-
ness of 160 nm, as revealed by TEM.
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Journal of The Electrochemical Society, 150 ͑9͒ G577-G580 ͑2003͒
It is interesting that nothing has been reported in the literature
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