Electrical conductivity measurement of silicon wire prepared by CVD
Hiroshi Suzuki, Hiroshi Araki, Masahiro Tosa, Tetsuji Noda
*
National Institute for Materials Science, Materials Reliability Center, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
article info
Article history:
Received 11 October 2008
In final form 27 November 2008
Available online 6 December 2008
abstract
The electrical resistivity of a silicon nanowire formed from Si
2
H
6
by CVD was measured using micro-
probes equipped with SEM. The resistivity of 6.58 Â 10
5
X
cm at room temperature was obtained from
the current–voltage (I–V) curve for the wire with both ends fused to the probes. The non-linear I–V curve
measured only by contacting the wire with the probes could be explained by the resistivity in a series of
silicon and dielectric thin oxide films formed on the silicon nanowires.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Silicon nanowires (SiNWs) are potential materials for nanoscale
electronic and chemical devices [1–4] due to their quantum con-
finement effects and peculiar structures with a high aspect ratio
in nano size. Most SiNWs have been formed by bottom-up pro-
cesses, such as laser ablation [5], thermal evaporation [6–8], solid
reaction [9], and chemical vapor deposition (CVD) [10,11]. In each
case, SiNWs are synthesized by essentially utilizing a vapor–

Pa, the substrate was heated at
569 K. A 10% Si
2
H
6
gas diluted with H
2
and argon gas were then
introduced into the chamber. The disilane and argon gases were
run at constant rates of 0.0167 cc/s and 0.33 cc/s, respectively.
The total pressure was set at 0.667 kPa. The disilane gas was chem-
ically decomposed to form SiNWs on the substrate through form-
ing a liquid Au–Si eutectic, followed by silicon nanowire growth
underneath the liquid eutectic droplet [12]. After the deposition
of SiNWs for 1200 s, the structure of SiNWs was examined with
a scanning electron microscope (JEOL JSM-6700F) equipped with
an energy dispersive X-ray spectrometer (EDS) and high-resolution
transmission electron microscope (JEOL JEM-3000F TEM) with a
Gatan GIF Tridiem energy-filter system. Plasmon-loss images were
taken using the Gatan GIF system.
2.2. Electrical resistance measurements
The SiNWs formed on the substrate were manipulated with a
Zyvex S100 system installed in LEO 1550 FE-SEM. The manipula-
tion system consisted of four tungsten probes. One nanowire was
picked up with the probes, and the current–voltage relations were
measured at room temperature using 4-terminal and 2-terminal
methods.
3. Results
3.1. Silicon nanowires
Fig. 1 shows the morphology of SiNWs formed on a silicon wa-

for the SiNW shown in Fig. 2; (c) and (d) are the intensity profiles of the boxed areas in (a) and (b), respectively.
212 H. Suzuki et al. / Chemical Physics Letters 468 (2009) 211–215
SiNWs [2,5,7,15] and after exposure to air [13] when the samples
were taken out of the chamber.
The chemical state of the SiNWs was examined using plasmon-
loss images. According to the electron energy loss spectra (EELS), a
peak at 16.7 eV corresponding to the first plasmon-loss peak of Si
appears for silicon crystal [16]. The peak for the first plasmon-loss
of SiO
2
is 22.4 eV [16]. Then, the plasmon-loss images were ob-
tained by placing the slit at 16.7 ± 2.5 eV and 22.4 ± 2.5 eV for the
SiNW shown in Fig. 2.
The Si and SiO
2
plasmon-loss images for the nanowire are
shown in Fig. 3a and b, respectively. The corresponding intensity
profiles from the boxed areas perpendicular to the wire axis, as
seen in Fig. 3a and b, were indicated in Fig. 3c and d, respectively.
The full-width at half-maximum (FWHM) value of SiO
2
is 3.5 nm
wider than that of Si. Consequently, the SiNW of around 22 nm
in diameter in Fig. 2 was estimated to consist of the core silicon
crystal of 19 nm covered with silicon oxide of around 1.8 nm in
thickness. Other SiNWs with different diameters were also exam-
ined by TEM. Most SiNWs were found to be covered with oxide
film with a thickness ranging from around 1 nm to several tens nm.
3.2. Electrical resistance measurements
One SiNW with length longer than 100

5
X
cm at 30–38 V.
The result of Fig. 5 seems to be caused by the effect of surface
oxide surrounding the silicon core. Then, the applied current in-
creased, resulting in the melting of the end of the SiNW contacting
the tungsten probe. In Fig. 4b, the features of the contacts of the
fused SiNW on the two tungsten probes are shown. It is evident
that the end of the SiNW contacting the probe was melted and cov-
ered the probe. According to the binary phase diagram for Si–W
system, W
2
Si phase appears at temperatures lower than the melt-
ing point of silicon [17].This means that the tungsten silicide could
be formed at the interface between the fused silicon wire and the
tungsten tip. The silicon wire therefore firmly adhered to the tung-
sten probe and the contact resistivity between the silicon wire and
the tungsten tip is considered to be negligibly small. The length of
the SiNW between the two probes was 38.6
l
m. The I–V relation
was then measured. The linear relation was obtained as shown in
Fig. 6. The slope gives a resistivity of 6.58 Â 10
5
X
cm for the SiNW
of 850 nm in diameter and 38.6
l
m in length. The present result is
in good agreement with an intrinsic resistivity in the order of

ity obtained from Fig. 6 was 6.58 Â 10
5
X
cm, which corresponds
to the resistivity of silicon crystal with a purity better than 11 N.
In Fig. 5, the resistivity decreases with increasing voltage and came
close to the value of that of Fig. 6. The value of 6.9 Â 10
5
X
cm at
30–38 V was described in the previous section. The surface silicon
oxide is essentially an insulator and has ionic conductive property.
By applying voltage, the ionic current gradually increases with
increasing the electric field, and finally, a dielectric breakdown
phenomenon occurs at high electric fields.
The ionic current, I, through a dielectric material is given as Eq.
(1) [20]:
Fig. 4. SEM micrographs of a SiNW contacted with 4 tungsten probes (a), and the
same SiNW of which both ends were fused to the probes (b). The electrical
measurements were conducted with the 4-terminal method (a) and the 2-terminal
method (b).
H. Suzuki et al. / Chemical Physics Letters 468 (2009) 211–215
213
I ¼ K sinhðk
q
E=2kTÞ; ð1Þ
where k is the ionic jump distance, q, the charge, E, the electric field,
k, the Boltzmann constant, and T, the temperature. K is a constant at
the measured temperature.
Fig. 7 is a schematic drawing of the electrical measurement of

and V
2
are applied voltages to the silicon wire and the surface
oxide layer, respectively.
Assuming T = 298 K, k = 0.491 nm of the lattice parameter for
the (1 0 0) plane of quartz [21], q = 3.2 Â 10
À19
C for the oxygen
ion, and the average thickness of oxide of 12.5 nm as a result of
the observation of the fractured surface of the SiNW described in
the previous section, the I–V relationship can be obtained by giving
proper K and V
2
values. In the present case, the ratio of V
2
/V
T
, r, was
assumed to be constant independently of V
T
.
Fig. 8 is the result of curve fitting using Eq. (3), where the K va-
lue and r were assumed to be 1.1 Â 10
À11
A and 0.055. A fairly good
coincidence between the experimental and simulation results is
observed. The electric field, E, estimated from the r value, was
changed from 4 kV/mm at VT of 1 V and 44 kV/mm at 15 V. The
reported breakdown electrical field for silicon oxide film is around
40 kV/mm [22]. As described in the previous section, the resistivity

contact points of the SiNW were fused to the tungsten probes.
The following conclusions were derived.
(1) The prepared SiNWs were single crystalline and grow paral-
lel to the h110i direction. The SiNWs were covered with thin
silicon oxide film.
(2) The I–V curve for the SiNW of 850 nm in diameter measured
with the 4-terminal method using micro probes showed a
non-linear relationship that might have been caused by
the effect of surface silicon oxide on the SiNWs.
(3) The 2-terminal measurement, in which both ends of a SiNW
were fused to the probes, indicated a linear I–V curve giving
the resistivity of 6.58 Â 10
5
X
cm. This value corresponds to
that of bulk silicon with purity better than 11 N.
(4) The I–V curve obtained with the 4-terminal method could be
explained by the resistivity in a series of silicon and thin
oxide film.
Acknowledgements
The authors are grateful to Zyvex Corp. for their cooperation in
the measurement of the electrical conductivity and Dr. K. Mitsuishi
for the TEM analyses.
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