先進材料原子尺度結構及動力學研究
Ni-doped TaSi2 Nanowires: Emitter, Interconnect and/or Contact for Future
Nanosystems
計劃編號：甲-91-E-FA04-1-4
執行期限:94 年 4 月 1 日至 95 年 3 月 31 日
周立人教授
TaSi2 nanowires have been synthesized on Si substrate by annealing NiSi2 films at 950 ℃ in an
ambient containing Ta vapor. The TaSi2 nanowires would grow in length up to 13 μm. A vapor-solid
growth mechanism is proposed. Ni content in TaSi2 nanowires was found to increase with annealing
time and eventually reach a saturation value. Field-emission measurements show that the turn-on field
is low at 4.5 V/μm and the threshold field is down to 5.5 V/μm with the field enhancement factor as
high as 1700. The metallic TaSi2 nanowires exhibit excellent electrical properties with remarkable high
failure current density of 3×108 Acm-2. This simple approach promises future applications in
nano-electronics and nano-optoelectronics
Introduction
One-dimensional (1D) systems such as
nanowires, nanorods, nanobelts and nanotubes
have attracted much attention owing to their
unique optical, electronic, and mechanical
properties.1,2 The cathode materials, such as
carbon-based materials, carbon nanotubes
(CNTs), diamond films, and sp2-sp3 hybridized
carbon materials, can be applied in
field-emission, mainly due to their metallic
characteristic and high thermal stability.1-3 On
the other hand, the requirements for interconnect
and contact
in the next generation
nanoelectronics are low resistivity, good ohmic
contact to both p- and n-type semiconductor,
high temperature stability, low cost and
compatibility with the processing of Si
complementary
metal-oxide-semiconductor
(CMOS) devices.4 Refractory metal silicides are
a group of silicides that possess satisfactory
properties and may be used in nanoelectronics.
Nanowires are the building blocks for any
nanoelectronic devices. A number of silicides
have been grown by self-assembly. The
challenges for self-assembled silicide NWs are
control of aspect ratio and location. In addition,
the self-assembly of nanowires usually requires
that the substrate be crystalline precluding their
use for many potential applications.4,
Only a few alternative approaches have been
adopted to grow nanowires without relying on
the mismatch between the nanowires and the
substrate. For the growth of NiSi nanowires, Ni
film was deposited on Si nanowires via the
chemical vapor deposition (CVD) process to
form the NiSi nanowires after annealing.5 Others
included preparing the carbon-coated nickel
silicide nanowires (C-coated NiSi NWs) via a
radio-frequency-induction heating chemical
vapor deposition (RF-CVD) reactor.6 Nickel
silicide nanowires were also grown on Ni
surfaces by decomposition of silane at 320-420
o
C. Depending the growth conditions,
single-phase Ni2Si, Ni3Si2 and NiSi nanowires
were formed. It has been demonstrated that
directed growth of silicide nanowires can be
achieved with the aid of applied electric field.7
Xiang et al. used a vapor-phase deposition
method to grow TiSi2 nanowires on silicon
wafers. Field emission and cathodoluminescence
measurements reveal the potential applications
in vacuum microelectronics.8 TaSi2 nanowires
have been synthesized by annealing FeSi2 thin
film and nanodots grown on Si substrate in an
ambient containing Ta vapor. Strong field
emission properties promise future electronics
and optoelectronics applications.9
In the present study, we utilized an
innovative method to synthesize the Ni-doped
TaSi2 nanowires by annealing NiSi2 films on Si
substrate in an ambient containing Ta vapor in a
vacuum better than 1×10-6 Torr. The strong
field-emission and excellent electrical transport
behaviors demonstrate that a promising metallic
nanowire can be used in a field-emission device
or as an interconnect and/or contact in
nanoelectronic circuit.
Experimental Procedures
Single crystal (001) Si wafers (1-30 Ω-cm)
were cleaned by the standard cleaning process.
30-nm-thick Ni film was deposited on Si
substrate by an ultra-high vacuum e-beam
deposition system at room temperature. The
as-deposited samples were annealed at 850 ℃
for 30 min without breaking the vacuum
chamber to form the NiSi2 thin film on the Si
1
substrate. As-annealed samples were transferred
into a Ta filament heating chamber for annealing
at a pressure of lower than 1×10-6 Torr at
850-950 ℃ for different time. The Ta atoms were
vaporized constantly as the supplementary
source for the growth of nanowires [Figure 1S].
The grazing incidence X-ray diffractometry
(GIXRD) with a fixed incident angle of 0.5° was
carried out to identify the phases. Field-emission
transmission electron microscope (JEM-3000F,
operated at 300 kV with point-to-point
resolution of 0.17 nm) equipped with an energy
dispersion spectrometer (EDS) and a high angle
annular dark-field (HAADF) detector were used
to obtain the information of the microstructure
and the chemical composition. The surface
morphology was examined with a field-emission
scanning electron microscope (JSM-6500F,
operating at 15 kV). The electron field-emission
property was measured in a vacuum at a pressure
of 1×10-7 Torr using a spherical stainless-steel
probe (1mm in diameter) as the anode. The
lowest emission current was recorded on the
level of nA. The measurement distance between
the anode and emitting surface was fixed at 100
μm. Electrical measurements were performed by
sequential procedures including electron-beam
lithographical
defined
electrodes,
metal
evaporation, and device evaluation. The 30 keV
cold field emission scanning electron
microscope (SEM FEI-SIRION) with nano
pattern generation system (NPGS) was utilized
for these purposes. A LabView program was
used to control the I-V testing process.
card, No-38-0483). It indicates that the Si atoms
have been vaporized or migrated out of the Si
substrate to react with Ta to form TaSi2 during
the growth of the nanowires. In contrast, TaSi2
nanowires synthesized by annealing FeSi2 film
in the same conditions, the lengths are about
100-200 nm, about 1 to 2 order of magnitude
shorter than the present instance [Figure 2S]. 9
On the other hand, the diameters are about 20-40
nm, similar to the NiSi2-catalyzed growth.
Results and Conclusion
Figure 1(a) shows the SEM image of
nanowires synthesized by annealing the NiSi2
thin film on Si substrate at 950 ℃ for 16 h in a
Ta ambient. The diameters of the nanowires are
about 20-30 nm. The nanowires were found to
grow to 4-6 μm in length and the aspect-ratios
are estimated to be about 100 to 300. The
nanowires can grow to over 13 μm in length
with an aspect-ratio of about 650 when the
annealing time was extended to 32 h, as marked
by the white arrows in Fig. 1 (b). The inset
shows the corresponding side-view SEM image,
revealing that the average length of the
nanowires is about 7 μm after 32 h annealing.
Pinholes are seen elsewhere. The nanowires are
grown uniformly on the Si substrate and the tip
regions are semi-spherical in shape without the
presence of metal-catalyst. The GIXRD
spectrum, as shown in Fig. 1(c), reveals that the
nanowires are TaSi2. The TaSi2 is hexagonal in
structure [P222 (180)point group] with a lattice
constant of a = 0.48 nm and c = 0.66 nm (JCPSD
Fig 1. (a) Top-view SEM images of nanowires synthesized
by annealing the NiSi2 film on Si substrate at 950 ℃ for (a)
16 h and (b) 32 h in a Ta ambient. The dark contrast regions
correspond to the pinholes. Upper insets show the
corresponding side-view SEM images. (c) The GIXRD
spectrum corresponding to that of (a) shows the presence of
TaSi2 phase. and Ni. Upper inset shows the corresponding
EDS spectrum
Figure 2(a) shows TEM image of two TaSi 2
nanowires with diameters of 20 nm and 25 nm.
The tips are semi-spherical in shape without the
trace of metal catalyst. The corresponding
selected area diffraction (SAD), as shown in Fig.
2
2 (b), again confirms that the phase of nanowires
is TaSi2. Point defects marked by circles, as
shown in Fig. 2 (c), are induced with the
presence of Ni atoms, since thedefects are not
present in the TaSi2 nanowires catalyzed by the
FeSi2.9 Figure 2 (d) shows the high annular angle
dark-field
(HAADF)
image
and
the
corresponding EDS line-scan profiles. The bright
image is surrounded by the dark thin layer which
standard NiO sample and Ni atoms in TaSi2
nanowires is observed in the EELS spectrums.
Nevertheless, it may suggest that the Ni atom in
TaSi2 nanowries is of nature type (Ni0) by
determining the intensity ratio of L3/L2 edges,
which is smaller than that of NiO standard
sample.10
For I-V measurement, Cr was selected as the
contact metal with a work function of 4.4 eV,
which is smaller than that of TaSi2 (4.7 eV). It
provides a better Ohmic contact compared to the
other metals, such as Au (~5.1 eV) or Pt (~5.7
eV). In the present study, the four-probe
measurements were performed but failed, owing
to the high contact resistance between electrode
1 and the nanowire, as shown in Fig. 4(a).
Instead, the resistance of 25 nm TaSi2 nanowire
was obtained to be 2.47 kΩ at 300 K by
three-probe measurements (2-2-3-4) minus the
contact resistance of electrode 2, as shown in Fig.
4(b).11 The linear I-V behavior at 300 K shows
that electrical characteristic of TaSi2 nanowire is
metallic with low resistivity at about 114 μΩ-cm,
although it is 2 times higher than that of its
bulk.12 The higher measured resistivity is
attributed to the presence of doping Ni atoms in
the nanowires, oxide coating layer and
contaminations around the surface and contact
regions which were unavoidably generated
Fig. 2 (a) TEM image of nanowires synthesized by annealing
NiSi2 film at 950 ℃ for 16 h in a Ta ambient. (b) The
corresponding diffraction pattern revealing the nanowire is of
TaSi2 phase with [1233] zone axis. (c) High-resolution
TEM image of TaSi2 nanowire indicating the growth
direction is along [2110] . The defect structures are evident,
as marked by circles. (d) The corresponding EDS elemental
line-profiles show that the nanowire is composed of Ta, Si
correspond to the TaSi2 nanowire and the surrounding
1-nm-thick amorphous oxide layer. The EDS
line-profiles indicate that the nanowire consist of
Ta, Si and Ni with the atomic concentrations of
Ta = 32 %, Si = 64 %, and Ni = 4 %. The inset
shows the corresponding EDS spectrum.
Fig. 4 (a) SEM image of the I-V measurement configuration,
(b) I-V curve at 300 K. The resistance was estimated to be
about 2.47 kΩ. (c) I-V curve at 70 K. The resistance was
estimated to be about 2.29 kΩ. (d) The resistivity as a
function of temperature.
Fig. 3 EELS spectra of Ni atoms in TaSi2 nanowires after
background subtraction.
during the device processing. In addition, defects
and anisotropy in electron transport (along
[2110] growth direction) in TaSi2 nanowires may
also increase the measured resistivity in the case.
From the previous reports, the elastic scatting
mean free path of the TaSi2 nanowire can be
calculated and estimated at about 8 nm at 300
K,13 indicating that the TaSi2 nanowire retains
the attractive metallic transport property in
The EELS spectra of L2,3 edge for the Ni
atoms in TaSi2 nanowires and standard NiO
sample are shown in Fig. 3. In general, Ni may
possess two valent states (Ni0, and Ni+) in
different conditions. L3 and L2 edge peaks were
found at positions of 855 and 873 eV for TaSi2
nanowires. No chemical shift in L3 edge between
3
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nanosize. It suggests that TaSi2 nanowire has
great potential to be used as contact and
interconnect for future nano-electronics. As the
measured temperature was reduced to 70 K, the
I-V characteristic still kept the linear behavior as
well [Fig. 4 (c)]. The resistivity as a function of
temperature was estimated, as shown in Fig. 4
(d). The resistivity was decreased with the
temperature and saturated at the temperature
below 30 K. The residual resistivity was found at
about 107 μΩ-cm, which is also higher than that
of its bulk.13 Again, the oxide coating layer may
be the detrimental factor. The negative curvature
(d2ρ/dT2 < 0) found in Fig. 4 (d) was similar to
bulk behavior, which suggest that the
electron-phonon scatting is dominant for the
transport mechanism.14 The results of durability
and reliability test of the TaSi2 nanowire were
obtained by performing I-V measurements under
very high applied current and voltage. The
nanowire can endure a current of up to 2.2 mA
under high voltage stress before failure. Note
that the high current density of 3×108 Acm-2 was
estimated in this case. The high failure current
density is an important feature of TaSi2 nanowire,
essential if used as an interconnect in future
nanodevices and nanosystems. 15-17
In summary, Ni-doped TaSi2 nanowires have
been synthesized successfully by annealing
NiSi2 films at 950 ℃ in a Ta ambient. The Ni
content in TaSi2 nanowires was found to increase
with annealing time and eventually reach a
saturation value. The growth mechanism of
TaSi2 nanowires is suggested to be VS growth.
Detailed microstructures and compositions
analysis of these unique TaSi2 nanowires are
presented. Field-emission measurements show
that the turn-on field is low at 4.5 V/μm and the
threshold field is down to 5.5 V/μm, the field
enhancement factor is as high as 1700. The
electrical transport properties show that the
metallic TaSi2 nanowires can endure a current of
up to 2.2 mA with the calculated current density
of 3×108 Acm-2.
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