World Journal Of Engineering
Formation of InAs Nanowire by Solid Source Reaction: A promising material as the
high mobility channel for nanoelectronics
Yu-Lun Chuch§,*, Alexandra C. Ford†, and Ali Javey†
Abstract- InAs nanowires have been actively explored as the
channel material for high performance transistors owing to
their high electron mobility and ease of ohmic metal
contact formation. In present study, the InAs NWs are
simply synthesized by using solid source reaction
approaches. The nanowires show superb electrical
properties with electron mobility >3000 cm2/Vs and
ION/IOFF ~104. The detailed current-voltage (I-V) for
individual InAs NWs with different radius at different
temperatures is reported, enabling the direct assessment of
electron mobility as a function of NW diameter. The
fundamental kinetics of the Ni/InAs alloying reaction
is explored with the Ni diffusion reported as the rate
determining step with diffusion activation energy of
~1 eV/atom. The uniformity of the grown InAs nanowires
is further demonstrated by large-scale assembly of parallel
arrays of nanowires on substrates for high performance.
BACKGROUND1
Semiconductor nanowires, especially for Si1 and Ge2
nanowires, have been spent much attention on acquiring the
best performance. However, low electron mobility of Si limits
the device performance. Recently, by using the bandgap lineup concept, Ge/Si core-shell, which confines electrons
transported along the axial direction, opens a route of realizing
the highest device performance for Si-based nanowire.3 III-V
compound semiconductor nanowires are the best candidate
materials for high speed and low power devices due to the
extremely high electron mobility.4 InAs nanowires have been
reported to have promising high speed nanodevices application
due to (1) low effective mass and small bandgap, resulting in
the high electron mobility and (2) high electron gas
accumulated at the surface of NW owing to Fermi level
pinning above the conduction band after contacting with metal
electrode.5 In present study, InAs NWs are simply synthesized
by using solid source reaction approaches. The detailed
microstructure and I-V behavior will be analyzed. Of particular
interest is the dependence of the carrier mobility on InAs NW
radius, especially since smaller NWs are highly attractive for
§
Department of Materials Science and Engineering, National Tsing Hua
University, Hsinchu, Taiwan 300, ROC .
†
Department of Electrical Engineering and computer Sciences, University of
California at Berkeley, Berkeley, CA, 9720, USA.
*contacting
author,
ylchueh@mx.nthu.edu.tw).
phone:+886-3-5715131ext33965;E-mail:
the channel material of nanoscale transistors as they enable
improved electrostatics and lower leakage currents. Therefore,
the detailed current-voltage (I-V) for individual InAs NWs with
different radius at different temperatures is reported, enabling
the direct assessment of electron mobility as a function of NW
diameter. On the other hand, one of the major challenges
associated with nanowire devices, and all nanoscale devices in
general, is the development of nanoscale and ohmic
Source/Drain (S/D) contacts. To address this challenge,
developing and characterizing metal/InAs alloys with low
resistivity and abrupt interfaces as the contact material to InAs
is of major interest. Here, we report the formation and
materials properties of NixInAs/InAs/NixInAs heterojuction
and NixInAs NWs by using a simple solid source reaction of Ni
with InAs NWs at annealing temperatures of 220-300 ℃ in
different ambient conditions. The diffusion kinetics of Ni
atoms inside InAs NWs and the electrical properties of the
fabricated devices with ohmic NixInAs contacts are
investigated in detail.
MATHERIALS AND METHODS
The growth furnace consisted of two independently controlled
temperature zones, one for the solid source and the other for
the sample. Ni nanoclusters used for InAs nanowire growth
were obtained by thermal annealing (800-900ºC) of thin Ni
films (thermally evaporated) on Si/SiO2 (50 nm thermally
grown) substrates in a hydrogen environment. After the
thermal annealing process, the sample temperature was
decreased to 470-550 ºC, and InAs NWs were grown for ~1 hr
by vaporization of InAs solid source (source temperature
720ºC. The pressure was maintained constant at ~1 torr. InAs
NWs were harvested in an ethanol solution by a gentle
sonication process, and drop-casted on a p+Si/SiO2 (~50 nm,
thermally grown) substrate. Metal source/drain (S/D) contacts
were then defined by photolithography, Ni evaporation (~50
nm thick), and lift-off. To ensure an ohmic contact formation, a
5 sec HF etch (~0.1%) was applied immediately prior to the Ni
contact evaporation to remove the native oxide on the exposed
NW surfaces. The surface morphology was examined by a
field-emission scanning electron microscope (JSM-6500F),
operated at 15 kV. In order to prepare the TEM specimen, all
samples were sonicated in ethanol and then dispersed in copper
grid supported by a holy carbon film. A field-emission
transmission electron microscope (JEM-2100 F), operated at
200 kV, with a point-to-point resolution of 0.17 nm and
equipped with an energy dispersion spectrometer (EDS) was
used to characterize the microstructures and chemical
compositions.
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World Journal Of Engineering
RESULTS
dNP ~ 10 nm
(from 1.5 nm Ni film)
(d)
(c)
dNP ~ 27 nm
dNW ~ 38 nm
(from 3 nm Ni film)
(e)
(f)
Figure 1 (a) AFM images and NP diameter distribution histograms for Ni
particles resulting from the thermal anneal of (a) ~0.5 nm, (c) ~1.5 nm, and (e)
~3 nm Ni films at 850 ºC for 10 min. All AFM images show an area of 1 μm x
1 μm. (b) SEM images and nanowire diameter distribution histograms for InAs
nanowires grown using Ni catalyst particles produced by the thermal anneal of
(b) 0.5 nm, (d) 1.5 nm, and (f) 3 nm Ni films. SEM image insets clearly show
the Ni catalyst tips at the ends of the nanowires, depicting the tip-based growth
mechanism.
(a)
(b)
20000
2
~422 (cm /Vs)/nm
7000
Mobility (cm /Vs)
6000
5000
2
2
8000
4000
3000
2000
1000
T =298 K
T =50 K
16000
12000
8000
4000
0
0
5
10
15
Radius (nm)
20
5
10
15
Radius (nm)
20
Figure 2 (a) Mobility as a function of radius for more than 50 different devices
with NWs ranging from 7-18 nm in radius post oxide subtraction. Over this
NW radius range, the mobility linearly increases with radius, closely fitting the
linear expression µn=422r-1180. (b) Temperature dependent electron transport
properties for four NWs of different radius at temperatures of 50 and 298 K.
(a)
NiXInAs InAs
250 nm
(b) 10
-5
10
-6
10
-7
10
-8
10
-9
10
(c) Ni
NiXInAs
L: 280 nm
D: 30 nm
16
12 VDS=0.3 V
8
4
0
-4 -2 0 2 4
VGS (V)
X Axis Title
-4
-2
0
VGS(V)
2
Ni
200 nm
Vd=0.3 V
Vd=0.1 V
Vd=10 mV
-10
InAs
4
(d) 10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
-6
L: 400 nm
D: 36. 5 nm
Vd=0.3 V
Vd=0.1 V
Vd=10 mV
12
8
4
0
IDS (μA)
[5]
dNW ~ 26 nm
IDS(A)
[4]
(b)
dNP ~ 14 nm
IDS (A)
[3]
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D. Wang, Q. Wang, A. Javey, R. Tu, H. Dai, H. Kim, P. C. Mclntyre, T.
Krishnamohan, K. Saraswat, C. Appl. Phys. Lett. 2003, 83, 2432.
G. Liang, J. Xiang, N. Kharche, G. Klimeck, C. M. Lieber, M.
Lundstrom, Nano Lett. 2007, 7, 642.
Y. Li, J. Xang, F. Qian, S. Gradecak, Y. Wu, H. Yan, D. Blom, C. M.
Lieber, Nano Lett, 2006, 6, 1468.
T. Beyllert, L. E. Wernersson, , L. E. Froberg, L. Samuelson, IEEE
Electr Device 2006, 72, 323.
(a)
IDS (μA)
REFERENCES
[1]
[2]
dNW ~ 23 nm
(from 0.5 nm Ni film)
Field-Effect Mobility (cm /Vs)
Direct annealing Ni film with controllable thickness is a
remarkably effective approach to control the diameter of Ni
NPs with discrepancy below ~10 % for the growth of InAs
NWs. Figure 1 shows the atomic force microcopy (AFM)
images and corresponding particle diameter distributions of Ni
particles formed by the thermal annealing of 0.5 nm (Fig. 1a),
1.5 nm (Fig. 1c), and 3 nm (Fig. 1e) Ni films at 850 ºC for 10
min. The NP diameters obtained from the 0.5, 1.5, and 3 nm
films are 10±2, 14±3, and 26±5 nm, respectively. After InAs
growth, Ni NPs can be clearly observed at the tip of most
nanowires (Fig. 1), which is a distinct characteristic of the tipbased, VLS/VSS growth mechanism. The NW diameter shows
a direct correlation with the catalytic NP diameter. Nanowire
diameters of 23±6, 26±8, and 38±9 nm were obtained for 10,
14, and 26 nm NPs, respectively. The variation for the grown
nanowires is 25-32% which is slightly larger than that of the
NP distribution. Figure 2a illustrates the peak mobility as a
function of InAs NW radius for more than 50 different FETs
with r=7-18 nm. Over this NW radius range, the mobility
linearly increases with radius with a slope of ~422
(cm2/Vs)/nm. In an effort to shed light on the source of
mobility degradation for smaller NWs, temperature-dependent
electron transport measurements were conducted, as shown in
Figure 2b, indicating a linear enhancement of the peak electron
mobility from ~6,000 to 16,000 cm2/Vs as the temperature is
reduced from 298 K to 200 K. Below ~200K, minimal change
in the mobility is observed, which is attributed to the surface
roughness scattering caused by acoustic phonon and/or
surface/interface trap states. Figure 3 shows how the device
improvement as the bulk Ni contacts (fabricated by e-beam
lithography) replaced by NixInAs NW contacts. Data clearly
illustrates the advantage of using nanoscale contacts for
improved electrostatics.
Vd=0.3 V
-4 -2 0 2 4 6
VGS (V)
X Axis Title
-3
0
VGS (V)
3
6
X Axis Title
Figure 3. (a) SEM image of a short channel, back-gated FET formed by using
the InAs metallization approach, with nanoscale NixInAs contacts. The channel
length is ~280 nm. (b) The corresponding IDS-VGS behaviors at VDS=10 mV, 0.l
V and 0.3 V. Inset shows the linear scale IDS-VGS at VDS=0.3V. (c) SEM image
of a short channel, back-gated FET fabricated by electron-beam lithography,
with bulk Ni contacts. The channel length is ~400 nm. (d) The corresponding
IDS-VGS behaviors at VDS=10 mV, 0.1 V, and 0.3 V. Inset shows the linear scale
of IDS-VGS at VDS=0.3V.
210