UC Irvine
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Title
ZnO nanowire field-effect transistor and oxygen sensing property
Permalink
https://escholarship.org/uc/item/3qk17435
Journal
Applied Physics Letters, 85(24)
ISSN
0003-6951
Authors
Fan, Z Y
Wang, D W
Chang, P C
et al.
Publication Date
2004-12-01
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
Fan et. al., Appied Physics Letters, 85, 5932 (2004).
ZnO nanowire field Effect transistor and oxygen sensing
property
Zhiyong Fan, Dawei Wang, Pai-Chun Chang, Wei-Yu Tseng, Jia G. Lu
*
Department of Chemical Engineering and Materials Science &Department of
Electrical Engineering and Computer Science
University of California, Irvine, CA 92697
ABSTRACT
Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor
deposition method and configured as field-effect transistors. Electrical transport studies
show n-type behavior with a carrier concentration of ~10
7
cm
-1
and an electron mobility
of ~17 cm
2
/V s. The contact Schottky barrier between the Au/Ni electrode and nanowire
is determined from the temperature dependence of the conductance. Thermionic emission
is found to dominate the transport mechanism. The effect of oxygen adsorption on
electron transport through the nanowires is investigated. The sensitivity to oxygen is
demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection
sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds
high potential for nanoscale sensing applications.
*
correspondence author: jglu@uci.edu
1
Fan et. al., Appied Physics Letters, 85, 5932 (2004).
Quasi-one-dimensional ZnO nanostructures, such as nanowires, nanobelts, and
nanoneedles, are attracting tremendous research interests.
1–3
As a II–VI compound
semiconductor with a wide band gap (3.37 eV), ZnO nanowires (NWs) are emerging as
candidates for nanoscale ultraviolet (UV) lasers, light-emitting diodes, photodetectors,
and chemical sensors.
4-7
Although investigations of electrical transport through individual
ZnO NWs were recently reported,
8,9
carrier concentrations and mobilities in such NWs
have not been explored in detail. In our work, ZnO NWs were synthesized via a vapor
trapping chemical vapor deposition (CVD) method.
10
Individual NWs were then
configured as field-effect transistors (FETs), and the electron concentration and mobility
were determined. Furthermore, the effects of oxygen adsorption on the NW electrical
behavior were investigated and the oxygen sensing characteristics were quantified. The
results show that ZnO NWs can serve as potential building blocks for nanoscale
electronic and sensing devices.
ZnO NWs have been synthesized by other groups using various methods, such as
CVD, physical deposition, and electrodeposition.
11–13
In our work, we have modified the
CVD synthesis approach with a vapor trapping method. Details of the synthesis are
described in Ref. 10. Field emission scanning electron microscopy (FE-SEM) images
[Fig. 1(a)] show that the synthesized ZnO NWs are uniformly distributed in the area
seeded with gold nanoparticles. NWs were formed with an average diameter of 60 nm,
and lengths up to several tens of microns. Most of the NWs were observed to terminate
with gold nanoparticles [see inset of Fig. 1(a)] suggesting that the synthesis mechanism is
mainly the well-documented nanoparticle catalytic vapor–liquid–solid mechanism.
4,11,14
High-resolution transmission electron microscopy (HRTEM) images and electron
diffraction patterns [see Fig. 1(b)] confirm that the ZnO NWs are single-crystalline. The
2
Fan et. al., Appied Physics Letters, 85, 5932 (2004).
distance between the adjacent lattice planes is 0.506 nm, in good agreement with the c-
axis lattice constant of hexagonal ZnO (c=0.5195 nm). This finding suggests that the
synthesized ZnO NWs are grown along the [001] direction.
To fabricate FETs, ZnO NWs were first removed from the substrate by sonicating the
Si chip in isopropyl alcohol. The resulting NW suspension was then deposited onto
another silicon substrate - a degenerately doped p-type substrate capped with a 500 nm
oxide layer. Photolithographic masking techniques were utilized to define a square array
of 100 µm
2
areas with 3–5 µm distance between neighboring squares. An array of 100
µm
2
electrodes was formed by a bilayer evaporation of 10 nm of Ni followed by 100 nm
of Au. Individual NWs with good contacts on both ends were located using a high
magnification optical microscope. ZnO NW FETs were thus obtained, with metal
contacts functioning as source and drain electrodes and with the Si substrate as a back
gate. The inset of Fig. 2(a) shows a scanning electron microscopy (SEM) image of such a
ZnO NW FET. Note that the channel length between the electrodes is 7.0 µm. Figure 2(a)
displays seven current vs drain–source bias (I
-
V
ds
) curves obtained under different gate
voltages (V
g
) varying from −6 V to 6 V. The conductance obtained from the linear region
of I
-
V
ds
curves, in the drain–source bias range of ±100 mV, increased from 5.7×10
−9
S at
V
g
= −6 V to 2.12×10
−7
S at V
g
= +6 V. This behavior shows that the ZnO NW FET is an
n-channel device. The I
-
V
ds
curves are not symmetric and saturation regions exist at
positive values of drain–source bias. When the source is grounded and the drain is
positively biased, electrons will be locally depleted near the drain, giving rise to a pinch-
off effect restricting the drain–source current and the observed saturation. As shown in
Fig. 2(b), transfer characteristics of this NW FET were obtained under different biases
3
Fan et. al., Appied Physics Letters, 85, 5932 (2004).
varying from 100 mV to 25 mV, and an on/off ratio for this device at 100 mV bias
exceeds 10
4
(comparing V
g
=15 V and −10 V). According to Ref. 15, the charge carrier
concentration, n, for a quasi-one-dimensional system is expressed as
))/2ln(/2()/(
0
rheVn
gt
π
εε
×=
, where V
gt
is the gate threshold voltage obtained from
transconductance, ε=3.9, h is the gate oxide layer thickness, and r is the nanowire radius.
Using h =500 nm, r=60 nm, and a V
gt
value obtained from Fig. 2(b), the charge carrier
concentration was estimated to be 4.0 ×10
7
cm
−1
. In addition, an electron mobility of
e
µ
=17.2 cm
2
/V s can be derived using
))/2ln(/2/()/(
0
rhLVdVdI
dsge
π
ε
ε
µ
=
, where the
transconductance dI/dV
g
=1.9×10
−9
A/V was obtained from the linear region (−5V to 0V)
of Fig. 2(b) (at V
ds
=100 mV) and L =7.0 µm is the NW channel length. It is known that
native defects of zinc interstitials contribute to the n-type semiconducting behavior of
ZnO, and these defects serve as shallow donors with a binding energy of 30–60 meV.
16
The vapor trapping synthesis introduces a large number of zinc interstitials into the ZnO
NWs and enhances the n-type behavior. To characterize the contact barrier between the
electrodes and ZnO NW, I–V
ds
curves were obtained under a vacuum at different
temperatures, as shown in Fig. 2(c). Electrical current was observed to increase
monotonically with temperature. The semilogarithmic plot of the conductance versus
reciprocal temperature (I /T) [inset of Fig. 2(c)] agrees well with the thermionic emission
model, in which the current
]1)/)[exp(/exp(
−
−
∝ TkqVTkI
BdsBb
φ
. The effective energy
barrier height was extracted from the slope
b
φ
= 0.30 eV.
It has been observed that the ambient oxygen partial pressure has a considerable
effect on the performance of ZnO NW FETs. This is mainly due to conductivity changes
caused by surface band bending, induced by O
2
molecule adsorption. It is known that
4