Pt/WO3 Nanoplatelet/SiC Schottky Diode Based …eprints.qut.edu.au/87264/1/ICNS4, WO3 sensing paper...
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Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran
Pt/WO3 Nanoplatelet/SiC Schottky Diode Based Hydrogen Gas Sensor
M. Shafieia,b*
, J. Yub,c
, Abu Z Sadekd, N. Motta
a, K. Kalantar-zadeh
b, W. Wlodarski
b
a School of Engineering Systems, Queensland University of Technology, Brisbane, QLD 4001, Australia b School of Electrical and Computer Engineering, RMIT University, Melbourne, VIC 3001, Australia
c Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong SAR
d School of Applied Sciences, Applied Physics, RMIT University, Melbourne, VIC 3001, Australia
Abstract: This paper presents the fabrication and study of a Schottky diode based on Pt/WO3 nanoplatelet/SiC for H2
gas sensing applications. The nanostructured WO3 films were synthesized from tungsten (sputtered on SiC) via an acid-
etching method using a 1.5 M HNO3 solution. Scanning electron microscopy of the developed films revealed platelet
crystals with thicknesses in the order of 20-60 nm and lengths between 100-700 nm. The current-voltage characteristic
and dynamic response of the diodes were measured in the presence of air and 1% H2 gas balanced in air from 25 to
300°C. Upon exposure to 1% H2, voltage shifts of 0.64, 0.93 and 1.14 V were recorded at temperatures of 120, 200 and
300°C, respectively at a constant forward bias current of 500 µA.
Keywords: Sensor; Hydrogen, Schottky diode; WO3 nanoplatelet; Acid etching
Introduction
Recently, a much attention has been directed to examine
the unique properties exhibited by tungsten trioxide
(WO3) films in their nanostructured form. These
properties (such as electrical, optical, defect and
structural) have reported to benefit a wide range of
applications [1] including: gas sensors [2], electrochromic
devices [3] and batteries [4]. Nanostructured WO3 films
can increase the sensitivity of gas sensors [5, 6], which is
caused by their morphology with a large surface to
volume ratio. To date, a number of techniques for the
synthesis of nanostructured WO3 films have been
developed. The techniques include: chemical vapour
deposition [7], electrodeposition [8], sol-gel [9], thermal
evaporation [10], and liquid synthesis (anodization [11-
13] and high-temperature acid-etching [14]) techniques.
Among these, acid-etching has been recognised as a fast,
inexpensive process, first reported by Widenkvist et al.
[14]. This technique holds the capacity to produce
nanostructured WO3 on large substrate areas.
Reports in literature have begun to examine Schottky
diode based sensors using an embedded nanostructured
metal-oxide layer and have found them to exhibit high
sensitivity towards H2 [5, 15, 16]. Herein, the authors
present the development of a novel Pt/WO3
nanoplatelet/SiC Schottky diode for sensing H2 gas. The
electrical characteristics and dynamic response of the
diode are studied and presented at elevated temperatures
up to 300°C.
Experimental
In this work, the Pt/WO3 nanoplatelet/SiC Schottky diode
was fabricated using n-type 6H-SiC substrates
(Tankeblue). A metallic layer of Pt/Ti with 100/40 nm
thickness was deposited on the backside of the 5×5 mm2
SiC substrates via e-beam evaporation. The substrates
were annealed at 500°C for 30 min in N2 to form the
ohmic contact. Tungsten thin films were RF sputtered
onto the SiC substrates. The sputtering power was
maintained constant at 60 W for 45 min and the substrate
temperature was 300°C. The thickness of the deposited
tungsten thin films was measured as ~1 µm.
The tungsten coated SiC substrates were then placed into
a reflux apparatus containing 200 mL of 1.5 M HNO3
solution at 50°C for 2 hrs. The as-deposited sample was
subsequently annealed at 500°C in 90% O2 in Ar
atmosphere for 4 hrs to obtain the WO3 films. Finally, a
circular pad of Pt (~25 nm thickness) was deposited onto
the WO3 layer to form the Schottky contact.
The electrical properties and gas sensing performance of
the sensor was measured using a fully automated multi-
channel gas testing system. The experimental set-up and
procedure can be referred to our previous studies [16].
Results and Discussion
The morphological characterisation of the sputtered
tungsten layer and the annealed nanostructured WO3 were
investigated by scanning electron microscopy (SEM).
Fig. 1a shows a micrograph of uniformly dispersed
tungsten nano-grains on SiC with diameters of ~20-
70 nm. SEM micrograph of nanostructured WO3 revealed
randomly oriented nanoplatelets with lengths of 100-
700 nm and thicknesses in the order of 20-60 nm
(Fig. 1b). A SEM image taken at 45° rotation (Fig. 1c)
shows the thickness of the annealed acid-etched films as
~3.3 µm, which consist of two layers: (a) nanoplatelets
layer on top and (b) non-porous layer at below.
The current-voltage (I-V) characteristics and dynamic
response of the sensor were measured in the presence of
air and 1% H2 at different temperatures from 25 to 300°C.
Fig. 2 shows the voltage shifts of the sensor towards H2 at
a constant current of 500 µA as a function of temperature.
The largest voltage shift was observed at 300°C. It was
observed that the lateral voltage shift of the sensor in the
forward bias condition was significantly larger than that
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Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran
under the reverse bias condition. This contrasts with the
sensing performance reported from other nanostructured
Schottky diode based sensors [15-18]. This observation
may be attributed to the partial conversion of tungsten to
WO3 and the presence of non-porous layer as well as
protonated metal-oxide (H0.23WO3) (as shown in XRD
analysis in [17]).
Fig. 1 Surface morphology of (a) RF sputtered tungsten
on SiC substrate, (b) annealed nanostructured WO3 and
(c) SEM taken at 45° rotation.
Fig. 3 illustrates the forward I-V characteristics of the
sensor at 300°C towards H2 (with different
concentrations). The lateral voltage shift in the I-V curves
upon exposure to H2 gas is caused by following sensing
mechanism in terms of a change in the barrier height [19].
The H2 molecules that adsorb on the catalytic Pt surface
dissociate into H atoms. These H atoms diffuse through
the thin Pt layer and induce dipole charged layer at the
Pt/WO3 interface [19]. This directly causes the lowering
of the barrier height and therefore leads to an increase in
the bias current intensity.
In semiconductor theory, the forward Schottky J-V
characteristic is given by [20]:
⋅
⋅−⋅⋅=
kT
Vq
kT
qTAJ FB
F
.expexp2** φ for
qkTVF
3> (1)
where, JF is the forward current density, VF is the forward
applied voltage, A** is the effective Richardson constant,
T is the absolute temperature, q is the charge constant, φB
is the barrier height, and k is the Boltzmann’s constant.
It can be seen from Fig. 3, as the concentration of H2 gas
increases, the lateral voltage shift in the I-V curves
increases, which can be explained by the presence of a
greater number of H atoms being adsorbed onto the WO3
resulting in the increase of the free carrier concentration.
This phenomenon occurs in conjunction with the
lowering of barrier height. Using Eq. 1, the barrier height
change (∆φB) of 93.9 meV was calculated upon exposure
of the sensor to 1% H2 at 300°C.
0 50 100 150 200 250 300
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Vo
ltag
e S
hift (V
)
Temperature (°C)
1% hydrogen 500 µA constant current bias
FB
RB
Fig. 2 Plot of voltage shifts vs temperatures in the
presence of 1% H2 at a constant bias current of 500 µA.
0.0 0.5 1.0 1.5 2.0 2.5
0
200
400
600
800
1000
0%
0.06%
0.125%
0.25%
0.5%
1%
Voltage (V)
Curr
ent
(µA
)
Fig. 3 Forward I-V characteristics of the sensor towards
H2 with different concentrations at 300°C.
The dynamic response of the sensor towards H2 with
different concentrations at 300°C is shown in Fig. 4. The
sensor was biased at constant forward current of 500 µA.
Voltage shifts of 0.07, 0.20, 0.49, 0.84 and 1.14 V were
recorded towards 0.06, 0.125, 0.25, 0.5 and 1% H2,
respectively.
The gas sensing performance of the WO3 based sensors
was not affected after continuously testing towards H2 (at
elevated temperatures up to 300°C) over three week
period. However, SEM of the nanostructured WO3 layer
revealed substantial changes in the morphology as shown
in Fig. 5. It was observed that many of the nanoplatelets
were cracked and damaged. Thus, it is suggested that this
sensor should be operated at an optimum temperature
range below 200°C to minimise long term degradation
[17].
Conclusions
In this work, a Schottky diode based on nanostructured
WO3 was fabricated and its sensing properties were
examined towards H2 gas at elevated temperatures up to
300°C. After multiple testing the nanostructured WO3
films towards H2 at elevated temperatures, degradation in
the nanostructures morphology was observed. The
experimental results exhibit that the development of WO3
nanostructured Schottky diodes using a fast, inexpensive
and simple synthesis process is promising for hydrogen
sensing applications at temperatures below 200°C. Future
work can focus on optimising the WO3 nanostructures
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Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran
films making them fully oxidized in order to enhance the
sensor performance.
0 50 100 150 200 250 3000.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.06% 1%0.5%0.25%0.125%
500µA constant forward
bias current at 300°C
Vo
lta
ge
(V
)
Time (min)
Fig. 4 Dynamic response of the sensor
towards H2 with different concentrations at 300°C.
Fig. 5 SEM image of WO3 nanoplatelets after testing.
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