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

*mahnaz.shafiei@qut.edu.au

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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