wo3 sensor response according to operating temperature experiment and modeling

WO 3 sensor response according to operating temperature: Experiment and modeling Laboratoire Mat´eriaux et Micro´electronique de Provence, L2MP-CNRS, Universit´e Paul C´ezanne, Aix-Marse

WO 3 sensor response according to operating temperature: Experiment and modeling

Laboratoire Mat´eriaux et Micro´electronique de Provence, L2MP-CNRS, Universit´e Paul C´ezanne,

Aix-Marseille III, Facult´e des Sciences et Techniques de St J´erˆome, France

Received 10 July 2006; received in revised form 22 November 2006; accepted 22 November 2006

Available online 20 December 2006

Abstract

WO3-based sensors are realized in the aim to detect ozone The thin film of WO3is sputtered on a SiO2/Si substrate with Pt micro-electrodes In a previous work, the sensor response dependence on processing parameters has been studied Now operating temperature of the sensor is investigated and a theoretical model developed by our team confirms experimental measurements

The interaction between the gas and the surface was modeled by Langmuir isotherm and the electrical resistivity was evaluated by solving the transport equations

© 2006 Elsevier B.V All rights reserved

Keywords: WO3 ; Gas sensor; Modeling

1 Introduction

Electrical properties of semiconductor oxides depend on the

composition of the surrounding gas atmosphere The surface

conductivity of the sensor is modified by adsorption of gas

species and related space charge effects In oxidizing

atmo-sphere, the oxide surface is covered by negatively charged

oxygen adsorbates and the adjacent space charge region is

electron-depleted: the oxide layer presents therefore a high

resis-tance Under reducing conditions, the oxygen adsorbates are

removed by reaction with the reducing gas species and the

elec-trons are re-injected into the space charge layers: as a result, the

oxide layer resistance decreases

Recently, gas sensing properties of simple binary metal

oxides, such as tin oxide (SnO2) and tungsten trioxide (WO3)[1]

have been tested for monitoring pollutant components of

atmo-sphere for improving quality of life and enhancing industrial

processes[2–4] Tungsten oxide is an n-type metal oxide

semi-conductor with oxygen vacancies, which act as donors Because

the electron density depends on the density of oxygen vacancies,

∗Corresponding author Tel.: +33 4 91288973; fax: +33 4 91288970.

E-mail address:marc.bendahan@L2MP.fr (M Bendahan).

the vacancies play a significant role in the detection mechanism

as in SnO2sensors[5]

Many techniques are being used for the fabrication of WO3

films, including thermal evaporation[6,7], sol–gel[7]and sput-tering[8–10]

Table 1summarizes the responses (S = Rgas/Rair) of various

WO3-based ozone sensors and fabrication methods For exam-ple, Qu and Wlodarski[6]studied WO3ozone sensors deposited

on sapphire substrates by thermal evaporation The working tem-perature of the sensors was 573 K and the film thickness was about 150 nm Cantalini et al.[7]reported a study of WO3ozone sensors realized on alumina substrates, by sol–gel, sputtering and thermal evaporation techniques The operating temperature range was 473–673 K The best ozone sensitivity is obtained with the WO3sensors prepared using reactive magnetron sputtering, with an operated temperature of 523 K[8], a film thickness of about 40 nm and a grain size of 40 nm These results show clearly that the preparation method influences the sensor response It is now well known that the sensor response depends on physical and chemical properties of sensitive films In fact, morphology, thickness, chemical composition, and microstructure of WO3

thin films are very important parameters to obtained stable and sensitive sensors We have shown that sensor response depends essentially on the grain size and film porosity[9] These proper-ties can be controlled during film deposition, using rf sputtering

0925-4005/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2006.11.036

Table 1

Gas sensor responses S vs ozone concentration and different fabrication methods

technique The influence of processing parameters on the sensor

response, such as oxygen partial pressure in an Ar–O2gas

mix-ture used during the sputtering process[8]or self bias voltage

[10], has been studied by our team The sputtering parameters

have been optimized to obtain sensors which exhibit the best

performance

In the present work we report on electrical responses of WO3

-based sensors for ozone detection WO3thin films are deposited

by rf reactive magnetron sputtering on a SiO2/Si substrate with

interdigitated platinum micro-electrodes (Fig 1) Here,

operat-ing temperature of the sensor is investigated and the results are

compared with a theoretical model developed by our team

2 Experimental

WO3thin films were prepared by reactive radio frequency

(13.56 MHz) magnetron sputtering, using a 99.9% pure tungsten

target The vacuum chamber was evacuated to 5.0× 10−10bar

by a turbo molecular pump The films were sputtered in a reactive

atmosphere under an oxygen–argon mixture Both argon and

oxygen flow were controlled by mass flow controllers The total

gas flow was maintained constant at 10 sccm, keeping the total

pressure in the deposition chamber at 3.0× 10−3mbar Oxygen

content in the gas mixture, defined as the ratio of oxygen flow

to the total flow, was maintained at 50%[8]

As WO3layers are highly resistive, interdigitated electrodes

were used in order to reduce the sensor resistance The distance

between the electrodes was 50␮m They were obtained from a

sputtered Pt film, using photolithography and lift off processes The samples were kept in dry air and no conditioning step was carried out before testing

To investigate the ozone sensing properties of WO3films, the sensors were introduced in a test chamber allowing the con-trol of the sensor temperature under variable gas concentrations Dry air was used as a reference gas Ozone gas was generated

by oxidizing oxygen using a pen-ray UV lamp (Stable Ozone Generator UVP/185 nm) The intensity of the UV radiation was varied by shifting a shutter around the lamp The different ozone concentrations are obtained in the range of 0.03–0.8 ppm with a flow rate of dry air maintained at 30 l/h

The operating temperature of the sensors was adjusted between 423 and 673 K The applied dc voltage was 50 mV and the current was measured using a computerised HP4140B source/pico-ammeter The sensor response was defined as

S = Gair/Ggas, where Gair and Ggas are the conductance of the sensor in air and in tested gas, respectively

3 Sensor response versus operating temperature

3.1 Measurements

Fig 2illustrates a typical isothermal kinetic sensor response

at 523 K for various ozone concentrations The response is plot-ted versus time for ozone concentrations varying from 0.03 to 0.8 ppm The recording cycle for each concentration is 6 min Sensitivity, stability, reversibility, reproducibility and response

Fig 1 WO sensor design.

Fig 2 Isothermal kinetic sensor response at 523 K to various ozone

concentra-tions.

time are very important parameters for evaluating sensor

per-formance We can notice that the present sensor exhibits very

attractive performances: very high sensitivity to ozone

concen-tration at ppb levels, total reversibility, good reproducibility and

good stability of the baseline

The sensor response in the presence of ozone can be

inter-preted by considering that the oxygen species interact with the

surface oxygen vacancies Without ozone the density of surface

oxygen vacancies corresponds to an equilibrium established for

the oxygen partial pressure above the surface and to the

con-ductance in air With ozone the oxygen species given by the

dissociative adsorption interact with the surface oxygen

vacan-cies according to

(O3)g→ 3Oads,

3Oads+ 6e−+ 3VO2+→ 3Olat.

As a result, oxygen vacancies and the corresponding free

elec-trons are annihilated and the conductance decreases In this

model six free electrons disappear when an ozone molecule

adsorbed on the WO3surface, so the electrical conductance will

be very sensitive to the ozone interaction with the sensing

sur-face It is evident that the small grained films, which have a

large ratio of surface area to volume, will have a better

sensitiv-ity performance and the grain size appears as a very important

parameter for the sensitivity of undoped WO3thin films used as

a chemical sensing material

In order to check the effect of operating temperature on the

sensor response, WO3sensors are maintained at fixed

tempera-tures from 423 to 673 K.Fig 3illustrates the response to 0.8 ppm

of ozone versus operating temperature for the sensor realized

with 50% O2in the oxygen–argon mixture during sputtering It

shows a systematic increase of response with increasing

operat-ing temperature below 523 K, but reverse tendency is observed

above 523 K We can also notice that the response and

recov-ery time decrease when temperature increases The response

and recovery time are directly related to the adsorption and

des-orption activation energies, respectively This can be explained

by considering the temperature dependence of the surface

cov-erage of chemisorbed species At low temperature, there is

physisorption, but the rate of chemisorption is negligible At

Fig 3 WO 3 sensor response at different operating temperatures (800 ppb of

O 3 ).

high temperature, the equilibrium chemisorption is possible but the coverage decreases with increasing temperature because the desorption rate rises faster than the adsorption rate So, the cov-erage of chemisorbed species shows a maximum with increasing temperature[11] At T = 423 K the gas desorption kinetic is slow, which results in a high recovery time The shortest times in response and in recovery are obtained at 523 K We can thus conclude that the optimal operating temperature of the WO3

thin film for ozone detection is about 523 K This behaviour is confirmed by the theoretical model developed in the following section

3.2 Theoretical model

In the last years, many authors have developed models for the response of metal oxide gas sensors[12–14] In these studies, the sensitive layers are mainly tin oxide The authors of these papers

have based their works on a potential barrier (Vs) theory model

[15] In fact, according to this theory, conduction electrons can be trapped by surface states driven by the energy difference between the conduction band and surface states The conductance of the SnO2layer can then be expressed in a function of the potential

barrier (Vs)[14] Nevertheless, this equation is available for a porous layer and large grains with small contact regions (mean diameter∼1 ␮m) These models were developed to determine the response of a temperature modulated sensor in the presence

of CO[12,13], NO2[13], and O2[12]

No model has been developed concerning tungsten oxide-based sensors in the presence of ozone So, in our work a theoretical model has been developed to compute the WO3 sen-sor responses in the presence of different ozone concentrations and for various working temperatures The results are compared with experimental results

Interaction between a thin film and environment is modeled

by the Langmuir theory of the adsorption–desorption balance The analysis of the gas sensor operation using a semiconductor metallic oxide thin layer can be simplified by considering the effect of surrounding gases on the surface of the grains con-stituting the sensitive layer on the one hand and the electronic transport mechanisms in and between the grains on the other hand

The surface of each grain is bathed by surrounding gas and

adsorption is uniform, leading to a radial establishment in

elec-tric field In addition, because of the interdigitated structure of

the sensor electrodes, the applied electric field is axial As

tung-sten trioxide is a wide gap semiconductor (2.7 eV), its electrical

conductivity is induced by the oxygen vacancies acting as n-type

impurities Two different densities of vacancies associated with

two different donor levels are introduced to take into account the

two types of W–O bonds of the WO3crystalline structure The

electrical charge of the transition zone that lies at the periphery

of the grains is induced by the environment of the layer

The equations retained in the numerical model for the

calcu-lation of conduction are

• the Poisson’s equation related to the intrinsic potential:

where n and p are the densities of carriers described by the

Fermi–Dirac statistics;

• the continuity equations for the electrons and the holes:

where U is the generation recombination rate derived from

the Shockley–Read–Hall model:

2

i

• the transport equations (drift diffusion model) for n and p:

J n = −nqμ n Grad ϕ n , J p = −pqμ p Grad ϕ p (4)

These equations must be supplemented by a model of

mobil-ity:

μ(T ) = μ0

300

The complex adsorption mechanisms of ozone and atmospheric

oxygen by a surface can be schematized by the following four

chemical irreversible (or weakly reversible) reactions:

Each reaction, thermally activated, is affected by each kinetic

constant:

k i = k i0 exp



−E ai

kt



When oxygen and ozone are simultaneously present, the

equa-tion of evoluequa-tion is written:

d[N]

dt = k1N∗[O2]+ k2N∗[O3]− k3N, (11)

Table 2 Model parameters

Nmax ( ×10 13 cm −2) 0.8

kcin= k30/k20 ( ×10 7 ) 0.2

kox= k10/k20 ( ×10 −8) 0.1702

Nmax: density of adsorption sites; εas : acceptor level of adsorbed oxygen atoms;

kcin, kox: kinetic constants associated to their activation energies Eact; Ndi, εdi:

vacancies densities and donor levels used in the conduction model.

where N and N*= Nmax− N are the densities of occupied and free

adsorption sites, respectively, and [O2] and [O3] the oxygen and ozone concentrations This equation shows that in a stationary state, the density of occupied sites is related to the concentrations

by the relation:

(k3/(k2[O3]+ k1[O2]))+ 1. (12) The adsorbed atoms are partially ionized according to the reac-tion:

The ionization rate α is deduced from the acceptor level εasby the Fermi–Dirac statistics[16] So, it is possible to determine the density of electrical charge on the surface of each grain The numerous parameters of the model could not be found in literature They were thus optimized mainly from thermoelectric characterizations carried out in the laboratory.Table 2gives the main values of the parameters used in the simulations We can notice that the response of complete sensor (i.e its variation of resistivity) results from the composition of

• the adsorption mechanism with respect to oxygen and ozone;

• the modification of the electrical charge distribution in the grains of the layer

If the first process, described by the previous formulation of

N, is easy to be analysed, the second one cannot be modeled by

a simple analytical expression but it influences considerably the total response

Fig 4shows the theoretical evolution of the adsorption

effi-ciency σ = dθg/d[O3] versus operating temperature in dry air

with various ozone concentrations (where θg= N/Nmax is the covering rate) The maximum efficiency of the adsorption pro-cess with respect to ozone detection is obtained at the point

where the slope σ = dθg/d[O3] is maximal When ozone concen-tration increases, this maximum occurs at higher temperatures and its magnitude is smaller

This can be explained by considering that at low tempera-ture, the desorption is weak and the adsorption sites are almost entirely saturated with oxygen; thus, a change in ozone concen-tration does not produce significant effect Conversely, at high

Fig 4 Adsorption efficiency in a mixture air/ozone vs temperature for different

ozone concentrations (0.1, 0.3, and 1 ppm).

temperature, the desorption increases because of the high value

of the activation energy and the density of adsorbed species

becomes very weak; so, the sensitivity to ozone decreases

Between these two opposite cases, for each ozone concentration

there is a temperature value for which the adsorption efficiency

is maximal When the temperature rises, this optimum value

decreases

We can also notice that the adsorption efficiency maximum

is shifted toward high temperatures with increasing the ozone

concentration Indeed, when temperature increases, the

desorp-tion rate increases too So, significant surface covering can be

reached only for higher ozone concentrations

Fig 5 shows the computed response of the sensor defined

by the ratio of resistances (or resistivities) in a dry air–ozone

mixture and dry air only: S = ρgas/ρair Calculation is carried

out in two steps First, the carrier densities and the

electri-cal potential of a set of two adjacent grains in thermodynamic

equilibrium surrounded by the gas mixture are computed using

Poisson equation Then, a voltage is applied between the

cen-tre of the grains and the electrical current induced is calculated

using the transport equation The resistivity is finally deduced

We can then notice that there is an optimal operating temperature

which provides the highest response, as suggested byFig 4 The

simulation results are in good agreement with the experimental

measurements

Fig 5 Calculated sensor response in a mixture air/ozone vs temperature for

different ozone concentrations (0.1, 0.3, and 1 ppm).

4 Conclusion

Tungsten oxide thin films are prepared by reactive magnetron sputtering A model of resistivity based on the existence of an accumulation or a depletion layer induced by the surrounding atmosphere has been elaborated and the simulations have been compared to the experimental data The interaction between the gas and the surface was modeled by Langmuir isotherm and the electrical resistivity was evaluated by solving the transport equations

We have shown that the sensor response to ozone depends on the working temperature and that the adsorption efficiency in a mixture air–ozone is also dependent on temperature We can now conclude that the variation of the sensor response with temper-ature is linked to the tempertemper-ature dependence of the adsorption efficiency

Acknowledgments

The authors gratefully acknowledge the fruitful collaboration with many colleagues throughout this work We want to mention particularly the contribution by A Combes (L2MP, Marseille) for technical support

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Biographies

M Bendahan is a researcher at the Paul CEZANNE, Aix-Marseille III

Univer-sity (France) He is also lecturer in electronics at the Institute of Technology

of Marseille He was awarded his PhD degree from the University of

Aix-Marseille III in 1996 with a thesis on shape memory alloys thin films He is

specialized in thin films preparation and characterization for applications in

microsystems Since 1997, he is interested in gas microsensors and he developed

a selective ammonia sensor based on CuBr mixed ionic conductor He currently

works at Laboratoire Materiaux & Microelectronique de Provence

(L2MP-CNRS) Marseille (France), on WO 3 gas sensors and selectivity enhancement

strategies.

J Gu´erin received his engineering diploma in electronics and

radio-communication at the Institut National Polytechnique of Grenoble (INPG) in

1972 and his PhD from the University of Aix-Marseille III (Paul Cezanne) with a thesis on spatial silicon solar cells for observation satellites After vari-ous research and engineering developments (thermionic conversion, electronic

power devices, ), he joined the Sensors Group of the Laboratoire Materiaux

& Microelectronique de Provence (L2MP-CNRS) Marseille (France) in 2002 Its principal research interests are now directed towards WO 3 gas sensors and selectivity enhancement strategies, conduction and adsorption mechanisms and modelling of sensor responses.

R Boulmani obtained his PhD degree in physics and material science in the

L2MP laboratory at the Paul Cezanne Aix Marseille III University (France) His research interest is the study and realization of microsensors based on tungsten trioxide for the ozone detection.

K Aguir is professor at Paul CEZANNE, Aix Marseille III University (France).

He was awarded his Doctorat d’Etat `es Science degree from Paul Sabatier Uni-versity Toulouse (France) in 1987 He is currently head of Sensors Group at Laboratoire Mat´eriaux & Micro´electronique (L2MP-CNRS) Marseille (France) His principal research interests are directed towards microsystems, gas sensors and selectivity enhancement strategies including multivariable analysis, noise spectroscopy and modelling of sensor responses.