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Sensors and Actuators B 124 (2007) 24–29

WO3 sensor response according to operatingtemperature: Experiment and modeling

M. Bendahan ∗, J. Guerin, R. Boulmani, K. AguirLaboratoire Materiaux et Microelectronique de Provence, L2MP-CNRS, Universite Paul Cezanne,

Aix-Marseille III, Faculte des Sciences et Techniques de St Jerome, France

Received 10 July 2006; received in revised form 22 November 2006; accepted 22 November 2006Available online 20 December 2006

bstract

WO3-based sensors are realized in the aim to detect ozone. The thin film of WO3 is sputtered on a SiO2/Si substrate with Pt micro-electrodes. In arevious work, the sensor response dependence on processing parameters has been studied. Now operating temperature of the sensor is investigated

nd 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 theransport equations.

2006 Elsevier B.V. All rights reserved.

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eywords: WO3; Gas sensor; Modeling

. Introduction

Electrical properties of semiconductor oxides depend on theomposition of the surrounding gas atmosphere. The surfaceonductivity of the sensor is modified by adsorption of gaspecies and related space charge effects. In oxidizing atmo-phere, the oxide surface is covered by negatively chargedxygen adsorbates and the adjacent space charge region islectron-depleted: the oxide layer presents therefore a high resis-ance. Under reducing conditions, the oxygen adsorbates areemoved by reaction with the reducing gas species and the elec-rons are re-injected into the space charge layers: as a result, thexide layer resistance decreases.

Recently, gas sensing properties of simple binary metalxides, such as tin oxide (SnO2) and tungsten trioxide (WO3) [1]ave been tested for monitoring pollutant components of atmo-

phere for improving quality of life and enhancing industrialrocesses [2–4]. Tungsten oxide is an n-type metal oxide semi-onductor with oxygen vacancies, which act as donors. Becausehe 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).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.11.036

he vacancies play a significant role in the detection mechanisms in SnO2 sensors [5].

Many techniques are being used for the fabrication of WO3lms, including thermal evaporation [6,7], sol–gel [7] and sput-

ering [8–10].Table 1 summarizes the responses (S = Rgas/Rair) of various

O3-based ozone sensors and fabrication methods. For exam-le, Qu and Wlodarski [6] studied WO3 ozone sensors depositedn sapphire substrates by thermal evaporation. The working tem-erature of the sensors was 573 K and the film thickness wasbout 150 nm. Cantalini et al. [7] reported a study of WO3 ozoneensors realized on alumina substrates, by sol–gel, sputteringnd thermal evaporation techniques. The operating temperatureange was 473–673 K. The best ozone sensitivity is obtained withhe WO3 sensors prepared using reactive magnetron sputtering,ith an operated temperature of 523 K [8], a film thickness of

bout 40 nm and a grain size of 40 nm. These results show clearlyhat the preparation method influences the sensor response. It isow well known that the sensor response depends on physicalnd chemical properties of sensitive films. In fact, morphology,hickness, chemical composition, and microstructure of WO3

hin films are very important parameters to obtained stable andensitive sensors. We have shown that sensor response dependsssentially on the grain size and film porosity [9]. These proper-ies can be controlled during film deposition, using rf sputtering

M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 25

Table 1Gas sensor responses S vs. ozone concentration and different fabrication methods

Sensors Fabrication method Operating temperature (K) O3 (ppb) S Reference

WO3 Thermal evaporation 573 50 1.25 [6]175 2.25

WO3 Thermal evaporation 473 80 15 [7]WO3 Sol–gel 473 80 19 [7]WO3 rf sputtering 473 80 3 [7]

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O3 rf magnetron sputtering 523

echnique. The influence of processing parameters on the sensoresponse, such as oxygen partial pressure in an Ar–O2 gas mix-ure used during the sputtering process [8] or self bias voltage10], has been studied by our team. The sputtering parametersave been optimized to obtain sensors which exhibit the besterformance.

In the present work we report on electrical responses of WO3-ased sensors for ozone detection. WO3 thin films are depositedy rf reactive magnetron sputtering on a SiO2/Si substrate withnterdigitated platinum micro-electrodes (Fig. 1). Here, operat-ng temperature of the sensor is investigated and the results areompared with a theoretical model developed by our team.

. Experimental

WO3 thin films were prepared by reactive radio frequency13.56 MHz) magnetron sputtering, using a 99.9% pure tungstenarget. The vacuum chamber was evacuated to 5.0 × 10−10 bary a turbo molecular pump. The films were sputtered in a reactivetmosphere under an oxygen–argon mixture. Both argon andxygen flow were controlled by mass flow controllers. The totalas flow was maintained constant at 10 sccm, keeping the totalressure in the deposition chamber at 3.0 × 10−3 mbar. Oxygenontent in the gas mixture, defined as the ratio of oxygen flow

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

As WO3 layers are highly resistive, interdigitated electrodesere used in order to reduce the sensor resistance. The distanceetween the electrodes was 50 �m. They were obtained from a

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Fig. 1. WO3 sen

30 16 [8]400 263800 310

puttered Pt film, using photolithography and lift off processes.he samples were kept in dry air and no conditioning step wasarried out before testing.

To investigate the ozone sensing properties of WO3 films,he sensors were introduced in a test chamber allowing the con-rol of the sensor temperature under variable gas concentrations.ry air was used as a reference gas. Ozone gas was generatedy oxidizing oxygen using a pen-ray UV lamp (Stable Ozoneenerator UVP/185 nm). The intensity of the UV radiation wasaried by shifting a shutter around the lamp. The different ozoneoncentrations are obtained in the range of 0.03–0.8 ppm with aow rate of dry air maintained at 30 l/h.

The operating temperature of the sensors was adjustedetween 423 and 673 K. The applied dc voltage was 50 mVnd the current was measured using a computerised HP4140Bource/pico-ammeter. The sensor response was defined as= Gair/Ggas, where Gair and Ggas are the conductance of the

ensor in air and in tested gas, respectively.

. Sensor response versus operating temperature

.1. Measurements

Fig. 2 illustrates a typical isothermal kinetic sensor response

t 523 K for various ozone concentrations. The response is plot-ed versus time for ozone concentrations varying from 0.03 to.8 ppm. The recording cycle for each concentration is 6 min.ensitivity, stability, reversibility, reproducibility and response

sor design.

26 M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29

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ig. 2. Isothermal kinetic sensor response at 523 K to various ozone concentra-ions.

ime are very important parameters for evaluating sensor per-ormance. We can notice that the present sensor exhibits veryttractive performances: very high sensitivity to ozone concen-ration at ppb levels, total reversibility, good reproducibility andood stability of the baseline.

The sensor response in the presence of ozone can be inter-reted by considering that the oxygen species interact with theurface oxygen vacancies. Without ozone the density of surfacexygen vacancies corresponds to an equilibrium established forhe oxygen partial pressure above the surface and to the con-uctance in air. With ozone the oxygen species given by theissociative adsorption interact with the surface oxygen vacan-ies according to

O3)g → 3Oads,

Oads + 6e− + 3VO2+ → 3Olat.

s a result, oxygen vacancies and the corresponding free elec-rons are annihilated and the conductance decreases. In this

odel six free electrons disappear when an ozone moleculedsorbed on the WO3 surface, so the electrical conductance wille very sensitive to the ozone interaction with the sensing sur-ace. It is evident that the small grained films, which have aarge ratio of surface area to volume, will have a better sensitiv-ty performance and the grain size appears as a very importantarameter for the sensitivity of undoped WO3 thin films used aschemical sensing material.

In order to check the effect of operating temperature on theensor response, WO3 sensors are maintained at fixed tempera-ures from 423 to 673 K. Fig. 3 illustrates the response to 0.8 ppmf ozone versus operating temperature for the sensor realizedith 50% O2 in the oxygen–argon mixture during sputtering. It

hows a systematic increase of response with increasing operat-ng temperature below 523 K, but reverse tendency is observedbove 523 K. We can also notice that the response and recov-ry time decrease when temperature increases. The responsend recovery time are directly related to the adsorption and des-

rption activation energies, respectively. This can be explainedy considering the temperature dependence of the surface cov-rage of chemisorbed species. At low temperature, there ishysisorption, but the rate of chemisorption is negligible. At

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ig. 3. WO3 sensor response at different operating temperatures (800 ppb of

3).

igh temperature, the equilibrium chemisorption is possible buthe coverage decreases with increasing temperature because theesorption rate rises faster than the adsorption rate. So, the cov-rage of chemisorbed species shows a maximum with increasingemperature [11]. At T = 423 K the gas desorption kinetic is slow,hich results in a high recovery time. The shortest times in

esponse and in recovery are obtained at 523 K. We can thusonclude that the optimal operating temperature of the WO3hin film for ozone detection is about 523 K. This behaviour isonfirmed by the theoretical model developed in the followingection.

.2. Theoretical model

In the last years, many authors have developed models for theesponse of metal oxide gas sensors [12–14]. In these studies, theensitive layers are mainly tin oxide. The authors of these papersave based their works on a potential barrier (Vs) theory model15]. In fact, according to this theory, conduction electrons can berapped by surface states driven by the energy difference betweenhe conduction band and surface states. The conductance of thenO2 layer can then be expressed in a function of the potentialarrier (Vs) [14]. Nevertheless, this equation is available for aorous layer and large grains with small contact regions (meaniameter ∼1 �m). These models were developed to determinehe response of a temperature modulated sensor in the presencef CO [12,13], NO2 [13], and O2 [12].

No model has been developed concerning tungsten oxide-ased sensors in the presence of ozone. So, in our work aheoretical model has been developed to compute the WO3 sen-or responses in the presence of different ozone concentrationsnd for various working temperatures. The results are comparedith experimental results.Interaction between a thin film and environment is modeled

y the Langmuir theory of the adsorption–desorption balance.he analysis of the gas sensor operation using a semiconductoretallic oxide thin layer can be simplified by considering the

ffect of surrounding gases on the surface of the grains con-tituting the sensitive layer on the one hand and the electronicransport mechanisms in and between the grains on the otherand.

and Actuators B 124 (2007) 24–29 27

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Table 2Model parameters

Nmax (×1013 cm−2) 0.8εas (eV) 0.0kcin = k30/k20 (×107) 0.2Eact = Ea3 − Ea2 (eV) 0.8kox = k10/k20 (×10−8) 0.1702Eact = Ea1 − Ea2 (eV) 0.1Nd1 (×1013 cm−3) 0.9εd1 (eV) 0.672Nd2 (×1015 cm−3) 1.0εd2 (eV) 0.959

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M. Bendahan et al. / Sensors

The surface of each grain is bathed by surrounding gas anddsorption is uniform, leading to a radial establishment in elec-ric field. In addition, because of the interdigitated structure ofhe sensor electrodes, the applied electric field is axial. As tung-ten trioxide is a wide gap semiconductor (2.7 eV), its electricalonductivity is induced by the oxygen vacancies acting as n-typempurities. Two different densities of vacancies associated withwo different donor levels are introduced to take into account thewo types of W–O bonds of the WO3 crystalline structure. Thelectrical charge of the transition zone that lies at the peripheryf the grains is induced by the environment of the layer.

The equations retained in the numerical model for the calcu-ation of conduction are

the Poisson’s equation related to the intrinsic potential:

�ψ = q

ε(n− p−N+

d ), (1)

where n and p are the densities of carriers described by theFermi–Dirac statistics;the continuity equations for the electrons and the holes:

Div Jn = qU, Div Jp = −qU, (2)

where U is the generation recombination rate derived fromthe Shockley–Read–Hall model:

U = np− n2i

τn(p+ ni) + τp(n+ ni); (3)

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

Jn = −nqμn Grad ϕn, Jp = −pqμp Grad ϕp. (4)

These equations must be supplemented by a model of mobil-ty:

(T ) = μ0300

T. (5)

he complex adsorption mechanisms of ozone and atmosphericxygen by a surface can be schematized by the following fourhemical irreversible (or weakly reversible) reactions:

dsorption of oxygen : O2 → Oads + O, (6)

dsorption of ozone : O3 → Oads + O2, (7)

esorption of the sites : Oads → O, (8)

olecular recombination : O + O → O2. (9)

ach reaction, thermally activated, is affected by each kineticonstant:

i = ki0 exp

(−Eaikt

). (10)

hen oxygen and ozone are simultaneously present, the equa-ion of evolution is written:

d[N]

dt= k1N

∗[O2] + k2N∗[O3] − k3N, (11)

tet

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

cin, kox: kinetic constants associated to their activation energies Eact; Ndi, εdi:acancies densities and donor levels used in the conduction model.

here N and N* = Nmax − N are the densities of occupied and freedsorption sites, respectively, and [O2] and [O3] the oxygen andzone concentrations. This equation shows that in a stationarytate, the density of occupied sites is related to the concentrationsy the relation:

= Nmax

(k3/(k2 [O3] + k1 [O2])) + 1. (12)

he adsorbed atoms are partially ionized according to the reac-ion:

e− + Oads →αOads− + (1 −α)Oads. (13)

he ionization rate α is deduced from the acceptor level εas byhe Fermi–Dirac statistics [16]. So, it is possible to determinehe density of electrical charge on the surface of each grain.

The numerous parameters of the model could not be found initerature. They were thus optimized mainly from thermoelectricharacterizations carried out in the laboratory. Table 2 gives theain values of the parameters used in the simulations. We can

otice that the response of complete sensor (i.e. its variation ofesistivity) results from the composition of

the adsorption mechanism with respect to oxygen and ozone;the modification of the electrical charge distribution in thegrains of the layer.

If the first process, described by the previous formulation of, is easy to be analysed, the second one cannot be modeled bysimple analytical expression but it influences considerably the

otal response.Fig. 4 shows the theoretical evolution of the adsorption effi-

iency σ = dθg/d[O3] versus operating temperature in dry airith various ozone concentrations (where θg = N/Nmax is the

overing rate). The maximum efficiency of the adsorption pro-ess with respect to ozone detection is obtained at the pointhere the slope σ = dθg/d[O3] is maximal. When ozone concen-

ration increases, this maximum occurs at higher temperaturesnd its magnitude is smaller.

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

28 M. Bendahan et al. / Sensors and A

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ig. 4. Adsorption efficiency in a mixture air/ozone vs. temperature for differentzone concentrations (0.1, 0.3, and 1 ppm).

emperature, the desorption increases because of the high valuef the activation energy and the density of adsorbed speciesecomes very weak; so, the sensitivity to ozone decreases.etween these two opposite cases, for each ozone concentration

here is a temperature value for which the adsorption efficiencys maximal. When the temperature rises, this optimum valueecreases.

We can also notice that the adsorption efficiency maximums shifted toward high temperatures with increasing the ozoneoncentration. Indeed, when temperature increases, the desorp-ion rate increases too. So, significant surface covering can beeached only for higher ozone concentrations.

Fig. 5 shows the computed response of the sensor definedy the ratio of resistances (or resistivities) in a dry air–ozoneixture and dry air only: S = ρgas/ρair. Calculation is carried

ut in two steps. First, the carrier densities and the electri-al potential of a set of two adjacent grains in thermodynamicquilibrium surrounded by the gas mixture are computed usingoisson equation. Then, a voltage is applied between the cen-

re of the grains and the electrical current induced is calculatedsing the transport equation. The resistivity is finally deduced.

e can then notice that there is an optimal operating temperaturehich provides the highest response, as suggested by Fig. 4. The

imulation results are in good agreement with the experimentaleasurements.

ig. 5. Calculated sensor response in a mixture air/ozone vs. temperature forifferent ozone concentrations (0.1, 0.3, and 1 ppm).

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ctuators B 124 (2007) 24–29

. Conclusion

Tungsten oxide thin films are prepared by reactive magnetronputtering. A model of resistivity based on the existence of anccumulation or a depletion layer induced by the surroundingtmosphere has been elaborated and the simulations have beenompared to the experimental data. The interaction between theas and the surface was modeled by Langmuir isotherm andhe electrical resistivity was evaluated by solving the transportquations.

We have shown that the sensor response to ozone depends onhe working temperature and that the adsorption efficiency in a

ixture air–ozone is also dependent on temperature. We can nowonclude that the variation of the sensor response with temper-ture is linked to the temperature dependence of the adsorptionfficiency.

cknowledgments

The authors gratefully acknowledge the fruitful collaborationith many colleagues throughout this work. We want to mentionarticularly the contribution by A. Combes (L2MP, Marseille)or technical support.

eferences

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[2] X. Wang, N. Miura, N. Yamazoe, Study of WO3-based sensing materialsfor NH3 and NO detection, Sen. Actuators B 66 (2000) 74–76.

[3] D.S. Lee, S.D. Han, D.D. Lee, Nitrogen oxides-sensing characteristics ofWO3-based nanocrystalline thick film gas sensor, Sens. Actuators B 60(1999) 57–63.

[4] D. Manno, A. Serra, M. Di Giulio, G. Micocci, A. Tepore, Physical andstructural characterization of tungsten oxide thin films for NO gas detection,Thin Solid Films 324 (1998) 44–51.

[5] W. Gopel, K.D. Schierbaum, SnO2 sensors: current status and futureprospects, Sens. Actuators B 26–27 (1995) 1–12.

[6] W. Qu, W. Wlodarski, A thin film sensing element for ozone, humidity andtemperature, Sens. Actuators B 64 (2000) 42–48.

[7] C. Cantalini, W. Wlodarski, Y. Li, M. Passacantando, S. Santucci, E.Comini, F. Faglia, G. Sberveglieri, Investigation on the O3 sensitivity prop-erties of WO3 thin films prepared by sol–gel, thermal evaporation and rfsputtering techniques, Sens. Actuators B 64 (2000) 182–188.

[8] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization ofozone sensors based on WO3 reactively sputtered films: influence of O2

concentration in the sputtering gas, and working temperature, Sens. Actu-ators B 100 (2004) 320–324.

[9] M. Gillet, K. Aguir, M. Bendahan, P. Mennini, Grain size effect in sputteredtungsten trioxide thin films on the sensitivity to ozone, Thin Solid Films484 (2005) 358–363.

10] R. Boulmani, M. Bendahan, K. Aguir, Influence of rf sputtered parameterson tungsten trioxide response sensors, in: Proceedings of the EurosensorsXIX, Barcelona, Spain, September 2005.

11] M.J. Madou, S. Roy Morrisson, Chemical Sensing with Solid State Devices,chap3: Solid/Gas Interfaces, Academic Press, 1989, pp. 67–72.

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M. Bendahan et al. / Sensors

14] J. Ding, T.J. McAvoy, R.E. Cavicchi, S. Semancik, Surface state trappingmodels for SnO2-based microhotplate sensors, Sens. Actuators B 77 (2001)597–613.

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iographies

. Bendahan is a researcher at the Paul CEZANNE, Aix-Marseille III Univer-ity (France). He is also lecturer in electronics at the Institute of Technologyf Marseille. He was awarded his PhD degree from the University of Aix-arseille III in 1996 with a thesis on shape memory alloys thin films. He is

pecialized in thin films preparation and characterization for applications in

icrosystems. Since 1997, he is interested in gas microsensors and he developedselective ammonia sensor based on CuBr mixed ionic conductor. He currentlyorks at Laboratoire Materiaux & Microelectronique de Provence (L2MP-NRS) Marseille (France), on WO3 gas sensors and selectivity enhancement

trategies.

vLHas

ctuators B 124 (2007) 24–29 29

. Guerin received his engineering diploma in electronics and radio-ommunication at the Institut National Polytechnique of Grenoble (INPG) in972 and his PhD from the University of Aix-Marseille III (Paul Cezanne)ith a thesis on spatial silicon solar cells for observation satellites. After vari-us research and engineering developments (thermionic conversion, electronicower devices, . . .), he joined the Sensors Group of the Laboratoire Materiaux

Microelectronique de Provence (L2MP-CNRS) Marseille (France) in 2002.ts principal research interests are now directed towards WO3 gas sensors andelectivity enhancement strategies, conduction and adsorption mechanisms andodelling of sensor responses.

. Boulmani obtained his PhD degree in physics and material science in the2MP laboratory at the Paul Cezanne Aix Marseille III University (France). His

esearch interest is the study and realization of microsensors based on tungstenrioxide for the ozone detection.

. Aguir is professor at Paul CEZANNE, Aix Marseille III University (France).e was awarded his Doctorat d’Etat es Science degree from Paul Sabatier Uni-

ersity Toulouse (France) in 1987. He is currently head of Sensors Group ataboratoire Materiaux & Microelectronique (L2MP-CNRS) Marseille (France).is principal research interests are directed towards microsystems, gas sensors

nd selectivity enhancement strategies including multivariable analysis, noisepectroscopy and modelling of sensor responses.