modeling of the conduction in a wo3 thin film as ozone sensor

The interaction between the semiconductor surface and the gases is approached by means of the adsorption theory described by Wolkenstein in order to determine the equilibrium state of th

L2MP UMR-CNRS, F.S.T St J´erˆome, Service 152, Universit´e Paul C´ezanne, 13397 Marseille Cedex 20, France

Received 22 July 2005; received in revised form 21 November 2005; accepted 1 December 2005

Available online 20 January 2006

Abstract

In this paper we propose a model for ozone detection in atmospheric conditions The sensitive layer material used in this study is tungsten oxide The interaction between the semiconductor surface and the gases is approached by means of the adsorption theory described by Wolkenstein

in order to determine the equilibrium state of the grains The layer conductivity is then determined by computing the current flowing between the grains (in the spherical assumption) across the depletion layer induced by the adsorbed molecules and the semiconductor interaction This calculation is performed using the “drift diffusion” equation set

We have first analyzed the oxygen adsorption effect, then the ozone adsorption one and finally, the combined action of the two mixed gases on the sensor layer

This model takes into account the fundamental mechanisms implied in the gas detection and the results obtained are in good agreement with the experimental results

© 2005 Elsevier B.V All rights reserved

Keywords: Adsorption; Electrical conductivity; WO3 ; Gas sensors; Ozone; Modeling; Thin films

Contents

1 Introduction 327

2 Wolkenstein adsorption theory 328

2.1 Non-dissociative adsorption 329

2.2 Dissociative adsorption 329

3 Computation method 330

4 Results and discussion 330

4.1 Influence of oxygen 331

4.2 Influence of ozone 331

4.3 Simulation in presence of the two gases at operating conditions 331

4.4 Comparison between modeling and experimental sensor response 333

5 Conclusion 333

References 334

1 Introduction

The performances improvement of the microsensors requires

the control of technology as well as the knowledge of the

mecha-nisms of reactivity and conduction Most of them are made from

a metal oxide film used as a sensitive layer Tin oxide (SnO2),

∗Corresponding author Tel.: +33 4 91 28 85 10; fax: +33 4 91 28 89 70.

E-mail address: jacques.guerin@L2MP.fr (J Gu´erin).

titanium oxide (TiO2), zinc oxide (ZnO) and tungsten oxide (WO3), in microcrystalline state, are the most usual materials for this application[1–3] These metallic oxides are n-type large gap

under-stoichiometric semiconductors, with oxygen vacancies Their electrical conduction is explained by oxygen vacancies which induce defect states in the band gap and act as electron donors[4,5]

Electrical measurements based on the impedance spec-troscopy allow to understand the mechanisms involved in the

0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.12.005

change of the sensitive layer resistivity in presence of oxidizing

or reducing gas This method shows that the Schottky barrier

which is spread out between adjacent grains is increased by

oxi-dizing vapours and decreased in the opposite case[6–8], that

implies a variation of resistivity in the same way

Since 1980, many authors have developed models in order

to describe the functionality of gas sensors Most of them

are based on the same principle In a first time, the surface

of the grains adsorbs molecules or atoms (O2 or O) from

air which, because of their oxidizing properties, can ionize

negatively (O2 − or O−) and act as electron acceptors[3] A

depletion layer is then created in each grain and the

electri-cal conductivity of the sensitive film is decreased In a second

time, if an oxidizing gas is present in the atmosphere, this

new species is also adsorbed and the mechanism is

ampli-fied Conversely, if a reducing gas is present, a part of the

adsorbed oxygen is reduced by the gas and removed from the

surface, the depletion layer and the resistivity are then decreased

[9–12]

The numerous approaches differ by the adsorption isotherm

and the way of evaluating the microcrystalline film

resistiv-ity The most useful isotherms are Langmuir and Wolkenstein

isotherms

In the Langmuir model, the bonding energy between the

adsorbent and the adsorbate is supposed to be independent of

the covering rate It is easy to implement in a fully analytical

models and gives interesting results, particularly with metals

However the adsorption on semiconductors is more complicated

because the semiconductors can exchange electrons with the

adsorbate and thus create neutral and ionized species whereas

that model does not take account of these two species So it is

not very realistic when it is applied to semiconductors

mate-rials Despite its disadvantages, the Langmuir model is often

used with a good accuracy because for low covering rates

(less than 5× 10−3) the adsorbate is almost fully ionized and

the bonding energy can be compared to a constant: it is the

case at high temperature, low concentration, dynamic regime

[13,14]

The Wolkenstein model takes into account the electronic

cou-pling between the semiconductor and the adsorbate species but is

rather complex to implement because it needs the simultaneous

resolution with the Poisson’s equation[15–18]

The described mechanism is not the only one which is

pro-posed and other models, based on slightly different adsorption

isotherms[19–21]or oxidation of the surface vacancies by ozone

[22]are described in the literature

In a previous paper[23], a model of gas sensor based on the

conductivity decrease of a polycrystalline film of metal oxide

(WO3), in an oxidizing atmosphere, was already described The

interaction between the gas and the surface was modeled by

Langmuir isotherm and the electrical resistivity was evaluated

by solving the transport equations

This paper deals with ozone detection in the atmosphere,

the influence of another gas is not studied The action of

oxy-gen is first analyzed, then the ozone one The combined action

of the two mixed gases is finally studied at the end of this

article

In each case, the interaction between the semiconduc-tor material and the gases is approached by means of the adsorption theory of Wolkenstein in order to determine the equilibrium state of the grains The film conductivity is then determined by computing the current flowing between the grains (supposed spherical) across the depletion layer This is performed using the Shockley Read Hall generation recombination model and the “drift diffusion” equations set, largely used for the calculation of the semiconductor devices

[24] The simulation results are in good agreement with the exper-imental measurements

2 Wolkenstein adsorption theory

The Wolkenstein adsorption model[25,26]which introduces

in a natural way the reciprocal interactions between the adsor-bent and the adsorbate seems particularly well suited to the case of the semiconductors materials In this article one will

be interested in the case of an oxidizing vapour adsorbed on

a n-type semiconductor (oxygen or ozone on WO3sensor) but

it is clear that the other schemes can be analyzed in a similar way

In the Wolkenstein model, the adsorption of an oxidizing gas species is carried out with two successive steps: ‘weak or neutral chemisorption’ and ‘strong or ionized chemisorption’

[27] During the first step, the bond between the adsorbate and the substrate is weak and does not involve electronic trans-fer, the electrons of the atom or the molecule remain located

in the vicinity of the adsorbate involving a simple deforma-tion of the orbitals The binding energy of the adsorbate is

Ew and corresponds to the loss of free energy of the sys-tem during the adsorption process This neutral chemisorption does not change the electrical properties of the material but the perturbation created by the adsorbate induces surface state

Ess in the band gap This surface state acts as a trap for the electrons

The second step (strong chemisorption) occurs when an

elec-tron of the conduction band, whose energy is Ec, is transferred from the semiconductor to the adsorbed species The binding

energy of the adsorbate is increased by Es= Ec− Ess, that is the loss of free energy of the system during the ionization pro-cess This process involves the creation of a negative superficial

charge and a chemisorption induced surface potential barrier Vs

(Vs< 0)

Let us name Ecs= Ec− qVsthe surface conduction band level,

one can write Es= Ec− Ess= Ecs− Ess+ qVs The energy

dif-ference Ecs− Ess is also the differenceχads− χsc between the electronic affinities of the neutral adsorbate and the

semicon-ductor So, Es=χads− χsc+ qVs The binding energy of the

strongly adsorbed species write Ew+ Es= Ew+χads− χsc+ qVs This expression shows that the binding energy of the strongly adsorbed species decreases when the covering rate increases what facilitates the desorption

The neutral chemisorption mechanism is only limited by the number of adsorption sites at the surface of the material, while the strong one is limited by the upper band bending

For the diatomic gases as oxygen, the adsorption may be

non-dissociative (generally at low temperature) or dissociative

(at higher temperature) The corresponding chemical reactions

between a diatomic molecule O2and one or two free adsorption

sites S are:

S+ O2→ (S–O2); (S–O2)+ e−→ (S–O2)−

for the non-dissociative kinetics (1a)

2S+ O2→ 2(S–O); (S–O)+ e−→ (S–O)−

for the dissociative kinetics (1b)

For triatomic gases as ozone O3one considers only one

possi-bility:

S+ O3→ (S–O) + O2; (S–O)+ e−→ (S–O)− (2)

2.1 Non-dissociative adsorption

In stationary conditions, the value of the covering rate

θ = Nads/N *is determined by the adsorption and desorption

bal-ance:

p(1 − θ)

N∗√

2πmkT = ν exp

−Ew

kT

 

θ0+ θ−exp

−Es

kT



(3)

In the first term, p is the gas partial pressure and m is its molecular

mass, N * is the total density of adsorption sites, k the Boltzman

constant and T is the thermodynamic temperature.

In the second term,ν is the typical phonon frequency of the

lattice,θ0andθ−are the covering rates of the neutral and ionized

species, respectively

This equation means that a strongly chemisorbed species

must give again its trapped electron to the bulk and

return to the neutral state before desorbing and a neutral

chemisorbed species must release its bonding energy Ewbefore

desorbing

2.2 Dissociative adsorption

The presence of two free close sites is necessary so

that the reaction of adsorption occurs, conversely, the

reac-tion of desorpreac-tion requires the presence of two close atoms

The relation of balance is thus modified in the following

way:

p(1 − θ)2

N∗√

2πmkT = ν exp

−E

w

kT

 

θ2

0+ θ2

−exp

−E

s

kT



(4)

It is to be noticed that the adsorption kinetics of a triatomic gas

(expression 2), which utilizes only one adsorption site has the

same behavior than a non-dissociative kinetics

In any case,θ−andθ0are related to the total covering rateθ

by the Fermi–Dirac statistics:

η−= θ θ− = 1

1+ 2 expEss−EF

kT

(5)

η0=θ0

Eqs.(3)–(6)are related to superficial quantities, but they depend

on the band bending qVs This quantity must be calculated from the Poisson’s equation:

In this expression, V is the intrinsic potential, n and p are the

elec-tron and hole densities, respectively,Nd+the density of ionized oxygen vacancies andε is the permittivity.

n, p and Nd+are calculated by the set of classical drift diffusion equations using Fermi–Dirac statistics

Thus, the computation of the solution of Eqs.(3), (5) and (6)

or (4)–(6)must be performed simultaneously with that of the Poisson’s equation The boundary condition is given by Gauss law at the surface of each grain:

En=σ

ε =

−qN∗θ−

Enis the normal electric field andσ is the superficial density of

charge

When two species of oxidizing adsorbates are simultaneously

in the atmosphere, it is assumed that no interaction between these two species takes place in the gaseous phase and Eq.(3)

is replaced by two coupled equations:

k1p1(1− θ1− θ2)

= ν1exp

−Ew1

kT

 

θ10+ θ1 −exp

−Es1

kT



(9a)

k2p2(1− θ1− θ2)

= ν2exp

−Ew2

kT

 

θ20+ θ2 −exp

−Es2

kT



(9b) And the total covering rate writes:

θ = θ1+ θ2= k1A1(η20, η2 −)p1+ k2A2(η10, η1 −)p2

k1A1(η20, η2 −)p1+ k2A2(η10, η1 −)p2

+A1(η20, η2 −)A2(η10, η1 −)

(10)

With

A i(η i0 , η i−)= ν iexp

−E

wi kT

 

η i0 + η i−exp

−E

si kT

 (11)

In the dissociative adsorption case Eq.(4)related to oxygen must

be modified in the same way:

k1p1(1− θ1− θ2)2

= ν1exp

−Ew1

kT

 

θ2

10+ θ2

1 −exp

−Es1

kT



(12) whereas Eq.(9b)remains valid for ozone

In this case, the total covering rate writes:

θ = θ1+ θ2

2p2+ A1(η20, η2 −)

×

⎧

⎪

⎪

⎪

⎪

k2p2+

√

k1p1[A1(η20, η2 −)]2

√

k1p1A1(η20, η2 −)+ [k2p2

+A1(η20, η2 −)]

A1(η2

20, η2

2 −)

⎫

⎪

⎪

⎪

⎪ (13)

The previous set of Eqs (5)–(7) and (10)or (5)–(7) and (13)

cannot be analytically solved, but a numerical resolution is

pos-sible if one chooses grains of simple geometrical form[15] No

shape of grain is fully satisfactory to model a thin polycrystalline

layer made up of grains which have a great disparity of shape and

size In this work, the grains are supposed to be quasi-spherical,

identical in size, and single-crystal They are jointed, coupled the

ones with the others by a small contact surface allowing a great

porosity Each grain is bathed by the atmosphere to be controlled

This simplified model allows at the same time to easily

deter-mine the electrical features of a grain and to take into account the

various mechanisms taking part in electric conduction Indeed,

by supposing that the film consists of an homogeneous stacking

of identical spherical grains of known properties, the resistivity

of the layer results from the properties of only one grain

3 Computation method

Computation is carried out in two steps

The first step consists in determining the thermodynamic

equilibrium state (no bias) of the grains surrounded by their

environment The neutral and ionized covering rates (θ0 and

θ−), the electrons and holes densities (n and p) and the

intrin-sic potential V are simultaneously computed This calculation is

performed by solving the Poisson’s equation using a four points

Runge Kutta method

The second step consists in solving the set of drift

diffu-sion equations in the vicinity of the equilibrium solution This

solution is perturbed by a little bias current flowing between

two adjacent grains which induces perturbationsδV, δn and δp

on V, n and p, respectively The corresponding linear equations

are then derived and the matrix inversion is performed by

gaus-sian elimination Non-equilibrium values of electric field, Fermi

quasi-potentials, current densities, resistivity are then calculated

The simulation results presented in this paper are obtained

for WO3sensors with values usually quoted in the literature:

gap width, Eg= 2.7 eV and refractive index, n = 2.

Most of the other data are estimated from the literature related

to SnO2: effective mass of electrons and holes, me= mh= 0.3m0;

typical phonon frequency, ν = 1013Hz [26,10]; strong oxygen

and ozone chemisorption level depth, χads− χsc= 1 eV (S–O

or S–O2 occupied sites) [28,29] The desorption energy Ew

is equal to 0.1 eV and 0.35 eV for oxygen dissociative and

non-dissociative chemisorption, respectively and 1.2 eV for

ozone

4 Results and discussion

The stoichiometry, the grain size and the superficial density

of the adsorption sites are the only adjustable parameters of simulations The other parameters are related to the sensitive layer material

The oxygen vacancy density is assumed to be 1× 1019cm−3

(corresponding to a chemical composition WO2.99959) with a donor level located on the conduction band (quasi-total ion-ization) The grains are 20 nm radius, this size is comparable with the granularity of the layers carried out in the laboratory and moreover the reduction in this value would lead to nano-crystalline structure not compatible with the classical statistical analysis The superficial density of sites is 1× 1015cm−2.

For a sensor application, the film resistivity is the significant physical parameter It is also the last which can be computed because it is the result of a succession of different mechanisms Among the various intermediate parameters, the more interest-ing are the total and ionized coverinterest-ing rates of the grains due to the atmosphere interaction, the electrical potential induced by the surface electric charge and the resulting electronic density

Fig 1shows the influence of oxygen pressure on the elec-tron density and the potential distributions in the grains under bias The curves are drawn along an axis connecting the centers

of two adjacent grains between which the current flows When this current is null, these profiles are symmetrical but when the current increases, the balance between conduction and diffusion components involves a decrease of potential in the current direc-tion and a displacement of electrons in the reverse direcdirec-tion The curves are computed for two opposite operating conditions:

• under very low oxygen pressure (1 × 10−18bar), with a bias

current equal to 1× 108A/m2: the depletion zone is narrow and the resistivity is weak,

• under high oxygen pressure (1 bar), with a bias current equal

to 1 A/m2; the depletion zone is spread out until in the medium

of the grain and the resistivity is much higher

The operating conditions of the sensor are driven by atmo-spheric oxygen which determines its baseline It is thus useful to analyze separately the influences of the oxygen and ozone partial pressures before to take into account the ozone detection

Fig 1 Influence of oxygen pressure on electron density and potential at 473 K.

Fig 2 Layer resistivity vs oxygen or ozone pressure.

4.1 Influence of oxygen

The simulations are carried out in pure oxygen atmosphere

at different temperatures in the range of 473–673 K and variable

pressure

Fig 2 shows the behavior of the film resistivity

accord-ing to the oxygen pressure in the cases of non-dissociative

and dissociative adsorption On this figure and on the

fol-lowing ones, the unit of pressure is the atmospheric pressure:

1013 mb For very low oxygen pressure, the value of the

resis-tivity, approximately 2× 10−2

the temperature This is due for a part to the choice of the

null donor level of oxygen vacancies and to the weak

varia-tion of mobility (like 1/T) in the temperature range for the other

part

For very high oxygen pressure, the layer resistivity reaches a

limit value This is due to the saturation of the electrons trapping

process by the adsorbed species which limits the ionization rate

of the adsorbed layer at 4.8× 10−2 It should be noted that the

increase in temperature shifts the ascending part of the curves

towards the stronger pressures

The increase in resistivity is slower in the dissociative

adsorp-tion case but the extreme values remain the same ones, in

accor-dance with the saturation of the trapping process

4.2 Influence of ozone

Simulation was made in the presence of pure ozone, only

in the non-dissociative adsorption case in accordance with the

previous assumptions

The curves are drawn inFig 2which summarizes the results:

• the resistivity behavior is unchanged compared to oxygen,

• the extreme values of the resistivity are also unchanged

because of the same value of the strong chemisorption level

(1 eV),

• the scale of the pressures is eight decades shifted towards the

lower values, what highlights the greatest reactivity of the

ozone compared to the oxygen

4.3 Simulation in presence of the two gases at operating conditions

Now, the sensors response are simulated in presence of two oxidizing gases: oxygen, under a fixed partial pressure (0.2 on the reduced scale) and ozone, under variable partial pressure The partial pressure of oxygen determines the resistivity under airρ0whereas the mixture determines the resistivity under gasρgas

The response S is defined by the ratio S = ρgas/ρ0

Fig 3a shows the variation of the covering rates of the adsorp-tion sites by oxygen (molecular O2or ionized O2 −) and by the

ozone (atomic O or ionized O−) according to the partial

pres-sure of ozone in the non-dissociative chemisorption case For very low ozone pressure (1× 10−18), the covering rate is

prin-cipally due to atmospheric oxygen pressure (1× 10−2) since

the covering rate due to ozone is more than seven decades lower (1× 10−10to 1× 10−13) When this pressure increases,

the desorbed atoms or ions resulting from oxygen are gradually removed and replaced by those coming from ozone, until satu-ration of the layer occurs (θ = 1) Under strong pressure, there is

no more adsorbed oxygen, this is due to the greatest desorption energy of ozone (1.2 eV against 0.35 eV for non-dissociative oxygen)

The results obtained in the case of the dissociative adsorption

of oxygen are similar

Fig 3b shows the covering rates resulting from oxygen and from ozone, respectively Interpretation is more delicate since

in both cases the adsorbed species is always the same: atomic oxygen, ionized or not The ozone dissociation being easier than that of oxygen, one must think that when equilibrium is estab-lished, the atoms of oxygen resulting from the dissociation of oxygen have been replaced by other atoms of oxygen resulting from the dissociation of ozone

The oxygen adsorption on SnO2is generally non-dissociative for temperatures lower than 500 K and dissociative at higher temperature [17,18] Supposing that this assumption remains true for WO3, the values of the total covering rateθ0 (due to

Fig 3 (a) Total covering rate of adsorption sites induced separately by O 2 and

O 3 in the non-dissociative adsorption case (b) Total covering rate of adsorption

sites induced separately by O 2 and O 3 in the dissociative adsorption case.

species O2, O2 −, O or O−at 473 K and O or O− at 573 and

673 K) and ionized covering rateθ−(due to species O2 −or O−

at 473 K and O−at 573 and 673 K) are given inFig 4a and b,

respectively

We can notice that the low value of the adsorbate ionization

rate saturates at a value lower than 5.4× 10−3while the total

covering rate reaches the unit

Fig 5 shows the response of the layer S according to the

partial ozone pressure at the same time for non-dissociative and

dissociative adsorption of oxygen

It should be noted in Fig 5 that the increase in

tempera-ture which shifts the ascending part of the curves towards the

strongest pressures must result in a loss of sensitivity to the

low pressures This figure shows that for the non-dissociative

adsorption:

• the response is always very weak at the low temperatures

because of the weak desorption of the layer which is already

almost saturated,

• this response can become very high at high temperature with

the condition of having a strong ozone concentration, which

does not correspond to the normal conditions of a sensor

oper-ation,

Fig 4 (a) Total covering rate of adsorption sites induced both by O 2 and O 3 in the non-dissociative (at 473 K) and dissociative (at 573 and 673 K) adsorption case (b) Ionized covering rate of adsorption sites induced both by O 2 and O 3 in the non-dissociative (at 473 K) and dissociative (at 573 and 673 K) adsorption case.

• for an operation as sensor or detector, there exists for each pressure to be detected, an optimal temperature which pro-vides the highest sensitivityΣ = dS/dp.

The behavior of S in the case of dissociative adsorption is

slightly different: the response is significant at low temperature:

140 at 473 K against 50 for non-dissociative adsorption

Fig 6represents the computed response of a sensor placed

in normal operating conditions Two curves are calculated in

Fig 5 Response of the layer vs ozone partial pressure in the two adsorption cases.

Fig 6 Response of the layer vs ozone partial pressure.

non-dissociative adsorption mode at 423 and 473 K and four

curves are calculated in dissociative adsorption mode for the

higher temperature values In the considered range of partial

pressure, the optimum response is obtained around 523 K

4.4 Comparison between modeling and experimental

sensor response

WO3 sensors are prepared by reactive radio frequency

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

tung-sten target The vacuum chamber is evacuated to 5× 10−7mbar

by a turbo molecular pump The films are sputtered on SiO2/Si

substrates with platinum electrodes in a reactive atmosphere

under controlled oxygen–argon mixtures The total pressure is

stabilized at 3× 10−3mbar Both argon and oxygen flows are

controlled by mass flow controllers Oxygen content in the gas

mixture, defined as the ratio of oxygen flow to the total flow, is

fixed at 50% The R.F power applied to the target is 60 W and

the temperature substrate is 300 K

The WO3layers are highly resistive, so interdigitated

elec-trodes are used in order to reduce the sensor resistance The

distance between the electrodes is 50␮m They are obtained

from a sputtered Pt film, using photolithography and lift off

pro-cesses After WO3deposition on the Pt electrodes, the films are

annealed at 425◦C for 1 h 30 min in air in order to stabilize the

chemical composition and the crystalline structure

To investigate the ozone sensing properties of WO3films, the

sensors are introduced in a test chamber allowing to control the

sensor temperature under variable gas concentrations Dry air is

used as a reference gas Ozone is generated by oxidizing oxygen

molecules of a dry air flow exposed to a calibrated pen-ray UV

lamp

The resistance measurement is carried out by a picoammeter

HP 4140B

These sensors have a high sensitivity at ozone, the response is

typically 100–300 at 0.8 ppm However they are very dependent

on the variations of process, so the dispersion of characteristics

among the different manufacturing batches requires an

adjust-ment of the simulation parameters

Fig 7gives an example of response (experimental and

simu-lated) versus ozone pressure at 523 K The experimental curve is

Fig 7 Simulation of the response of the layer vs ozone partial pressure at 523 K

in the two adsorption cases and comparison with an experimental sensor.

Fig 8 Simulation of the response of the layer vs ozone temperature at 0.8 ppm ozone in the two adsorption cases and comparison with an experimental sensor.

obtained with a WO3sensor while the simulated one is computed

in the dissociative chemisorption case with desorption energies

Ew= 0.05 and 1.195 eV for O2and O3, respectively and a phonon frequencyν = 3 × 1013Hz The experimental dots are very close

to the simulated curve

Fig 8gives an example of response obtained under 0.8 ppm ozone concentration with a WO3sensor The curves correspond-ing to non-dissociative and dissociative chemisorption are high-lighted in their validity domain of temperature The simulation results are in good agreement with the experimental measure-ments, especially in the non-dissociative case

5 Conclusion

The model presented in this paper is built on an approach a little different from those which are usually described in the lit-erature It is based on the approximation of the sensitive film by a regular stacking of identical spherical grains It can be improved using a statistics related to the grain size The model supposes moreover that each grain is single-crystal, isotropic, and can

be described by a finished number of macroscopic parameters characterizing the material It supposes finally that the chemical mechanisms are located at the interfaces and are fully reversible

in the whole range of temperatures, what must be true in the real

sensors

The accurate determination of the macroscopic parameters

(or more generally mathematical expressions) which describe a

sum of physical mechanisms and geometrical properties of the

crystal still requires a theoretical and experimental significant

work

However, this model takes into account the fundamental

mechanisms implied in the detection of gas and the results

obtained confirm a non-dissociative behavior of the oxygen

adsorption at temperature lower than 550 K and a dissociative

behavior at higher temperature

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Biographies

Jacques Guerin was born in France in 1947 He received his

engineer-ing diploma in electronics and radio-communication at the Institut National Polytechnique of Grenoble (INPG) in 1972 and his PhD from the Univer-sity of Aix-Marseille III (Paul Cezanne) with a thesis on spatial silicon solar cells for observation satellites After various research and engineering devel-opments (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.

Khalifa Aguir was born in 1953 He is professor at Paul Cezanne, Aix-Marseille

III University (France) He was awarded his Doctorat d’Etat es Sciences degree from the Paul Sabatier University Toulouse (France) in 1987 He is cur-rently head of Sensors Group at Laboratoire Materiaux & Microelectronique (L2MP–CNRS) Marseille (France) His scientific interests are thin films prepa-ration and characterization for microsystems Since 1998, he is interested in gas microsensors and selectivity by signal treatment strategies, and electronic noses, physical and chemical properties of metal and oxides thin films, and applications in microelectronics He currently works on WO 3 gas sensors and selectivity enhancement strategies including PCA analysis, noise spectroscopy and modelling of sensor responses.

Marc Bendahan was born in 1967 He is a researcher at the Paul Cezanne,

Aix-Marseille III University (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 con-ductor He currently works at Laboratoire Materiaux & Microelectronique de Provence (L2MP–CNRS) Marseille (France), on WO 3 gas sensors and selectiv-ity enhancement strategies.