wo3 sensor response according to operating temperature experiment and modeling

Sensors and Actuators B 124 (2007) 24–29 WO 3 sensor response according to operating temperature: Experiment and modeling M. Bendahan ∗ ,J.Gu ´ erin, R. Boulmani, K. Aguir 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 WO 3 -based sensors are realized in the aim to detect ozone. The thin film of WO 3 is sputtered on a SiO 2 /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: WO 3 ; 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 (SnO 2 ) and tungsten trioxide (WO 3 ) [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 SnO 2 sensors [5]. Many techniques are being used for the fabrication of WO 3 films, including thermal evaporation [6,7], sol–gel [7] and sput- tering [8–10]. Table 1 summarizes the responses (S=R gas /R air ) of various WO 3 -based ozone sensors and fabrication methods. For exam- ple, Qu and Wlodarski [6] studied WO 3 ozone 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 WO 3 ozone 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 WO 3 sensors 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 WO 3 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 M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 25 Table 1 Gas sensor responses S vs. ozone concentration and different fabrication methods Sensors Fabrication method Operating temperature (K) O 3 (ppb) S Reference WO 3 Thermal evaporation 573 50 1.25 [6] 175 2.25 WO 3 Thermal evaporation 473 80 15 [7] WO 3 Sol–gel 473 80 19 [7] WO 3 rf sputtering 473 80 3 [7] WO 3 rf magnetron sputtering 523 30 16 [8] 400 263 800 310 technique. The influence of processing parameters on the sensor response, such as oxygen partial pressure in an Ar–O 2 gas 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 WO 3 - based sensors for ozone detection. WO 3 thin films are deposited by rf reactive magnetron sputtering on a SiO 2 /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 WO 3 thin 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 −10 bar 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 −3 mbar. Oxygen content in the gas mixture, defined as the ratio of oxygen flow to the total flow, was maintained at 50% [8]. As WO 3 layers 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 WO 3 films, 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 = G air /G gas , where G air and G gas are the conductance of the sensor in air and in tested gas, respectively. 3. Sensor response versus operating temperature 3.1. Measurements Fig. 2 illustrates 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 3 sensor design. 26 M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 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 (O 3 ) g → 3O ads , 3O ads + 6e − + 3V O 2+ → 3O lat . 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 WO 3 surface, 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 WO 3 thin films used as a chemical sensing material. In order to check the effect of operating temperature on the sensor response, WO 3 sensors are maintained at fixed tempera- tures from 423 to 673 K. Fig. 3 illustrates the response to 0.8 ppm of ozone versus operating temperature for the sensor realized with 50% O 2 in 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].AtT = 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 WO 3 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 SnO 2 layer 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],NO 2 [13], and O 2 [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 WO 3 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. M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 27 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 WO 3 crystalline 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: ψ = q ε (n − p − N + d ), (1) where n and p are the densities of carriers described by the Fermi–Dirac statistics; • the continuity equations for the electrons and the holes: Div J n = qU, Div J p =−qU, (2) where U is the generation recombination rate derived from the Shockley–Read–Hall model: U = np − n 2 i τ n (p + n i ) + τ p (n + n i ) ; (3) • 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 ) = μ 0 300 T . (5) 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: adsorption of oxygen : O 2 → O ads + O, (6) adsorption of ozone : O 3 → O ads + O 2 , (7) desorption of the sites : O ads → O, (8) molecular recombination : O + O → O 2 . (9) Each reaction, thermally activated, is affected by each kinetic constant: k i = k i0 exp  − E ai kt  . (10) When oxygen and ozone are simultaneously present, the equa- tion of evolution is written: d[N] dt = k 1 N ∗ [O 2 ] + k 2 N ∗ [O 3 ] − k 3 N, (11) Table 2 Model parameters N max (×10 13 cm −2 ) 0.8 ε as (eV) 0.0 k cin = k 30 /k 20 (×10 7 ) 0.2 E act = E a3 − E a2 (eV) 0.8 k ox = k 10 /k 20 (×10 −8 ) 0.1702 E act = E a1 − E a2 (eV) 0.1 N d1 (×10 13 cm −3 ) 0.9 ε d1 (eV) 0.672 N d2 (×10 15 cm −3 ) 1.0 ε d2 (eV) 0.959 N max : density of adsorption sites; ε as : acceptor level of adsorbed oxygen atoms; k cin , k ox : kinetic constants associated to their activation energies E act ; N di , ε di : vacancies densities and donor levels used in the conduction model. where N and N * = N max − N are the densities of occupied and free adsorption sites, respectively, and [O 2 ] and [O 3 ] 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: N = N max (k 3 /(k 2 [ O 3 ] + k 1 [ O 2 ] )) + 1 . (12) The adsorbed atoms are partially ionized according to the reac- tion: αe − + O ads → αO ads − + (1 − α)O ads . (13) The ionization rate α is deduced from the acceptor level ε as by 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 2 gives 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. 4 shows the theoretical evolution of the adsorption effi- ciency σ =dθ g /d[O 3 ] versus operating temperature in dry air with various ozone concentrations (where θ g = N/N max 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 =dθ g /d[O 3 ] 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 28 M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 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 = ρ gas /ρ air . 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 by Fig. 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 temperature 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. References [1] D.E. Williams, Semiconducting oxides as gas-sensitive resistors, Sens. Actuators B 57 (1999) 1–16. [2] X. Wang, N. Miura, N. Yamazoe, Study of WO 3 -based sensing materials for NH 3 and NO detection, Sen. Actuators B 66 (2000) 74–76. [3] D.S. Lee, S.D. Han, D.D. Lee, Nitrogen oxides-sensing characteristics of WO 3 -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 and structural characterization of tungsten oxide thin films for NO gas detection, Thin Solid Films 324 (1998) 44–51. [5] W. G ¨ opel, K.D. Schierbaum, SnO 2 sensors: current status and future prospects, Sens. Actuators B 26–27 (1995) 1–12. [6] W. Qu, W. Wlodarski, A thin film sensing element for ozone, humidity and temperature, 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 O 3 sensitivity prop- erties of WO 3 thin films prepared by sol–gel, thermal evaporation and rf sputtering techniques, Sens. Actuators B 64 (2000) 182–188. [8] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization of ozone sensors based on WO 3 reactively sputtered films: influence of O 2 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 sputtered tungsten trioxide thin films on the sensitivity to ozone, Thin Solid Films 484 (2005) 358–363. [10] R. Boulmani, M. Bendahan, K. Aguir, Influence of rf sputtered parameters on tungsten trioxide response sensors, in: Proceedings of the Eurosensors XIX, Barcelona, Spain, September 2005. [11] M.J. Madou,S. Roy Morrisson, Chemical Sensing withSolid State Devices, chap3: Solid/Gas Interfaces, Academic Press, 1989, pp. 67–72. [12] A. Fort, S. Rocchi, M.B. Serrano-Santos, M. Mugnaini, V. Vignoli, A. Atrei, R. Spinicci, CO sensing with SnO 2 based thick film sensors: surface state model for conductance responses during thermal-modulation, Sens. Actuators B 116 (2006) 43–48. [13] R. Ionescu, E. Llobet, S. Al-Khalifa, J.W. Gardner, X. Vilanova, J. Brezmes, X. Correig, Response model for thermally modulated tin oxidebased micro- hotplate gas sensors, Sens. Actuators B 95 (2003) 203–211. M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 29 [14] J. Ding, T.J. McAvoy, R.E. Cavicchi, S. Semancik, Surface state trapping models for SnO 2 -based microhotplate sensors, Sens. Actuators B 77 (2001) 597–613. [15] S.R. Morrison, The Chemical Physics of Surfaces, 1st ed., Plenum press, New York, NY, 1997. [16] R.C. Jeager, F.H. Gaensslen, Simulation of impurity freezout through numerical solution of Poisson’s equation and application to MOS device behaviour, IEEE Trans. Electron Dev. 27 (1980) 914–920. 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. . Sensors and Actuators B 124 (2007) 24–29 WO 3 sensor response according to operating temperature: Experiment and modeling M. Bendahan ∗ ,J.Gu ´ erin,. reproducibility and response Fig. 1. WO 3 sensor design. 26 M. Bendahan et al. / Sensors and Actuators B 124 (2007) 24–29 Fig. 2. Isothermal kinetic sensor response