Characterization and Electrical Properties of WO 3 Sensing Thin Films
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C. CANTALINI*, S. DI NARDO**, L. LOZZI**, M. PASSACANTANDO**, M. PELINO*, A. R. PHANI*, S. SANTUCCI**. *Department of Chemistry and Materials - 67040 Monteluco di Roio - L'Aquila -Italy **Department of Physics - 61010 - L'Aquila - Italy - canta@dsiaq 1.ing.univaq.it
ABSTRACT The microstructure and the electrical properties of thermally evaporated W0 3 thin films have been investigated by glancing angle XRD, atomic force microscopy AFM, X-ray photoelectron spectroscopy XPS and dc techniques. Thin films of 1500 A thickness have been obtained by evaporating high purity W0 3 powders by an electrically heated crucible at 5 x 10-4 Pa on sapphire substrates. The as-deposited films have resulted to be amorphous. After annealing at 500 'C in dry air for 6, 12 and 24 hours the films have shown well crystallized structures with preferential orientations of W0 3 in the (200) direction. The increase of the annealing time has shown marked influence on the microstructural features of the films surface, as highlighted by AFM investigations. The binding energies of W 4f 7/2 have been close to that of WO3, the 24 h annealed yielded an O/W ratio close to 2.9 which is in good agreement with the theoretical one. The gas sensitivity, selectivity and stability of the annealed films in presence of NO 2 gas (between 0.7 and 5 ppm) have been evaluated by measuring the electrical change of the film resistance in dry air and in gas atmosphere conditions. The influence of NO and Humidity interfering gases to the NO 2 electrical response has been also evaluated. The 500 'C annealed at 24 h has shown better electrical properties in terms of NO 2 sensitivity, stability and cross sensitivity effects. INTRODUCTION Among the transition metal oxides pure W0 3 [1] and metal tungstates [2] thick films have been reported to have outstanding sensitive properties towards nitrogen oxides at low and elevated operating temperatures respectively. In the case of air air-quality systems the TLV (Threshold Limit Value) limits for NO2 and NO are respectively 3 and 25 ppm, being NO 2 the main gas to be detected, mainly because in ambient atmosphere NO easily oxidize to NO 2. Market needs for airquality NO. sensor demand for high sensitivity to NO2 at low concentration (between 0 and 5 ppm), reduced cross sensitivity effects and long term stability of the electrical response. Thin WO 3 films, with respective advantages in quality and cost, have been prepared via physical and chemical routes, focusing on sol-gel coating [3], chemical vapor deposition [4], RF sputtering and thermal annealing [5-6]. Both thin and thick films are sensitive to NO. at elevated temperature, but thin film exhibits high performances including large gas sensitivity, fast response and low working temperature. Our previous research [7-8] has established that W0 3 thin-film prepared by high vacuum thermal evaporation of pure W0 3 powder and subsequent annealing for 1 h at 500 'C in static air, posses high sensitivity to sub-ppm levels of NO 2 gas. This paper outlines the results obtained fo
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