Sensing behavior of TiO2 thin films exposed to air at low temperatures

The sensing behavior of thin TiO2 films exposed to air pulses was studied by means of electrical conductivity measurements. The TiO2 rutile films, with a grain size of similar to 20-30 nm, were deposited on oxidized Si substrates by means of reactive sputtering and subsequently vacuum annealed (reduced) at 400 degrees C. The films were exposed to air pulses at temperatures between 100 degrees C and 325 degrees C and their electrical conductivity was measured with embedded interdigital Au electrodes. When air was introduced into the system, the conductivity decreased and vice-versa. The conductivity changes during identical pulses were the same and reproducible, though the conductivity underwent some drift. The kinetics of the conductivity response during a pulse of air followed a logarithmic law, typical of Elovich-Roginsky chemisorption kinetics, during the first stage of exposure and a modified parabolic law, typical of oxidation kinetics, afterwards. A two-stage model is proposed to describe the response kinetics which involves first surface processes where charge transfer to oxygen adsorbates dominates followed by oxidation of the pre-reduced nonstoichiometric TiO2-x film by diffusion of oxygen. (C) 2000 Elsevier Science S.A. All rights reserved.