Simulation of thin film gas sensors kinetics

The consistent approach to simulation of oxygen sensitivity of undoped tin dioxide thin films in transient case was developed. The fundamentals of the model are the physical ideas, which take into consideration the presence of chemisorbed particles in neutral and charged forms on the semiconductor surface. From the analysis of rate equations for dissociative adsorption, the general expression for surface concentration of chemisorbed oxygen was found and then its interconnection with thin film conductivity for two most important grain geometry was established. It was shown that film conductivity is described by power dependence on surface concentration of oxygen in neutral form. Also we have found that the kinetic processes of gas sensing are determined by second order reactions of adsorption/desorption of oxygen. Charging and neutralization of the chemisorbed oxygen takes place in the condition of equality of their rates. Transient curves of response/recovery were calculated for various temperatures and oxygen partial pressures. The main peculiarities of experimental transient curves are coincided with results of simulation. The explanation of transient curves for Pd surface doped films is proposed. Mechanism of gas sensitivity of SnO2:Pd films assumes the indirect participation of molecular oxygen in the surface band bending due to competition with atomic oxygen on SnO2 surface for adsorption centers. This mechanism also assumes the atomic oxygen spillover from catalyst to semiconductor oxide surface. (C) 1999 Elsevier Science S.A. All rights reserved.