On the temperature dependence of the resistive and surface ionisation response of SnO₂ gas sensing layers

Gas sensing experiments have been performed on SnO₂ thin films using a wide range of different analyte gases. In these experiments, the SnO₂ layers were specifically configured to observe the familiar resistive (RES) gas response alongside with the novel surface ionisation (SI) response. It is shown that the RES and SI responses, in general, occur in dissimilar temperature ranges and that both follow very different selectivity criteria. Microscopically, both kinds of response proceed through analyte-specific sequences of adsorption, surface reaction, charge transfer and desorption steps. A generalisation of the Ahlers model [1], originally developed to account for the bell-shaped temperature variation of the RES response of thin film metal oxide layers, is shown to quantitatively account for the whole range of SI data. Fits to experimental SI response vs. temperature curves allow the total energy input into the surface ionisation process to be determined and insights into the ionisation mechanism to be gained.