Operation of SnO₂ resistive gas sensors is mainly governed by the surface space charge region, which is formed due to both adsorption of gas molecules and ionization of surface states related to the oxygen vacancies at the surface. The surface-related phenomena become particularly important for nanostructures, which exhibit a much higher surface-to-volume ratio compared to SnO₂ thick films composed of micro-grains. Therefore, the understanding and control of the surface and near-surface electronic properties and their relationship with materials ones are key factors for technology optimization, and thus improvement of the sensitivity and selectivity of SnO₂ – based sensor devices. In addition, the modeling of sensor structures based on nanolayers needs to consider size and shape of grains, which decide about the layer conductance.

The purpose of this work is a computer analysis of an influence of oxygen adsorption at the SnO₂ surface on the electronic parameters of the induced depletion layer. The surface potential value and in-depth potential profiles were obtained by solving the Poisson-Boltzmann equation in the case of finite grains with slab geometry. The SnO₂ grain size was in the range from 15 nm to 150 nm. The surface energy barrier dependence on grain size was determined. The mobile singly or double ionised bulk oxygen vacancies (donors) were assumed. The effect of donor mobility and degree of donor ionisation on the electrical potential inside of grains with different thickness was analysed. Furthermore, the dependence of conductance per square on temperature (from 400 to 900 K) was rigorously computed. The temperature variations of the carrier mobility were also taken into account. It was found that the development of near-surface depletion region and thus modification of the layer electronic parameters are strongly dependent on the layer thickness, bulk donor concentration, and temperature.

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