Tin dioxide is one of the most studied semiconductor metal oxides because of its various applications in gas sensors, solar cells, optical devices, transparent electrodes etc. due to interesting optical, electronic and thermal properties of SnO₂. In most of these structures the surface electronic status of SnO₂ layers plays crucial role. However, the standard models of electrical effects in SnO₂ layers are usually simplified and neglect the contribution of surface states to charge transport phenomena [1].

The aim of this work is a rigorous theoretical analysis of the influence of surface states on the conductance of SnO₂ layers with a thickness larger than the Debye length. In the calculations different continuous and discrete surface state distributions in the energy gap were assumed. From the one-dimensional numerical solution of the Poisson equation, the detailed in-depth profiles of the potential barrier V(x) and carrier concentrations n(x) and p(x) were obtained. Then, the values of surface potential, surface charge, Fermi level position and layer conductance were determined. These magnitudes were calculated for different bulk doping and carrier mobility. In addition, the surface fixed charge representing adsorbed ions or surface delta-doping was introduced to modify the band bending and layer conductance. Furthermore, the influence of temperature on near-surface region and conductance with assumption of non-mobile bulk donors was studied. The temperature dependence of the effective density of states, carriers mobility and concentration of donors was included into account for temperature dependent modeling. The results of theoretical analysis were compared with experimental data obtained from measurements of the SnO₂ sensor response to the changes of NO₂ concentration in synthetic air.

[1] N.Barsan, U.Weimar, J. Electroceramics, 7 (2001) 143

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1. Theoretical analysis of oxygen adsorption at SnO₂ grains with slab geometry
2. Studies of gas sensing, electrical and chemical properties of n-InP epitaxial surfaces
3. Rigorous analysis of the contribution of surface states to the conductivity of semiconductor thin layers