Oxidation kinetics of nitrogen doped TiO_2-δ thin films: analysis on the basis of oxygen activity dependence of the chemical diffusion coefficient

The model explaining the occurrence of the electron concentration step front during oxidation of nitrogen-doped TiO_2-δ thin films is presented. This model is based on ambipolar chemical diffusion coefficient analysis. The diffusion species and oxygen activity (pressure) profiles are obtained by numerical and approximate analytical simulation of the chemical diffusion equation in the framework of a single acceptor level scheme. The profiles indicate the presence of two separate singularities: the electron concentration step front and the electron-hole recombination reaction front. The electron concentration step front relates to the sharp growth of the ambipolar chemical diffusion coefficient when passing from a region with high electron concentration to a region with low electron concentration.