Photo Induced Self-Diffusion and Viscosity in Amorphous Chalcogenide Films

Acceleration of the mass transport in amorphous chalcogenide films under band gap light illumination is usually attributed to the decrease of the film viscosity. However, our direct measurements of the film viscosity at various temperatures and light intensities, made by flattening of surface relief gratings, have shown that the viscosity did not vary under illumination and the acceleration of the mass transfer was caused by the contribution of photo-induced (PI) self-diffusion. The PI diffusion coefficient is not related to the viscosity coefficient by the Stokes-Einstein relation and PI diffusion should be considered as an additional mechanism of the overall mass transport. In this paper, using well-known models of self-trapped excitons, we present the first atomic interpretation of PI diffusion coefficients, explain their dependence on temperature and light intensity, and compare with our experimental data. For characterization of PI acceleration of the mass transfer we introduce the term 'diffusional viscosity', like it is used for description of diffusion creep in crystalline solids. We estimate the temperature dependence of diffusional viscosity and show that it noticeably depends on the distance over which the material is redistributed. Taking into account the diffusional viscosity allows an adequate general interpretation of many photo-induced phenomena observed in the literature.