Thermally-stimulated currents in thin-film semiconductors - computer modelling and experiment

We apply computer modelling to the multiple-trapping rate equations governing the time-evolution of non-equilibrium distributions of electrons and holes in extended and localised states in thin-film semiconductors, in thermally-stimulated conductivity measurements. We examine in detail the application of this method as a ‘spectroscopy’ of the energy distribution of localised states. Factors influencing energy and density scales are examined critically. Such factors include whether ‘weak’ or ‘strong’ re-trapping prevails during the measurement; the use of non-physical ‘effective’ attempt-to-escape frequencies; the effect of lifetime variation as distributions relax, and whether this can be measured accurately by supplemental steady-state photoconductivity. We show that in some circumstances, the energy scale can be approximated by the quasi-Fermi level position, and suggest further that the density of states may then be computed using the derivative of the quasi-Fermi energy with respect to temperature.