Density of band-gap traps in polycrystalline films from photoconductivity transients using an improved Laplace transform method

Slow relaxation of the photoconductivity over a period of days was studied in polycrystalline ZnO and ${\mathrm{TiO}}_2$ films prepared by spray pyrolysis. The phenomenon is described by a model involving deep sensitizing hole traps in the forbidden gap. The trap state distribution based on this model was calculated using an improved Laplace transform method. For ZnO and ${\mathrm{TiO}}_2$ films, the density of states was shown to have a peak-like distribution with a maximum near the lower third of the energy gap.