Investigation of the free charge carrier properties at the ZnO-sapphire interface in a-plane ZnO films studied by generalized infrared ellipsometry

The free charge carrier properties at the film-substrate interface of non-polar ZnO films were investigated by generalized infrared spectroscopic ellipsometry in dependence on film thickness and annealing time. For all samples a highly-conductive film-substrate interface layer was found. It was observed that the charge density at the interface increases with increasing film thickness and decreases with increasing annealing time. Possible explanations could be the diffusion of Al from the substrate into the ZnO film, the formation of zinc aluminate, or structural changes, respectively.