Optical and photoelectrical properties of ZnO thin films and the effects of annealing

Single-crystalline ZnO thin films were grown using plasma-assisted molecular beam epitaxy. The effect of post growth annealing was studied using photoluminescence and photoconductivity for characterisation. Photoluminescence of annealed samples shows a strong improvement in response between non-annealed and annealed samples. Sharp excitonic features as well as LO phonon replicas were observed for annealed samples, suggesting improved film quality, although the annealing temperature has to be carefully adjusted. Photoconductivity reveals strong multi-peak features at the bandedge and, therefore helps identifying the features in the photoluminescence spectra. Temperature and chopping speed dependencies show an interesting behaviour of the photoconductivity peaks observed for samples annealed at close to optimum temperatures.