Fast photoresponse and the related change of crystallite barriers for ZnO films deposited by RF sputtering

Hexagonal ZnO films were deposited on a glass substrate at room temperature by RF sputtering a ZnO target using a mixture of oxygen and argon. Polycrystalline ZnO films prepared in the presence of excess oxygen or at a high oxygen pressure tend to have crystallites with the orientation of the c-axis parallel to the substrate and a large and relatively fast photoresponse. The time constants of the photoresponse can be much reduced by doping the surface layer with nitrogen. This fast photoresponse of the resulting films is related to the changes of barrier heights of grain boundaries due to photodesorption of the chemisorbed oxygen on the grain boundaries when the films are irradiated with uv light.