Dependences of the surface and the optical properties on the O2/O2 + Ar flow-rate ratios for ZnO thin films grown on ZnO buffer layers

ZnO active layers on ZnO buffer layers were grown at various O2/O2 + Ar flow-rate ratios by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughnesses of the ZnO active layers grown on ZnO buffer layers decreased with decreasing O2 atmosphere, indicative of an improvement in the ZnO surfaces. The type of the ZnO active layer was n-type, and the resistivity of the layer increased with increasing O2 atmosphere. Photoluminescence spectra from the ZnO active layers grown on the ZnO buffer layers showed dominant peaks corresponding to local levels in the ZnO energy gap resulting from oxygen vacancies or interstitial zinc vacancies, and the peak positions changed significantly with the O2/O2 + Ar flow rate. These results can help improve understanding of the dependences of the surface and the optical properties on the O2/O2 + Ar ratio for ZnO thin films grown on ZnO buffer layers.