Porous/dense ZnO bilayer films grown by thermal oxidation of ZnS film with gallium

Porous/dense ZnO bilayer films were grown by thermal oxidation of ZnS films deposited with gallium in air. The microstructures and photoluminescent properties of the as-grown ZnO bilayer films were investigated by XRD, SEM, HRTEM, XPS and PL. It was found that Ga doping contributed to the growth of ZnO dense layer, which covers the ZnO porous layer formed due to the evaporation of sulfur oxides. The diffusion-limited Ga gradient distribution along the thickness of ZnO films was observed. The remained Ga on the surface region led to the formation of ZnGa2O4 layer covering the ZnO film. The growth mechanism and morphology of the ZnO film covered by ZnGa2O4 grown by physical vapor deposition of metal gallium are quite different from that of the ZnO film covering ZnGa2O4 grown by manual application of molten gallium. Meanwhile, the formation of ZnGa2O4 was prohibited by altering Ga deposition distance. Higher Ga doping decreased the concentration of oxygen vacancy, which reduced the visible emission from ZnO film in combination with existence of ZnGa2O4 layer.