Multi-layered ZnO thin films were prepared through a multi-step deposition process on (0001) sapphire substrates by radiofrequency magnetron sputtering method. For comparison, single-layered ZnO films were fabricated by single-step deposition process. The multi-layered ZnO films consisted of three layers: the bottom layer, the intermediate layer and the top layer. The crystalline quality and surface roughness of the multi-layered films were superior to those of the single-layered films. The effect of bottom layer thickness on the crystalline quality and surface roughness of ZnO film was also studied. As the bottom layer thickness decreased, the intensity of (002) peak in X-ray diffraction spectrum increased and a full-width at half maximum (FWHM) of the (002) peak became narrower, indicating the bottom layer thickness had a significant effect on the crystalline quality. The surface smoothness of the film was also improved with decreasing the bottom layer thickness. The optical bandgap was in substantial agreement with that of bulk ZnO with decreased bottom layer thickness.

Keywords: ZnO thin film, RF sputtering, Multistep deposition, Bottom layer thickness, Crystalline quality, Surface smoothness.