Variations of microstructure, conductivity and transparency of Al-doped ZnO thin films prepared by radio frequency magnetron sputtering with target-substrate distances

Al-doped ZnO films were deposited at different target-substrate distances by radio frequency magnetron sputtering. The crystallite size of the films is reduced with increasing the target-substrate distance, but the (002) preferential orientation of ZnO is observed for all the films. It is found that the target-substrate distance has a great influence on the carrier concentration in the films. Reduction of the target-substrate distance is favorable to obtain higher carrier concentrations. The lowest resistivity of 1.1 × 10−3 Ω cm is obtained for the film at target-substrate distance of 55 mm. The optical transmittance in the visible range remains higher than 90% for all the films, and the absorption edge shifts towards the shorter wavelength side with decreasing the target-substrate distance. The band gap was widened by 0.11 eV due to the Burstein-Moss (BM) shift from 3.33 eV to 3.44 eV with the reduction of the target-substrate distance from 60 mm to 55 mm.