Received August 26, 2016; Revised September 26, 2016; Accepted October 26, 2016.
Crystallographic and electro-optical characteristics were studied for the ZnO:Al transparent conducting thin films grown by co-sputtering with ZnO and Al targets. Al content in the films grown at 400oC with some constant ZnO target power was found to increase and the electrical resistivity decreased as Al target power increased up to some levels. It can be explained by the substitution of Al ions with Zn ions and the increase of free carriers. But above some power, Al ions would be located interstitially in the ZnO lattice and the quality of the films got worse. With increasing substrate temperature, the electron density increased and the electrical resistivity decreased in the films grown at fixed powers. The transmission in visible range for all samples was fairly good. Low temperature photoluminescence showed a strong near band-edge emission near 3.3 eV compared with ~2.9 eV green emission, which means a high free carrier density and a low deep level trap density. It reveals that as the substrate temperature increases, Al ions are effectively substituted with Zn ions and act as dopants to increase free electron density.