Chemical trends of n-type doping of Al, Ga, In, and Ti donors for ZnO polycrystalline films deposited by direct-current magnetron sputtering

Abstract

We investigated the effects of various donors such as Al, Ga, In, and Ti atoms on the properties of ZnO polycrystalline films. The contents of the dopants in the sintered targets ranged from 0.25 to 5.0 mol. %. To reduce the contribution of grain boundary scattering to the carrier transport, we used an amorphous glass substrate with 10-nm-thick Ga-doped ZnO films showing a texture with a well-defined (0001) orientation. 490-nm-thick n-type doped ZnO films were deposited at a substrate temperature of 200 °C by direct current magnetron sputtering with a power of 200 W. We found that Ga doping resulted in a high carrier concentration at any given donor content, whereas Al doping was an effective way of achieving ZnO polycrystalline films with a high Hall mobility. We also found that In-doped ZnO films have a high density of in-gap states, resulting in high optical absorption in the visible wavelength region. X-ray absorption spectroscopy measurement results clearly show the dependence of the occupation sites of incorporated Ti atoms on Ti contents.