Effect of vanadium doping on amorphization of ZnO thin films on c-plane sapphire substrate

The effect of vanadium (V) doping on amorphization of zinc oxide (ZnO) thin films deposited on c-plane sapphire substrate at room temperature by RF magnetron sputtering was investigated. From the in-plane X-ray diffraction measurements, the diffraction intensity from 30° rotated domains observed in the ZnO film weakened with increasing V concentration. Faint diffractions from both the 30° rotated and normal domains were seen in the V-doped ZnO (VZO) film of 1.7-at.% V, and relatively strong diffractions only from the normal domain were observed at 2.1-at.% V while the 30° rotated domains diminished. There were only weak normal domains for VZO over 2.4-at.% V. Concerning the c-axis orientation, the broad (0002) diffraction peaks were observed in all VZO films, and the peak was the widest for the 3.8-at.% VZO film. From the dependence of crystallinity on thickness for the highly-doped VZO (V: 3.8 at.%) films, the c-axis orientation deteriorated with increasing film thickness and no diffraction signal was seen for the VZO films over 35-nm thick. This means that the VZO film aligned by the c-plane sapphire grew only at the initial stage, but an accumulation of disordered layers worsened the crystallinity with thickening the film. From the above-mentioned results, even in the ZnO film in which the grains tend to align to the c-axis orientation preferentially, the doping of transition metals, like V in this study, plays an effective reaction to form the pseudo-amorphous thin film. It is considered that this phenomenon is attributed to the feature of transition metal in which the transition metal atoms can take a state of various valence or coordination numbers.