Correlation of conductivity enhancement and Al-site defects in nanocolumnar ZnO films under vacuum annealing by experimental and calculations

The variation of Al-site defects was experimentally and theoretically investigated to explain the significant conductivity enhancement of vacuum-annealed Al-doped nanocolumnar ZnO (AZO) films, as compared to as-deposited AZO films. From the analyses of XPS and XAS, several defects including Al substitution (AlZn), Al interstitial site (Ali), and Zn vacancy (VZn) were detected in the as-deposited AZO films. After annealing, the transition from 2AlZn-VZn-Ali to AlZn-Ali complex defects was found. Moreover, FT-EXAFS also indicated the significant decrease of VZn in the annealed films. The formation energy computed by DFT calculation suggested that the AlZn-VZn-Ali complex defects could be formed in the as-deposited films. By heat treatment, an increase of AlZn is expected because more Ali could substitute to the VZn site forming the AlZn-Ali complex defects in the annealed films. The DOS calculation, the presence of Ali defects induced donor states in the AZO structure while and additional VZn acted as an acceptor state which diminished the Ali states. Hence, the increase of carrier concentration in the annealed films can be explained by the reduction of VZn defects.