Structural and optical characterization of ZnO and AZO thin films: the influence of post-annealing

In the present study, ZnO and Al:ZnO (AZO) thin films were prepared by reactive RF sputtering on quartz substrates at a constant oxygen partial pressure and a typical sputtering power. Films were annealed at different temperatures in argon ambient in the oven to study their various structural and optical properties. It was understood that introducing Al into ZnO structure would affect the ZnO crystalline structure noticeably. It was observed that annealing had great influence on various properties of thin films while ZnO film showed low crystallinity, Al doping into ZnO structure pronounced significant improvement in both crystallinity and particle sizes. It was found that crystal structures, average crystalline sizes, and topology of all thin films were modified enormously by post-annealing. It was shown that films transparency fluctuated by annealing, in which the transparency of AZO thin film annealed at 500 °C was much greater than others. Annealing led to decrease optical band gap of all annealed films from 3.31 to 3.26 eV for ZnO and 4 to 3.4 eV for AZO films. Photoluminescence manifested that blue emission in as-deposited film led to two different blue and violet emissions in all AZO and ZnO films. It was identified that the emission intensity of AZO film annealed at 500 °C was 12 times more than other ZnO and AZO films.