(002) oriented ZnO and ZnO:S thin films by direct ultrasonic spray pyrolysis: A comparative analysis of structure, morphology and physical properties

In this study, we deposit sulfur incorporated ZnO thin films (ZnO:S) by ultrasonic deposition and analyze changes in the structure, morphology, optical and electrical properties of the films in comparison with the pristine ZnO thin films. Further, electronic band structures of ZnO and (ZnO:S) were theoretically calculated using DFT-VASP method. ZnO and (ZnO:S) thin films formed at different conditions showed hexagonal crystal structure with crystallites of 30–40 nm in size fully oriented along [002] direction perpendicular to substrate. The optical band gap of ZnO was 3.2 eV which was reduced to 2.8 eV due to sulfur incorporation. Vacuum annealed ZnO showed a very low sheet resistance of 67 Ω/□ and the value was increased to 470 Ω/□ in ZnO:S films, nearly metallic behavior. Hall effect measurements confirmed the n-type conductivity with a maximum carrier concentration of 5.6 × 1020 cm−3. These films can find potential applications as window layers with improved charge extraction contacts for solar cell applications.