Probing the effect of annealing on the structural and optical properties of CdS thin films: Combining experimental and first-principles study

Defects play a pivotal role in determining the performance of semiconductor materials, and their influence can be effectively adjusted through processing techniques. In this study, CdS thin films were prepared using a novel sputtering-evaporation-sputtering (SES) method with metal target followed with annealing. The effects of annealing temperature on the structural and optical properties of CdS film were thoroughly investigated by combining various experimental techniques with first-principles calculations. The results showed that the unannealed precursor of CdS film has poor crystallinity and low transmittance, with unreacted Cd present. Annealing significantly improves crystallinity of CdS films. The transmittance and stoichiometric ratio initially increase with annealing temperature but decrease at higher temperatures. Optimal annealing conditions enable a complete transformation of precursors into CdS films, while excessive temperatures lead to the escape of sulfur and the formation of sulfur vacancies. Sulfur vacancies, confirmed by first-principles calculations and positron annihilation Doppler broadening spectroscopy, significantly contribute to the observed transmittance degradation at elevated temperatures. This research is expected to provide insights into the thermal treatment and vacancy management of CdS thin films.