Exploring the impact of air annealing on zinc sulfide thin films for enhancing ultraviolet photodetection in environmental monitoring applications

Zinc sulfide (ZnS) thin films were deposited on p-type silicon substrates using RF magnetron sputtering and annealed in air between 125 °C and 500 °C. XRD and XPS confirmed the gradual formation of ZnS–ZnO mixed phases due to oxidation during and after deposition. AFM showed that the films retained a smooth morphology after post-annealing. The optical band gap increased from ∼3.54 eV (unannealed) to ∼3.89 eV at 250 °C, then dropped to ∼3.42 eV at 500 °C due to phase transformation and defect formation. Photodiodes fabricated from both unannealed and annealed films showed typical p–n junction behavior. The device annealed at 500 °C exhibited enhanced UV photodetection under 365 nm light exposure at 1.22 mW/cm², with a sensitivity of 1.16 × 10⁴ %, responsivity of 0.6 A/W, and rise/fall times of 0.17/0.26 s. These results demonstrate that air annealing is a simple and eco-friendly strategy to enhance the optoelectronic properties of ZnS films for UV photodetector applications.