Analysis of microstructural properties, energy band structure, and defect levels of ZnS films obtained from micro-controlled SILAR method

Zinc sulphide (ZnS) thin films were deposited on glass substrate by micro-controller-based successive ion layer adsorption and reaction method using ZnCl₂ as a precursor. The films were found to be polycrystalline. The crystallite size of the films increased with the increase in both precursor concentration and immersion cycles. The increased crystallite size resulted in the reduction of grain boundary defects which, in turn, improved the electrical properties of the films. The energy band structure of the films was analysed using absorbance and photoluminescence spectra. The films displayed strong absorption for wavelengths less than 400 nm indicating the presence of a wide bandgap with shallow defect levels near the band edges. The bandgap of the films was estimated to be between 3.3 and 3.5 eV. The photoluminescence spectra of the films exhibited a prominent excitonic peak at around 3.33 eV. The defect level emissions were observed at around 3.1, 2.92, 2.76, and 2.64 eV. These emissions were attributed to the transitions from localized zinc and sulphur vacancy levels and interstitial states. An energy band diagram of the films was drawn based on the result of optical and photoluminescence studies.