Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application

RF-sputtered ZnS thin film was grown under various substrate temperatures with the aim of investigating its effects on the structural, surface morphology and optical properties. Investigated substrate temperature in this study was in the range of 25 °C-300 °C and the structural and optical properties were investigated in order to elucidate the changes induced by the varying thermal energy during the growth process. Structural determination by XRD method indicates all sputtered films have cubic structure with (1 1 1) as the preferential orientation. However, higher substrate temperature up to 200 °C increases the film's crystallinity and grain size evident by the increase in peak intensity. Slight peak shift indicates ZnS lattice undergoes strain relaxation process mediated through the increase in the lattice constant from 5.32 Å to 5.40 Å. SEM image of surface morphology clearly shows the evolution of grain growth in which sputtered film at 200 °C has the largest grains with distinct grain boundaries. Calculation from the obtained transmission spectra indicates optical band gap is in the range of 3.6-3.9 eV. Theoretical analysis in terms of lattice parameter between ZnS with several upcoming photovoltaic absorber layers shows that lattice matched ZnS buffer layer can be grown by varying the substrate temperature.