Effect of substrate temperature on the optical properties of thermally evaporated ZnS thin films

Zinc Sulfide (ZnS) thin films were formed onto cleaned glass substrates using the thermal evaporation method in vacuum. The substrate temperature was varied between as- deposited and 150 °C, keeping the film thickness and the rate of evaporation fixed at 200 nm and 0.3 nm . s^-1, respectively. The film thickness was measured in situ by a quartz crystal thickness monitor. The structure of the films was ascertained by x-ray diffraction (XRD) method. The XRD spectra show that the films are polycrystalline in nature, which possess hexagonal structure. The optical properties of the films were ascertained by UV-VIS-NIR spectrophotometer (photon wavelength ranging between 300 and 2500 nm). The optical transmittance and reflectance were utilized to compute the absorption coefficient, band gap energy and refractive index of the films. Nature of the optical transition of the films has been observed in direct allowed with band gap energies between 3.37 and 3.46 eV depending on substrate temperatures. The refractive index was found to vary inversely with incident photon energy.