Preparation and characterisation of thickness dependent nano-structured ZnS thin films by sol-gel technique

Zinc sulfide (ZnS) thin films of different thickness were coated on glass substrates by the sol-gel dip-coating technique. Thickness dependent structural and optical properties of the films were studied in detail. X-ray diffraction (XRD) analysis indicated that the films had mixture of cubic (beta) and hexagonal (alpha) phases with cubic (beta) phase being predominant. Scanning electron microscope (SEM) showed that the film surfaces were smooth and crack free. Energy dispersive X-ray (EDX) measurement showed no impurity in the ZnS compound with elemental concentration of Zn/S being 50.38/49.62. Optical measurements showed that optical transmittance of the films were decreased in the visible range as the film thickness increased and band gap of the thin films were estimated to be about 3.61, 3.56, and 3.39 eV for the films with thickness of 100, 220, and 360 nm, respectively. Reactive indices and extinction coefficients of the films were measured by Spectroscopic Ellipsometer. Both the refractive index (n) and extinction coefficient (k) of the films were increased as the film thickness decreased. Electrical measurements showed that the resistivity of the films were decreased as the substrate temperature and film thickness increased.