Effect of [Zn]/[S] ratios on the properties of chemical bath deposited zinc sulfide thin films

ZnS thin films have been prepared by chemical bath deposition (CBD) technique onto glass substrates deposited at about 80 °C using aqueous solution of zinc sulfate hepta-hydrate, ammonium sulfate, thiourea, ammonia and hydrazine hydrate. Ammonia and hydrazine hydrate were used as complexing agents. The influence of the ratio of [Zn]/[S] on formation and properties of ZnS thin films has been investigated. The ratio of [Zn]/[S] was changed from 3:1 to 1:9 by varying volumes and/or concentrations of zinc sulfate hepta-hydrate and thiourea in the deposition solution. The structural and morphological characteristics of films have been investigated by X-ray diffraction (XRD), scanning electron microscope and UV-vis spectroscopic analysis. ZnS films were obtained with the [Zn]/[S] ratio ranged from1:1 to 1:6. In the cases of [Zn]/[S] ratio ≥ 3:1 or ≤1:9, no deposition was found. Transparent and polycrystalline ZnS film was obtained with pure-wurtzite structure at the [S]/[Zn] ratio of 1:6. The related formation mechanisms of CBD ZnS are discussed. The deposited ZnS films show good optical transmission (80-90%) in the visible region and the band gap is found to be in the range of 3.65-3.74 eV. The result is useful to further develop the CBD ZnS technology.