Effects of annealing on the structural and optical properties of zinc sulfide thin films deposited by ion beam sputtering

We report the structural and optical properties of ZnS thin films fabricated by ion-beam sputtering. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed a polycrystalline ZnS film with zinc blende phase as manifested by diffraction from the (111), (220) and (311) planes. Annealing resulted in the appearance of a metastable wurtzite phase with a concentration up to 26.6%. An energy bandgap, estimated from absorption spectra, was found to vary between 3.32 and 3.40 eV. The lower energy of this bandgap, as compared to bulk ZnS, is associated with the structural point defects along with mixed zinc blende and wurtzite phases of the polycrystalline ZnS films. Ion beam sputtering deposition can be used to tune the optical bandgap for potential applications in optoelectronic materials.