Structure and optoelectronic properties of ZnS1− x Se x films deposited by RF co-sputtering

In this study, ZnS1−xSex alloys were deposited by RF magnetron co-sputtering with varying ZnSe/ZnS ratios, followed by post-annealing in Ar-H2 at 300 °C. Structure and optoelectronic properties were evaluated. X-ray diffraction revealed that films crystallize in the cubic phase with high (111) orientation, along the 0 < x < 1 composition range indicating the formation of the pseudobinary alloy. UV-Vis spectroscopy confirmed the pseudobinary alloy formation with tunable bandgaps (3.57–2.74 eV) due to Se by S substitution. Tauc plots and Urbach tail parameter showed evidence of electronic states within the bandgap and disorder related to the different ion size in the substitutional alloy. Raman spectroscopy indicates that films have Zn–Se, Zn–S, S–Se bonds in the local-range ordering possibly due to the nature of co-sputtering method. The presence of Zn interstitials was evidenced by the Auger Zn L3M4.5M4 peak at ca. 494 eV from XPS. Photoluminescence in ZnSe revealed the presence of Zni, VZn, VSe, and VZn-VSe defects; in ZnS, the dominant defects were Zni and VS, while VSe and VZn-VSe defects predominate in the films with higher Se contents. Electrochemical impedance spectroscopy indicates a high sheet resistance of the films, while Mott–Schottky plots confirm n-type conductivity in all the films. Semiempirical band diagrams were built using experimental and reported data, indicating that films have a favorable bandgap and band positions for eventual photocatalytic H2 production; however, defects near the valence band could be deleterious for the potential application in photoelectrochemical cells and deserve further consideration.