Optical Study of High Quality c-ZnS Crystals for UV Photodiodes and Photoelectrochemical Applications

Wide band gap semiconductors are proper candidates for manufacturing UV-visible electronic devices, photocatalysts and power electronic components. Among them, II-VI ZnS has a high flexibility in tuning energy range as compared to ZnO because of a larger bandgap about 3.7eV. In this work, a high-quality {111}-oriented ZnS crystal of zinc-blende structure has been successfully grown by chemical vapor transport method using I-2 as the transport agent. High-resolution transmission electron microscopy verifies its crystallinity and crystal orientation, and energy dispersive X-ray analysis identifies its stoichiometry. Due to the coupling between crystal field and spin orbital in the high quality solid, three band-edge excitonic transitions of E-A = 3.745, E-B = 3.799 and E-C = 3.881 eV have been detected. An initial metal-semiconductor-metal (MSM) Schottky photodiode made by {111}-ZnS crystal has been assessed. The dark resistivity is about 6.13 M Omega-cm. Under the irradiation of a 266-nm laser (power similar to 1.6 mW), the photo-resistivity change (i.e., Delta rho/rho(dark)) reaches 88%. Photo-catalytic ability of the {111}-ZnS single crystal was also evaluated using Methyl-blue degradation. The degradation (normalized concentration change) after 1 hour can reach C/C-0 approximate to 0.47 under irradiation of Xe-arc lamp. All the experimental results reveal high responsivity of the {111}-ZnS photodiode and good photocatalytic function of the {111}-oriented crystal photocatalyst operated in the UV range.