Synthesis and optical characterization of a high-quality ZnS substrate for optoelectronics and UV solar-energy conversion

Abstract

ZnS is an environmentally friendly wide-band-gap semiconductor possessing a direct gap larger than ZnO, which makes it more promising for application in ultraviolet (UV) optoelectronics and solar-energy conversion applications. However, highly crystalline ZnS is usually obtained by elegant epitaxial growth, such as with pulsed laser vaporization, molecular beam epitaxy, and metal–organic chemical vapour deposition, in a thin-film form. A high-quality ZnS substrate crystal has been rarely achieved to date. Herein, we demonstrate a high-grade cubic ZnS (c-ZnS) substrate crystal with a longer range order grown by a chemical vapour transport (CVT) method. Photoluminescence (PL), transmittance, contactless electroreflectance (CER), and transmission electron microscopy were performed for qualification of the c-ZnS substrate. The transparency of the c-ZnS substrate crystal (∼0.6 mm thick) was about 81% at λ = 400 nm and 92% at λ = 600 nm. Strong and complete series excitonic emissions were detected from the ZnS substrate crystal by PL, indicating the high crystallinity of the CVT-grown ZnS. Band-edge free-exciton emission of the ZnS substrate showed an intense emission around 3.68 eV at 300 K. The temperature dependences of the band-edge excitonic emissions from 10 to 300 K were analyzed and are herein discussed in terms of sustaining the high crystalline quality of the c-ZnS substrate. An initially-formed Cu/ZnS Schottky solar cell was also tested using the c-ZnS substrate. Under the illumination of a 325 nm laser (i.e. a power density of P ∼ 48 mW cm⁻²), a significant photovoltage of ∼0.6 V could be generated from the original Cu/ZnS Schottky solar cell.