Properties of sputtered ZnS and ZnS:A (A = Er, Yb) films grown at low substrate temperatures

Abstract

Radio-frequency magnetron sputtering was used to deposit ZnS thin films doped with rare earth elements (Er and Yb) on glass substrates at two different temperatures (room temperature and 180 °C). The crystal structure, surface morphology, and optical properties of the deposited films were studied as a function of the deposition variables as a first step in the possible application of lanthanide-doped ZnS as buffer layer and/or window layer in thin film photovoltaic devices. From the x-ray diffraction patterns, it was found that the samples have preferred orientation depending on the substrate temperature and on the doping level. Moreover, optical constants were determined from spectroscopic ellipsometry measurements and found that samples of undoped ZnS, in particular, exhibit optical constants that are in good agreement with those reported in the literature for the two ZnS crystalline phases. The band gap (Eg) was calculated using the extinction coefficient (k), which was reduced according to the degree of doping. Also, based on ultraviolet-visible spectroscopy measurements, it was determined that the films have a transmittance of 80% for wavelengths above 350 nm, and a shift of the absorption edge to longer wavelengths when the degree of doping was increased. Finally, photoluminescence measurements were performed at room and low temperature using below bandgap excitation to probe defect states within the gap, as well as the luminescent characteristics of the Er3+ and Yb3+ ions in the ZnS host.