Effect of the growth conditions on infrared upconversion efficiency of CaS: Eu, Sm thin films

High-quality electron-trapping thin films CaS: Eu, Sm with red light output have been successfully deposited on quartz and single-crystal silicon substrates by electron beam evaporation (EBE) and radio frequency (RF) magnetron sputtering in vacuum and H₂S partial pressures. Infrared upconversion efficiency of CaS: Eu, Sm thin films under different growth conditions has been investigated by using ultrashort infrared (IR) laser pulse with 20 ps (full width at half-maximum (FWHM)). The results show that upconversion efficiency of CaS: Eu, Sm thin films depends strongly on growth conditions in spite of the existence of "exhaustion" phenomena. Microstructures identified by X-ray diffraction (XRD) indicate that crystallinity of CaS films relies on both substrate materials and growth conditions. The stoichiometric composition of CaS films was measured by secondary-ion mass spectrometry (SIMS). The post-annealing process was found to promote grain growth and activate strong luminescence so that it could obviously improve upconversion efficiency of CaS: Eu, Sm films, even though it had negative influence on transmittance and spatial resolution of these films.