Optical Properties and X-ray Absorption Fine Structure Analysis of ZnS:Cu,Cl Thin-Film Phosphors

ZnS:Cu,Cl thin-film phosphors have been studied for possible application to resin-free white light-emitting diodes (LEDs). The ZnS:Cu,Cl polycrystalline films grown by a molecular-beam deposition technique at relatively high substrate temperatures exhibit strong blue-green photoluminescence (PL). The films with a relatively high (${\sim}1$%) Cu concentration exhibit an enhanced near-UV excitation band as well as strong PL, making them suitable for excitation by UV LEDs; however, the transmission in the visible spectral range decreases. The X-ray absorption fine structure analysis of the phosphor films suggests that Cu atoms substitute for Zn sites at low Cu concentrations, whereas excess Cu atoms exist in a different form, possibly as CuS-like precipitates, at high Cu concentrations. The addition of Al donors to the phosphor films enhances the PL intensity and transparency, resulting from a reduction in the amount of the precipitates.