LUMINESCENCE IN ALKALINE-EARTH SULFIDES

The paper reviews the major developments over the past two decades of alkaline earth sulphides as luminescent emitting and energy storage materials. Over this period there has been a marked increase in the fundamental understanding of the defect states in these sulphides through studies of electron spin resonance, thermally stimulated luminescence and fluorescence emission spectra at low and elevated temperatures. Advances have been made on the theoretical front in the estimation of defect energies in CaS, SrS and BaS that afford close agreement between calculated and observed cohesive energies assuming an ionic model with an ionicity of 0.9. Preparative methods for polycrystalline powders, thin film layers and single crystals have been developed significantly. The use of the sulphurisation flux technique has resulted in highly stable CaS powders, whilst electron beam evaporation, RF sputtering and atomic layer epitaxial methods have been employed to produce thin film multilayer structures. The xenon melt growth technique has been applied to the production of single crystal rods of CaS. There has been a recent focus on full colour ac electroluminescence using CaS and SrS doped with rare earth ions in mixed and multilayer structures for displays. BaS has been developed particularly as a thermoluminescent dosimeter material. Enhancements in fluorescent emission characteristics have been investigated in CaS mixtures with non-isostructural sulphides. The increased understanding gained of alkaline earth sulphides has advanced their development particularly in the electroluminescence field. However, further work on the theoretical and preparative fronts is needed to achieve greater opportunities for commercial exploitation.