Rare-Earth Excitation Mechanism in Wide Band Gap H-VI Compounds

The excitation mechanism of rare-earth emission in Eu and Ce doped CaS and SrS is studied. It is proposed that the Eu and probably also Ce emission is induced by the photoionization transition of the rare-earth ion, which is followed by the carrier trapping via the excited state of the ion. At increased temperatures the efficiency of excitation is reduced. We explain this effect by the carrier emission from the excited core state of the rare-earth ion to the continuum of the conduction (valence) band states. It is also suggested that the charge transfer state of the rare-earth ion may act as the intermediate state in the carrier trapping.