Optical recombination mechanisms in Eu2+-doped CaS and SrS thin films

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Abstract
Excitation and deexcitation mechanisms of Eu2+ emission in CaS:Eu and SrS:Eu thin films have been studied. The photoluminescence spectra and the luminescence decay kinetics were examined as function of the temperature. It was found that both internal transitions within the Eu2+ ions and resonant energy transfer between different Eu centers are responsible for the Eu2+luminescence excitation in low temperatures. Moreover, at temperatures T≳100 K, electron transfer between different europium ions over the conduction band (CB) of the host lattice influences efficiency of the emission. This is possible since the Eu ion can change its charge state from 3+ to 2+ by trapping a CB electron via the 4f 65d 1 Eu2+excited state.