Optical absorption studies on the trapping states of CaS:Eu,Sm

Abstract

The optical absorption spectra of CaS:Eu,Sm at several concentrations of Eu (from 0 to 0.05 mol%) were examined to clarify the electron trapping mechanism for its photo-stimulated luminescence. Corresponding to the electron and hole trapping states, the difference between the absorption spectra after and before excitation was observed. The substructure of the infrared band of the absorption difference is eclipsed by the 4f-4f absorption of Sm3+ ions in accordance with the photo-stimulation spectrum. In comparison of Sm3+ with La3+ in Eu co-doped CaS, it was inferred that the Sm3+ ions are responsible for the storability of the traps instead of the direct capture of electrons as the previous investigators thought. Our results demonstrate that the Sm3+ ions induce defects in their neighbourhood which act as the trapping centres but the Sm3+ ions do not form Sm2+ ions by trapping electrons.