Photo-Induced Transferred Charge in Rare-Earth-Doped Alkaline-Earth Sulfide Electroluminescent Thin Films

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Abstract
The spectral response of the photo-induced transferred charge in Eu2+- or Ce3+-doped CaS and SrS thin-film electroluminescent (EL) devices has been studied. It is found that excitation spectra of the photo-induced transferred charge agree with those of photoluminescence. This result implies that the Eu2+ or Ce3+ luminescent centers are ionized by electric field when the centers are excited to the (4f) n-1(5d) excited state. The dynamics of the EL emission are controlled by field-dependent ionization of the luminescent centers and by field-dependent trapping at the ionized centers, rather than by impact excitation of the luminescent centers.