Excitation Mechanism of SrS Thin-Film Electroluminescence Devices Studied by Excitation Spectra of Photo-Induced Transferred Charge

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Rare-earth doped alkaline-earth sulfides have been receiving a great deal of attention as a phosphor materials for malti- and full-color thin-film ElectroLuminescent (EL) display panels. It has been found that CaS thin-films doped with Eu^<2+> and Ce^<3+> luminescent centers, which give rise to luminescence due to (4f)^<n-1>(5d)-(4f)^n transitions, Show the efficient EL. Based on several experimental results, it has been argued that the processes play an important role in that the ionization of luminescent centers first occurs, subsequent recombination with electrons follows, and finally luminescence arises. However no direct evidence of the ionization of luminescent centers has been observed until new. In this work, we studied the photo-induced transferred change in Eu^<2+> and Ce^<3+> doped CaS and SrS thin-film EL devices, which give us direct evidence of ionization of luminescent centers. The experimental results are described in the following.1. The spectral response of the photo- … More induced transferred charge DELTA Q in Eu^<2+> and Ce^<3+> doped CaS and SrS thin-film EL devices has been studied. For the Eu^<2+> doped EL devices, the increase in DELTA Q is observed for wide wave region from 400 to 600 nm. For the Ce^<3+> doped devices, the increase in DELTA Q is observed at around 440 nm.2. It is found that the excitation spectra of the photo-induced transferred charge agree with those of photoluminescence. This results implies that Eu^<2+> or Ce^<3+> 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 is controlled by field dependent ionization of the luminescent centers, and by field-dependent trapping at the ionized centers rather than impact excitation of the luminescent centers.3. We have calculated the ionization probability of the luminescent centers as a function of electric field. The ionization of the Eu^<2+> or Ce^<3+> luminescent centers takes place rather than luminescent transition under the strong electric field over 1X10^6 V/cm. EL characteristics of these EL devices are well explained by this excitation model. Less