AN XPS STUDY OF THE DOPANTS VALENCE STATES AND THE COMPOSITION OF CAS1-XSEXEU AND SRS1-XSEXCE THIN-FILM ELECTROLUMINESCENT DEVICES

X-ray photoelectron spectroscopy (XPS) was performed on CaS1-xSex:Eu and SrS1-xSex:Ce electroluminescent structures, in order to examine the composition and stoichiometry of the different layers in the thin film stack. The positive effect of selenium coevaporation on the quality of the active layer was clearly established. A detailed XPS study of the valences of europium and cerium doping elements, incorporated in the CaS1-xSex and SrS1-xSex host lattice, was completed. It was shown that Ce is in a 3+ state. The Eu valence determination was not as straightforward since divalent europium does not remain stable during the XPS measurements. The impact of aging on the electroluminescent emission and on the chemical state of the rare earth dopants was also studied.