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 a nd SrS1-xSex:Ce electroluminescent structures, in order to examine the composition and stoichiometry of the different layers in the thin fil m stack. The positive effect of selenium coevaporation on the quality of the active layer was clearly established. A detailed XPS study of t he valences of europium and cerium doping elements, incorporated in th e CaS1-xSex and SrS1-xSex host lattice, was completed. It was shown th at Ce is in a 3+ state. The Eu valence determination was not as straig htforward since divalent europium does not remain stable during the XP S measurements. The impact of aging on the electroluminescent emission and on the chemical state of the rare earth dopants was also studied.