An XPS study of SrS:Ce thin films for electroluminescent devices

Cerium-doped SrS thin films were analyzed using X-ray photoelectron spectroscopy (XPS) in order to optimize the doping conditions of real electroluminescent thin films. The films were deposited by atomic layer epitaxy (ALE) on soda lime glass substrates using β-diketonate complexes of the metals and H2S as precursors. The composition and the stoichiometry of the SrS:Ce films as well as the chemical state of strontium, sulfur and cerium were determined. In addition, the structure of the films was studied using X-ray diffraction (XRD) and the average cerium content of the films was measured by X-ray fluorescence spectroscopy. XRD measurements showed that the SrS:Ce thin films were well-crystallized having oriented cubic structure. According to the XPS measurements, cerium occurred as trivalent both at the surface and in the bulk of the films even in high concentrations. Strontium was mainly present as strontium sulfide at the film surface. In addition to this, strontium carbonate, strontium hydroxide and strontium sulfite were detected. Sulfur appeared mainly as sulfide at the surface but traces of oxidized sulfur were observed, too.