An X-Ray Photoelectron Spectroscopy Study of Elements' Chemical States in SrGa₂S₄:Ce Blue Electroluminescent Thin Films

SrGa 2 S 4 :Ce blue electroluminescent thin films with a (440) single orientation have been prepared by a multisource deposition method using a molecular beam epitaxy (MBE) apparatus. The elements' chemical states and the doped oxygen effects on them have been analyzed by X-ray photoelectron spectroscopy (XPS). A small variation in the elements' chemical state with depth indicates that the growth of the thin film is stable. Sr ions exhibit no chemical shift due to a Sr–O bond, but Ga ions exhibit a chemical shift due to a Ga–S bond and a larger shift due to a Ga–O bond, indicating that the oxygen ion does not substitute for a S site in which the S ion bonds with the Sr ion. No Ce 4+ ions have been detected and no oxidation of Ce 3+ ions occurs even if some of the S ligands of Ce 3+ are replaced by doped oxygen ions. The trivalent state of Ce is stable in the SrGa 2 S 4 crystal.