An X-ray photoelectron spectroscopy study of elements' chemical states in SrGa2S4:Ce blue electroluminescent thin films

SrGa2S4: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 stares 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 Ca 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 bands with the Sr ion. No Ce4+ ions have been detected and no oxidation of Ce3+ ions occurs even if some of the S ligands of Ce3+ are replaced by doped oxygen ions. The trivalent state of Ce is stable in the SrGa2S4 crystal.