Chromium-Doped MgO Thin Films: Morphology, Electronic Structure, and Segregation Effects

Incorporation of Cr into a crystalline MgO(001) thin film has been investigated by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and diffraction. For this purpose, samples of different Cr content (0-33 at %) and postannealing treatments (300-1050 K) have been analyzed. The Cr impurities mainly adopt a 3+ oxidation state, which renders the formation of compensating Mg vacancies necessary to maintain charge neutrality. Only at 33 at % doping amount are Cr6+ species detected in the as-grown films, indicating the development of a metastable CrO3 phase. At low doping level, the Cr ions fully dissolve in the MgO lattice, while segregation toward the surface is observed for Cr-rich films. In the latter case, a new surface oxide develops that is characterized by a lower binding energy of the Mg is and Cr 2p core levels and a (2 x 1) superstructure with respect to weakly doped MgO. In combination with a distinct 4-fold symmetric XPD pattern, the new surface phase resembles Mg chromite (MgCr2O4), growing on top of the heavily doped MgO film. Our study provides insight into the technologically relevant transition region between doped binary and ternary oxides.