In situ XPS analysis of various iron oxide films grown by NO2- assisted molecular-beam epitaxy

We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., α−Fe2O3, γ−Fe2O3, Fe3−δO4, and Fe3O4. All iron oxide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and α−Al2O3(0001) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the NO2 pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe 3d to O 2p hybridization and the d−d electron-correlation energy. The small difference in the satellite structures between α−Fe2O3 and γ−Fe2O3 resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in γ−Fe2O3 compared to α−Fe2O3. With increasing reduction in the γ−Fe2O3−Fe3O4 system, the satellite structures in XPS became unresolved. This was not only due to the formation of Fe2+ ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral Fe3+ ions