In situ xps analysis of various iron oxide films grown by (formula presented)-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., (Formula presented) (Formula presented) (Formula presented) and (Formula presented) All iron oxide films were prepared epitaxially by (Formula presented)-assisted molecular-beam epitaxy on single crystalline MgO(100) and (Formula presented) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the (Formula presented) 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 (Formula presented) to O (Formula presented) hybridization and the (Formula presented) electron-correlation energy. The small difference in the satellite structures between (Formula presented) and (Formula presented) resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in (Formula presented) compared to (Formula presented) With increasing reduction in the (Formula presented) system, the satellite structures in XPS became unresolved. This was not only due to the formation of (Formula presented) ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral (Formula presented) ions.