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 oxid e films, i.e., alpha-Fe2O3, gamma-Fe2O3, Fe3-deltaO4, and Fe3O4. All iron o xide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and alpha-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 s atellite structures. These satellite structures were simulated using a clus ter-model calculation, which could well reproduce the observed structures b y considering the systematic changes in both the Fe 3d to O 2p hybridizatio n and the d-d electron-correlation energy. The small difference in the sate llite structures between alpha-Fe2O3 and gamma-Fe2O3 resulted mainly from c hanges in the Fe-O hybridization parameters, suggesting an increased covale ncy in gamma-Fe2O3 compared to alpha-Fe2O3. With increasing reduction in th e gamma-Fe2O3-Fe3O4 system,the satellite structures in XPS became unresolve d. This was not only due to the formation of Fe2+ ions, but also to nonhomo geneous changes in the hybridization parameters between octahedral and tetr ahedral Fe3+ ions. [S0163-1829(99)02704-6].