In situ XPS analysis of various iron oxide films grown by ${\mathrm{NO}}_2$-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., $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3,$ $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3,$ ${\mathrm{Fe}}_{3\mathrm{-}\mathrm{\delta }}{\mathrm{O}}_4,$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4.$ All iron oxide films were prepared epitaxially by ${\mathrm{NO}}_2$-assisted molecular-beam epitaxy on single crystalline MgO(100) and $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3\mathrm{}\left(0001\right)\mathrm{}$ substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the ${\mathrm{NO}}_2$ 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 $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ compared to $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3.$ With increasing reduction in the $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3{-\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4$ system, the satellite structures in XPS became unresolved. This was not only due to the formation of ${\mathrm{Fe}}^{2+}$ ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral ${\mathrm{Fe}}^{3+}$ ions.