Atomic-oxygen-assisted MBE growth of α${-\mathrm{F}\mathrm{e}}_2$${\mathrm{O}}_3$ on α${-\mathrm{A}\mathrm{l}}_2$${\mathrm{O}}_3$(0001): Metastable FeO(111)-like phase at subnanometer thicknesses

The growth of $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ on $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3\mathrm{}\left(0001\right)\mathrm{}$ by atomic oxygen—molecular-beam epitaxy has been studied by real time reflection high-energy electron diffraction, low energy electron diffraction, and x-ray photoelectron spectroscopy. For equivalent thicknesses below 2 ML, epitaxial islands of an uncommon iron oxide are evidenced. This metastable FeO(111)-like phase exhibits a strongly expanded in-plane parameter, and contains ferric ${\mathrm{Fe}}^{3+}$ ions instead of ferrous ${\mathrm{Fe}}^{2+}$ ones. This phase seems to be specific to a given thickness (∼2 ML), independently of the substrate. For higher equivalent thicknesses, epitaxial islands of hematite $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3\mathrm{}\left(0001\right)\mathrm{}$ begin to nucleate. The amount of the hematite phase increases whereas that of the FeO(111)-like phase decreases with deposition time. For a deposit equivalent to one hematite unit cell, islands coalesce and $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3\mathrm{}\left(0001\right)\mathrm{}$ grows in a two-dimensional mode, with the hematite bulk in-plane parameter.