Atomic-oxygen-assisted MBE growth of α−Fe2O3on α−Al2O3(0001): Metastable FeO(111)-like phase at subnanometer thicknesses

Abstract

The growth of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ on $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ 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 (\ensuremath{\sim}2 ML), independently of the substrate. For higher equivalent thicknesses, epitaxial islands of hematite $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ 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 $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ grows in a two-dimensional mode, with the hematite bulk in-plane parameter.