Atomic-oxygen-assisted MBE growth of α-Fe2O3 on α-Al2O3(0001):: Metastable FeO(111)-like phase at subnanometer thicknesses

The growth of alpha-Fe2O3 on alpha-Al2O3(0001) by atomic oxygen-molecular-beam epitaxy has been studied by real time reftection 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 Fe3+ ions instead of ferrous Fe2+ ones. This phase seems to be specific to a given thickness (similar to 2 ML), independently of the substrate. For higher equivalent thicknesses, epitaxial islands of hematite alpha-Fe2O3(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 alpha-Fe2O3,(0001) grows in a two-dimensional mode, with the hematite bulk in-plane parameter. [S0163-1829(99)01543-X].