Structural properties of epitaxial nanometric iron oxide layers on α-Al 2O 3(0001): an in situ RHEED study during growth

Abstract

By means of atomic oxygen molecular beam epitaxy (MBE), nanometric layers of different iron oxide phases have been epitaxially grown on the α-Al 2O 3(0001) surface. The evolution of their structural properties during growth has been studied in situ by real time reflection high-energy electron diffraction (RHEED). Independently of the substrate temperature, we observe the stabilisation of an uncommon iron oxide phase which seems to be specific to a subnanometric thickness (∼2 equiv. ML). This phase exhibits the fcc symmetry and a significantly expanded in-plane lattice parameter, compared with that of standard iron oxides. Two possibilities for the nature of this phase are compatible with our RHEED results: (a) an FeO(111) phase; (b) an iron oxide phase presenting an ordered oxygen fcc sublattice and an incomplete occupation of the Fe sites. From thicknesses beyond 2 equiv. ML, either haematite α-Fe 2O 3(0001) or magnetite Fe 3O 4(111) begins to nucleate, depending on the substrate temperature (respectively 250°C and 450°C). From a deposit equivalent to roughly one iron oxide unit cell (∼9 equiv. ML), haematite or magnetite phases exhibiting the standard cell parameter are largely prevalent.