Structural properties of epitaxial nanometric iron oxide layers on alpha-Al2O3(0001): an in situ RHEED study during growth

By means of atomic oxygen molecular beam epitaxy (MBE), nanometric layers o f different iron oxide phases have been epitaxially grown on the alpha-Al2O 3(0001) surface. The evolution of their structural properties luring growth has been studied in situ by real time reflection high-energy electron diff raction (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 (similar to 2 equiv. ML). This phase exhibits the fee symmetry a nd 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 oxid e phase presenting an ordered oxygen fee sublattice and an incomplete occup ation of the Fe sites. From thicknesses beyond 2 equiv. ML, either haematite alpha-Fe2O3(0001) or magnetite Fe3O4(111) begins to nucleate, depending on the substrate tempera ture (respectively 250 degrees C and 450 degrees C). From a deposit equival ent to roughly one iron oxide unit cell (similar to 9 equiv. ML), haematite or magnetite phases exhibiting the standard cell parameter are largely pre valent. (C) 2000 Elsevier Science B.V. All rights reserved.