Nanometric films of iron oxides (Fe_3O_4, #alpha# and #gamma# Fe_2O_3) of high crystallien order and purity are epitaxially grown on #alpha#-Al_2O_3(0001) by atomic oxygen assisted MBE. A complete characterization of the film structure has been performed by in situ LEED and RHEED, and ex situ GIXRD using synchrtron radiation. The films grown at room temperautre and post annealed at 400 deg C and 700 deg C (p_o2=10~-6 Torr) are respectively metastable #gamme#-Fe_2O_3 (111) and #alpha#-Fe_2O_3(0001). For a substrate temperature of 450 deg C during growth, Fe_3O_4 (111) is directly obtained. GIXRD shows an in-plane expansion of the films, which decreases with thickenss (0.8 and 0.2
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