Synthesis of epitaxial films of Fe₃O₄ and $α$-Fe₂O₃ with various low-index orientations by oxygen-plasma-assisted molecular beam epitaxy

Epitaxial thin films of pure-phase Fe₃O₄(110), Fe₃O₄(111), $α$-Fe₂O₃(112̄0), and $α$-Fe₂O₃(11̄02) have been grown on MgO(110), $α$-Al₂O₃(0001), $α$-Al₂O₃(112̄0), and $α$-Al₂O₃(11̄02) substrates, respectively, by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. Characterization of the crystal structures, chemical states, and epitaxial relationships was carried out using a variety of techniques, including in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, x-ray photoelectron spectroscopy/diffraction, and ex situ x-ray reflectivity and diffraction. Real-time RHEED reveals that Fe₃O₄ growth on MgO appears in a step-flow fashion, whereas the growth of Fe₃O₄(111) on $α$-Al₂O₃(0001) occurs initially by island formation, and then island coalescence. However, the growth of $α$-Fe₂O₃ on $α$-Al₂O₃ appears to follow an intermediate growth mode. The formation of pure-phase films is controlled largely by oxygen partial pressure, plasma power, and growth rate, but appears to be independent of growth temperature, at least from 250 to 550 °C. The present study demonstrates that selective growth of pure-phase iron oxides with various low-index orientations can be achieved by controlling the growth conditions and selecting suitable substrates.