Epitaxial growth of cobalt oxide phases on Ru(0001) for spintronic device applications

Cobalt oxide films are of technological interest as magnetic substrates that may support the direct growth of graphene, for use in various spintronic applications. In this work, we demonstrate the controlled growth of both Co₃O₄(111) and CoO(111) on Ru(0001) substrates. The growth is performed by Co molecular beam epitaxy, at a temperature of 500 K and in an O₂ partial pressure of 10^-4 Torr for Co₃O₄(111), and 7.5 × 10^-7 Torr for CoO(111). The films are distinguished by their dissimilar Co 2p x-ray photoemission (XPS) spectra, while XPS-derived O/Co stoichiometric ratios are 1.33 for Co₃O₄(111) and 1.1 for CoO(111). Electron energy loss (EELS) spectra for Co₃O₄(111) indicate interband transitions at ∼2.1 and 3.0 eV, while only a single interband transition near 2.0 eV is observed for CoO(111). Low energy electron diffraction (LEED) data for Co₃O₄(111) indicate twinning during growth, in contrast to the LEED data for CoO(111). For Co₃O₄(111) films of less than 20 Å average thickness, however, XPS, LEED and EELS data are similar to those of CoO(111). XPS data indicate that both Co oxide phases are hydroxylated at all thicknesses. The two phases are moreover found to be thermally stable to at least 900 K in UHV, while ex situ atomic force microscopy measurements of Co₃O₄(111)/Ru(0001) indicate an average surface roughness below 1 nm. Electrical measurements indicate that Co₃O₄(111)/Ru(0001) films exhibit dielectric breakdown at threshold voltages of ∼1 MV cm^-1. Collectively, these data show that the growth procedures yield Co₃O₄(111) films with topographical and electrical characteristics that are suitable for a variety of advanced device applications.