Epitaxy of polar semiconductor $Co_{3}O_{4}$ (110): growth, structure, and characterization

The (110) plane of catalytic $Co_{3}O_{4}$ exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co$^{3+}$ species at the surface. However, experimental studies of $Co_{3}O_{4}$ (110) surfaces and interfaces have been limited due to the difficulties in growing high-quality films. In this paper, we present thin (1- 25nm) $Co_{3}O_{4}$ films grown by molecular beam epitaxy in the polar (110) direction on $MgAl _{2}O_{4}$ substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. We note that the film surface roughens at intermediate thickness, but slowly smoothens as growth continues, returning to an RMS surface roughness less than 1 {\AA}. Furthermore, we investigate the electronic structure and optical properties of this material by core level and valence band x-ray photoelectron spectroscopy, first-principles density functional theory calculations, and ellipsometry. A valence band offset of 3.5 eV is measured for the $Co_{3}O_{4}/MgAl _{2}O_{4}$ heterostructure. Magnetic measurements show the signature of antiferromagnetic ordering at 46 K.