Experimental study of the interfacial cobalt oxide in Co3O4/α-Al2O3(0001) epitaxial films

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular-beam epitaxy on alpha-Al2O3(0001) single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low-energy electron-diffraction patterns suggest that the unit cell remains similar to that of alpha-Al2O3(0001) up to a thickness of 17 angstrom, while at larger thicknesses a pattern identified with that of Co3O4(111) becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the Co 2p lines from 3.4 to 17 angstrom cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer toward [111]-oriented Co3O4. In particular, the absence of characteristic satellite peaks in the Co 2p lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the Co2O3 corundum phase.