Electron-energy-loss spectroscopy studies of Cu-α-Al₂O₃ interfaces grown by molecular beam epitaxy

Abstract

The electron-energy-loss near-edge structure (ELNES) of a Cu-α-Al₂O₃ interface grown by molecular beam epitaxy has been studied using spatial difference electron-energy-loss spectroscopy in the scanning transmission electron microscope. Interpretation of the interface-specific Cu L_2,3, Al L_2,3 and O K ELNES components implies the existence of Cu-O bonding at the interface together with the retention of a local octahedral oxygen coordination of aluminium atoms and hence their non-participation in the interface plane. There is significant evidence for interfacial charge transfer from the copper layer to the oxygen layer, resulting in the existence of one monolayer of copper at the interface nominally in the Cu⁺ oxidation state. Furthermore, the ELNES studies reveal that the interfacial Cu-O bonds are of mixed ionic-covalent character. This becomes apparent when considering the O K ELNES where the presence of hybridized Cu 3d-O 2p states is indicated. The basal plane of α-Al₂O₃ at the interface is terminated by a layer of oxygen atoms which is directly bonded to the copper lattice. This chemical information was employed as an initial assumption in the subsequent simulation of experimental high-resolution transmission electron microscopy (HRTEM) images of the same interface. A structural model for the atomistic arrangement at the interface was derived which exhibited a projected Cu-O distance of 0.2 nm. Since the O K ELNES provides information on medium-range order, the HRTEM-derived model of the interface was then used as input for the simulation of the experimental interface-specific O K ELNES component using multiple-scattering calculations. Excellent agreement between the theoretical and experimental ELNES results confirms the HRTEM model and rules out the possibility of an aluminium-terminated α-Al₂O₃ basal plane at the interface. This study attempts to demonstrate the synergism between HRTEM and spatially resolved electron-energy-loss spectroscopy measurements for structure determination and highlights the self-consistent aspect of such combined information.