ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDIES OF CU-ALPHA-AL2O3 INTERFACES GROWN BY MOLECULAR-BEAM EPITAXY

The electron-energy-loss near-edge structure (ELNES) of a Cu-alpha-Al2 O3 interface grown by molecular beam epitaxy has been studied using sp atial difference electron-energy-loss spectroscopy in the scanning tra nsmission electron microscope. Interpretation of the interface-specifi c Cu L-2,L-3, Al L-2,L-3 and O K ELNES components implies the existenc e of Cu-O bonding at the interface together with the retention of a lo cal octahedral oxygen coordination of aluminium atoms and hence their non-participation in the interface plane. There is significant evidenc e for interfacial charge transfer from the copper layer to the oxygen layer, resulting in the existence of one monolayer of copper at the in terface nominally in the Cu+ oxidation state. Furthermore, the ELNES s tudies reveal that the interfacial Cu-O bonds are of mixed ionic-coval ent character. This becomes apparent when considering the O K ELNES wh ere the presence of hybridized Cu 3d-O 2p states is indicated. The bas al plane of alpha-Al2O3 at the interface is terminated by a layer of o xygen atoms which is directly bonded to the copper lattice. This chemi cal information was employed as an initial assumption in the subsequen t simulation of experimental high-resolution transmission electron mic roscopy (HRTEM) images of the same interface. A structural model for t he atomistic arrangement at the interface was derived which exhibited a projected Cu-O distance of 0.2 nm. Since the O K ELNES provides info rmation on medium-range order, the HRTEM-derived model of the interfac e was then used as input for the simulation of the experimental interf ace-specific O K ELNES component using multiple-scattering calculation s. Excellent agreement between the theoretical and experimental ELNES results confirms the HRTEM model and rules out the possibility of an a luminium-terminated alpha-Al2O3 basal plane at the interface. This stu dy attempts to demonstrate the synergism between HRTEM and spatially r esolved electron-energy-loss spectroscopy measurements for structure d etermination and highlights the self-consistent aspect of such combine d information.