STRUCTURE OF V-AL2O3 INTERFACES GROWN BY MOLECULAR-BEAM EPITAXY

V films were deposited on the (1102BAR) R-plane of sapphire by molecular beam epitaxy. Cross-sectional and plan-view specimens were prepared and the V-Al2O3 interface was investigated from different directions by conventional and high-resolution electron microscopy (HREM). It was found that the film grows in an epitaxial manner with respect to the substrate with a tilt angle of about 2-degrees. Using selected-area diffraction patterns and HREM, the orientation relationship between V and Al2O3 was determined and the geometrical misfit dislocations and interfacial stacking faults were analysed. The misfit dislocations are a geometrical consequence of the large mismatch between interatomic spacings at the two sides of the V-Al2O3 interface and exist from the start of deposition. Because of the large lattice mismatch, the V-Al2O3 interface is expected to be incoherent; however, because of bonding between the atoms on the two sides of the interface, the atoms in the vicinity of the interface are displaced from their bulk positions, giving rise to very localized strains at the interface. These strains result in dislocation-like contrast in micrographs obtained by conventional electron microscopy. The V film is tilted because it grows so as to maintain a unique three-dimensional orientation relationship between V and Al2O3. Basically the V atoms follow the same spatial orientation as the Al cations in the underlying sapphire substrate. As a result of the tilt, periodic stacking faults at the interface are observed in plan-view electron microscopy specimens. A comparison of HREM image simulations with experimental images of the interface indicates that the V atoms may be bonded to O atoms on the R-plane of Al2O3; these O atoms, however, are not from the original O-terminated R-plane but may have formed as an extra monolayer on this plane just prior to deposition.