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

V films were deposited on the (1102BAR) R-plane of sapphire by molecul ar 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 di ffraction patterns and HREM, the orientation relationship between V an d Al2O3 was determined and the geometrical misfit dislocations and int erfacial stacking faults were analysed. The misfit dislocations are a geometrical consequence of the large mismatch between interatomic spac ings at the two sides of the V-Al2O3 interface and exist from the star t of deposition. Because of the large lattice mismatch, the V-Al2O3 in terface is expected to be incoherent; however, because of bonding betw een the atoms on the two sides of the interface, the atoms in the vici nity 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 e lectron microscopy. The V film is tilted because it grows so as to mai ntain a unique three-dimensional orientation relationship between V an d Al2O3. Basically the V atoms follow the same spatial orientation as the Al cations in the underlying sapphire substrate. As a result of th e tilt, periodic stacking faults at the interface are observed in plan -view electron microscopy specimens. A comparison of HREM image simula tions with experimental images of the interface indicates that the V a toms 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 f ormed as an extra monolayer on this plane just prior to deposition.