High-resolution transmission electron microscopy study of defects and interfaces in epitaxial TiO₂ films on sapphire and LaAlO₃

TiO₂ thin films grown on sapphire (α-Al₂O₃) (1102) and on LaAlO₃ (001) at 800° C by pulsed-laser deposition have been characterized by transmission electron microscopy. The TiO₂ thin films grown on LaAlO₃ (001) crystallize in anatase, while those on sapphire (1102) crystallize in rutile. The epitaxial relationship between the anatase thin films and the LaAlO₃ substrate is (004)[110]_A//(001)[110]_L, where the subscripts A and L indicate anatase and LaAlO₃ respectively, and that between the rutile thin films and the sapphire substrate is (011)[111]_R//(1102)[202 1]_S, where the subscripts R and S indicate rutile and sapphire respectively. The rutile thin films contain a high density of growth twins with (101) and (011)_T being the twin planes and [101] being the twinning direction, where the subscript T represents a twinning plane. The atomic structure of the twin boundary, the anatase-LaAlO₃ and the rutile-sapphire interfaces were investigated by high-resolution transmission electron microscopy in combination with image simulations. Neither of the two epitaxial systems had a good lattice match between film and substrate. Growth mechanisms are discussed, on the basis of information on the atomic structure of the interfaces. Investigation also revealed that substantial similarity existed in the local atomic patterns of the substrate and the film for both systems, which is attributed to the formation of different TiO₂ phases in different substrates.