Growth of anatase films on vicinal and flat LaAlO3 (110) substrates by oxygen plasma assisted molecular beam epitaxy

The heteroepitaxial growth and surface structures of TiO2 films on vicinal and flat LaAlO3 (110) were characterized. The films were grown by oxygen plasma assisted molecular beam epitaxy at low growth rates and temperatures between 825 and 875 K. Bulk characterization by low energy electron loss spectroscopy indicated that only the anatase structure formed. Both X-ray diffraction and scanning transmission electron microscopy indicated that the films grew with anatase (102) planes parallel to the interface. The surfaces of the films were characterized by reflection high energy electron diffraction (RHEED) during growth, and photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM) following growth. The photoelectron spectra were consistent with the growth of stoichiometric TiO2. Both RHEED and LEED, however, showed patterns expected of anatase (101) surfaces, not anatase (102). Further, STM images revealed the oblique unit cell expected of anatase (101) along with many parallel steps. The (101) surface is the lowest energy anatase surface and it is suggested that the film surface facets towards (101) to reduce the surface energy. Anatase (101) is also a much better geometric match to LaAlO3 (110) than anatase (102); however, unlike the [102] orientation, the [101] orientation places Ti cations directly above substrate cations increasing the interfacial energy. Thus the film structure and surface morphology is determined by the interplay between geometric and electronic matching at the interface and the energy of the free surface.