HAADF STEM and Ab Initio Calculations Investigation of Anatase TiO2/LaAlO3 Heterointerface

The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO remains a challenging issue. In particular, in TiO ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO, from extrinsic effects, such as those arising from structural defects, dislocations, and the presence of competing phases at the film/substrate interface. It is, therefore, mandatory to unambiguously ascertain the structure of the TiO/substrate interface. In this work, by combining high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), density functional theory calculations, and multislice image simulations, we have investigated the nature of strainless anatase TiO thin films grown on LaAlO substrate. In particular, the presence of oxygen vacancies in anatase TiO has been proved to stabilize the formation of an extra alloy layer, TiAlO, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO directly on LaAlO substrate has required the deposition of a TiO extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO/LAO heterointerfaces to engineer the 2DEG formation in anatase TiO for tailored applications.