HAADF STEM and Ab Initio Calculations Investigation of Anatase TiO 2 /LaAlO 3 Heterointerface.

The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO₂ remains a challenging issue. In particular, in TiO₂ ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO₂, 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 TiO₂/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 TiO₂ thin films grown on LaAlO₃ substrate. In particular, the presence of oxygen vacancies in anatase TiO₂ has been proved to stabilize the formation of an extra alloy layer, Ti₂AlO₄, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO₂ directly on LaAlO₃ substrate has required the deposition of a TiO_x extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO₂ to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO₂/LAO hetero-interfaces to engineer the 2DEG formation in anatase TiO₂ for tailored applications. [ABSTRACT FROM AUTHOR]