Orbital mapping of the LaAlO3-TiO2 interface by STEM-EELS

Oxide interfaces can give rise to and exhibit interesting phenomena, like magnetism,ferroelectricity and superconductivity. For the particular example of the interface between anataseTiO2 and lanthanum aluminate LaAlO3 also the presence of a two-dimensional electron gas (2DEG)has been discussed. [1] Different studies attribute the mechanism for 2DEG formation to aninternal electrical potential, which requires a critical thickness, or to structural imperfections byoxygen vacancies. Despite of the technical potential, however, such emergent phenomena at theTiO2/LaAlO3 interface are still not fully understood.The LaAlO3-TiO2 system exhibits two kinds of interfaces, which have different electronic propertiespredicted by first-principles calculations. The La-terminated interface shows metallic character inthe first two TiO2-layers (Figure 1 (a)), whereas the Al-terminated one remains semiconducting [1].Both atomic structures have been observed by aberration corrected scanning transmissionelectron microscopy (STEM). However, for mapping out the electronic information from theinterface, electron energy loss spectroscopy (EELS) is required.Löffler et al. [2] and Bugnet et al. [3] have demonstrated the real-space mapping of individualelectronic states in bulk materials by STEM-EELS. Although they could prove feasibility in rutileand graphene, the inherently poor signal-to-noise ratio (SNR) for such experiments imposes amajor challenge in terms of general applications. Apart from instrumental parameters, the reasonfor this is the small usable integration window for orbital mapping with EELS signals. The relevantenergy region for 2DEG related phenomena in LaAlO3-TiO2 is around the onset of the titaniumcore-loss edge, which marks the electronic states near the Fermi-level (Figure 1 (b)).Despite the low intensity at the onset, we were able to map individual electronic states at theLaAlO3-TiO2 interface by using a direct electron detection camera and special post-processingprocedures that included multicell averaging and denoising via principal component analysis. Forthe La-terminated interface, two layers near the titanium are visible, which might indicate thepresence of a 2DEG (Figure 1 (c)), as predicted by [1].This contribution aims to discuss the details of acquisition and data processing of the interface ofTiO2 and LaAlO3 to yield spatially resolved orbital information. Such experiments, combined withsimulations, can help clarifying the mechanisms behind 2DEG formation and related phenomena,potentially applicable to other complex oxide heterostructures as well.