Scanning Transmission Electron Microscopy Investigation of LaAlO3/SrTiO3 Bi‐Interfaces

Abstract

The interfaces between complex oxides can generate fascinating properties that are not observed in the single compounds. A significant example is the high‐mobility 2‐dimensional electron liquid (2DEL) detected at the interface between two good band‐gap insulators, a LaAlO 3 (LAO) thin film grown epitaxially on (001) TiO 2 ‐terminated SrTiO 3 (STO) single crystal [1]. The 2DEL formation is understood in the framework of the polar catastrophe scenario for which electrons are transferred at the interface in order to minimize the built‐in potential generated by the contact between the polar planes of LAO and the neutral ones of STO. According this model a fraction of Ti 3+ , with 3d 1 configuration, should be stabilized in proximity of the interface. LaAlO 3 /SrTiO 3 bi‐Interfaces, here discussed, are multilayer structures with a STO film and a second LAO thin film subsequently grown on the top of the first LAO thin film. Such system displays three inequivalent interfaces ‐ two of which are conducting: LAO / STO substrate and LAO / STO film, for STO thickness ≥ 8 nm [2,3]. Our work is driven by the effort to understand the 2DEL formation at the LAO / STO film interface. For this purpose bi‐interfaces with thick (12 nm ≈ 30 unit cells (uc)) and thin (6 nm) STO film were investigated and discussed in parallel. High‐angle annular dark‐field (HAADF) imaging as well as electron energy‐loss spectroscopy (EELS) were performed in an aberration corrected Nion UltraSTEM TM Scanning Transmission Electron Microscope (STEM). The possibility to combine HAADF, an incoherent and Z‐sensible technique ideal to investigate distortions and defects, and EELS, a spectroscopy capable to probe valence states with atomic spatial resolution, makes STEM a powerful tool to understand interfaces. Specifically in STO the hybridization between the 3d band of Ti and the 2p of O results in a features‐rich spectroscopy. According the HAADF images collected in STO thick bi‐Interfaces, coherent growth, with no obvious defects or dislocations, was observed at the bottom and the middle interface whereas a periodic network of edge dislocations were identified at the top interface pointing out to a relaxed LAO / STO film and to a strained LAO / STO substrate interface. Ti fine structure corroborates the HAADF observations since evidences of orbital reconstruction i.e. a shift of ≈ 60 meV towards higher energy of the orbital‐field edge L 3 ‐e g , are observed at the LAO / STO substrate and not at the LAO / STO film interface. Generally Ti‐L 2,3 fine structure is known to be a spectroscopic fingerprint of the strain state of the interfaces. Besides strain, roughness and polarity of the interfaces are key features. In order to determine the termination plane sequences, a large energy range (1.9 keV) for the EELS data was used collecting simultaneously all the meaningful edges from Ti‐L 2,3 (at ca. 450 eV) to Sr‐L 2,3 (at ca. 1950 eV). These atomically resolved elemental maps show that the insulating interface(s) is(are) the sharpest, indicating that the cation intermixing may play a role in the response of the system to the occurrence of the 2DEL.