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 LaAlO3 (LAO) thin film grown epitaxially on (001) TiO2-terminated SrTiO3 (STO) single crystal . 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 Ti3+, with 3d1 configuration, should be stabilized in proximity of the interface.
LaAlO3/SrTiO3 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 UltraSTEMTM 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 L3-eg, are observed at the LAO / STO substrate and not at the LAO / STO film interface. Generally Ti-L2,3 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-L2,3 (at ca. 450 eV) to Sr-L2,3 (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.
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“A/B” indicates that the material “A” is grown on the top of the material “B”.
To cite this abstract:Giulio Tieri, Alexandre Gloter, Danfeng Li, Stefano Gariglio, Jean-Marc Triscone, Odile Stéphan; Scanning Transmission Electron Microscopy Investigation of LaAlO3/SrTiO3 Bi-Interfaces. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/scanning-transmission-electron-microscopy-investigation-of-laalo3srtio3-bi-interfaces/. Accessed: February 25, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/scanning-transmission-electron-microscopy-investigation-of-laalo3srtio3-bi-interfaces/