Studies of Local Structural Distortions in Strained Ultrathin BaTiO_3 Films Using Scanning Transmission Electron Microscopy

摘要

Ultrathin ferroelectric heterostructures (SrTiO_3/BaTiO_3/BaRuO_3/SrRuO_3) were studied by scanning transmission electron microscopy (STEM) in terms of structural distortions and atomic displacements. The TiO_2-termination at the top interface of the BaTiO_3 layer was changed into a BaO-termination by adding an additional BaRuO_3 layer. High-angle annular dark-field (HAADF) imaging by aberration-corrected STEM revealed that an artificially introduced BaO-termination can be achieved by this interface engineering. By using fast sequential imaging and frame-by-frame drift correction, the effect of the specimen drift was significantly reduced and the signal-to-noise ratio of the HAADF images was improved. Thus, a quantitative analysis of the HAADF images was feasible, and an in-plane and out-of-plane lattice spacing of the BaTiO_3 layer of 3.90 and 4.22 ? were determined. A 25 pm shift of the Ti columns from the center of the unit cell of BaTiO_3 along the c-axis was observed. By spatially resolved electron energy-loss spectroscopy studies, a reduction of the crystal field splitting (CFS, ΔL_3 = 1.93 eV) and an asymmetric broadening of the eg peak were observed in the BaTiO_3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO_3 film.