Structural investigation of interface and defects in epitaxial Bi3.25La0.75Ti3O12 film on SrRuO3/SrTiO3 (111) and (100)

Abstract

The structure of La-doped bismuth titanate (BLT), Bi3.25La0.75Ti3O12, is investigated with atomic resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy. The images reveal evidence of the tilting of TiO6 octahedra within the perovskite-like layers of the BLT unit cell. The tendency of La ions to substitute Bi ions and occupy the top part of the (Bi2O2)2+ layer, previously observed from electron energy loss spectroscopy (EELS) mapping experiments, is explained based on the tolerance factors and stress relief mechanism. The atomic resolution HAADF images also reveal the presence of the out-of-phase boundaries (OPBs). The role of OPBs in BLT is discussed in terms of its fatigue resistance as the OPBs provide extra nucleation sites for ferroelectric domains during polarization reversals. Further, we show evidence that the first deposited atomic layer at the interface also governs the subsequent film growth, resulting in the modulation of the “defect-free” and the “defected” regions throughout the BLT film in the lateral direction, parallel to the film-substrate interface. As demonstrated from atomic-resolved elemental mapping with EELS, the generation of the defect-free and regions with defect in the BLT film are likely to be induced by the presence of surface steps and dislocations at the film-substrate interface. These effects are discussed in terms of the large remanent polarization and enhanced fatigue resistance in BLT.