Studies of Barium Titanate on the Atomic Scale using Transmission Electron Microscopy

Ferroelectric barium titanate thin films can be used for non-volatile memories. Although below the critical thickness the ferroelectric polarisation is suppressed, many theoretical calculations predicted an enhancement of ferroelectricity in BaTiO3 (BTO) thin filmswith a compressive strain and proper termination. Despite these evidences based ontheoretical calculations, the corresponding experimental results are still missing.An aberration-corrected transmission electron microscope gives access to structural and chemical information of materials on the atomic scale. High-angle annular dark field (HAADF) imaging allows to directly observe the atomic structures with Z-contrast. To study and optimise the terminations of the ferroelectric thin films, HAADF imaging wasapplied for various epitaxially grown heterostructures. However, owing to a relatively long acquisition time, which is required to obtain a sufficient signal-to-noise ratio, sample and probe drifts are unavoidable and the precise atomic column positions cannot be obtained. Thus, a different approach in HAADF imaging is suggested to reduce inevitable artefacts caused by drift. The suggested drift-corrected HAADF imaging technique allows a precise determination of the atomic column positions with significantly reduced drift. The positions of the Ti atomic columns and the structural deformation were quantitatively analysed in the ferroelectric heterostructure at the atomic level. StripeSTEM was applied to observe a variation of the fine structures on the Ti-L23 and O-K edges. To reach a deeper understanding of the fine structures, the corresponding fine structures were calculated based on real space multiple scattering theory. BaTiO3 ceramics are also commonly used in multilayer ceramic capacitors to increase capacitance. In spite of the relatively high permittivity of BTO, its permittivity varies in a non-linear manner with temperature and shows a rapid increase in the vicinity of the ferroelectric transition temperatures. To produce temperature stable capacitors, BTOceramics are doped by rare-earth elements together with various additives. By doping with Dysprosium (Dy), the permittivity is flattened over a wide range of temperatures and some significant improvements for capacitor applications are obtained. Since these improvements are directly linked with the microstructure, an investigation on the doping behaviour of Dy in BTO ceramics was performed using transmission electron microscopy (TEM). Energy filtering transmission electron microscopy (EFTEM) was mainly applied to reveal the spatial distribution of Dy and the incorporation of Dy into the perovskite structure of BTO. In addition, energy dispersive X-ray (EDX) spectroscopy and electron energy-loss spectroscopy (EELS) are performed to obtain the local chemical informationof Dy-doped BTO. Drift-corrected HAADF imaging and StripeSTEM showed the presence of ferroelectricity in the compressively strained 2.8 nm thick BTO thin film with a proper termination. EFTEM revealed the core-shell structure of the Dy-doped BTO ceramic