|Author:||Leung, Man Ho|
|Title:||Atomic scale structure variations at ferroelectric domain walls in multiferroic BiFeO₃|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Ferroelectric thin films
|Department:||Department of Applied Physics|
|Pages:||xiv, 106 pages : color illustrations|
|Abstract:||Ferroelectric materials have been intensively explored because of their attractive future in electronic industries. Among all kinds of ferroelectric materials, a considerable amount of researches have been conducted on the applications and the natures of BiFeO3 owing to its improved ferroelectric properties. It has been demonstrated that the electrical properties of the ferroelectric domain walls such as the electrical conductivity can be different from that of the bulk ferroelectric domains. Recently, 109° ferroelectric domain walls have been widely examined but inadequate attention has been paid to the promising future of the 180° ferroelectric domain walls. It has been stated that ferroelectric 180° domain walls may contribute to the polarization response and possess enhanced electrical properties. Therefore, the study on 180° ferroelectric domain walls is critical. However, the detailed atomic structure at the ferroelectric 180° domain walls has not been fully established. Recent studies have focused on the polarization configurations near the domain walls without clarifying the detailed atomic structure. In this work, we examined the atomic scale structure and the polarization configurations using the aberration-corrected scanning transmission electron microscopy Z-contrast imaging with sub-Angstrom resolution. Results implied that 45° inclined ferroelectric 180° domain walls were obtained above the SrRuO₃ bottom electrode instead of the 71° and 109° domain walls which may be due to the effect of BiFeO₃ thin film thickness. Besides, the customized Matlab image analysis demonstrated that the lattice parameters depend on the polarization configurations. The observed variations in lattice parameters across the domain walls are attributed to the displacement of the Bi cations induced by the ferroelectric spontaneous polarization. It is also demonstrated that the orientation of the ferroelectric 180° domain walls will be affected by the monoclinic distortion of the substrate. More importantly, substrate interface terminations also play a critical role on the formation of 180° domain walls by switching the sign of the built-in electric field at the interface. Considering the polarization discontinuity model at the heterostructure interface, the polarization configurations of the domains will be strongly influenced by the terminating layer of the substrate. Specifically, spatially varying the interfacial atomic termination can produce both positive and negative built-in electric fields along the interface, and the boundaries across which the built-in field switches sign favor the formation of 180° domain walls in ferroelectric BiFeO₃. This indicates an effective strategy using precisely patterned heterostructure to 'plant' ferroelectric domain walls at the designated sites and to make artificial domain structure with desirable polarization configuration, as demonstrated by theoretical calculations. This work provided the preliminary understanding about the relationship between polarization configuration and atomic scale structure at BiFeO₃ ferroelectric 180° domain walls and filled the knowledge gap in the field of ferroelectric materials and pointed the way to optimize the multiferroic properties of BiFeO₃. Finally, the demonstrated polarization configuration and atomic structure mapping approach can be applied to other ferroelectric materials, to explore the promising correlation between atomic structure and ferroelectric property. Therefore, it could provide critical insights into the application of ferroelectric materials in different electronic devices.|
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