The Nanoscale Structure and Dynamic Properties of Ferroelectric Films

Ferroelectric Materials;Transmission Electron Microscopy;Multiferroic Materials;Materials Science and Engineering;Engineering;Materials Science and Engineering

Nelson, Christopher TobiasSun, Kai ;

University of Michigan

关键词: Ferroelectric Materials;    Transmission Electron Microscopy;    Multiferroic Materials;    Materials Science and Engineering;    Engineering;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/89859/ctnelson_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Ferroelectric oxides possess a spontaneous polarization which can be oriented by an applied electric field making them attractive for non-volatile memories and a variety of other applications harnessing the coupling between the polarization and other material properties.Switching between polarizations is governed by nanoscale defects, however these are generally of unknown type and mechanism since they are below the spatial resolution of contemporary ferroelectric characterization techniques.This limitation has also precluded study of the switching behavior along dimensionally confined structures such as thin films, despite the prominence of these geometries in devices.In this dissertation I employ novel in situ characterization using atomic-resolution transmission electron microscopy (TEM) to study the static and dynamic ferroelectric properties in thins films.This work focuses on the prototypical tetragonal and rhombohedral ferroelectric materials: PbZr0.2Ti0.8O3 (PZT) and BiFeO3 (BFO). Using sub-Angstrom resolution TEM imaging we develop a technique to map the atomic-scale polarization distribution in the ferroelectric films which are combined with other analytical techniques to determine local structure-property relationships in the ferroelectric films.Most notably we observe several nanoscale interface phenomenon including non-polar layers induced by structural distortion from the boundary material and the formation of vortex flux closure domains to reduce electrostatic energy at insulating interfaces.The latter is akin to the larger scale closure domains commonly observed in ferromagnetic materials but heretofore unknown for ferroelectric films.Nanoscale ferroelectric switching was inducing using a conductive surface probe.In situ characterization by TEM revealed the growth and failure modes of the hysteretical switching along with several unexpected behaviors.In both BFO and PZT the nucleation site and the switching properties are dictated by the built-in electric fields formed by Schottky junctions. In BFO the initial phase of low voltage switching is found to be thermodynamically limited.Furthermore independent nanoscale switching occurs at the BFO interface.This has significant import on heterostructure devices which require deterministic control of the interface polarization such as the BiFeO3 / La0.7Sr0.3MnO3 magnetoelectric bilayer studied here.These results demonstrate integrated SPM-TEM as an ideal platform to study the nanoscale structure which dictates macroscale ferroelectric behavior.