Orientation dependence of dielectric susceptibilities and switching characteristics in epitaxial ferroelectric thin films

Xu, Ruijuan

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https://hdl.handle.net/2142/50487 Copy

Description

Title

Orientation dependence of dielectric susceptibilities and switching characteristics in epitaxial ferroelectric thin films

Author(s)

Xu, Ruijuan

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Martin, Lane W.

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• ferroelectrics
• epitaxial thin films
• dielectric permittivity
• piezoresponse force microscopy
• ferroelectric domain structure

Abstract

When subjected to low electric field, domain walls play an important role in determining the response of ferroelectrics to applied stimuli. While contributions to ferroelectric properties from the intrinsic response of the polarization within a domain and so-called extrinsic contributions from the motion of domain walls have been studied, the non-motional or stationary contribution from the material within the finite width of the domain wall itself has been particularly difficult to understand. When subjected to high electric field, switchable polarization makes ferroelectrics a critical component in memories, actuators, and electro-optic devices and potential candidates for nanoelectronics. Although many studies of ferroelectric switching have been undertaken, much remains to be understood about switching in complex domain structures and in devices. In the present work, in order to investigate the low field dielectric response, using a combination of phenomenological models, thin-film growth, and multi-scale characterization we have probed model versions of (001)-, (101)-, and (111)-oriented epitaxial films. In particular, we observe that (111)-oriented films, in which the extrinsic contributions from the high density of 90° domain walls are frozen out, exhibit permittivity values approximately 3-times larger than that expected from the intrinsic response alone. This discrepancy can only be accounted for by considering a stationary contribution to the permittivity from the domain wall volume of the material that is 6-77.5-times larger than the bulk response and is consistent with recent predictions of the enhancement of susceptibilities within 90° domain walls. In order to investigate the high field switching characteristics, a combination of thin-film epitaxy, macro- and nanoscale property and switching characterization, and molecular dynamics simulations are used to elucidate the nature of switching in PbZr0.2Ti0.8O3 thin films. Differences are demonstrated between (001)-/(101)- and (111)-oriented films, with the latter exhibiting complex, nanotwinned ferroelectric domain structures with high densities of 90° domain walls and considerably broadened switching characteristics. Molecular dynamics simulations predict both 180° (for (001)-/(101)-oriented films) and 90° multi-step switching (for (111)-oriented films) and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. These results have implications for our understanding of ferroelectric switching and offer opportunities to change domain reversal speed. Our work also offers new insights into the microscopic origin of dielectric enhancement and provides a pathway to engineer the dielectric response of these materials.

Graduation Semester

2014-08

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http://hdl.handle.net/2142/50487

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Copyright 2014 Ruijuan Xu

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