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Title:New modalities for strain engineering of lead-free perovskite ferroelectric thin films
Author(s):Rama Damodaran, Anoop
Director of Research:Martin, Lane W.
Doctoral Committee Chair(s):Martin, Lane W.
Doctoral Committee Member(s):Cahill, David G.; Zuo, Jian-Min; Cooper, S. Lance
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):bismuth ferrite (BiFeO3)
thin films
electromechanical responses
phase transitions
barium titanate (BaTiO3)
Crystal structure
Abstract:Over the last few decades, considerable attention has been given to the development of lead-based ferroelectric systems such as PbZr1-xTixO3 due to their robust high temperature ferroelectric properties, as well as the presence of the so-called morphotropic phase boundary (MPB) ̶ a temperature-independent composition-driven structural instability that results in superior dielectric and piezoelectric properties. However, increasing environmental concerns are driving efforts towards the development of lead-free ferroelectrics such as BiFeO3, BaTiO3, and others. Previous work on epitaxial BiFeO3 thin films have shown that large compressive strains can drive the formation of complex mixed-phase structures with enhanced electromechanical responses (4-5% strains). In this work, we probe the nanoscale distribution of phases present in these mixed-phase structures using a combination of epitaxial thin-film growth and characterization techniques such as x-ray diffraction and piezoresponse force microscopy. We show, for the first time, the presence of monoclinic distortions and intermediate phases (akin to conventional MPB systems) in the mixed-phase films that are crucial for enhanced electromechanical responses. We then present thickness- and temperature- dependent phase-evolution studies that indicate the presence of a strain-spinodal between the various structural polymorphs of BiFeO3 to be the origin of mixed-phase formation. Finally, we discuss limitations due to a breakdown in epitaxy that occurs in thicker films and present chemical alloying-based approaches to mitigate these challenges. Having highlighted strain relaxation with increasing film thickness as a fundamental limitation to epitaxial-strain control, we explore an alternative route involving the use of a combination of defect-engineering and epitaxial strain to stabilize enhanced deformation states in materials. For this, we present a systematic study of BaTiO3 thin films, and show that epitaxial strain can be used to control the ordering of growth-induced defects driving deterministic additional out-of-plane strains that can enhance the ferroelectric Curie temperature to values exceeding 800°C without any need to change substrates. Such a combined control of epitaxial strain and engineered defect-structures provides a new pathway to extend the limits of strain-control of materials and properties. Lastly, we investigate a new route involving the use of epitaxial strain in conjunction with controlled composition- and strain- gradients to tune the thermal stability of dielectric responses of ferroelectric thin films. We present preliminary studies that reveal enhanced relative dielectric permittivity values of ~750, that change by less than 10% over a wide temperature range from 25-350ºC in compositionally-graded epitaxial BaxSr1-xTiO3 thin films, which is promising for next-generation microwave applications.
Issue Date:2014-09-16
Rights Information:Copyright 2014 Anoop Rama Damodaran
Date Available in IDEALS:2014-09-16
Date Deposited:2014-08

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