New modalities for strain engineering of lead-free perovskite ferroelectric thin films

bismuth ferrite (BiFeO3);thin films;electromechanical responses;phase transitions;barium titanate (BaTiO3);BaxSr1-xTiO3;Lead-free;Epitaxy;Crystal structure

Rama Damodaran, Anoop

关键词: bismuth ferrite (BiFeO3);    thin films;    electromechanical responses;    phase transitions;    barium titanate (BaTiO3);    BaxSr1-xTiO3;    Lead-free;    Epitaxy;    Crystal structure;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/50388/Anoop_Rama%20Damodaran.pdf?sequence=1&isAllowed=y

美国|英语

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

【 摘 要 】

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.