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Oriented Epitaxial Bismuth Ferrite Thin-Films with Robust Ferroelectric Properties

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Oriented Epitaxial Bismuth Ferrite Thin-Films with Robust Ferroelectric Properties Development of Chemical Solution Deposition Derived (001)-Oriented Epitaxial Bismuth Ferrite Thin-Films with Robust Ferroelectric Properties Qi Zhang A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Materials Science and Engineering Faculty of Science The University of New South Wales April 2015 Originality Statement ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed....................................... Date........................................... i Acknowledgement First, I wish to thank my supervisor, Dr Owen Standard, for his patient guidance and insightful advice during my thesis. His critical thinking and professional character regarding experiment design, data analysis and thesis writing have impacted immensely on my research training. I also would like to express my appreciation to my co- supervisor, Prof. Nagarajan Valanoor for the fabulous opportunity to be a part of his research group. His professional and enthusiastic guidance, as well as friendly personality, have made a positive influence on me as well as the whole group. Next, I express my gratitude to many people who helped me over the course of the research work. In particular, I would like to thank Prof. Chris Sorell for using his spin coater, Dr Yu Wang for his help on XRD analysis, Dr Chris Marjo and Dr Anne Rich for their expert opinions on FTIR and Raman analysis, Dr Donald Thomas for his time and effort on NMR analysis, Dr Nadia Court and Dr Fay Hudson from ANFF for their help on device preparation, Mr. Bill Joe for his support on ferroelectric testing system maintenance, Dr Rahmat Kartono for his furnace maintenance, Dr George Yang for his assistance in microstructural sample preparation, Mr Anthony Zhang and Ms Soo Chong for their lab assistance, and Ms Jane Gao and Mr Danny Kim for their IT service support. I also would like to thank my group members and friends who supported me for my project. Miss Hsin-Hui Huang and Miss Yanyu Zhou for their help on TEM, Mr Jeffrey Cheung for his patient technical support on thin film characterization, Mr Ruoyu Li for his help on data analysis, Mr Rui Ding for his help on film property measurement, Mr Guangqing Liu for his help on film buffer layer optimization and deposition, Dr Dewei Chu for his help and advice on film resistivity measurement, Miss Xuan Cheng ii for her help on my thesis and paper proof reading, Miss Cong Chen for the happiness she brings to my research life as the best neighbour. I also would like to thank Ruoming Tian, J.J.Lee, Xing Xing, Qinghua Cao, for their help, support and encouragement over the past four-year of my PhD study. Last but not least, I am deeply grateful to my husband, Mr Ming Xu for his trust and encouragement of me all these years, to my parents for their understanding and support on my decision in pursuing PhD degree abroad, and to all my friends who encouraged and supported me. Without them, I would have gone insane on this seemingly endless endeavour. iii Abstract Bismuth ferrite (BiFeO3, BFO) has attracted recent attention due to its multi- functional properties, including multiferroism, resistive switching and photovoltaic effects. In particular, epitaxial BFO has been shown to demonstrate giant polarization, polarization-mediated resistive switching and unique magnetic properties. Until now, the most popular methods to obtain epitaxial BFO films with robust properties have been pulsed laser deposition (PLD) and radio frequency (RF) sputtering. Films made using these methods have been reported to have a high spontaneous polarization of up to 130 µC/cm2 and a switchable diode effect. Chemical deposition techniques, such as chemical solution deposition (CSD), have attracted recent interest for the preparation of BFO films and owing to them offering a cost-effective and more convenient manufacturing method compared with PLD and RF sputtering, an aspect of particular importance in an industrial context. However, the large scale adoption of CSD-derived BFO thin films for a variety of applications has been stymied by a number of significant limitations and challenges including: (1) the imprecision of the starting chemical composition and the subsequent volatilisation of Bi during the annealing step leading to the formation of secondary phases and/or highly conductive films with very poor leakage resistance; (2) variable sintering and densification behaviour leading to films having porosity and poor microstructures; and, (3) limited epitaxy between the film and substrate. Collectively, these dramatically impair the structural and electromechanical properties of the BFO films rendering them unsuitable for practical application. Thus, there is the important need to optimize the CSD preparation process for obtaining pure-phase epitaxial BFO. iv In this thesis, a non-aqueous CSD route was developed and studied with the aim to optimise it for the preparation of epitaxial (001) BFO thin films with robust (square) polarization hysteresis loops, high dielectric constant, strong piezoelectric response and distinct diode behavior. Molecular changes in the organic precursors on heating (determined by NMR and FTIR) and the effects of gelation temperature–time and thickness on film morphology were studied to develop an optimal deposition–gelation process for the synthesis of homogenous, defect-free gel films suitable for subsequent crystallization. The key to obtaining a homogenous gel was control of the delicate balance between gelation and salt (metal nitrate) precipitation through solvent evaporation. The optimized synthesis route consists of spin-coating 0.25 M precursor on 70°C preheated substrate at 3000 rpm for 30 seconds then gelating at 90°C then drying at 270°C. The crystallization of optimized gel films was studied as a function of Bi/Fe concentration and stoichiometry in the precursor solution, film thickness and single versus multiple depositions, crystallization temperature and atmosphere. Oxygen atmosphere was found to be essential for suppression of Bi volatilization and promotion of film epitaxial orientation. Pure-phase, epitaxial BFO thin film on (001)-strontium titanate (STO) substrate was obtained by rapidly heating the thin film to 650°C in an oxygen atmosphere and holding at the temperature for 30 minutes. A multi-layer deposition process for fabrication of films of various thicknesses was optimised by study of the deposition-heating sequence. The ferroelectric properties of pure-phase, epitaxial BFO thin films on lanthanum strontium manganite buffered (001)-STO substrates were studied as a function of thickness (40, 70, and 150 nm). The 70 and 150 nm films exhibited v exhibited square hysteresis loops at room temperature with high remanent polarization 2 (2Pr up to 100 μC/cm ), low coercive field (2Ec down to 193 kV/cm), and high relative dielectric constant (up to 613). High-cycle fatigue tests showed that these films are resistant to polarization fatigue (up to 108 cycles). All thicknesses showed resistive switching behaviour and a polarization-mediated diode effect both of which became more pronounced with decreasing thickness. The CSD technique developed in this work yielded high-quality BFO thin films and offers a viable low-cost alternative to current BFO deposition techniques. vi List of Publications 1. Q. Zhang, V. Nagarajan, and O. Standard, Chemical solution deposition derived (001)- oriented epitaxial BiFeO3 thin films with robust ferroelectric properties using stoichiometric precursors (invited manuscript), Journal of Applied Physics, 2014, 116, 066810. 2. A. Rana, H. Lu, K. Bogle, Q. Zhang, R. Vasudevan, V. Thakare, A. Gruverman, S. Ogale and V Nagarajan, Scaling behavior of the resistive switching in epitaxial bismuth ferrite heterostructures, Advanced Functional Materials, 2014, 24 (25), 3962. 3. Q. Zhang, V.Nagarajan, and O.Standard, Epitaxial (001) BiFeO3 thin-films with excellent ferroelectric properties by chemical solution deposition-the role of gelation, Journal of Materials Chemstry C, 2015,3, 582 vii Table of Contents Originality statement ................................................................................ i Acknowledgement .................................................................................... ii Abstract .................................................................................................... iv List of Publications ................................................................................. vii Table of Contents ................................................................................... viii List of Figures ......................................................................................... xii List of Tables ........................................................................................... xx Chapter 1. ............................................................................................
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