PREPARATION AND CHARACTERIZATION OF FERROELECTRIC THIN FILMS FOR TUNABLE AND PYROELECTRIC APPLICATIONS WANG SHIJIE (M.S.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS It‟s my pleasure to take this opportunity to acknowledge all the support, encourage, joy and love got from many people, without which it would have been impossible for me to complete this thesis in such a pleasant way I would firstly like to express my sincere gratitude and heartfelt appreciation to my supervisor, Prof Lu Li, for his strong support and guidance, as well as ever-lasting encouragement throughout the course of my Ph.D research I benefited from all the discussion we had and enjoyed the freedom he gave me The same gratitude goes to A/Prof Lai Man On, who served as my co-supervisor, for his continuous support and encouragement I would like to thank Dr Shu Miao and Prof Ian M Reaney from Department of Materials Science and Engineering, the University of Sheffield, for their collaboration on the TEM analysis and paper construction I specially thank Dr Shu Miao for all the valuable discussions we had I would like to thank all the technician staff of the Materials Science Lab for their kind assistance and generous help to let me complete the experiments well They are Mr i Thomas Tan Bah Chee, Mr Ng Hong Wei, Mr Abdul Khalim Bin Abdul, Mr Maung Aye Thein, and Mdm Zhong Xiang Li Many thanks also go to my colleagues and friends in Materials Science group I really appreciate Dr Zhang Zhen for his kind help to tell me how to use PLD and other equipments when I just came to the Lab, and the later discussions we held on both academic and life issues made me learn a lot I‟m also grateful to Dr Xia Hui, who shared his knowledge and experience and helped me a lot in the four years Other members, Mr Wang Hailong, Mr Yan Feng, Mr Xiao Pengfei, Mr Ye Shukai, Mr Song Bohang and Ms Zhu Jing, also thank you It‟s your friendships that make my Ph.D study more fun I always remember the time we spent together Finally, I would like to express my deepest gratitude to my family, for their constant support and love Especially, to my wife, Wang Yu, for her deep-felt love, persistent encouragement and understanding throughout the course of my Ph.D study ii TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS iii ABSTRACT viii LIST OF TABLES xi LIST OF FIGURES xii LIST OF PUBLICATIONS xvii Chapter I Introduction 1.1 Overview & Motivations 1.2 Scope and Organization of Thesis Chapter II Literature Review 2.1 Introduction to Ferroelectrics 10 2.1.1 Ferroelectricity and Perovskite Ferroelectrics 10 2.1.2 Characteristics of Perovskite Ferroelectric Materials 13 iii 2.2 Ferroelectric Ba(Ti1-xSnx)O3 Materials 17 2.2.1 Structure and Phase Diagram of Ba(Ti1-xSnx)O3 17 2.2.2 Applications of Ba(Ti1-xSnx)O3 18 2.3 Thin Film Devices 21 2.3.1 Pyroelectric Infrared Detectors 21 2.3.2 Microwave Tunable Devices 25 2.4 Pulsed Laser Deposition Method 27 2.5 Conclusions 33 Chapter III Growth Optimization of BTS Thin Films 34 3.1 Introduction 35 3.2 Experimental 38 3.3 Growth of BTS Thin Films 40 3.3.1 Oxygen Pressure Effect 40 3.3.2 Thickness Effect 51 3.3.3 Temperature Effect 64 iv 3.4 Conclusions 70 Chapter IV Leakage Characteristics of BTS Thin Films 72 4.1 Introduction 73 4.2 Experimental 74 4.3 Results and Discussion 76 4.3.1 Microstructure Analyses 76 4.3.2 Leakage of Pt/BTS/LNO/SiO2/Si Structure 77 4.3.3 Leakage of Pt/BTS/Pt/Ti/SiO2/Si Structure 83 4.4 Conclusions 900 Chapter V Structural Modification: BTS/BZN Heterostructures 92 5.1 Introduction 93 5.2 Experimental 95 5.3 BZN/BTS Thin Films 96 5.3.1 Microstructure Analysis 96 v 5.3.2 Electrical Behaviors 100 5.3.3 Tunable and Pyroelectric Performance 101 5.4 Conduction Mechanisms of BTS/BZN Heterostructure 105 5.5 Conclusions 111 Chapter VI Compositional Modification: La doped-BTS Thin Films 112 6.1 Introduction 113 6.2 Experimental 115 6.3 Results and Discussion 117 6.3.1 Microstructure Analyses 117 6.3.2 Polarization 123 6.3.3 Dielectric Properties 125 6.3.4 Pyroelectric Properties 128 6.4 Conclusions 132 Chapter VII Conclusions and Future Work 133 vi 7.1 Conclusions 134 7.2 Future Work 138 REFERENCE 139 vii ABSTRACT Ferroelectric thin films have been extensively studied for their wide applications in pyroelectric detectors and tunable devices In the present work, pulsed laser deposition (PLD) technique has been employed to deposit ferroelectric Ba(Ti0.85Sn0.15)O3 (BTS) thin films and heterostructures BTS thin films have been successfully deposited on LaNiO3 (LNO)/SiO2/Si substrates by PLD The role of oxygen pressure and the effect of thickness on the microstructure, electrical and pyroelectric properties of BTS thin films have been systematically studied BTS thin films deposited at higher oxygen pressures are found to possess better electrical properties The study on the thickness dependence of dielectric and pyroelectric properties shows that both LNO and BTS thin films are under tensile stress and they decrease with increasing thickness of the BTS films Larger dielectric constant and higher pyroelectric coefficient are obtained for BTS thin films with higher thickness, and the effect of stress is considered to be the dominant factor The substrate temperature is also found to play an important role in structural evolution of BTS thin films In addition, Pt and LNO are used as bottom electrodes to investigate their influences on conduction mechanisms For the Pt/BTS/LNO structure, the leakage current viii shows bulk-limited space-charge-limited-current (SCLC) behavior at positive bias while interface-limited Fowler-Nordheim (FN) tunneling at negative bias For the Pt/BTS/Pt structure, the dominant conduction mechanism is mainly controlled by the bulk-limited SCLC and/or Poole-Frenkel (PF) emission We have studied Bi1.5Zn1.0Nb1.5O7 (BZN) -buffered BTS heterostructures deposited on Si-based substrates The BZN layer has been proven to be a high-quality growth template and effective diffusion barrier to reduce the dielectric loss and leakage current of the BTS films Improved tunable and pyroelectric properties of BTS films have been achieved by controlling the thickness of the BZN layer The leakage mechanism of the Pt/BTS/BZN/LNO heterostructure has been studied at the temperature range from 303 to 403 K At positive bias and high electric fields, the conduction mechanism is controlled by SCLC; while at negative bias and high electric fields, FN tunneling is the dominant conduction mechanism At low electric fields, the leakage is controlled by the Ohmic contact irrespective of the sign of the bias field La has been selected as a dopant to tailor BTS thin films through the effect of compositional modification mol % La-doped BTS (BLaTS) thin films have been successfully deposited on LNO/SiO2/Si substrates by PLD It is found that BLaTS films show highly (h00) textured orientation Higher crystallization quality is obtained at ix Chapter VII Conclusions and Future Work curves The bulk-limited SCLC is operative when electrons are injected from the LNO layer into the BTS bulk thin film (at positive biases) This implies that the interface barrier height is not the critical factor when conductive oxides are used as electrodes The dominant interface-limited FN tunneling mechanism at negative biases suggests the formation of a partial depletion layer at the Pt/BTS interface, thus carrier charges (electrons) transport through the depletion layer are facilitated by high electric fields For the Pt/BTS/Pt structure, the dominant conduction mechanism depends on temperature and the electric field, and it is mainly controlled by the bulk-limited SCLC and/or PF emission, reflecting the decisive influence of defects (e.g., oxygen vacancies) in the bulk BTS thin films (3) BZN buffered-BTS heterostructures have been deposited on Si-based substrates The BZN layer serves as a high-quality growth template and effective diffusion barrier and reduces the dielectric loss and leakage current of the BTS films Improved tunable and pyroelectric properties of the BTS films have been achieved by controlling the thickness of the BZN layer At 303 K, the dielectric loss, tunability and the tunable figure of merit of a BZN buffered-BTS film can achieve 0.009, 47.9% and 68.4, respectively Furthermore, a promising 136 Chapter VII Conclusions and Future Work pyroelectric property, i.e., pyroelectric coefficient (24.7×10-4 C/m2 K) and figure of merit (16.3×10-5 Pa-1/2) can be obtained at 293K (4) The leakage mechanism of the BTS/BZN heterostructure has been studied over the temperature range from 303 to 403 K Under high positive biases, the conduction mechanism is controlled by SCLC Under high negative biases, FN tunneling is the dominant conduction mechanism At low electric fields, the leakage is controlled by the Ohmic contact irrespective of the sign of the bias field (5) La has been selected as a dopant to tailor BTS thin films through the effect of compositional modification mol % La-doped BTS (BLaTS) thin films have been successfully deposited on LNO/SiO2/Si substrates using PLD All thin films grow with a highly (h00) textured orientation Higher crystallization quality is obtained at higher deposition temperatures Sharp interface between the as-deposited BLaTS thin film and the bottom LNO layer is confirmed La possesses a pure +3 valence and the valence state of other elements is not affected after La doping It is found that BLaTS thin film shows reduced dielectric loss which is attributed to a reduction in defects such as oxygen vacancies and an improvement in insulating behavior by La doping In addition, 137 Chapter VII Conclusions and Future Work La dopant intensifies the relaxor behavior of BTS thin films as reflected by the more diffused phase transition between the ferroelectric and paraelectric states The present study is expected to help better understand the potential of BTS thin films The efforts toward improving the tunable and pyroelectric properties of BTS thin films have demonstrated the appealing potential applications of BTS thin films in the relevant fields 7.2 Future Work Although this work has succeeded in giving a clearer picture on the growth and performance improvements of BTS thin films using PLD, future work is still needed to clarify some problems as listed below: (1) The evolution of film morphology deposited at medium oxygen pressure is still unknown More detailed investigations may be needed (2) La doping level should be further optimized in La-doped BTS thin films to achieve a balance between low loss and high pyroelectric properties 138 REFERENCE [1] J Valasek, Physical Review 17 475 (1921) [2] J Scott and C Paz de Araujo, Science 246 1400 (1989) [3] Y Xu, Ferroelectric materials and their applications, North-Holland, New York (1991) [4] P Muralt, Reports on Progress in Physics 64 1339 (2001) [5] A Tagantsev, V Sherman, K Astafiev, J Venkatesh, and N Setter, Journal of Electroceramics 11 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