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Bistability and Electrical Characterisation of Two Terminal Bistability and Electrical Characterisation of Two Terminal Non-Volatile Polymer Memory Devices. Zahra Alhalafi A thesis submitted to De Montfort University in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) Emerging Technologies Research Centre De Montfort University, Leicester, UK January 2018 Author’s declaration I declare that the work in this thesis is in accordance with the regulations of De Montfort University. No part of this thesis has been submitted for any other degree or qualification at De Montfort University, or any other academic institution. Permission to copy or use whole or part of the work contained herein must be solicited except for the purpose of private study or academic purposes in which case the author must be explicitly acknowledged. The work contained in this thesis is as a result of my own effort unless stated otherwise. [ii] Acknowledgements I would like to offer my gratitude to my first supervisor Professor Shashi Paul for his guidance and support which has been invaluable. My gratitude also goes to Dr. Richard Barrie Michael Cross, my second supervisor, for his invaluable comments. I would also like to thank Mr. Paul Taylor, senior technician at EMTERC for his assistance throughout my research and other staff and students at EMTERC who provided me with advice and support. Special thanks go to my husband Dr. Dhafer Alhalafi and my children for their support, understanding and patience throughout these years. Finally, special and heartfelt thanks to my mother for her motivation and to my late father who hoped for me a great future and encouraged me to study, and to my brother and sister for their support and encouragement throughout. [iii] Abstract Polymer blended with nanoparticle and ferroelectric materials in two terminal memory devices has potential for electronic memory devices that may offer increased storage capacity and performance. Towards understanding the memory performance of a combination of an organic polymer with a ferroelectric or unpolarised material, this research is concerned with testing the memory programming and capacitance of these materials using two-terminal memory device structures. This research contributes to previous investigation into the internal working mechanisms of polymer memory devices and increases understanding and verifies the principles of these mechanisms through testing previously untested materials in different material compositions. This study makes a novel contribution by testing the electrical bistability of new materials; specifically, nickel oxide, barium titanate and methylammonium lead bromide and considers their properties which include nanoparticles, ferroelectric, perovskite structures and organic-inorganic composition. Due to their material properties which have different implications for internal switching and memory storage. Nanoparticles have a greater band gap between the valence band and conduction band compare to bulk material which be exploited for memory storage and ferroelectric properties and perovskite materials have non-volatile properties suitable for switching mechanisms. Specific attributes of memory function which include charging mechanism, device programming, capacitance and charge retention were tested for different material compositions which included, blend and layered with a PVAc polymer, and as a bulk material with a single crystal structure using MIM memory devices and MIS device structures. The results showed that nickel oxide was the most effective material as a blend with the polymer for memory performance, this was followed by barium titanate, however, methylammonium [iv] lead bromide performed poorly with polymer but showed promise as a single crystal structure. The results also showed that an increase in concentration of the tested material in a blend composition resulted in a corresponding increase in memory function, and that blend compositions were much more effective than layered compositions. [v] Table of Contents Author’s Declaration ......................................................................................................... ii Acknowledgements .......................................................................................................... iii Abstract ............................................................................................................................ iv Table Of Contents ............................................................................................................ vi Table Of Figures .............................................................................................................. xi Chapter 1 Overview Of Research ................................................................................ 1 1.1 Introduction ........................................................................................................ 1 1.2 Aims And Objective ........................................................................................... 4 1.3 Contributions Of Study ...................................................................................... 5 1.4 Thesis Organisation ............................................................................................ 5 1.5 Thesis Outcomes ................................................................................................ 5 1.6 Work Published And/Or Presented At Conference ............................................ 7 Chapter 2 Overview Of Memory Devices ................................................................... 8 2.1 Introduction ........................................................................................................ 8 2.2 Types Of Memory .............................................................................................. 9 2.2.1 Non-Volatile Memories (ROM-Hybrid) ....................................................... 11 2.3 Memory Devices Structures ............................................................................. 15 2.3.1 Capacitor-Type ............................................................................................. 15 2.3.2 Resistor-Type ................................................................................................ 16 2.4 Bistability ......................................................................................................... 19 [vi] 2.5 Memory Device Characteristics ....................................................................... 20 2.5.1 Memory Cell ................................................................................................. 20 2.5.2 Cycle Endurance ........................................................................................... 22 2.5.3 Memory Speed .............................................................................................. 23 2.5.4 Retention Time ............................................................................................. 24 2.6 Summary .......................................................................................................... 25 Chapter 3 Two Terminal NVM Devices And Charge Transport Mechanisms ......... 26 3.1 Introduction ...................................................................................................... 26 3.2 Memristor ......................................................................................................... 28 3.3 Oxide Based-Memory ...................................................................................... 29 3.4 Organic NVM ................................................................................................... 31 3.4.1 Use Of Polymers In Two-Terminal NVM .................................................... 32 3.5 Phase Change Memory ..................................................................................... 33 3.6 Magnetoresistive RAM .................................................................................... 35 3.7 Charge Transport Mechanisms ......................................................................... 36 3.7.1 Poole-Frenkel ................................................................................................ 36 3.7.2 Schottky Mechanisms ................................................................................... 37 3.7.3 Fowler-Nordheim And Direct Tunnelling .................................................... 38 3.7.4 Space Charge Limited Current (SCLC) ........................................................ 39 3.7.5 Ionic And Ohmic Conduction ....................................................................... 39 3.8 MIM Memory ................................................................................................... 41 3.8.1 Resistive Switching In MIM ......................................................................... 41 3.8.2 Structure Of Mim Devices ............................................................................ 43 3.8.3 MIM Current-Voltage Characteristic (Resistive Qualities) .......................... 44 [vii] 3.9 Principles Of Switching Mechanisms And Electrical Bistability..................... 49 3.10 Summary .......................................................................................................... 51 Chapter 4 Fabrication, Characterisation And Experiment Techniques Of Memory Devices…………………. ............................................................................................... 52 4.1 Introduction .....................................................................................................
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