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Resistive Switching in Core-Shell Nanowires for Applications in Neuromorphic Architectures

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Resistive Switching in Core-Shell Nanowires for Applications in Neuromorphic Architectures RESISTIVE SWITCHING IN CORE-SHELL NANOWIRES FOR APPLICATIONS IN NEUROMORPHIC ARCHITECTURES By SHANGRADHANVA ESWARA VASISTH A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2021 © 2021 Shangradhanva Eswara Vasisth To family and friends ACKNOWLEDGMENTS Primarily, I would like to express my sincerest gratitude towards my graduate advisor, Dr. Juan C. Nino. With his understanding, guidance, and constant support, I have been able to complete this work and earn my degree. High standards in safety and work set by him has made me a better academic scholar and researcher. Leading by example, the work ethic he has instilled in me will stay the course of my life. I would also like to thank Dr. Anthony Brennan, Dr. Christopher Batich, Dr. Brent Gila, and Dr. David Mazyck, for being on my adivisory committee and providing me their time and advice. At the Nino Research Group, I have been glad to have worked and interacted with some of the smartest, kind, and diverse group of people. I would like to thank all the past and present members of the group for their support and help when I needed it. I would like to specially thank Dr. Hiraku Maruyama for constantly motivating me to work, Amalie Atassi and Marshall Frye for taking the initiative to help complete the work on functionalized nanocellulose, Marc Martorell for helping me build the setup for noise amplification, and Mustafa Siddiqi for the many hours he has spent in sample preparation. I would also like to thank Andres Trucco and other engineers at the Nanoscale Research Facility at the University of Florida for their assistance with e-beam lithography and overall guidance towards the fabrication of the core-shell nanowires. I would like to thank my family the most as they have played a significant role in making me the person I am today. Finally, I would like to thank my roommates (past and present) and friends who are in different parts of the world, for beilieving in me and helping me unwind. A part of this dissertation is based upon work supported by the National Science Foundation Grant No. ECCS-1709641. Any opinions, findings, conclusions, or 4 recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation. 5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 9 LIST OF FIGURES ........................................................................................................ 10 LIST OF ABBREVIATIONS ........................................................................................... 14 ABSTRACT ................................................................................................................... 16 CHAPTER 1 INTRODUCTION .................................................................................................... 20 1.1. Statement of Problem and Motivation .............................................................. 20 1.2. Scientific Approach .......................................................................................... 22 1.3. Organization of Dissertation ............................................................................. 24 1.4. Contributions to the Field ................................................................................. 25 2 BACKGROUND ...................................................................................................... 27 2.1. Memristive or Resistive Switching.................................................................... 27 2.1.1. Types of Resistive Switching .................................................................. 30 2.2.2. Mechanisms of Resistive Switching in Metal Oxides .............................. 32 2.2. Resistive Switching in HfO2.............................................................................. 35 2.3. Charge Transport Mechanisms ........................................................................ 40 2.4. Organic and Inorganic Synapses ..................................................................... 43 2.5. Noise in Networks ............................................................................................ 47 2.6. Impedance Spectroscopy ................................................................................ 50 3 EXPERIMENTAL PROCEDURES AND TECHNIQUES ......................................... 54 3.1 Fabrication Techniques ..................................................................................... 54 3.1.1. Lithography ............................................................................................. 54 3.1.1.1. Resist coat (spinners) ................................................................... 55 3.1.1.2. Bake .............................................................................................. 56 3.1.1.3. Exposure by e-beam ..................................................................... 56 3.1.1.4. Development ................................................................................. 58 3.1.2. Deposition Techniques ........................................................................... 58 3.1.2.1. Sputter deposition ......................................................................... 58 3.1.2.2. Atomic layer deposition (ALD) ....................................................... 59 3.1.3. Other Techniques Used .......................................................................... 60 3.1.3.1. Reactive ion etching (RIE) ............................................................ 60 3.1.3.2. PR stripping and plasma ashing ................................................... 60 6 3.2. Electrical Characterization ............................................................................... 61 3.2.1. Setup ...................................................................................................... 61 3.2.2. DC I-V Measurement .............................................................................. 62 3.2.2.1. I-V sweep mode ............................................................................ 62 3.2.2.2. I-V pulse mode and synaptic pulse modes .................................... 62 3.2.3. AC Impedance Measurement ................................................................. 63 3.2.4. Low-frequency Noise (LFN) Measurement ............................................. 63 3.3. Microstructural Characterization ...................................................................... 64 4 FABRICATION OF CORE-SHELL NANOWIRES ................................................... 66 4.1. Prototypes Fabricated ...................................................................................... 66 4.1.1. Prototype 1 ............................................................................................. 67 4.1.2. Prototype 2 ............................................................................................. 67 4.1.3. Prototype 3 ............................................................................................. 69 4.1.4. Crossbar Architectures ........................................................................... 69 4.1.5. Prototype 4 ............................................................................................. 70 4.1.5.1. Layer 1 .......................................................................................... 70 4.1.5.2. Layer 2 .......................................................................................... 71 4.1.5.3. Layer 3 .......................................................................................... 72 4.1.5.4. Layer 4 .......................................................................................... 73 5 COMPLEMENTARY RESISTIVE SWITCHING IN CORE-SHELL NANOWIRES ... 77 5.1. Electroforming .................................................................................................. 77 5.2. Bipolar Resistive Switching (BRS) ................................................................... 78 5.2. Complementary Resistive Switching (CRS) ..................................................... 84 5.3. Conclusion ....................................................................................................... 90 6 IMPLICATIONS OF RESISTIVE SWITCHING MECHANISMS IN CORE-SHELL NANOWIRES .......................................................................................................... 91 6.1. TEM of Core-shell Nanowires .......................................................................... 91 6.2. Bipolar Resistive Switching .............................................................................. 96 6.3. Complementary Resistive Switching ................................................................ 99 6.4. Conclusion ..................................................................................................... 103 7 ARTIFICIAL SYNAPTIC RESPONSE AND EFFECT OF TEMPERATURE ON BIPOLAR RESISTIVE SWITCHING IN CORE-SHELL NANOWIRES .................. 104 7.1. Artificial Synaptic Response in BRS and CRS Mode ..................................... 104 7.1.1. Potentiation and Depression in BRS Mode .......................................... 104 7.1.2. Synaptic Behavior in CRS Mode .........................................................
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