NUMERICAL INVESTIGATION OF CARBON NANOTUBE THIN-FILM COMPOSITES AND DEVICES A Dissertation Presented to The Academic Faculty By Man Prakash Gupta In Partial Fulfillment Of the Requirements for the Degree Doctor of Philosophy in Mechanical Engineering Georgia Institute of Technology May 2015 Copyright © Man Prakash Gupta 2015 NUMERICAL INVESTIGATION OF CARBON NANOTUBE THIN-FILM COMPOSITES AND DEVICES Approved by: Dr. Satish Kumar, Adviser Dr. Abhijit Chatterjee School of Mechanical Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Dr. Alexander Alexeev Dr. Manos Tentzeris School of Mechanical Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Dr. Baratunde Cola School of Mechanical Engineering Georgia Institute of Technology Date Approved: December 5, 2014 ACKNOWLEDGEMENTS The period of last five years during my PhD program has been profoundly transformational for me in many ways. Looking back, I find that it has been truly an enjoyable and intellectually satisfying journey which was made possible by the help and support of a number of people. Among them first and foremost, I would like to thank my mentor and adviser Dr. Satish Kumar for providing the necessary guidance and constant encouragement. He has been incredibly patient and supportive during the entire program. Needless to say, he has been a role model for me and I consider it my privilege to be one of the first graduate students in his research group. I am grateful to my collaborators Dr. Saibal Mukhopadhyay, Dr. Eric Pop, Dr. Minki Cho, Dr. David Estrada and Dr. Ashkan Behnam for allowing me to work with them and having fruitful discussions. I am very thankful to my colleagues Liang, Owen, Matt, David, Ben, Nick, Zhequan and Jialuo for providing a great company and keeping the atmosphere at office fully relaxed, friendly and enjoyable. I am also thankful to my room-mates Joseph Jaoudi, Vikas, Sagar and Rajat for making me feel home away from home. I cannot thank enough my close friends Dr. Vivek Tiwari and Dr. Abhik Kumar Das since undergraduate days and Dr. Pawan Goyal since school days for being an important part of my life. Last but not the least, I am deeply indebted to my parents Murari Lal Gupta and Bhagwan Devi Gupta and my siblings Beena, Jai Prakash and Mukesh for their unconditional love and support throughout my life. I dedicate this thesis to them. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ............................................................................................... iii LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES .............................................................................................................x LIST OF SYMBOLS AND ABBREVIATIONS ........................................................... xvii SUMMARY .................................................................................................................... xxii CHAPTER 1: INTRODUCTION ........................................................................................1 1.1 Carbon Nanotube Thin Film Transistors (CNT-TFTs) ............................................. 2 1.2 Motivation and Scope of Current Work .................................................................... 5 1.2.1 Heat Dissipation and Thermal Reliability of CN-TFTs ..................................... 5 1.2.2 Electro-Thermal Cross-talk in Aligned CNT TFTs ............................................ 7 1.2.3 Hysteresis and the Role of Trap Charges in Aligned CNT TFTs ....................... 8 1.2.4 Alignment Control in CNT-Liquid Crystal Composites .................................... 9 CHAPTER 2: BACKGROUND AND OVERVIEW OF CURRENT WORK .................12 2.1 Synthesis of CNT Networks and Aligned Arrays ................................................... 12 2.2 Performance Assessment of CNT-TFTs ................................................................. 14 2.3 Applications and Devices ........................................................................................ 17 2.3.1 Flexible circuits ................................................................................................ 17 2.3.2 Frequency Range .............................................................................................. 21 iv 2.3.3 Optoelectronics ................................................................................................. 23 2.4 Tuning of Properties of CNT-TFTs ........................................................................ 24 2.4.1 Chirality ............................................................................................................ 24 2.4.2 Electrical Contacts/Junctions ............................................................................ 26 2.4.3 Hysteresis ......................................................................................................... 27 2.4.4 Thin-film Fabrication ....................................................................................... 28 2.4.5 Miscellaneous ................................................................................................... 29 2.5 Modeling and Simulation ........................................................................................ 30 2.5.1 CNT Network and Aligned Array TFTs .......................................................... 30 2.5.2 Liquid Crystal - CNT Composites .................................................................... 34 2.6 Research Contributions of the Current Work.......................................................... 37 CHAPTER 3: METHODOLOGY .....................................................................................41 3.1 Modeling of CNT network TFTs ............................................................................ 41 3.1.1 Thermal Transport ............................................................................................ 41 3.1.2 Electrical Transport .......................................................................................... 43 3.2 Modeling of Aligned CNT TFTs ............................................................................ 46 3.3 Modeling of Hysteresis in Aligned CNT TFTs ....................................................... 48 3.4 Modeling of Carbon Nanotube –Liquid Crystal Composites ................................. 48 3.4.1 Dissipative Particle Dynamics (DPD) Method ................................................. 48 3.5 Closure .................................................................................................................... 53 v CHAPTER 4: ROLE OF CONTACT RESISTANCE AND JUNCTION TOPOLOGY IN HIGH FIELD BREAKDOWN OF CN-TFT .....................................................................55 4.1 Modeling Parameters and Experimental Conditions ............................................... 55 4.1 Model Validation and Estimation of Interfacial Conductances .............................. 59 4.2 Role of CNT-Substrate and CNT-CNT Interfacial Thermal Conductance ............. 62 4.3 Effect of Variation in Network Density .................................................................. 65 4.4 Impact of CNT buckling at Junctions ..................................................................... 67 4.5 Closure .................................................................................................................... 69 CHAPTER 5: ROLE OF CHANNEL DIMENSIONS AND CNT NETWORK MORPHOLOGY IN HEAT DISSIPATION AND THERMAL RELIABILITY .............71 5.1 Channel Length and Width Dependence ................................................................. 71 5.2 Network Morphology .............................................................................................. 77 5.2.1 Variable Alignment Angle with Constant CNT Length ................................... 77 5.2.2 Variable alignment angle with log-normal distribution of CNT length ........... 80 5.3 Closure .................................................................................................................... 86 CHAPTER 6: ELECTRO-THERMAL CROSS-TALK IN ALIGNED CNT TFTs ........88 6.1 Transfer and Output Characteristics ........................................................................ 90 6.2 Array Density and Electrostatic Screening ............................................................. 91 6.3 Gate Coupling and Electrostatic Screening of CNTs .............................................. 93 6.4 Power Dissipation and Temperature Profile ........................................................... 94 vi 6.5 Closure .................................................................................................................... 99 CHAPTER 7: HYSTERESIS IN ALIGNED CNT TFTs ...............................................101 7.1 Impact of Sweep Rate (SR) ................................................................................... 102 7.1.1 Interface Trap Charges ................................................................................... 103 7.1.2 Bulk Trap Charges .......................................................................................... 104 7.2 Impact of Interface Trap Charges Concentration .................................................. 105 7.3 Impact of Oxide Thickness ................................................................................... 106 7.4 Impact