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Device Engineering of Organic Field-Effect Transistors DEVICE ENGINEERING OF ORGANIC FIELD-EFFECT TRANSISTORS TOWARD COMPLEMENTARY CIRCUITS A Dissertation Presented to The Academic Faculty by Xiaohong Zhang In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Electrical and Computer Engineering Georgia Institute of Technology May 2009 DEVICE ENGINEERING OF ORGANIC FIELD-EFFECT TRANSISTORS TOWARD COMPLEMENTARY CIRCUITS Approved by: Dr. Bernard Kippelen, Advisor Dr. Ajeet Rohatgi School of Electrical and Computer School of Electrical and Computer Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Oliver Brand Dr. Samuel Graham School of Electrical and Computer School of Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Shyh-Chiang Shen School of Electrical and Computer Engineering Georgia Institute of Technology Date Approved: March 13, 2009 ACKNOWLEDGEMENTS I would like to express my sincerest appreciation to my research advisor, Prof. Bernard Kippelen, for his mentorship, encouragement, and guidance during my time at the Georgia Institute of Technology (Gatech) for my PhD study in ECE and at the University of Arizona for my Master study in Optical Sciences. I also would like to thank all my committee members, Dr. Oliver Brand, Dr. Shyh-Chiang Shen, Dr. Ajeet Rohatgi, and Dr. Samuel Graham, for their guidance and participation in the review and evaluation of this work. I would like to thank all the past and present group members for their great assistance and friendship. Especially, I thank Dr. Josh Haddock for taking his initiative in the setup of the OFET testing systems, Dr. Benoit Domercq, Dr. Andreas Haldi, and William Potscavage for deposition using the Spectros, and Jungbae Kim for the discussion and testing of organic devices. I also thank Dr. Xudong Wang from Prof. Zhonglin Wang's group for collaborating on the work using ALD-deposited Al2O3 as gate dielectrics for OFETs and Dr. Soo Young Kim from Prof. Seth R. Marder’s group for the XRD measurement on pentacene films. This work was funded by the STC Program of the National Science Foundation and the Office of Naval Research. This work was performed in part at the Microelectronics Research Center at Georgia Institute of Technology, a member of the National Nanotechnology Infrastructure Network, which is supported by NSF. Finally, my deepest gratitude goes to my family, without whose love and support I would not be who I am today. Special thanks go to my mom for her continuous support iv and inspiration in my life and to my husband Dr. James Rigsby for always being there for me. This dissertation is dedicated to all of them. v TABLE OF CONTENTS ACKNOWLEDGEMENTS.............................................................................................. IV LIST OF TABLES............................................................................................................ IX LIST OF FIGURES ........................................................................................................... X SUMMARY.....................................................................................................................XV CHAPTER 1 INTRODUCTION ........................................................................................ 1 1.1 Organic Semiconductors and Organic Field-Effect Transistors............................ 1 1.1.1 Organic Semiconductors................................................................................ 1 1.1.2 OFETs vs. Inorganic Thin-Film Transistors.................................................. 4 1.2 Review of Progress in OFETs and OFET-Based Complementary Inverters ........ 5 1.2.1 Progress in OFETs ......................................................................................... 5 1.2.2 Progress in Organic Complementary Inverters.............................................. 7 1.3 Current Issues in the Development of Organic Complementary Inverters............ 9 1.4 Objectives and Organization of the Dissertation................................................. 13 CHAPTER 2 EXPERIMENTAL METHODS ................................................................. 16 2.1 OFET Device Structures and Operation.............................................................. 16 2.1.1 Device Structures of OFETs ........................................................................ 16 2.1.2 Device Operation of OFETs ........................................................................ 19 2.2 Electrical Characterization of OFETs.................................................................. 22 2.2.1 Current-Voltage (IV) Characteristics........................................................... 22 2.2.2 Extraction of Electrical Parameters ............................................................. 25 2.2.3 Schottky Contact and Calculation of Contact Resistance............................ 26 2.3 OFET Fabrication ................................................................................................ 32 2.3.1 Purification and Deposition of Organic Semiconductors ............................ 32 2.3.2 Preparation of Gate Dielectrics and Interfacial Control .............................. 36 2.3.3 Patterning and Deposition of Metal Contacts .............................................. 42 2.4 Setup of Electrical Characterization .................................................................... 44 CHAPTER 3 P-CHANNEL PENTACENE OFETS ........................................................ 47 vi 3.1 Pentacene OFETs with Al2O3 Gate Dielectrics Prepared by ALD...................... 47 3.1.1 Deposition and Characterization of Al2O3 Gate Insulators.......................... 47 3.1.2 Fabrication and Characterization of OFETs with Al2O3 Gate Insulators .... 52 3.1.3 Conclusions.................................................................................................. 59 3.2 Dielectric Interface Engineering with Polymeric Buffering Layers: Performance and Stability............................................................................................................... 59 3.2.1 Experimental Details.................................................................................... 61 3.2.2 Operational Stability and Electrical Characterization.................................. 65 3.2.3 Bias Stress Effect ......................................................................................... 70 3.2.4 Conclusions.................................................................................................. 75 CHAPTER 4 N-CHANNEL C60 OFETS ......................................................................... 76 4.1 Polymeric Smoothing Layer as Surface Modification of Gate Dielectrics ......... 78 4.1.1 Experimental Details.................................................................................... 78 4.1.2 Electrical Characterization........................................................................... 81 4.1.3 Operational Stability Under dc Stress.......................................................... 83 4.1.4 Uniformity.................................................................................................... 84 4.1.5 Summary and Conclusions .......................................................................... 85 4.2 Reduction of Contact Resistance......................................................................... 86 4.2.1 Experimental Details.................................................................................... 87 4.2.2 Electrical Characterization........................................................................... 89 4.2.3 Channel Length Scaling and Contact Resistance......................................... 93 4.2.4 Conclusions.................................................................................................. 98 CHAPTER 5 INTEGRATION OF N- AND P-CHANNEL OFETS: COMPLEMENTARY INVERTERS ..................................................................... 100 5.1 Introduction........................................................................................................ 101 5.1.1 Device Structure and Design ..................................................................... 101 5.1.2 Device Operation ....................................................................................... 104 5.1.3 Characterization of a Static Complementary Inverter ............................... 107 5.2 Organic Complementary Inverters on Flexible Substrates................................ 111 5.2.1 Experimental Details.................................................................................. 111 5.2.2 Electrical Characterization......................................................................... 113 vii 5.2.3 Bending Experiment .................................................................................. 117 5.2.4 Conclusions................................................................................................ 118 5.3 Encapsulation of Organic Complementary Inverters: Preliminary Study ......... 119 5.3.1 Introduction to Multilayer Thin-Film Encapsulation................................. 119 5.3.2 Experimental Details.................................................................................. 124 5.3.3 Results and Discussions............................................................................. 125 5.3.4 Conclusions................................................................................................ 127 CHAPTER
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