Molecular-Scale Organic Electronic Devices for Integrated Nonvolatile Memory Application
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Molecular-scale Organic Electronic Devices for Integrated Nonvolatile Memory Application Troy Graves-Abe A DISSERTATION PRESENTED TO THE FACULTY OF PRINCETON UNIVERSITY IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY RECOMMENDED FOR ACCEPTANCE BY THE DEPARTMENT OF ELECTRICAL ENGINEERING JUNE 2006 © Copyright 2006 by Troy Graves-Abe All rights reserved. Abstract Self-assembly techniques that allow the controlled growth of nanometer-scale organic molecular films present new opportunities to develop electronic devices with dimensions much smaller than those of current technologies. In this thesis we address several of the challenges to realizing this goal, and demonstrate a molecular-scale programmable-resistance memory device. Although technologically attractive, field-effect transistors (FETs) with a self- assembled organic channel are difficult to realize due to the poor gate-channel coupling. We have used electrostatic modeling to determine guidelines that allow the maximum gate modulation of the channel potential in these devices. Molecular-scale devices with integrated metal wiring are desirable for practical application. However, this typically requires the vacuum deposition of metal electrodes, which can damage the thin organic layer. We developed several approaches to fabricate two-terminal molecular-scale devices with vacuum-deposited metal electrodes and minimal defects in the organic layer. To demonstrate these device structures, we used films consisting of 1 to 12 self-assembled layers of 11- mercaptoundecanoic acid (MUA). The device structures that we developed included two large-area planar structures as well as a minimal-area structure that utilizes an insulating film to limit the device area to the edge of a metal layer. These structures allowed the first study of the mechanism of conduction in MUA films, which was characterized as Richardson-Schottky emission. Finally, we found that these devices could be operated as a programmable memory by applying voltage pulses to increase or decrease the conductivity over a range of 103. The conductivity of the stored state could be read non-destructively with low-voltage pulses. Devices had remarkably large conductance (in the low-resistance state) of up to 106 S/cm2 at 1 V, programmed states remained stable for many months, and devices were functional for more than 104 programming cycles. The likely mechanism for the programmable resistance was the formation and destruction of conducting paths due to metal injected into the MUA film. We discuss their practical application and show that because of their high conductance these devices are uniquely promising among organic memories for use in dense, high-speed memory arrays, where large conductance is required to minimize resistance-capacitance delays. iii Acknowledgements I am very grateful to the many people who have been so helpful to me during my time at Princeton University. First of all is my advisor, Prof. Jim Sturm. His excitement for new ideas, insight, and exceptional ability to instruct were invaluable in motivating, directing, and challenging me. The opportunity to learn from him was one for which I will always be grateful. I was privileged to work with a number of remarkable graduate students whose camaraderie and advice were invaluable. Within Prof. Sturm’s group, these included Florian Pschenitzka, Mike Lu, Eric Stewart, Xiang-Zheng Bo, Haizhou Yin, Ke Long, Hongzheng Jin, Kun Yao, David Inglis, Keith Chung, Suberr Chi, and Bahman Hekmatashoar. I especially thank John Davis and Rebecca Peterson, who joined the group at the same time as me- their friendship was greatly appreciated. It was also a pleasure to get to know Alan Wan, Rabin Bhattacharya, and Philo Juang when studying for generals our first year, and I enjoyed their friendship throughout my time at Princeton. In particular I’d like to thank Alan for all of the time spent discussing technical problems, as well as all of our (occasionally successful) efforts to make it to the gym. Other students that I would like to thank include Toh Hean Ch’ng, Rob Ellis, Joe Valentino, Russell Holmes, Chris Keimel, Brent Bollman and Joe Peach. I’m very grateful to Professors Zhenan Bao, Rick Register, Sandra Troian, and Antoine Kahn for taking time from their busy schedules to help me with this work. I am indebted to Professors Sigurd Wagner and Antoine Kahn for reading this thesis and offering helpful comments. I enjoyed discussions with Prof. C. R. Selvakumar during his sabbatical here. I would also like to thank the many staff members of the ELE department and PRISM for all of their help, especially Barbara Varga, Carolyn Arnesen, Sheila Gunning, Jamie Kubian, Karen Williams, Sarah Braude, Joe Palmer, Jane Woodruff, and Dr. Nan Yao. Jinsong Pei is a close friend whose support and advice were greatly appreciated. I have been blessed with a wonderful family that has been an unending source of support and encouragement. I especially thank my mom and dad, whose faith has encouraged my own and who taught me so many things, most recently exemplifying optimism and perseverance. Finally and most importantly, my deepest thanks to my wife Katie, for her amazing love and support. iv Contents Abstract ................................................................................................................. iii Acknowledgements .............................................................................................. iv List of Figures ....................................................................................................... viii List of Tables ......................................................................................................... xii 1. Introduction 1.1 Inorganic Crystalline Semiconductors................................................... 1 1.2 Opportunities for Organic Semiconductors in Highly Scaled Applications ........................................................................................ 2 1.3 Electrical Properties of Metal-Molecule Junctions................................. 3 1.4 Conduction in Organic Solids................................................................ 8 1.5 Molecular Films by Self-Assembly ........................................................ 9 1.6 Summary and Thesis Organization ...................................................... 10 2. Electrostatic Modeling of Molecular-Scale FETs 2.1 Introduction ........................................................................................... 16 2.2 Setup of Electrostatic Simulations......................................................... 19 2.3 Simulation Results................................................................................. 20 2.4 Summary and Conclusions ................................................................... 26 3. Growth and Characterization of MUA Self-Assembled Multilayer Films 3.1 Introduction ........................................................................................... 30 3.2 Deposition of Gold Substrates............................................................... 32 3.3 Growth of MUA Multilayer ..................................................................... 34 3.4 Characterization of MUA Multilayers and Gold Substrates 3.4.1 Ellipsometry............................................................................ 36 3.4.2 Rutherford Backscattering...................................................... 39 3.4.3 Atomic Force Microscopy ....................................................... 41 3.4.4 Ultraviolet Photoelectron Spectroscopy.................................. 46 3.5 Summary............................................................................................... 52 4. Electrical Characteristics of MUA Films in a Two-Terminal Planar Device Structure 4.1 Introduction ........................................................................................... 57 4.2 Device Fabrication................................................................................. 59 4.3 Electrical Measurements ...................................................................... 65 4.3.1 Device Area and Fabrication Dates........................................ 68 4.3.2 Device Structure, Gold Topology, and Number of MUA layers ................................................................................... 71 4.3.3 Deposition of the Top Metal Electrode ................................... 73 4.4 Discussion of Electrical Characteristics................................................. 75 4.4.1 Role of the Fabrication Conditions in Device Performance .... 76 4.4.2 Summary of Optimum Conditions to Minimize Electrical Defects................................................................................. 81 v 4.4.3 Comparison to Previous Results ............................................ 83 4.4.4 Conduction Mechanism in Low-Defect Devices ..................... 84 4.5 Summary............................................................................................... 93 5. Edge Device Fabrication and Electrical Measurements 5.1 Introduction ........................................................................................... 99 5.2 Development of the Edge-Structure Device.......................................... 100 5.3 Fabrication of Edge Devices ................................................................. 102 5.4 Edge Device Characteristics ................................................................. 106 5.5 Discussion............................................................................................