View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Digital Repository @ Iowa State University Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations 2010 Fabrication and modeling of thin-film anodic titania memristors Kyle James Miller Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/etd Part of the Electrical and Computer Engineering Commons Recommended Citation Miller, Kyle James, "Fabrication and modeling of thin-film anodic titania memristors" (2010). Graduate Theses and Dissertations. 11426. https://lib.dr.iastate.edu/etd/11426 This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Fabrication and modeling of thin-film anodic titania memristors by Kyle Miller A thesis submitted to the graduate faculty in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Major: Electrical Engineering Program of Study Committee: Nathan Neihart, Co-Major Professor Sumit Chaudhary, Co-Major Professor Ayman Fayed Iowa State University Ames, Iowa 2010 Copyright © Kyle Miller, 2010. All rights reserved. ii TABLE OF CONTENTS LIST OF FIGURES ..................................................................................................... iv ACKNOWLEDGEMENTS ......................................................................................... vi ABSTRACT ................................................................................................................ vii CHAPTER 1. INTRODUCTION ................................................................................ 1 CHAPTER 2. REVIEW OF LITERATURE ............................................................... 3 2.1 Introduction ........................................................................................................ 3 2.2 Switching Mechanisms ...................................................................................... 6 2.3 Fabrication ....................................................................................................... 12 2.4 Modeling .......................................................................................................... 14 2.5 Applications ..................................................................................................... 18 2.6 Summary .......................................................................................................... 20 CHAPTER 3. METHODS AND PROCEDURES .................................................... 22 3.1 Introduction ...................................................................................................... 22 3.2 Fabrication ....................................................................................................... 22 3.3 Testing.............................................................................................................. 24 3.4 Modeling .......................................................................................................... 26 3.5 Summary .......................................................................................................... 28 CHAPTER 4. RESULTS ........................................................................................... 29 iii 4.1 Keithley Measurement Results ........................................................................ 29 4.2 Atomic Force Microscopy Results................................................................... 34 4.3 Ideal Matlab Modeling Results ........................................................................ 35 4.4 ORCAD Pspice Memristor Model ................................................................... 36 4.5 Summary .......................................................................................................... 40 CHAPTER 5. FUTURE WORK ............................................................................... 42 5.1 Memristor Fabrication ..................................................................................... 42 5.2 Memristor Modeling ........................................................................................ 43 5.3 Summary .......................................................................................................... 43 APPENDIX A: MATLAB MEMRISTOR MODEL CODE ..................................... 44 A.1 Main Body of Code ......................................................................................... 44 A.2 ODE45 Function Code .................................................................................... 45 APPENDIX B: VERILOGA MEMRISTOR CODE ................................................. 46 BIBLIOGRAPHY ....................................................................................................... 47 iv LIST OF FIGURES Figure 1. Figure showing ideal memristor behavior. OFF and ON states shown by (b) and (c) respectively. The operation between states is illustrated in (a). ................. 5 Figure 2. (a) Unipolar switching and (b) bipolar switching curve examples. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials [4], © 2007. ................................................................................................................... 6 Figure 3. Formation of a filament in the switching medium for (a) vertical and (b) horizontal structures. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials [4], © 2007. ............................................................................... 7 Figure 4. Test vehicle to test TiO 2 memristor switching mechanisms. Reprinted by permission from Macmillan Publishers Ltd: Nature Nanotechnology [6], © 2008............................................................................................................................. 11 Figure 5. Close-up of each junction and a plot of its work function. Reprinted by permission from Macmillan Publishers Ltd: Nature Nanotechnology [6], © 2008............................................................................................................................. 11 Figure 6. SPICE Macro Model of a Memristor. .................................................................... 18 Figure 7. Anodization equipment. ......................................................................................... 23 Figure 8. Example of a finished sample on a titanium film. .................................................. 24 Figure 9. Example of sample made with titanium deposited on a glass slide. ...................... 24 Figure 10. Probe station with sample under test. ................................................................... 25 Figure 11. Keithley 4200 Semiconductor Characterization System. ..................................... 26 Figure 12. Samples that were not annealed. The number above each plot states how many seconds the sample was anodized for. All currents have been normalized and arrows denote the direction of the sweep. ......................................... 30 Figure 13. Annealed samples. The anodization time for each sample is above the respective plot. All currents have been normalized. ................................................... 31 v Figure 14. Three consecutive sweeps on the not-annealed ten second sample. .................... 32 Figure 15. "Soft-Switching" sweeps for the not-annealed samples. ...................................... 33 Figure 16. "Soft-Switching" sweeps for annealed samples. .................................................. 34 Figure 17. AFM images for not-annealed samples. Left to Right: 60s, 10s, 3s, 1s. ............. 34 Figure 18. Plot made with Matlab memristor model. ............................................................ 36 Figure 19. Comparison of Biolek Pspice model to 10 second sample................................... 38 Figure 20. Pspice V vs time memristor model curve. ............................................................ 39 Figure 21. Pspice I vs time memristor model curve. ............................................................. 40 vi ACKNOWLEDGEMENTS I would like to thank Dr. Neihart and Dr. Chaudhary for giving me the opportunity to do research on a very cutting-edge and interesting topic. Memristors did not exist as an area of research at Iowa State, and both of them supported me in the creation of this new research. Without their help and support, this work would not have been possible. I would also like to thank my parents who supported me financially and emotionally through college. They instilled the notion of the importance of higher level education in my life. vii ABSTRACT A new method for the fabrication of memristors is exhibited involving the electrochemical anodization of titanium. This is an inexpensive, room temperature alternative to the current methods of fabrication. Two sets of devices were fabricated with varying anodization times and the devices were characterized. One set of the devices was annealed before characterization to evaluate the importance of annealing as stated in papers. The devices not annealed yielded memristive behavior due to oxygen vacancies created at the titanium-TiO 2 junction buried below the surface of the device. The annealed devices behaved as resistors