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Growth, Characterization, and Applications of Vanadium Dioxide Thin Films

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Growth, Characterization, and Applications of Vanadium Dioxide Thin Films Growth, Characterization, and Applications of Vanadium Dioxide Thin Films by Keller Andrews, M.Sc. A Dissertation In Physics Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved Luis Grave De Peralta Chair of Committee Stefan Estreicher David Lamp Ayrton Bernussi Sergey Nikishin Mark Sheridan Dean of the Graduate School August 2019 c 2019, Keller Andrews Texas Tech University, Keller Andrews, August 2019 ACKNOWLEDGMENTS There are many people that supported the completion of this dissertation. With- out them, it would not have been finished. I would like to thank the Texas Tech Department of Physics and Astronomy for the support they have provided during my time in graduate school at TTU. I would also like to thank the physics shop for their training and assistance in the construction of several of the apparatuses used in this dissertation. In addition, I would like to acknowledge that this work was assisted by Dr. Daniel K. Unruh in the TTU Department of Chemistry and Bio- chemistry X-ray diffraction facility as well as many of my colleagues in the Nano Tech Center, in particular Ishtiaque Ahmed and Ahmad Jafari. I would like to acknowledge the help of many fellow members of the research groups I have been a part of. The equipment and methods are maintained and developed by many hands working together. I would like to acknowledge the assistance of my committee for their contribu- tions to this dissertation. Their input and guidance helped form some of the theory in this dissertation. I would like to thank Dr. Anthony Kaye beginning my research down this path, and guiding my research for many years. I would also like to thank Dr. Luis Grave de Peralta for guiding my research after Dr. Kaye and mentoring me while I finished these projects. Lastly, I would like to thank my wife for helping me through this while writing a dissertation herself. For being a sounding board, a brainstorming partner, and a constant source of support, I will ever be grateful. ii Texas Tech University, Keller Andrews, August 2019 TABLE OF CONTENTS Abstract ......................................v List of Tables ................................... vi List of Figures ................................... vii 1. Introduction ..................................1 1.1 A general description of the phase transition in vanadium dioxide . .2 1.2 This Dissertation . .5 1.2.1 My Contribution to the Field . .6 2. The state of Vanadium Dioxide .......................8 2.1 The Metal-Insulator Transition in Vanadium Dioxide . .8 2.1.1 Hysteresis in Vanadium Dioxide . .9 2.1.2 Inducing a Metal-Insulator Transition in VO2 ........ 11 2.1.3 Proposed Models of the Metal-Insulator Transition in VO2 13 2.2 A Selection of Past Proposed Applications of Vanadium Diox- ide................................... 16 3. Methods of Growth and Characterization of Materials ......... 20 3.1 Growth of thin films by two primary methods . 20 3.1.1 Film Growth by Pulsed Laser Deposition . 20 3.1.2 Film Growth by Electron-beam Deposition . 28 3.2 Characterization Techniques . 29 3.2.1 Thermal control of samples during characterization . 30 3.2.2 General Spectroscopy . 31 3.2.3 Pump-Probe Technique . 34 3.2.4 Four-point Probe Technique . 36 3.2.5 X-Ray Diffraction . 37 3.2.6 Atomic Force Microscopy . 39 4. Proposed New Applications for Vanadium Dioxide-based Systems .. 40 4.1 Performance of Vanadium dioxide . 40 4.2 Tungsten-Doping of Vanadium Dioxide . 45 4.3 VO2 as a tunable absorber at targeted wavelengths . 52 iii Texas Tech University, Keller Andrews, August 2019 5. The Path Forward From Here ........................ 61 5.1 Potential Applications of Vanadium Dioxide Thin Films . 62 5.2 Concluding Remarks . 64 Appendix A. Matlab R scripts for the processing of spectral data ..... 74 A.1 readin.m . 74 A.2 Spectra processing.m . 76 Appendix B. Derivation of Quarter-Wavelength Thickness for Reflec- tion Minimization ............................. 81 iv Texas Tech University, Keller Andrews, August 2019 ABSTRACT Vanadium dioxide (VO2) exhibits a solid-solid phase transformation, transition- ing from an insulator to a metal when heated above its transition temperature, or by application of incident light, electric fields, or mechanical strain. Across this transition, many of the properties of VO2 change in potentially useful ways, in- cluding the response of VO2 to light. A set of versatile growth procedures have been developed for the fabrication of vanadium dioxide thin films by Pulsed-Laser Deposition (PLD), and these procedures have been shown to be compatible with a wide array of substrates. Several methods have been developed and implemented for the characterization and study of these thin films, and new, more versatile form of PLD has been implemented for the more widely applicable growth of doped VO2 films as well as some more complex films not easily fabricated by standard PLD. In addition, a new application of vanadium dioxide has been developed to controllably modify the reflectivity of metal surfaces. v Texas Tech University, Keller Andrews, August 2019 LIST OF TABLES 2.1 Properties of vanadium dioxide below and above the metal-insulator transition . .8 4.1 Change in TMIT of V1−xWxO2 as x changes. 47 vi Texas Tech University, Keller Andrews, August 2019 LIST OF FIGURES 1.1 The crystalline structure of VO2......................4 2.1 Example of hysteresis in vanadium dioxide . .9 2.2 1D chain with Peierls distortion . 15 3.1 PLD illustration . 20 3.2 Comparison of variations of pulsed laser deposition . 23 3.3 Model of vacuum furnace for annealing . 26 3.4 E-beam illustration . 28 3.5 Copper sample holders for characterization . 30 3.6 Spectroscopy illustration . 31 3.7 State machine flow . 32 3.8 Pump-probe configuration . 34 3.9 Pump/probe waveform of optimized VO2 growth procedures . 36 3.10 Four-point probe apparatus . 38 4.1 Hysteresis loops for various pressures . 41 4.2 Change in transmission for various target-substrate separations . 42 4.3 XRD of VO2 ................................. 43 4.4 Hysteresis loops for RT-VO2 and HT-VO2 ................ 44 4.5 Transmission spectra of VO2 ........................ 44 4.6 Change in transmission for VO2 on various substrates . 45 4.7 Percentage tungsten doping vs. insulator-metal transition temperature 48 4.8 Change in resistivity of V1−xWxO2 films . 49 4.9 Change in transmission of V1−xWxO2 films . 50 4.10 XRD diffraction of standard and alternating-target growth of VO2 films ..................................... 50 4.11 XRD diffraction of alternating-target grown V1−xWxO2 films . 51 4.12 AFM images of tungsten-dopes VO2 films . 52 4.13 Reflection, transmission, and absorption of VO2 film on glass . 54 vii Texas Tech University, Keller Andrews, August 2019 4.14 Two views of the reflection from a metal-under-insulator . 55 4.15 The measured reflectivity of three metal films, at a 20◦ angle of inci- dence. 57 4.16 Reflectivity for the insulating and metallic states of VO2 on various metals . 59 5.1 Illustrations of dielectrophoresis-based optoelectronic traps . 63 B1 Effect of angle of incidence on wavelengths at which destructive in- terference occurs. 83 viii Texas Tech University, Keller Andrews, August 2019 CHAPTER 1 INTRODUCTION The transformation of matter between one phase and another has been of in- terest to the scientific community due to both the underlying physics of phase transitions in different materials as well as the ways we might put that physics to use. We see many applications of phase changing materials. Looking at phase transitions in water, transitions between solid and liquids are remarkably useful and widely used, as most people in the developed world enjoy ice to keep their beverage cold. In addition liquid-to-gas phase transitions were the driving force behind the industrial revolution, with the steam engine harnessing the expansion of water as it transforms into a gaseous state. Of course, phase transition are not the only thing of any scientific or techno- logical importance. Many materials or phenomena that have found widespread use and applications which are not strict phase transitions, but instead may have some reversible change in their physical properties caused by an external stim- ulus. For example, photochromism is a molecular transformation where, when exposed to certain wavelengths of light, the structure of a photochromic molecule changes, which in turn changes how the molecule interacts with light, causing a color change in a material. A widespread example of this is seen in photochromic eyeglass lenses which darken when exposed to sunlight, but become clear again when taken back indoors. Perhaps the most impactful phenomenon we have been able to exploit in re- cent history came with the advent of semiconductor-based technologies. Since the invention of the transistor, semiconductors have added uncountable electronic benefits to the world. Semiconductor-based technologies do not utilize material phase transitions to accomplish their change in electrical properties, but instead in semiconductors, the density of charge carriers (electron or holes) and therefore the electrical conductivity are controlled by doping. Based on examples like these, it is fair to say that when nature gives us a “han- dle” by which we can control some mechanism, humanity can find a way to put 1 Texas Tech University, Keller Andrews, August 2019 it to use. Vanadium dioxide is a material for which it seems we have several such methods of control. Vanadium dioxide is a crystalline material that exhibits a phase transition, dis- tinct from all the others mentioned here. This phase transition is between two solid states and is stimulated by several means. When this transition occurs, the prop- erties of the vanadium dioxide (VO2) change, some properties showing a larger change than others.
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