Growing Anatase and Rutile Titania on c-cut Sapphire using Pulsed-Laser Deposition by Alexandra V. Gordienko, inener-fizik po special~nosti fizika kondensirovannogo sostoni vewestva i nanosistem A Thesis In Physics Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Anthony B. Kaye Chair of Committee David Lamp Charles Myles Mark Sheridan Dean of the Graduate School December, 2016 Copyright 2016, Alexandra Gordienko Texas Tech University, Alexandra V. Gordienko, December 2016 Acknowledgments This research made use of the TTU College of Arts & Sciences Microscopy Center. I would like to thank Dr. D. Unruh for his assistance with a number of aspects related to XRD. I would like to thank Keller Andrews for his thoughtful discussions, and Dr. A. Kaye for his mentorship and encouragement throughout the thesis process. Finally, I would like to thank Dennis and Brooke Harris as well as Vladimir and Svetlana Gordienko for their daily support. ii Texas Tech University, Alexandra V. Gordienko, December 2016 Contents Acknowledgments ................................ ii Abstract ...................................... v List of Tables ................................... vi List of Figures .................................. vii 1. Introduction .................................. 1 1.1 Introduction . 1 1.2 Overview . 3 1.3 My Contribution to the Field . 4 2. Producing the Different Phases of TiO2 ................ 6 2.1 Major Production Methodologies . 6 2.1.1 Techniques in Common . 6 2.1.2 Sol-gel techniques . 8 2.1.3 Sputtering . 10 2.1.4 Chemical Vapor Deposition . 11 2.1.5 Pulsed-Laser Deposition . 13 2.1.6 Other Techniques . 15 2.2 Film Growth Considerations . 17 2.2.1 Crystallographic Match and Cost . 18 2.3 A Brief Review of Titania Production Techniques . 22 2.4 Pulsed-Laser Deposition: Our System, Methods, and Results . 23 2.4.1 Our System . 23 iii Texas Tech University, Alexandra V. Gordienko, December 2016 2.4.2 Methodology . 24 2.4.3 Characterization . 28 2.4.3.1 Atomic Force Microscopy . 28 2.4.3.2 Scanning Electron Microscopy . 31 2.4.3.3 Raman Spectroscopy . 31 2.4.3.4 XRD . 35 2.4.4 Final Production Protocols . 39 3. Summary and Future Work ........................ 40 3.1 Summary . 40 3.2 Future Work . 40 Bibliography ................................... 42 iv Texas Tech University, Alexandra V. Gordienko, December 2016 Abstract In this thesis, I present a review of the growth of tetragonal phases of titanium dioxide on different substrates. I also report a pulsed-laser deposition growth proto- col that facilitates the growth of both anatase and rutile phases of titania without changing the substrate or target material. Finally, I also demonstrate the develop- ment of the first recipe for growth of anatase titania on a sapphire substrate. v Texas Tech University, Alexandra V. Gordienko, December 2016 List of Tables 2.1 Pulsed-Laser Deposition Variables . 14 2.2 Comparison of Thin-Film Growth Techniques . 16 2.3 Lattice Matches to Typical Substrate Materials . 20 2.4 Example Thin Film Results . 26 2.5 Raman Shift Peak Identification . 34 2.6 TiO2 film growth conditions . 39 vi Texas Tech University, Alexandra V. Gordienko, December 2016 List of Figures 2.1 The general process of film producing via the sol-gel technique . 9 2.2 Schematic of sputtering system used to create thin films. 10 2.3 Schematic of a typical low pressure hot wall chemical vapor deposition reactor used in coating silicon substrates. 12 2.4 Schematic representation of the fundamental transport and reaction steps underlying metalorganic chemical vapor deposition. 12 2.5 Schematic representation of the Kaye Research Group pulsed-laser depo- sition system. 14 2.6 Sketch of how an atomic force microscope works . 30 2.7 Raman spectra of thin-film samples. 33 2.8 X-ray diffraction patterns for the rutile (top, red) and anatase (bottom, blue) films. 36 2.9 X-ray diffraction pattern of the mixed-phase film from Sample 5. 38 vii Texas Tech University, Alexandra V. Gordienko, December 2016 Chapter 1. Introduction 1.1 Introduction Titanium dioxide (titania; TiO2) is a material that has been studied carefully over the last 100 years (Vegard, 1916). Since its first production, titania has become one of the most widely used white pigments. Pearlescent-effect pigments are based on TiO2, and when combined with metallic pigments, it is often used in car paints, since the combination of titania-based pearlescent pigments and metallic pigments creates an illusion of optical depth. Titania pigments are also used in decorative objects that are intended to imitate natural pearls, cosmetics, and in critical areas of security printing. Titania pigments are also used in almost all white paint and most red-colored candy. With so many applications, TiO2 has become a popular material to study. In 1972, Fujishima and Honda discovered the possibility to split water using TiO2 electrodes. Since its initial publication (Fujishima and Honda, 1972), the Fujishima and Honda paper has been cited over 18,000 times, and it has changed the landscape of photocatalytic science and industry: a whole range of new applications has now been discovered. Among new applications are: using TiO2 as self-cleaning coatings (i.e., when the coating breaks down the organic dirt after exposure to UV light and makes the surface hydrophilic so water spreads evenly on the glass (Roméas et al., 1999); as coatings that are used in environmental applications to clean both air and water (Di Fonzo et al., 2008; Lin et al., 2008); as components of various sensor devices (Bao et al., 2008; György et al., 2005); as a gate dielectric in MOSFET technologies 1 Texas Tech University, Alexandra V. Gordienko, December 2016 (Kim et al., 2006; Xie et al., 2010); and, as the basis for energy-efficient solar cells (Mincuzzi et al., 2009; Park et al., 2000). Titanium dioxide has three stable crystallographic forms, two of which, rutile and anatase, are tetragonal; the third phase (brookite) is orthorhombic. It is well established that properties of titania, and hence the performance of coatings and devices based on it, are different depending upon the crystallographic phase of the titania. For instance, the rutile phase is a more stable form and it scatters light more efficiently (Thiele and French, 1998). Anatase is used in production of solar cells because of its surface chemistry (Hoffmann et al., 1995; Park et al., 2000), and because this phase has roughly twice the photocatalytic power of the rutile phase (Luttrell et al., 2014). For that same reason, anatase is the preferred crystallographic phase for gas- sensing applications since photocatalytic activity is shown to enhance the sensitivity of gas sensors (Yang et al., 2003). Rutile titania could potentially become a better choice for use in solar cells because it is a less expensive material to produce [in fact, most production techniques of synthetic titania yield rutile as a result (Mo and Ching, 1995)], and it scatters light more effectively than anatase. However, the annealing procedure(s) can significantly affect the photocatalytic properties of rutile films (Luttrell et al., 2014), so the growth protocols and the substrate materials selected for the production of titania are critically important. Additionally, the dielectric properties of titania are directly related to the ratio between anatase and rutile phases present in the film. Specifically, dielectric constants increase with the increase of rutile to anatase ratio (Kim et al., 2006), which is significant when titania is used in MOSFET technologies. 2 Texas Tech University, Alexandra V. Gordienko, December 2016 1.2 Overview Over the last 100 years, a number of methods have been developed to produce titania, each one optimized for the final form required. Each of these methods will be summarized and compared to each other in Chapter 2., below. We used pulsed laser deposition as our growth method, and the principal goal of this study was to grow pure samples of both tetragonal phases of titania – anatase and rutile – using the same substrate and the same laser target. Because c-cut sapphire is one of the most commonly used in optical applications, and because sapphire is an excellent crystallographic match with rutile titania, we elected to use sapphire as our substrate. This obviously means that because the anatase phase has significantly different crystallographic parameters, it requires significantly different growth conditions compared to those used to grow the rutile phase. The impetus of this study was two-fold: First, a colleague has suggested growing layered films of TiO2, VO2, and TiO2 to studying how the optical and electrical properties of stack varied from simple VO2 films. From prior work, we know that c-cut sapphire is a substrate that shows both excellent switching amplitude and a very sharp (almost square) transition, so it was natural to choose this substrate for the production of the layered composite material. Since we do not know how each crystallographic phase of titania would affect the properties of the resulting system, we were interested in growing both phases of titania on this substrate. A literature review suggested that even though there are certain parameters that are known to be ideal for anatase and rutile growth on different substrates (e.g., glass, Si, and SiO2), growth of titania films on sapphire always lead the growth of the rutile phase. The second reason for this study was to test our understanding of crystallographic 3 Texas Tech University, Alexandra V. Gordienko, December 2016 growth using the pulsed-laser deposition technique, and specifically, if we were able to selectively grow each phase without changing the substrate or the laser target. Once proven, these techniques and procedures could be applied to other, more complex materials in which the crystallographic phase plays an even more critical role (e.g., La2CuO4 and similar materials).