Metal Oxide-Based Transparent Conducting Oxides Meagen Anne Gillispie Iowa State University

Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 2006 Metal oxide-based transparent conducting oxides Meagen Anne Gillispie Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Materials Science and Engineering Commons Recommended Citation Gillispie, Meagen Anne, "Metal oxide-based transparent conducting oxides " (2006). Retrospective Theses and Dissertations. 1891. https://lib.dr.iastate.edu/rtd/1891 This Dissertation 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 Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. UMI Number: 3243848 UMI Microform 3243848 Copyright 2007 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, MI 48106-1346 Metal oxide-based transparent conducting oxides by Meagen Anne Gillispie A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Materials Science and Engineering Program of Study Committee: David Cann, Co-major Professor Xiaoli Tan, Co-major Professor Mufit Akinc Vikram L. Dalal Mani Mina Iowa State University Ames, Iowa 2006 Copyright © Meagen Anne Gillispie, 2006. All rights reserved. ii Table of Contents Chapter 1: Introduction 1 1.1 Dissertation Organization 1 1.2 Overview of Transparent Conducting Oxides 1 1.2.1 Device Applications 2 1.2.2 Design Considerations 2 1.2.3 Current Materials 4 1.2.3.1 Electrical Properties 5 1.2.3.2 Optical Properties 6 1.2.4 Limitations of Existing TCOs 6 1.3 Doped Anatase TiO2 8 1.3.1 Structural Overview and Stability 8 1.3.2 Crystal Chemistry 10 1.3.3 Electrical Properties 13 1.3.3.1 Defect Chemistry 13 1.3.3.2 Undoped Anatase TiO2 15 1.3.3.3 Doped Anatase TiO2 17 1.3.4 Optical Properties 18 1.4 Delafossite Ceramics 19 1.4.1 Historical Perspective 19 1.4.2 Structural Overview 20 1.4.3 Phase Stability 21 1.5 Crystal Chemistry of Delafossites 26 1.6 Electrical Properties of Delafossites 28 1.6.1 Band Structure 28 1.6.2 Carrier Transport 30 1.6.3 Defect Chemistry 33 1.7 Optical Properties of Delafossites 34 1.8 Synthesis of Delafossites 35 1.8.1 Low Temperature Synthesis Techniques 36 1.8.2 High Temperature Solid State Synthesis 37 1.8.3 Thin Film Deposition Techniques 37 1.9 Bipolar Doping 38 1.9.1 Codoping ZnO 39 1.9.2 Delafossites 43 1.10 Mixed B Cation Substitution 47 2 + 1.10.1 CuB 2/3Sb1/3O2 47 1.10.2 Even 2+/4+ Substitution 48 1.10.3 Trivalent Mixture 49 1.11 Research Plan 49 1.12 References 50 Chapter 2: RF Magnetron Deposition of Transparent Conducting Nb-doped TiO2 Films on SrTiO3 55 Abstract 55 Figure List 60 References 64 iii Chapter 3: Sputtered Nb- and Ta-doped TiO2 Transparent Conducting Oxide Films on Glass 66 Abstract 66 I. Introduction 66 II. Experimental Procedure 68 III. Results and Discussion 69 IV. Conclusions 73 Acknowledgements 74 Figure Captions 74 References 80 Chapter 4: Crystal Chemistry and Electrical Properties of the Delafossite Structure 82 Abstract 82 1. Historical Background 83 2. Crystal Structure of Delafossite 83 3. Crystal Chemistry of the Delafossite Structure 84 4. Crystal Chemistry of Complex Delafossite Compounds 85 4.1 AB2 + B5+ O Compounds 86 2/3 1/3 2 2 + 4 + 4.2 AB 0.5B0.5O2 Compounds 86 4.3 A(B 3 + ,B 3 + )O Compounds 87 2 5. Synthesis of Delafossites 88 5.1 Low Temperature Synthesis Techniques 88 5.2 Solid State Synthesis 90 5.3 Thin Film Synthesis 90 6. Electrical Properties of Delafossites 91 7. Magnetic Properties of Delafossites 94 8. Conclusions 94 Acknowledgements 95 Figure List 95 Tables 99 References 103 Chapter 5: Effects of Codoping in CuAlO2 and CuGaO2 Delafossite Compounds 106 Abstract 106 1. Introduction 106 2. Experimental Procedure 108 3. Results and Discussion 109 4. Conclusions 116 References 116 iv Chapter 6: Synthesis and Electrical Properties of CuNi0.5Ti0.5O2 118 Abstract 118 1. Introduction 118 2. Experimental Procedure 121 3. Results and Discussion 122 4. Conclusions 124 Tables 125 References 125 Chapter 7: Stability Criteria for Complex Delafossite Compounds 127 Abstract 127 1. Introduction 127 2. Experimental Procedure 128 3. Results and Discussion 129 4. Mixed B Cation Delafossite Stability 132 5. Conclusions 134 Tables 134 Figure Captions 135 References 138 Chapter 8: General Conclusions 140 8.1 Doped Anatase TiO2 140 8.1.1 General Discussion 140 8.1.2 Recommendations for Further Study 142 8.2 Delafossite Ceramics 143 8.2.1 General Discussion 143 8.2.2 Recommendations for Further Study 145 8.3 References 145 Acknowledgements 147 1 Chapter 1: Introduction 1.1 Dissertation Organization This dissertation is composed of a number of journal articles published or submitted for publication. This introductory chapter contains background information and a comprehensive literature review of the topics addressed in subsequent chapters. Chapters 2 and 3 contain journal articles that have been submitted to Journal of Applied Physics and Journal of Materials Research, respectively. Chapter 7 is a journal article prepared for submission to Journal of Materials Science. Chapters 4 and 6 contain journal articles that have been published in Thin Solid Films and Materials Letters, respectively. For all journal articles, the author was the primary researcher and author. Listed co- authors provided some assistance in collecting data and edited papers for content and clarity. In addition to the journal articles, Chapter 5 presents research results not published and Chapter 8 contains general conclusions and recommendations for further study. Works cited are listed at the end of each chapter in which they appear. 1.2 Overview of Transparent Conducting Oxides New consumer electronic devices are developed to be ultra-portable and efficient, combining communication, storage, and multimedia technologies in one package. These so-called smart devices require transparent electrodes and circuitry in order to retain portability, thus much research has been conducted in the broad field of transparent conducting oxides (TCOs) [1-7]. TCOs are unique oxide materials because depending on the doping scheme used, they can behave as insulators, semiconductors, or metals. In general, the term TCO is used to describe a wide bandgap semiconductor (Eg > 3.1 eV) doped (sometimes to degeneracy) through the introduction of native or substitutional dopants, which provide high carrier concentration and high mobility [2, 6, 7]. A wide bandgap ensures transparency through the visible portion of the electromagnetic (EM) spectrum (400 nm < > 700 nm). High mobility ensures that the plasma absorption edge of the optical window is deep into the infrared portion of the EM spectrum. These important properties make TCOs widely applicable for any device that requires optical access behind electrical circuitry. In this section, the device applications of TCOs will be introduced followed by a brief discussion of the design considerations of TCOs. Next, current TCO materials will 2 be discussed, with extra attention paid to their electrical and optical properties. Finally, the limitations of current TCO materials will be addressed. 1.2.1 Device Applications The first device application of a TCO material was as a de-icer for WWII bomber windows [6]. In addition to being used in automobile and supermarket freezer display windows, TCOs are now used in a variety of applications that exploit certain aspects of the unique combination of electrical and optical properties they possess. Some applications, such as low emissivity and electrochromic windows, are more passive device applications in which the high IR reflectivity (controlled by the location of the plasma absorption edge) is utilized to reflect heat back into or out of certain spaces. Other applications, such as state-of-the-art flat panel displays, push the envelope of existing TCO properties such as conductivity and transmissivity. Specific passive device applications for TCOs include energy-conserving windows for aircraft, automobiles, household appliances, and buildings, as well as electromagnetic shielding and antistatic coatings for cathode-ray tubes and copy machines [1, 2, 6-9]. These kinds of applications require high optical transparency but not necessarily high conductivity. There are many more active device applications for TCOs that push the limits of current technology. These include flat screen high-definition televisions, high resolution flat panel liquid crystal displays, touch-sensitive displays, solar cells, electrochromic windows, and other thin film photovoltaics [1, 3, 6, 8, 9]. In all of the preceding applications, degenerately doped TCO materials are used as a transparent electrode. Technological advances continually put pressure on finding TCO materials that exhibit higher carrier concentration and mobility while maintaining high optical transmission through the visible EM range, an important issue that will be further addressed in following sections. 1.2.2 Design Considerations Many important factors must be considered when selecting a TCO material for each of the applications identified above. Haacke [8, 10] introduced a figure of merit, TC, to evaluate transparent conducting materials. In general, TC is defined as seen in Equation 1.1. 3 T10 = , Equation 1.1 TC R S where T is optical transmission (fraction) and RS is sheet resistance. Transmission is related to absorption coefficient () and film thickness (t) as shown in Equation 1.2, while sheet resistance can be related to electrical conductivity () and film thickness (t) as shown in Equation 1.3.

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