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Cu-Based Transparent Conducting Oxides Based on the Delafossite Structure

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Cu-Based Transparent Conducting Oxides Based on the Delafossite Structure Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1-1-2003 Cu-based transparent conducting oxides based on the delafossite structure Robert Bruce Gall Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Recommended Citation Gall, Robert Bruce, "Cu-based transparent conducting oxides based on the delafossite structure" (2003). Retrospective Theses and Dissertations. 19966. https://lib.dr.iastate.edu/rtd/19966 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 Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Cu-based transparent conducting oxides based on the Delafossite structure by Robert Bruce Gall A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Major: Materials Science and Engineering Program of Study Committee: David P. Cann, Major Professor Steve Martin Gordon Miller Iowa State University Ames, Iowa 2003 11 Graduate College Iowa State University This is to certify that the master's thesis of Robert Bruce Gall has met the thesis requirements for Iowa State University Signatures have been redacted for privacy TABLE OF CONTENTS CHAPTER 1. INTRODUCTION i 1.1 Background 1 1.1.1 Transparent Conducting Oxides and Coatings: Historical 1 Perspective 1.1.2 Transparent Conducting Oxides and Coatings: Material 3 Evaluation 1.1.3 Transparent Conducting Oxides: Device Possibilities 7 1.1.4 Transparent Conducting Oxides: Requirements 10 1.1.5 Transparent Conducting Oxides: Relationship between 17 Electrical and Optical Properties 1.2 Delafossites Structure 21 1.2.1 AB02 Overview 21 1.2.2 Overview of the Delafossite Structure 23 1.2.3 Crystal Chemistry in Delafossites 31 1.2.4 Electronic Transport Properties 35 1.2.5 Potential Bipolar Conductivities 36 1.3 Synthesis of Cu Based Delafossites 39 1.3.1 Overview of Delafossite Synthesis 39 1.3.2 Solid State Synthesis 41 1.4 Research Plan 42 CHAPTER 2. HIGH TEMPERATURE PHASE EQUILIBRIA 44 2.1 Introduction 44 2.2 Experimental 45 2.2.1 Synthesis 45 2.2.2 Crystal Structure Analysis 47 2.3 Results and Discussion 48 2.4 Conclusions 62 CHAPTER 3. DOPING METHODS 63 3.1 Experimental 63 3.1.1 Synthesis 63 3.1.2 Crystal Structure Analysis 65 3.2 Results and Discussion 68 CHAPTER 4. DEFECT CHEMISTRY 72 4.1 Introduction 72 4.2 Delafossite Doping 74 4.2.1 Undoped 74 4.2.2 Donor Doped 84 4.2.3 Acceptor Doped 88 1V CHAPTER 5. ELECTRICAL AND OPTICAL PROPERTIES 91 5.1 Introduction 91 5.2 Experimental 92 5.2.1 Synthesis 92 5.2.2 Crystal Structure Analysis 94 5.2.3 Density 94 5.2.4 Electrical Characterization 94 5.2.5 Optical Characterization 96 5.3 Results and Discussion 97 5.3.1 Synthesis 97 5.3.2 Density 102 5.3.3 Undoped CuGa02 Electrical Properties 102 5.3.4 Donor Doped CuGa02 Electrical Properties 107 5.3.5 Acceptor Doped CuGa02 Electrical Properties 108 5.3.6 CuGa~l_X~InX0 2 Electrical Properties 110 5.3.7 CuGa02 Optical Properties 111 5.4 Conclusions 113 CHAPTER 6. CONCLUSIONS AND FUTURE WORK 114 6.1 Synthesis and Phase Equilibria 114 6.1. l Synthesis and Phase Equilibria: Conclusions 114 6.1.2 Synthesis and Phase Equilibria: Future Work 116 6.2 Electrical and Optical Properties 117 6.2.1 Electrical and Optical Properties: Conclusions 117 6.2.2 Electrical and Optical Properties: Future Work 118 APPENDICES 120 Appendix I: Unit Cell Regression Diagnostics 120 Appendix II: Rietveld Refinement 121 Appendix III: Seebeck Measurements 122 REFERENCES 139 ACKNOWLEDGEMENTS 1a5 1 CHAPTER 1. INTRODUCTION 1.1 Background l.l.l Transparent Conducting Coatings: Historical Perspective Transparent conducting materials, both oxides and coatings, have been known for a long time. Transparent conducting oxides (TCO) were first reported by K. Badeker in 1907 when he published that thin films of cadmium metal deposited in a glow discharge chamber could become transparent through oxidation and still remain electrically conducting [1 ] . Since then, TCO's use in technological applications has dramatically increased. Currently, TCO's are used in solar cells, heat reflecting "smart" windows, flat-panel and liquid crystal displays, and gas sensors. Architectural applications and flat-panel displays (FPD's) are the two dominant markets for TCO's. The architectural applications for TCO's are for energy efficient windows. These windows, with tin oxide coatings, are efficient in preventing radiative heat loss because tin oxide has a low emissivity, ~ 0.16 [2]. Thus, these "low e" windows are ideal for cold to moderate climates [2]. In 1996, the annual consumption of TCO-coated glass in the U.S. was 7.3 x 10~ m2, but this number continues to grow as the demand for displays and photovoltaics increases dramatically. With the flat-panel display technology industry alone undergoing a rapid growth, 28% of the display market with $17 billion in 2000 and a prof ected $27 billion by 2005 [2], it 2 is apparent that the development of new and improvement of old TCO materials becomes increasingly important. Similar to TCO's, transparent conducting coatings (TCC) have been utilized since the last century. They first appeared in the form of thin film electrodes of silver and platinum on selenium photoelectric coatings [2]. TCC's entered the commercial market in the 1930's with the first solid-state photodetectors, which were Schottky barrier cells with transparent metal front electrodes. When wide-band-gap semiconductors were discovered, practical applications of TCC's increased significantly [3] . During WWII, for example, tin oxide _coatings were introduced into commercial Nesa glass, a transparent electrically conductive coating with de-icing and de-fogging capabilities on commercial and military aircraft. Thin films, approximately 100-200 ~ in thickness, of some metals have been found to have similar properties to TCC's. Their advantages lie in their chemical compatibility with substrates, their small work function, and their ease in deposition; however, these films are not very stable and their properties are time dependent [4]. Thin metal films also lack the superior stability and mechanical hardness that the transparent coatings possess [4]. The intensity of the transmitted light through a material is governed by the following equation: Equation 1.1 I = I o exp~— ax~, where Io is the initial light intensity hitting the surface, I is the final intensity of light emitted through the material, a is the absorption coefficient (cm-1), and x is the distance the transmitted light travels in the material. With absorption coefficients on the order of 109, thin 3 metal films must have an approximate thickness of 10 ~ to have a high transparency in the visible region of the light spectrum [5]. However, if a metal film approaches this thickness, its mean free path, or average distance between collisions, is reduced due to scattering of the charge carriers at the surface, and thus a dramatic decrease in conductivity is observed [5]. 1.1.2 Transparent Conducting Coatings: Materials Evaluation For non-metallic materials, the optical properties are dictated by both the electronic band gap and the charge carrier density [3]. Figure 1.1 illustrates a schematic of the transmission spectrum for a given material. The electronic band gap determines the lower wavelength limit (~,g), for optical transmission, ~300nm, below which absorption takes place from the excitation of electrons in the valence band to the conduction band. Equation 1.2 illustrates the relationship between ~,g and a materials band gap: _ he Equation 1.2 ~bg — ~ Eg where h is Planck's constant (4.136 x 1015 eV s), c is the speed of light (2.99 x 108 m/s), and Eg is the materials band gap. Typically a band gap greater than approximately 3.0 eV is required in order for transparency over the visible spectrum to occur [3]. 1 Figure 1.1: Typical transmission spectra for a hypothetical TCO material S The upper wavelength limit, ~Ip, for transparency is determined by free carrier absorption according to the charge carrier density, Equation 1.3. 1 2 mc2 Equation 1.3 ~p = 2 ~z i 4~ y e2 where m is the mass of an electron (9.11 x 10-31 kg), (N/V) is the charge carrier density, and e is the charge of an electron (1.602 x 10-19 C). This upper limit is usually in the infrared for semiconductors and ultraviolet for metals [3]. Typical materials that are both transparent and conducting include In20 3, Sn02, CdZSn04, and ZnO; however indium tin oxide (ITO, In20 3 doped with 9 mole % Sn) remains as the current standard [4]. Table 1.1 illustrates some properties of these materials. Table 1.1: Properties of TCO's at room temperature f 4 Crystal Conductivity Band-Gap Dielectric Refractive Compound Structure (S/cm) (eV) Constant Index Sn02 Rutile 10-1000 3.7-4.6 9.4-12 1.8-2.2 In20 3 C-rare earth 10-1000 3.5-3.75 8.9 2.0-2.1 ITO C-rare earth 100-1000 3.5-4.6 — 1.8-2.1 Cd2SnOa Sr2Pb04 100-1000 2.7-3.0 — 2.05-2.1 Zn0 Wurtzite 1-1000 3.1-3.6 8.5 1.85-1.90 With its high mobility ( 160 cm2/Vsec), high conductivity ( 104 SZ -1 cm-1), ease of processing, and its high transparency above 480 nm, ITO has many advantages [6].
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