Novel Ultra-Violet/Blue Optoelectronic Materials and Devices Based on Copper Halides (CuHa) By Aidan James Cowley B.Sc in Computer Applications, M.Eng in Electronic Systems Thesis Submitted for the degree of Doctor of Philosophy Research Supervisor Prof. Patrick J. McNally Dublin City University School of Electronic Engineering 2011 Declaration I hereby certify that this material, which I now submit for assessment on the programme of study leading to the award of PhD is entirely my own work, that I have exercised reasonable care to ensure that the work is original, and does not to the best of my knowledge breach any law of copyright, and has not been taken from the work of others save and to the extent that such work has been cited and acknowledged within the text of my work. Signed: ____________ ID No.: ___________ Date: _______ Acknowledgments I am lucky in having benefited from the support, teaching and guidance of some truly excellent people during the time taken for this PhD. It is a pleasure, therefore, to accord those involved (directly and otherwise) a grateful and heartfelt acknowledgment. None of this work would have been possible were it not for the opportunity many years ago to become involved with the Copper Halide materials during my M.Eng degree. Little did I realize then that, in addition to destroying many perfectly fine printer cartridges with acid, I would be most fortuitous in having Prof. Patrick McNally as my supervisor. Having the opportunity to do a PhD years later under the same supervisor would allow me to fulfill this great personal ambition and has been an awesome and enlightening experience. Thank you kindly for all the great support, advice and supervision over the years (and for the simple opportunity to allow this erstwhile programmer the chance to do something completely different). So much background work involving the procedures for dealing with, depositing and investigating CuCl material was previously established by the excellent work of NPL alumni. I am thusly indebted to Dr. Lisa O’Reilly and Dr. Francis Olabanji Lucas for laying so much of the groundwork for my own research and providing many valuable insights. I am also grateful to have worked alongside fellow Copper Halide enthusiasts and NPL/NCPST colleagues Rajani, Monjarul and Barry. Many thanks to Dr. Andreas Danilewsky of Freiburg University, for the chance to visit his faculty in order to advance my knowledge of LPE as well as being kindly generous with his time in visiting our own labs to assist with our own LPE system. I am also indebted to Dr. Stephen Daniels for his sage advice and assistance over the years. My thanks to the NPL/NCPST technical staff, particularly Mr. Billy Roarty for his continuously educational engineering solutions, as well as Mr. Robert Clare, Mr. Conor Murphy and Ms. Jennifer Stopford for their technical support and friendship. Likewise, to my NPL/NCPST colleagues Ken, Nick, Dave, Evgueni, Niall and Ameera for making the lab a great place to work in to this day. To my friends, who endured many, many rants about various esoteric aspects of my work. Particular thanks to my good friend Brian and Dr. Stuart Corr, for being excellent sounding boards for all sorts of ‘out-there’ ideas. To my girlfriend, Kimberly, for always being there with love and support. To my family; my brother John - you may still call me ‘legi’ but we’re two volumes of the same book. Lastly, but most importantly, I am eternally grateful to my mother and father, Phyl and Jim Cowley. Their unwavering support, encouragement and love has been the foundation for all my endeavors. To them, I dedicate this thesis. Table of Contents Abstract v Chapter 1 - Introduction 1 1.1 Introduction 1 1.2 Wide Band-gap Materials 2 1.3 Issues with Existing Materials 5 1.4 Short Wavelength Optoelectronics on Silicon 6 1.5 Liquid Phase Epitaxy (LPE) 7 1.6 Liquid Phase Epitaxy for Wide Band-gap Semiconductors 8 1.7 Copper Halides 10 1.7.1 CuBr Devices 12 1.7.2 Solid State Chemical Interaction of CuHa and KHa Materials 13 1.8 Thesis Objective 13 1.9 Layout of Thesis 15 1.10 References 17 Chapter 2 - Theory & Characterization Techniques 21 2.1 Introduction 21 2.1.1 Semiconductors 21 2.1.2 Energy-Band Theory of Solids 22 2.2 Photoluminescence in Semiconductors 25 2.2.1 Excitons 26 2.2.2 PL Experimental Setup 28 2.3 UV-Vis Spectroscopy (UV-Vis) 32 2.4 X-Ray Diffraction (XRD) 35 2.4.1 Glancing Incidence X-Ray Diffraction (GIXRD) 37 2.4.2 Crystallite Size Calculation using Scherrer Formula 38 2.5 Atomic Force Microscopy (AFM) 38 2.6 Scanning Electron Microscope (SEM) 39 i Table of Contents 2.6.1 Energy Dispersive X-Ray Spectroscopy (EDX) 40 2.7 X-Ray Enhanced Optical Luminescence (XEOL) 41 2.8 Secondary Ion Mass Spectrometry (SIMS) 43 2.9 References 45 Chapter 3 - Epitaxial Methods 47 3.1 Introduction 47 3.2 Epitaxy 47 3.3 Liquid Phase Epitaxy 51 3.3.1 LPE Process & Equipment 55 3.3.2 Tipping System 55 3.3.3 Sliding Boat 57 3.4 LPE Growth Techniques & Theory 58 3.4.1 Wetting 61 3.5 LPE Apparatus Development 63 3.6 Physical Vapour Deposition (PVD) 65 3.7 References 70 Chapter 4 - Copper Halides 73 4.1 Introduction 73 4.2 Structural Properties 73 4.2.1 X-Ray Diffraction Characteristics 75 4.2.2 Phase Changes for CuHa 78 4.2.3 Stability 79 4.3 Band Structure & Electronic Properties 79 4.4 Optical Properties 84 4.5 Growth of CuHa Materials 86 4.6 References 89 Chapter 5 - Liquid Phase Epitaxy of γ-CuCl on Silicon 91 ii Table of Contents 5.1 Introduction 91 5.2 Methodology 91 5.2.1 The CuCl/KCl Eutectic System 92 5.3 Experimental setup for LPE 94 5.3.1 Materials & LPE System Preparation 95 5.3.2 Substrate Cleaning & Preparation 96 5.4 LPE growth using CuCl/KCl Eutectic Melts 101 5.5 Structural Properties of LPE Deposited Samples 105 5.5.1 CuCl-Si Interface Reactions 108 5.5.2 Hybrid Layer Liquid Phase Epitaxy 112 5.6 Optical Characterization 114 5.6.1 XEOL Measurements 121 5.7 Difficulties with LPE 125 5.7 Conclusions 131 5.8 References 133 Chapter 6 - γ-CuBr Thin Films and Electroluminescent Devices 135 6.4 Structural Properties of Evaporated γ-CuBr Films 137 6.5 Optical Properties of Evaporated γ-CuBr Films 147 6.5.1 Low-K PL Measurements of γ-CuBr Thin Films 149 6.5.2 XEOL of γ-CuBr Thin Films 152 6.6 Stability of γ-CuBr Thin Films 153 6.6.1 Electrolytic Decomposition of γ-CuBr Film under AC/DC Applied Voltage 156 6.7 Towards a γ-CuBr based EL Device 160 6.8 Conclusions 164 6.9 References 166 Chapter 7 - CuBr/KBr Microdots 168 7.1 Introduction 168 iii Table of Contents 7.2 General Fabrication 168 7.2.1 Formation of Intermixed Microdot 171 7.3 Structural Characterization of CuBr/KBr Microdot Arrays 174 7.4 Growth Mechanics 178 7.4.1. Conventional Vapor Liquid Solid Growth 179 7.4.2 VLS growth of CuBr using KBr 180 7.5 Optical Characterization 185 7.6 Conclusions 191 7.6 References 192 Chapter 8 - Conclusions & Further Research 193 8.1 Liquid Phase Epitaxy 193 8.3 CuBr Thin Films & TFELD 195 8.5 CuBr/KBr Microdots 196 8.4 CuHa Optoelectronics 197 8.5 References 198 Appendix A - XRD Diffraction Data for CuHa and Related Compounds 199 Appendix B - Additional AFM Images of CuBr/KBr Microdots 204 Appendix C - Publications 207 iv Abstract Considerable research is being carried out in the area of wide band gap semiconductor materials for light emission applications in the UV/Blue (300-400 nm) spectral range. This project explores the novel use of the Copper Halides (CuHa), specifically γ-CuCl and γ-CuBr, I–VII wide band gap mixed ionic–electronic semiconducting materials with light emitting properties suitable for novel UV/blue light applications. This project details novel research carried out towards achieving single crystal growth of γ-CuCl from solution via Liquid Phase Epitaxy (LPE) based techniques. LPE growth runs are undertaken using an alkali halide flux compound (KCl) to depress the liquidus temperature of CuCl below its solid phase wurtzite-zincblende (β → γ) transition temperature for solution based epitaxy on lattice matched Si substrates (lattice constant of γ-CuCl (0.541 nm) is closely matched to that of Si (0.543 nm). Results show that the resulting KCl flux-driven deposition of CuCl onto the Si substrate has yielded superior photoluminescence (PL) and X-ray excited optical luminescence (XEOL) behaviour relative to comparatively observed spectra for GaN or polycrystalline CuCl. The resulting deposited material is a textured CuCl/K CuCl polycrystalline intermix, with strong broad 2 3 luminescence and novel luminescent characteristics not previously observed in CuCl. Difficulties inherent to LPE with CuCl/KCl melts, particularly with the CuCl/KCl eutectic system and the CuCl/Si surface reaction, are detailed. The use of γ-CuBr for thin film based blue light emitting devices is investigated. Its structural and physical properties allow for vacuum deposition on a variety of substrates and herein we report on the deposition of γ-CuBr on Si, glass and indium tin oxide coated glass substrates via vacuum evaporation with controllable film thickness from 100 to 500 nm. Temperature dependent v photoluminescence characteristics of these γ-CuBr films on Si substrates reveal familiar Z and I f 1 excitonic features. Work towards the development of a thin film electroluminescent device using a γ-CuBr active layer is outlined. Recently, dramatic improvements in the luminescent intensity of CuBr generated by the chemical interaction between CuCl films and KBr substrates have been demonstrated. The potential improvements in excitonic PL that can be gained from novel approaches to film preparation involving KBr and existing CuBr deposition techniques is promising. We report on the one such novel approach, the vacuum deposition of KBr spots (~30 µm radius) onto similarly deposited γ- CuBr epitaxial layer on a Si substrate. Post-deposition annealing of the samples at 220 °C in conjunction with a small CuBr flux from a target source leads to the formation of intermixed CuBr/ KBr microdots. PL characterisation reveals enhanced UV-Blue excitonic emission centered on the Z free exciton peak at ~418 nm, far superior to Z emission from γ-CuBr films deposited previously. f f An overview of the deposition process involving shadow masks to lay down an ordered array of KBr spots onto a γ-CuBr vacuum evaporated layer is presented, and the samples are characterised using XRD, EDX and spatially resolved room temperature PL. vi