LEDs for Lighting Applications

Light Emitting Diodes (LEDs) are no longer confined to use in commercial signage and have now moved firmly, and with unquestioned advantages, into the field of commercial and domestic lighting. This development was prompted in the late 1980s by the invention of the blue LED, a wavelength that had previously been missing from the available LED spectrum and which opened the way to providing white light. Since that point, LED performance (including energy efficiency) has improved dramatically, and now compares with the performance of fluorescent lights - and there remain further performance improvements yet to be delivered. The book begins with the principles of LED lighting, then focuses on issues and challenges. Chapters are devoted to key steps in LED manufacturing: substrate, epitaxy, process and packaging. Photoelectric characterization of LEDs, Lighting with LEDs and the imposition of a certain level of color quality, are the subject of later chapters, and finally there is a detailed discussion of the emergence of OLEDs, or organic LEDs, which have specific capabilities of immediate interest and importance in this field.

Table of Contents:
Foreword xi Introduction xv Chapter 1. Light-Emitting Diodes: Principles and Challenges 1 Chapter written by Georges ZISSIS 1.1. History of a revolution in the world of the light sources 1 1.2. LEDs and lighting 3 1.3. Principle of operation, color, efficiency, lifetime and quality of LEDs 11 1.3.1. White light production from LEDS: principles and challenges 15 1.3.2. Lifetime 19 1.3.3. Quality of LEDs 21 1.4. Challenges facing LEDs 22 1.5. Bibliography 26 Chapter 2. Substrates for III-Nitride-based Electroluminescent Diodes 29 Chapter written by Philippe DE MIERRY 2.1. Introduction 29 2.2. Crystal structure and epitaxial relation with 6H-SiC and Al2O3 33 2.3. Defects and constraints due to heteroepitaxy 38 2.3.1. Dislocations 38 2.3.2. Disorientation of the substrate 41 2.3.3. Epitaxial stress 43 2.3.4. Thermal stress 43 2.4. MOVPE growth of GaN on sapphire 45 2.4.1. GaN growth 45 2.4.2. Standard 2D epitaxy 48 2.4.3. 3D epitaxial growth 49 2.4.4. Epitaxial lateral overgrow (ELO 1S) 51 2.4.5. Anisotropic growth 53 2.4.6. Two stage ELO GaN growth (ELO 2S) 55 2.4.7. GaN growth using pendeo-epitaxy 57 2.4.8. Nano epitaxy 59 2.5. Bulk nitride substrates 61 2.5.1. HNPS (high nitrogen pressure solution method) for the fabrication of crystalline GaN 62 2.5.2. Ammonothermal synthesis of GaN 63 2.5.3. Halide vapor phase epitaxy (HVPE) of GaN 64 2.6. Conclusion 67 2.7. Bibliography 68 Chapter 3. III-Nitride High-Brightness Light-Emitting Diodes 75 Chapter written by Amélie DUSSAIGNE and Nicolas GRANDJEA 3.1. Introduction 75 3.2. p-n junction in GaN 77 3.3. Active region: InGaN/GaN quantum well 80 3.3.1. Growth and structure 81 3.3.2. Optical properties 83 3.4. Radiative efficiency 91 3.5. Conclusion and prospects 95 3.6. Bibliography 96 Chapter 4. Diode Processing 99 Chapter written by Philippe GILET 4.1. Introduction 99 4.2. Orders of magnitude 100 4.3. Diode configurations 103 4.3.1. Conventional chip (CC) 105 4.3.2. Flip chip (FC) 105 4.3.3. Vertical thin film (VTF) 106 4.3.4. Thin film flip chip (TFFC) 107 4.4. Light extraction at wafer level 108 4.5. Diode processing, etching, contact deposition 111 4.5.1. N-type contacts 113 4.5.2. P-type contacts 113 4.6. Etching 116 4.7. Substrate removal 117 4.8. Potential evolutions 118 4.9. Bibliography 119 Chapter 5. Packaging 123 Chapter written by Adrien GASSE 5.1. Introduction 123 5.2. Different packaging processes 124 5.2.1. Historical background 124 5.2.2. From the wafer to the chip 125 5.2.3. Components with connection pins 128 5.2.4. SMT leadform components 129 5.2.5. SMT “leadless” components 133 5.2.6. Other technologies 134 5.2.7. Conclusion 136 5.3. Thermal management 136 5.3.1. Motivations 136 5.3.2. Heat dissipation modes 137 5.3.3. Thermal dissipation in LEDs 139 5.3.4. Comparison of different packaging processes 141 5.3.5. Conclusion 145 5.4. Light extraction in LEDs 146 5.4.1. Lateral light extraction in LEDs 146 5.4.2. Vertical light extraction through a lens 147 5.4.3. Lens/encapsulant materials 149 5.4.4. Lenses and encapsulant implementation 153 5.5. LED component characteristics 153 5.5.1. Thermal and electrical characteristics 153 5.5.2. Optical characteristics 154 5.5.3. Binning 156 5.5.4. Reliability 157 5.6. Conclusion and trends 158 5.7. Appendix160 5.7.1. Physical properties of materials 160 5.8. Bibliography 163 Chapter 6. Photoelectric Characterization of Electroluminescent Photodiodes 165 Chapter written by Christian EUGÈNE and Jean-Michel DESWERT 6.1. Photometry of LEDs 165 6.1.1. Recap of fundamental knowledge 166 6.1.2. Parameters of interest 171 6.1.3. Required properties of photometers/radiometers 171 6.1.4. Measurement of luminous intensity 176 6.1.5. Measurement of luminous flux 179 6.1.6. Spectral measurements 188 6.2. Electrical characteristics of LEDs 191 6.2.1. Forward voltage 191 6.2.2. Temperature effect 192 6.2.3. Operating conditions of LEDs for photometric measurements 194 6.2.4. Stand of the normalization 195 6.3. Bibliography 196 Chapter 7. Quality of White Light from LEDs 197 Chapter written by Françoise VIÉNOT 7.1. Introduction: white light and visual quality 197 7.1.1. White light 197 7.1.2. A few ideas on the quality of light 198 7.1.3. The human visual function: receptors, retina, brain 199 7.1.4. Chapter presentation 200 7.2. Notions of colorimetry and photometry 201 7.2.1. Colorimetry 201 7.2.2. Photometric quantities 206 7.3. Obtaining white light with LEDs 211 7.3.1. White light diodes based on short wavelength emission 211 7.3.2. White light diodes based on the UV diode 212 7.3.3. Combining red, green and blue 212 7.3.4. Examples of combining many LEDs, spectrum optimization 213 7.3.5. Normalization of the color of white diodes 214 7.4. Color rendering of sources 215 7.4.1. The CRI of the CIE 216 7.4.2. Calculation details 219 7.4.3. Update of the CIE position to take the observer’s judgment into account 220 7.5. Works on quality of light from LEDs 220 7.5.1. Models 220 7.5.2. Color simulations 224 7.5.3. Experimental validations 224 7.5.4. Conclusion on the complexity of visual judgment 228 7.6. Applications of LEDs to lighting 228 7.7. Conclusion: advantages, precautions and perspectives 229 7.8. Acknowledgements 230 7.9. Bibliography 230 Chapter 8. OLED Technology 233 Chapter written by Tony MAINDRON and David VAUFREY 8.1. Introduction 233 8.1.1. Organic materials: a history 233 8.1.2. Birth of the first OLED device 234 8.2. Electroluminescent diodes 234 8.2.1. Organic semiconductor categories 236 8.2.2. Deposition technique description 238 8.3. Organic semiconductors: theory 239 8.3.1. Introduction to semiconductivity in organic chemistry 239 8.3.2. Electronic transport model in amorphous organic solids 242 8.4. OLED electrical characteristics 245 8.4.1. Charge carriers injection models 245 8.4.2. Charge carriers transport models 246 8.5. Different structure types of OLEDs 249 8.5.1. Direct and inverted diodes 249 8.5.2. Using the substrate emitting diode and the top surface emitting diode 250 8.5.3. Heterojunction diode and band engineering 250 8.5.4. Light extraction 252 8.5.5. Fluorescence versus phosphorescence 253 8.6. OLED lighting dedicated architectures 255 8.6.1. Single emitting layer structure 255 8.6.2. Double emitting layer structures 257 8.6.3. n-emitting layer structures (n˃3) 258 8.6.4. Stacked OLEDs and tandem structures 258 8.6.5. Converters (down conversion) 259 8.7. OLED stability and lifetime: encapsulation issue 259 8.8. OLEDs for lighting 262 8.9. Bibliography 264 List of Authors 267 Index 269

About the Author :
Patrick Mottier is currently head of lighting programs at CEA-LETI, Grenoble, France.