Handbook of Solid-State Lighting and LEDs

Handbook of Solid-State Lighting and LEDsSERIES IN OPTICS AND OPTOELECTRONICS
Series Editors: E Roy Pike, Kings College, London, UK Robert G W Brown, University of California, Irvine, USA
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Handbook of Solid-State Lighting and LEDs
Edited by Zhe Chuan Feng
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Library of Congress Cataloging-in-Publication Data
Names: Feng, Zhe Chuan, editor. Title: Handbook of solid-state lighting and LEDs / edited by Zhe Chuan Feng. Other titles: Series in optics and optoelectronics ; 25. Description: Boca Raton, FL : CRC Press, Taylor & Francis Group, [2017] | Series: Series in optics and optoelectronics ; 25 Identifiers: LCCN 2016052771| ISBN 9781498741415 (hardback ; alk. paper) | ISBN 149874141X (hardback ; alk. paper) Subjects: LCSH: Light emitting diodes. | LED lighting--Materials. | Solid state electronics. Classification: LCC TK7871.89.L53 H364 2017 | DDC 621.32--dc23 LC record available at https://lccn.loc.gov/2016052771
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Section i overview
1 From the Dawn of GaN-Based Light-Emitting Devices to the Present Day ................ 3 Hiroshi Amano
2 Spectrum-Related Quality of White Light Sources ......................................... 13 Yue Jun Sun, Dragan Sekulovski, Mart Peeters, Kees Teunissen, Remy Broersma, and Rene Wegh
3 Nanofabrication of III-Nitride Emitters for Solid-State Lighting ................... 31 Tao Wang and Yaonan Hou
4 III-Nitride Deep-Ultraviolet Materials and Applications .............................. 67 Jianwei Ben, Xiaojuan Sun, and Dabing Li
Section ii Gan-Based LeDs for Lighting
5 Efficiency Droop of Nitride-Based Light-Emitting Diodes ........................... 99 Chia-Yen Huang and Hao-Chung Kuo
6 Design and Fabrication of Patterned Sapphire Substrates for GaN-Based Light-Emitting Diodes ................................................................................... 123 Guoqiang Li, Haiyan Wang, and Zhiting Lin
7 Surface Plasmon–Coupled Light-Emitting Diodes ....................................... 141 Chia-Ying Su, Chun-Han Lin, Yang Kuo, Yu-Feng Yao, Hao-Tsung Chen, Charng-Gan Tu, Chieh Hsieh, Horng-Shyang Chen, Yean-Woei Kiang, and C.C. Yang
8 Deep Level Traps in GaN Epilayer and LED .................................................. 161 Xuan Sang Nguyen and Soo Jin Chua
vi Contents
Section iii Deep Ultraviolet LeDs and Related technologies
10 Technological Developments of UV LEDs ..................................................... 219 Ching-Hsueh Chiu, Po-Min Tu, Tzu-Chien Hong, Chien-Chung Peng, Chien-Shiang Huang, and Shih-Cheng Huang
11 Influence of Carrier Localization on Efficiency Droop and Stimulated Emission in AlGaN Quantum Wells ............................................................. 243 Gintautas Tamulaitis
12 Solar-Blind AlGaN Devices ........................................................................... 285 Jiangnan Dai, Jingwen Chen, Jun Zhang, Wei Zhang, Shuai Wang, Feng Wu, and Changqing Chen
Section iV Laser Diodes
13 Laser Diode–Driven White Light Sources .................................................... 299 Faiz Rahman
14 InGaN Laser Diodes by Plasma-Assisted Molecular Beam Epitaxy .............. 321 Czeslaw Skierbiszewski, Muziol Grzegorz, Turski Henryk, Siekacz Marcin, and Marta Sawicka
15 GaN-Based Blue and Green Laser Diodes .....................................................361 Jianping Liu and Hui Yang
Section V nano and other types of LeDs
16 Photonic Crystal Light-Emitting Diodes by Nanosphere Lithography ........ 393 Kwai Hei Li and Hoi Wai Choi
17 ZnO-Based LEDs ........................................................................................... 437 Hao Long
18 Natural Light-Style Organic Light-Emitting Diodes .....................................481 Jwo-Huei Jou, Meenu Singh, and Yi-Fang Tsai
Section VI Novel Technologies and Developments
19 III-Nitride Semiconductor LEDs Grown on Silicon and Stress Control of GaN Epitaxial Layers .................................................................................. 517 Baijun Zhang and Yang Liu
20 Hole Accelerator for III-Nitride Light-Emitting Diodes .............................. 557 Zi-Hui Zhang, Yonghui Zhang, Xiao Wei Sun, and Wengang Bi
viiContents
22 Terahertz Spectroscopy Study of III–V Nitrides ...........................................617 Xinhai Zhang and Huafeng Shi
23 Internal Luminescence Mechanisms of III-Nitride LEDs ............................. 651 Shijie Xu
24 Fabrication of Thin-Film Nitride-Based Light-Emitting Diodes ................. 677 Ray-Hua Horng, Dong-Sing Wuu, and Chia-Feng Lin
Index .......................................................................................................................691
ix
Preface
Economic and industrial developments have advanced the world economy and culture to extreme levels, which are having a high cost in terms of overuse and starvation of energy sources leading to potential harm and destruction to the future of human existence. This threat has raised major concerns in all countries irrespective of wealth or richness of energy sources. Clearly, the promotion of new, advanced technologies aimed at saving precious energy resources is the best and only way to combat the harmful effects. Among various options, solid-state lighting (SSL), especially semiconductor-based light-emitting diodes (LEDs), should prove to be a reliable direction for the development of energy optoelectronics technologies.
More than 100 years ago, lighting technology had its major breakthrough based on the efforts of Thomas Edison and others in the invention of the electric light bulb. The recent breakthrough and development in semiconductor LEDs—in particular based on gallium nitride (GaN) and related wide gap semiconductors— have opened a new century of LED lighting, changing our daily lives, industry, and society.
The 2014 Nobel Prize in Physics was awarded jointly to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy- saving white light sources.” Energy-efficient and environmentally friendly solid-state light sources, in particular GaN-based LEDs, are revolutionizing an increasing number of applications and bring clear, widespread benefits in areas such as lighting, communications, biotechnology, imaging, and medicine. It is expected that LEDs may replace traditional light bulbs and tubes to achieve a new lighting paradigm. The III-nitride-based industry has now formed and with it there are rapid developments across the world. It is expected that III-nitride-based LEDs may replace traditional light bulbs to realize a revolution in lighting similar to the impact of Edison’s invention of the electric light bulb more than 100 years ago.
Tremendous research and industry achievements and developments have been made in recent years. Handbook of Solid-State Lighting and LEDs reviews many of these significant results and progress, covering basic concepts and critical aspects, and serves professors, scientists, engineers, and students in the field. The handbook is organized for a wide range of audiences and covers major aspects of SSL and LED science and technology. Each chapter, written by experts in the field, reviews important topics and achievements in recent years, discusses progress made by different groups, and suggests further work needed. The handbook provides useful information on SSL–LED materials and devices; nanoscale processing; fabrication of LEDs, light diodes, photodetectors, and nanodevices; and characterization, application, and development of the various SSL and LED devices and nanoengineering.
This handbook consists of 24 review chapters, led by Prof. Hiroshi Amano, and divided into six main sections: “Overview,” “GaN-Based LEDs for Lighting,” “Deep UV LEDs and Related Technologies,” “Laser Diodes,” “Nano and Other Types of LEDs,” and “Novel Technologies and Developments.” The contents present the key properties of SSL and LEDs, describing current technologies and demonstrating the remaining challenges facing R&D in the twenty-first century. This book serves a diverse audience of material growers and evaluators, device design and processing engineers, potential users and newcomers, postgraduate students, engineers, and scientists in the field. If readers are interested, they are welcome to explore a recently published sister book I edited, III-Nitride Materials, Devices and Nanostructures.
x Preface
Developments in SSL and LED materials and devices happen so quickly that invariably the handbook will miss some advances. Nonetheless, it captures the current state of the art, in particular those made from the start of this century up to the recent years. We look forward to the future when, even if the pure science remains the same, further strides will be made on the engineering and materials side.
Zhe Chuan Feng
xi
Editor
Professor Zhe Chuan Feng earned his PhD in condensed matter physics from the University of Pittsburgh in 1987. Previously, he received his BS (1962–1968) and MS degrees (1978–1981) from the Department of Physics at Peking University. He has had positions at Emory University (1988–1992), National University of Singapore (1992–1994), Georgia Tech (1994–1995), EMCORE Corporation (1995–1997), Institute of Materials Research & Engineering, Singapore (1998–2001), Axcel Photonics (2001–2002), and Georgia Tech (2002–2003). In 2003, Professor Feng joined National Taiwan University as a professor at the Graduate Institute of Photonics & Optoelectronics and Department of Electrical Engineering, focusing on materials research and MOCVD growth of LED, III-nitrides, SiC, ZnO, and other semiconductors/oxides. He is currently a distinguished professor at the Laboratory of Optoelectronic Materials and Detection Technology, Guangxi Key Laboratory for Relativistic Astrophysics in the School of Physical Science and Technology at Guangxi University, Nanning, China.
Professor Feng has edited nine review books on compound semiconductors and microstructures, porous Si, SiC and III-nitrides, ZnO devices, and nanoengineering and has authored or coauthored more than 570 scientific papers with more than 220 indexed by the Science Citation Index and cited more than 2540 times. He has been a symposium organizer and invited speaker at different international conferences and universities and has been a reviewer for several international journals including Physical Review Letters, Physical Review B, and Applied Physics Letters. He has served as a guest editor for special journal issues and has been a visiting or guest professor at Sichuan University, Nanjing Tech University, South China Normal University, Huazhong University of Science & Technology, Nankai University, and Tianjin Normal University. He is a member of the International Organizing Committee for the Asia-Pacific Conferences on Chemical Vapor Deposition and International Conference for White LEDs and Solid- State Lighting and is on the Board of Directors for the Taiwan Association for Coating and Thin Film Technology. Professor Feng is an elected fellow of SPIE (2013).
xiii
Contributors
Electronics Institute of Materials and Systems for
Sustainability Nagoya University Nagoya, Japan
Jianwei Ben State Key Laboratory of Luminescence and
Applications Changchun Institute of Optics, Fine Mechanics
and Physics Chinese Academy of Sciences Changchun, People’s Republic of China
Wengang Bi Institute of Micro-Nano Photoelectron and
Electromagnetic Technology Innovation School of Electronics and Information
Engineering Hebei University of Technology and Key Laboratory of Electronic Materials and
Devices of Tianjin Tianjin, People’s Republic of China
Remy Broersma Philips Lighting Eindhoven, The Netherlands
Changqing Chen Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
Hao-Tsung Chen Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Horng-Shyang Chen Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Jingwen Chen Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
Hoi Wai Choi Department of Electrical and Electronic
Engineering The University of Hong Kong Pokfulam, Hong Kong
Ching-Hsueh Chiu Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Soo Jin Chua Singapore-MIT Alliance for Research and
Technology and Department of Electrical and Computer
Engineering National University of Singapore Singapore, Singapore
xiv Contributors
Zhe Chuan Feng Laboratory of Optoelectronic Materials &
Detection Technology Guangxi Key Laboratory for Relativistic Astrophysics School of Physical Science & Technology Guangxi University Nanning, People’s Republic of China
Eugene A. Fitzgerald Singapore-MIT Alliance for Research and Technology Singapore, Singapore
and
Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, Massachusetts
Muziol Grzegorz Institute of High Pressure Physics Polish Academy of Sciences Warsaw, Poland
Turski Henryk Institute of High Pressure Physics Polish Academy of Sciences Warsaw, Poland
Tzu-Chien Hong Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Ray-Hua Horng Institute of Electronics National Chiao Tung University Hsinchu, Taiwan, Republic of China
Yaonan Hou Department of Electronic and Electrical
Engineering University of Sheffield Sheffield, United Kingdom
Chieh Hsieh Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Chien-Shiang Huang Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Chia-Yen Huang Department of Photonics & Institute of Electro-
Optical Engineering National Chiao Tung University Hsinchu, Taiwan, Republic of China
Shih-Cheng Huang Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Jwo-Huei Jou Department of Materials Science and Engineering National Tsing-Hua University Hsin-Chu, Taiwan, Republic of China
Yean-Woei Kiang Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Hao-Chung Kuo Department of Photonics & Institute of
Electro-Optical Engineering National Chiao Tung University Hsinchu, Taiwan, Republic of China
Yang Kuo Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Kenneth E. Lee Singapore-MIT Alliance for Research and
Technology Singapore, Singapore
xvContributors
Lighting and its Informationisation of Guangdong Province
and Department of Electronic Materials School of Materials Science and Engineering South China University of Technology Guangzhou, People’s Republic of China
Kwai Hei Li Department of Electrical and Electronic
Engineering The University of Hong Kong Pokfulam, Hong Kong
Chia-Feng Lin Department of Materials Science and Engineering National Chung Hsing University Taichung, Taiwan, Republic of China
Chun-Han Lin Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Tao Lin Laboratory of Optoelectronic Materials &
Detection Technology Guangxi Key Laboratory for Relativistic Astrophysics School of Physical Science & Technology Guangxi University Nanning, People’s Republic of China
Zhiting Lin State Key Laboratory of Luminescent Materials
and Devices and Engineering Research Center on Solid-State
Lighting and its Informationisation of Guangdong Province
South China University of Technology Guangzhou, People’s Republic of China
Jianping Liu Suzhou Institute of Nano-Tech and Nano-Bionics Key Laboratory of Nanodevices and Applications Chinese Academy of Sciences Suzhou, People’s Republic of China
Yang Liu School of Electronics and Information Technology State Key Laboratory of Optoelectronic Materials
and Technologies Sun Yat-Sen University Guangzhou, People’s Republic of China
Hao Long School of Electronics and Information
Engineering South-Central University for Nationalities Wuhan, People’s Republic of China
Siekacz Marcin Institute of High Pressure Physics Polish Academy of Sciences and TopGaN Ltd Warsaw, Poland
Xuan Sang Nguyen Singapore-MIT Alliance for Research and
Chien-Chung Peng Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Faiz Rahman School of Electrical Engineering and Computer
Science Russ College of Engineering and Technology Ohio University Athens, Ohio
Marta Sawicka Institute of High Pressure Physics Polish Academy of Sciences and TopGaN Ltd Warsaw, Poland
xvi Contributors
Engineering Southern University of Science and Technology Shenzhen, People’s Republic of China
Meenu Singh Department of Materials Science and Engineering National Tsing-Hua University Hsin-Chu, Taiwan, Republic of China
Czeslaw Skierbiszewski Institute of High Pressure Physics Polish Academy of Sciences and TopGaN Ltd Warsaw, Poland
Chia-Ying Su Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Xiao Wei Sun Department of Electrical and Electronic
Engineering College of Engineering Southern University of Science and Technology Guangdong, People’s Republic of China
Xiaojuan Sun State Key Laboratory of Luminescence and
Yue Jun Sun Philips Lighting Eindhoven, The Netherlands
Gintautas Tamulaitis Semiconductor Physics Department Vilnius University Vilnius, Lithuania
Kees Teunissen Philips Lighting Eindhoven, The Netherlands
Yi-Fang Tsai Department of Materials Science and Engineering National Tsing-Hua University Hsin-Chu, Taiwan, Republic of China
Charng-Gan Tu Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Po-Min Tu Advanced Optoelectronic Technology Inc. Hsinchu, Taiwan, Republic of China
Haiyan Wang State Key Laboratory of Luminescent Materials
and Devices and Engineering Research Center on Solid-State Lighting
and its Informationisation of Guangdong Province South China University of Technology Guangzhou, People’s Republic of China
Shuai Wang Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
Tao Wang Department of Electronic and Electrical
Engineering University of Sheffield Sheffield, United Kingdom
Rene Wegh Philips Lighting Eindhoven, The Netherlands
Feng Wu Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
xviiContributors
Dong-Sing Wuu Department of Materials Science and Engineering National Chung Hsing University Taichung, Taiwan, Republic of China
Shijie Xu Department of Physics Shenzhen Institute of
Research and Innovation HKU-CAS Joint Laboratory on New Materials The University of Hong Kong Pokfulam, Hong Kong
C.C. Yang Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Hui Yang Suzhou Institute of Nano-tech and Nano-bionics Chinese Academy of Sciences Suzhou, People’s Republic of China
Yu-Feng Yao Department of Electrical Engineering Institute of Photonics and Optoelectronics National Taiwan University Taipei, Taiwan, Republic of China
Baijun Zhang School of Electronics and Information Technology State Key Laboratory of Optoelectronic Materials
and Technologies Sun Yat-Sen University Guangzhou, People’s Republic of China
Jun Zhang Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
Li Zhang Singapore-MIT Alliance for Research and
Wei Zhang Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, People’s Republic of China
Xinhai Zhang Department of Electrical and Electronic
Engineering Southern University of Science and Technology Shenzhen, People’s Republic of China
Yonghui Zhang Institute of Micro-Nano Photoelectron and
Zi-Hui Zhang Institute of Micro-Nano Photoelectron and
1
I Overview
1 From the Dawn of GaN-Based Light-Emitting Devices to the Present Day Hiroshi Amano ................................................................................................................................ 3 Introduction • History of GaN-Based Blue LEDs • Summary • References
2 Spectrum-Related Quality of White Light Sources Yue Jun Sun, Dragan Sekulovski, Mart Peeters, Kees Teunissen, Remy Broersma, and Rene Wegh ...............................................13 Introduction • Methods of Measuring and Specifying Light Quality • White Light • Conclusion • References
3 Nanofabrication of III-Nitride Emitters for Solid-State Lighting Tao Wang and Yaonan Hou ....................................................................................................................................31 Introduction • Growth and Fabrication of III-Nitride LEDs with Nanostructures • Fabrication of Hybrid White LEDs • Fabrication of Phosphor- Free White LEDs • Summary and Outlook • Acknowledgment • References
4 III-Nitride Deep-Ultraviolet Materials and Applications Jianwei Ben, Xiaojuan Sun, and Dabing Li ................................................................................................................................ 67 Introduction • Growth of AlN • Growth of AlGaN • Doping of AlGaN • AlGaN Low- Dimension Structure • Applications of AlGaN-Based Materials • Summary • References
3
1
1.1 Introduction
According to the Swedish Royal Academy of Sciences, the light-emitting diode (LED) lamp holds great promise for increasing the quality of life for over 1.5 billion people around the world who lack access to an electricity grid.
On February 7, 2015, Luvsannyam Gantumur, the Minister of Science and Education of Mongolia, visited Nagoya University. He gave his hearty appreciation to researchers of LEDs for their social implementation of new artificial lighting systems. He said that a nomadic life was the traditional lifestyle of the Mongolian people. However, many young Mongolians do not want to continue a nomadic life and tend to move to urban areas such as Ulan Bator and live in permanent houses. He told me that the emergence of LED lamps has changed the attitudes of the Mongolian people and some young people are returning to a nomadic life.
On March 27, 2016, I had the opportunity to visit a Mongolian family, who lived at an hour’s drive from Ulan Bator. They lived in a ger, a Mongolian tent, as shown in Figure 1.1a. In their ger, they had an LED lamp as shown in Figure 1.1b, and they used solar panels and a battery as the electricity source. It was reassuring to learn that LED lighting can not only contribute to modern life but also help maintain the traditional lifestyles of people trying to coexist with the natural environment.
LEDs are considered to be the fourth generation of artificial light sources. Before the nineteenth century, fire was the only light source, based on the chemical reaction or oxidation of flammable materials.
From the Dawn of GaN-Based Light-Emitting Devices to the Present Day
Hiroshi Amano Nagoya University
1.1 Introduction ..........................................................................................3 1.2 History of GaN-Based Blue LEDs ......................................................5 1.3 Summary ...............................................................................................9 References ........................................................................................................ 10
Abstract It is believed that group III nitride semiconductors are among the most promising materials for solving global problems and realizing a sustainable society. GaN and related nitride materials have had a major impact on human lives. Their most important application is blue LEDs. Portable games machines and cellular or smart phones are very familiar items, especially to young people. Until the end of the 1990s, all the displays of portable games machines and cellular phones were monochrome. The younger generation can now enjoy full-color portable games such as Pokemon Go because of the emergence of blue LEDs. Today, the applications of blue LEDs are not limited to displays. In combination with phosphors, blue LEDs can act as a white light source and are also used in general lighting. This chapter describes the development of GaN-based blue LEDs from the early 1960s to the 1990s.
4 Handbook of Solid-State Lighting and LEDs
However, CO2 emission was inevitable because most flammable materials are composed of carbon and hydrogen. In 1879, the incandescent lamp was invented after a long history of development. Joseph Swan in the United Kingdom and Thomas Edison in the United States first demonstrated the use of the incandescent lamp, for which both submitted patents that were granted in 1880 [1]. The mechanism of light emission from incandescent lamps involves blackbody emission, meaning that it is based on classical quantum theory. It is not clear who invented the fluorescent light bulb, but in 1934, Arthur Compton in the United Kingdom and George Inman in the United States demonstrated the use of fluorescent light bulbs. These bulbs comprise a vacuum glass tube that is coated on the inside with several fluorescent materials. Electrons are emitted from a cathode in the tube and excite Hg vapor. The excited Hg vapor emits UV photons that excite the fluorescent materials. Then visible light is emitted from the fluorescent materials.
(a)
(b)
FIGURE 1.1 (a) Mongolian ger and person living in a ger. The panel to the left of the ger is a solar panel. The para- bolic antenna to the right is for satellite TVs and smart phones. (b) LED lamp used in a ger.
5From the Dawn of GaN-Based Light-Emitting Devices to the Present Day
The mechanism of the fluorescent bulb involves energy transfer from energetic electrons to UV photons and from UV photons to visible photons. Therefore, the mechanism of light emission is actually based on quantum mechanics. Unfortunately, a fragile glass tube and Hg, which is an environmentally hazardous element, are necessary. Therefore, I believe LEDs are the ultimate light source because light emission is based on energy through band-to-band transitions in a solid, which are also a phenomenon based on quantum mechanics. In addition, no environmentally hazardous substances are necessary.
Today, portable game machines and cellular or smart phones are very familiar items, and not only young people but people of all ages can enjoy beautiful full-color displays. In the United States, portable game machines based on liquid crystal displays were first released in 1979 by the Milton Bradley Company, then cellular phones became commercially available in 1983 by Advanced Mobile Phone System, but until the end of the 1990s, the displays of all portable game machines and cellular phones were monochrome. So it should be emphasized that with the emergence of blue LEDs people can now enjoy full-color portable games and cellular/smart phones. Today, the applications of blue LEDs are not limited to displays. In combination with phosphors, blue LEDs can act as a white light source and also be used in general lighting. In 2015, the number of LED packages produced worldwide was more than 304 billion, nearly 41 times the world’s population, about 54% of which were GaN-based LEDs [2]. Nitride-based devices have become a key part of human lives and are used as general lighting in the backlight units of TVs, PCs, and smart phones/cellular phones; traffic signals and street lights; huge displays and billboards in stadiums; optical storage; and broadband wireless communications.
In the following section, I will describe how blue LEDs were developed, especially in the mid-1980s and early 1990s. The contribution of blue LEDs to saving energy and the environment and their future prospects for society are also discussed.
1.2 History of GaN-Based Blue LEDs
Red, yellow, and green LEDs were commercialized in the early 1960s and 1970s using GaAsP [3] and GaP:N [4], respectively. Considering that As-based III–V compounds are used in red LEDs and P-based III–V compounds are used in green LEDs, it should be easy to predict which colors can be obtained from which element from the periodic table. At that time, many researchers believed that blue LEDs could be realized using N-based III–V compounds, that is, GaN.
To grow bulk GaN crystals from a solution, we need very high pressure and temperature, comparable to those needed for the growth of diamonds or even higher-quality crystals [5,6]. However, because it is unlikely that GaN crystals can be grown under such extreme conditions, a chemical reaction should be used to reduce the pressure and temperature required for GaN crystal growth. We also have to use a foreign substrate, that is, GaN must be grown on a different material. For GaN synthesis, we use ammonia as the nitrogen source [7] because nitrogen molecules are inert and do not react with metallic Ga. In contrast, ammonia is very reactive at around 1000°C, the temperature at which GaN can be synthesized. Therefore, a limited numbers of materials can be used as the substrate.
Sapphire is a promising substrate material because it is stable at high temperatures and does not strongly react with ammonia [8]. The most serious problem with sapphire is its large mismatch with GaN of up to 16% for the (0001) planes. Some researchers believe that for heteroepitaxial growth, the lattice mismatch should not exceed 1% [9], so a mismatch of 16% would make heteroepitaxial growth virtually impossible.
GaN powder was first synthesized in 1932 by the reaction of metallic Ga with NH3 at high temperature [10]. However, no description of the expected applications of GaN was given in that study. In 1959, H. G. Grimmeiss and H. Koelmans from Philips Forschungslaboratorien at Aachen and Eindhoven, respectively, first reported the low-temperature luminescence properties of GaN powder grown by the same method, showing the potential of GaN as a light-emitting material [11]. This group also submitted a patent on GaN as a light-emitting material in 1960 [12].
Following these findings, in 1971, the RCA group led by J. I. Pankove developed the first GaN-based blue LEDs. These were metal-insulator-semiconductor (MIS)-type LEDs fabricated by halide vapor phase
epitaxy or hydride vapor phase epitaxy or halogen vapor phase epitaxy (HVPE), which involved the chemical reaction of Ga and hydrogen chloride to form GaCl and ammonia [13]. Unfortunately, these LEDs were MIS-type, rather than pn-junction-type, and therefore their efficiency was limited, ranging only from 10−5 to 3 × 10−4. Nevertheless, in the 1970s, following the success of the RCA group, many of the world’s top electrical companies, such as RCA, Philips, Oki Electric, Hitachi, and Matsushita Research Institute Tokyo (MRIT), tried to commercialize MIS-type blue LEDs, but all their efforts failed [14–18]. At that time, it was believed impossible to grow p-type GaN because of self-compensation [19], which means that if we dope acceptors as an impurity, the same number of intrinsic donors such as nitrogen vacancies is generated to compensate for the doped acceptors.
In 1972, Osamura et al. deposited InGaN, a compound related to GaN, by the electron beam evapora- tion of metal sources in DC discharge nitrogen plasma over the full compositional range from InN to GaN [20]. They reported a large absorption edge of InN of 1.9 eV, rather than the currently accepted bandgap of 0.65 eV, which might have been caused by the unintentional formation of InON. No luminescence data were reported. It took 19 years for the first luminescence data of InGaN to be reported [21] and 30 years for the bandgap of InN to reach the present value [22].
Many researchers and companies gave up on GaN and started research on other materials, such as ZnSe and SiC. The manager of MRIT also decided to abandon the project on GaN. However, the leader of the GaN research team, Dr. Isamu Akasaki, could not abandon GaN and moved from MRIT to Nagoya University in 1981, where he became a professor. I joined his laboratory in 1982 as an undergraduate student and was highly motivated by my graduate dissertation on GaN-based blue LEDs. At that time, I was interested in microcomputer systems, now called personal computers (PCs). In 1975, Bill Gates and Paul Allen established Micro-Soft. One year later, the Apple I computer was developed by Steve Jobs and Stephen Wozniak. From these successes, the personalization of computer systems progressed rapidly and I wanted to contribute to the further development of microcomputer systems, particularly their displays. At that time, Braun tubes were used in all displays, which were large, heavy, and had high energy consump- tion. So, I thought that if I could make blue LEDs, I would help change the world. That is why I was so motivated and focused on my research subject of GaN.
The problem in fabricating MIS-type blue LEDs using HVPE was that the growth rate was so high that it was difficult to control the thickness of the insulating layer in the MIS-type structure. Therefore, the operat- ing voltage could not be controlled. Professor Akasaki also noted the difficulty of growing GaN by molecular beam epitaxy (MBE) from his experience in the 1970s. Therefore, he decided to use metalorganic chemical vapor deposition (MOCVD) or metalorganic vapor phase epitaxy (MOVPE) for GaN growth.
The first report of the formation of III–V compounds by MOCVD was in 1960. A group from Union Carbide reported the formation of an In(CH3)3–PH3 adduct [23]. The first report of the epitaxial growth of III–V compounds including group III nitrides on a substrate was in a patent filed by Monsanto Co. [24]. The North American Rockwell group first reported the experimental deposition of GaN on sapphire and SiC substrates by MOCVD in 1971 [25].
At that time, funding for research at Japanese universities was insufficient. Also, there was no commercially available MOVPE system especially designed for GaN growth. Thus, it was impossible to purchase an MOVPE system. In 1982, a Master’s student 2 years older than me developed the first vertical-type MOVPE reactor [26]. At that time, the flow rate was so low that I could not grow GaN using hydrogen as the carrier gas. I tried to visualize the flow pattern by using the reaction between TiCl4 and H2O to form TiO2 powder and found that the flow rate would be insufficient if I used hydrogen as the carrier gas [26]. In 1984, Y. Koide, a PhD student, joined Professor Akasaki’s laboratory and started research on AlGaN and AlN, while I focused on growing GaN. Together, we modified the MOVPE reactor. I visited several laboratories and found that the flow rate would be insufficient if I used the old configuration of gas supply tubes in the reactor. So I merged all the gas lines into one line, which increased the flow rate from a few centimeters per second to more than 4 m/s [27]. Then, I successfully grew polycrystalline GaN on a sapphire substrate despite using hydrogen as the carrier gas. However, the surface was rather rough and the quality was very poor. I tried to grow GaN many times while varying the growth temperature, the flow
rates of the source and carrier gases, the configuration of the linear tubes, the susceptor shape, and other parameters, but I could not grow high-quality GaN with a smooth surface. The large lattice mismatch (16%) was too great a problem for an inexperienced student like me to overcome. Therefore, almost 3 years passed without any success. I measured the x-ray diffraction, photoluminescence, the Hall effect, and so forth, but all the properties were poor and no promising new results were obtained.
In February 1985, I was almost nearing the completion of my Master’s degree. I had decided to start a PhD program in April, and while all the other students went on a graduation trip, I conducted experi- ments on my own. At that time, Koide was growing Al-containing nitrides, such as AlN and AlGaN, and I was growing GaN. When I compared his Al-containing crystals with my GaN, the surface of his crystals seemed to be smoother. I concluded that AlN could be used to effectively grow GaN with a better surface morphology, so I tried to grow a thin AlN layer on a sapphire substrate immediately before the growth of GaN. At that time, I knew that the epitaxial temperature of AlN was higher than 1200°C. However, because the old oscillator did not work well, I could not get the temperature to reach 1200°C. Then I suddenly remembered a discussion in the laboratory with Dr. Sawaki, an associate professor, who had explained that the growth process of boron phosphide (BP) on Si [28], for which the lattice mismatch is as large as 16%, was similar to that of GaN and sapphire. Before Professor Akasaki started his laboratory at Nagoya University in 1981, Dr. Tatau Nishinaga studied the epitaxial growth of BP on Si by vapor phase epitaxy. Dr. Sawaki mentioned the effectiveness of using a preflow of phosphorus as a source gas immediately before BP growth on Si and pointed out that the phosphor atoms appeared to act as nucleation centers. From this, I imagined that if I supplied a small amount of AlN at a low temperature, it should provide nucleation centers. Normally, I used to look inside the reactor during growth to see whether there was an interference pattern on the substrate, whereby I could check that the source gas had been properly supplied. That time, however, I was so tired that I forgot to check the interference pattern, so when I removed the sample from the reactor and saw that it had a highly smooth surface which was perfectly transparent, I thought that I had made a mistake and forgotten to supply trimethylgallium. But thinking it over, I realized that I had not made a mistake, so I checked the surface using a Nomarski-type microscope and found that I had actually succeeded in growing atomically flat GaN. Following the advice of Professor Akasaki, I further characterized the film by determining its crystalline, optical, and electrical properties, all of which turned out to be superior to those reported in previous papers. This process is known as “low- temperature- deposited buffer layer technology” and it has been used by many researchers worldwide [27,29–45]. At that time, the majority of university professors in Japan had a negative impression of the submission of patents by university professors. However, Professor Akasaki came from a company and was aware of the importance of intellectual property rights. I was also interested in submitting a patent. So, I prepared the draft of a patent for low-temperature-deposited buffer layers. To ensure that the patent would be granted, I carefully considered similar previous results. For example, Dr. Akiyama from Oki Electric used a low- temperature-deposited GaAs buffer layer for the growth of GaAs on Si [46], so I focused on the novelty of using an AlN buffer layer, and not a GaN buffer layer. Dr. Yoshida from the Electrotechnical Laboratory used single-crystalline AlN for the growth of GaN on a sapphire substrate [47], so I focused on the novelty of low-temperature deposition, which implies that the layer is not single-crystalline but polycrystalline or amorphous. The patent was submitted in 1985 [48] and was granted in 1986. In 1989, Nichia Chemicals submitted a patent for a low-temperature-deposited buffer layer with a much wider range of compositions from GaN to close to AlN but not AlN [49]. I also submitted a paper on our breakthrough to the journal Applied Physics Letters [27], which accepted it for publication. I thought that the paper would attract the interest of other researchers, but I was wrong. At that time, the majority of blue LED researchers were interested in ZnSe, and GaN researchers were in the minority, although this allowed us to concentrate on research without the high stress of competition from other groups.
Anyway, I thought that our next task should be to realize p-type GaN. At that time, experienced mem- bers of the laboratory thought that nitrogen vacancies were the origin of the high donor density in nomi- nally undoped GaN films and that by doping GaN with an acceptor impurity such as Zn, the same number of shallow donors would be automatically generated. Therefore, it appeared that p-type GaN would be
impossible to realize. This mechanism is called self-compensation, and even today it is a hot topic [50]. I had a different impression. I was unconvinced by the mechanism of self-compensation and I believed that p-type GaN could be realized. We were very lucky that a government organization, Japan Science and Technology (JST) Agency, was interested in our achievement of growing high-quality GaN crystals on a sapphire substrate by MOVPE using a low-temperature-deposited AlN buffer layer and decided to support us. As a result, Akasaki Laboratory was able to buy a new MOVPE reactor from ULVAC, which is shown in Figure 1.2. More importantly, JST encouraged us to start a collaboration with a private company. As a result, the rubber company Toyoda Gosei decided to collaborate with us. This partnership between the industry, the government, and academia is thought to be one of the most successful projects supported by JST.
Although we grew many Zn-doped GaN samples, they were all highly resistive or even n-type. In 1987, during my PhD program, I observed very sharp exciton emission at a cryogenic temperature from Zn-doped GaN grown on c-plane and a-plane sapphire [30]. I also measured the deformation potential of GaN. I was excited by the results and presented them at the annual fall meeting of the Japan Society for Applied Physics, held at Nagoya University in 1987. However, I was a bit disappointed to see that there were only four people in the room for my presentation: the chairman, Professor Akasaki, another guy, and myself. As I mentioned earlier, researchers were more interested in other compound semiconductors and GaN researchers were in significant minority. Moreover, in 1987, during my internship as part of my PhD program, I found that Zn-related blue emission was irreversibly enhanced during cathodoluminescence measurement [51], a process I referred to as low-energy electron beam irradiation (LEEBI) treatment. However, even after LEEBI treatment, Zn-doped GaN did not show p-type conductivity. Nevertheless, I was excited with this result, which I presented at an international conference on luminescence held at Beijing, China, in 1987 [51]. I also hoped to write a journal paper on this treatment. At that time, PhD candidates of the Department of Electric Engineering in Nagoya University were required to write at least three journal papers to obtain a PhD in addition to a doctoral thesis. However, I found that the LEEBI effect itself had been observed by researchers at Moscow University 4 years earlier [52]. Because I was unable to write a third journal paper, I abandoned the plan of obtaining a PhD within 3 years of my doctoral course, and in 1989, I became a research associate at the Akasaki Laboratory of Nagoya University
FIGURE 1.2 Second MOVPE reactor for GaN growth in Akasaki Laboratory, Nagoya University.
without a PhD. While reading the textbook Bonds and Bands in Semiconductors by Dr. Phillips [53], I found one graph particularly interesting because it showed that Mg is better than Zn for the activation of acceptors in GaP, the material used in green LEDs. Although the Mg source, bis-Cp2Mg, was very expen- sive, I implored Professor Akasaki to let me purchase some. He kindly gave me permission, and after wait- ing several months for it to arrive, I was finally able to grow many Mg-doped samples with my laboratory partner, Kito, a Master’s student. Here, I would like to mention the pioneering work of Dr. Maruska in 1972 [54], who at the time was a PhD student at Stanford University. He succeeded in fabricating the world’s first MIS-type violet LED using Mg-doped GaN. All our Mg-doped GaN samples were highly resistive in the as-grown state, but after LEEBI treatment, some samples showed p-type behavior when subjected to hot probe measurement. I knew that hot probes were not considered to be reliable and that no one would believe that p-type conduction had been achieved. Kito then performed Hall effect measurements on the samples, by which we finally established that we had achieved p-type GaN for the first time in the world. We also fabricated p–n junction ultraviolet/blue LEDs [55–58]. Soon after that, Dr. Nakamura’s group also used LEEBI treatment [59,60]. In 1992, Dr. Nakamura claimed that p-type GaN could be obtained by simple thermal annealing [61]. Today, almost all LED companies use thermal annealing. The mechanism of p-type conduction involves the desorption of hydrogen near Mg acceptors, as first pointed out by Professor van Vechten [62], and confirmed experimentally by Dr. Nakamura [61].
For us, another important task was to realize true blue emission using a band-to-band transition, so from 1986 onward we tried to grow InGaN. However, this was also very difficult and we only succeeded in growing InGaN with an In composition of less than 1.7% [63]. In 1989, Dr. Matsuoka’s group at NTT reported the successful growth of InGaN under an extremely high ammonia supply while using nitrogen as a carrier gas [64]. The only difference between our work and that of the NTT group was the carrier gas; we used hydrogen as the carrier gas. They also reported blue-violet photoluminescence at 77 K, indicating the incorporation of a high In content. However, at room temperature, deep-level-related yellow emission was observed. The mechanism of In incorporation in InGaN has been elucidated through thermodynamic analysis by Professor Koukitu et al. [65,66].
Finally, by combining high-quality-crystal growth technology using a low-temperature-deposited buffer layer, p-type growth technology by Mg doping with LEEBI or thermal annealing, and InGaN growth technology, Nichia Corporation succeeded in commercializing double-heterostructure-type InGaN blue LEDs for the first time in the world in 1993 [67]. They also fabricated single-quantum-well LEDs in 1995 [68], which are also a very important technology for enhancing the efficiency of nitride LEDs because a very narrow quantum suppresses the quantum-confined Stark effect [69], thus increasing the transition probability [70].
Let me explain how InGaN LEDs can contribute to improving the electricity situation, especially in Japan. Many people will remember the great earthquake of east Japan and the meltdown of the nuclear power plants in March 2011. For the next 3 years, none of the 48 nuclear electricity generators in Japan were in operation [71]. Although three nuclear power plants restarted operation, they were not enough to make up for the loss of 30% of Japan’s generating capacity of all the nuclear power plants before 2011. The U.S. Department of Energy predicted that three quarters of lighting will have been replaced with LED lighting systems by the year 2030 in the United States, resulting in a 7% reduction in electricity use [72]. In the case of Japan, the penetration of LED lighting systems into the market is expected to be much faster. A research company in Japan has predicted that by 2020, more than 95% of the sales of general lighting systems will be LED lighting systems [73]. More importantly, we can develop and supply compact lighting systems to the younger generation, especially children in remote areas without access to electricity.
1.3 Summary
This chapter mainly gives a history of the development of GaN-based blue LEDs. I wrote this manu- script as a message to young scientists and engineers with the desire to invent something that will benefit humans. I sincerely hope that this message will encourage the next generation to tackle some of the world’s problems through science and engineering.
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Spectrum-Related Quality of White Light Sources
Yue Jun Sun Philips Lighting
Dragan Sekulovski Philips Lighting
Mart Peeters Philips Lighting
Kees Teunissen Philips Lighting
Remy Broersma Philips Lighting
Rene Wegh Philips Lighting
2.1 Introduction ........................................................................................ 14 2.2 Methods of Measuring and Specifying Light Quality ................... 15 2.3 White Light ......................................................................................... 15
Chromatic Aspects of White Light • Color Rendition of Light • Light Preference • Crisp White • Perceived Brightness of White Objects
2.4 Conclusion ..........................................................................................28 References ........................................................................................................29
2.1 Introduction
Visible light, or often simply called light, consists of a band of electromagnetic waves that human eyes can detect and that create a visual sensation. Vision is created by a series of complex processes that transduce these electromagnetic waves into neural signals. Intensity and colors are perceived by people as a result of the physical stimulation of light photons on the photoreceptor cells in the retina. In many applications, the preference of perceived light is caused by the physical properties of the emitted light and the reflectance of the objects and their complex interactions with individual psychological experience. In our visual system, the objective quantities are then entangled with the subjective feeling. So the visual perception is the result of processed information, deduced from individual sensations.
The influence of light on psychological mood has been studied to be systematic [1,2]. A bad lighting environment may subconsciously affect one’s comfort level or lower his or her spirit. Conversely, people can be emotionally stimulated when they are immersed in an appealing lighting atmosphere.
Just recently in 1998, melanopsin was first discovered from the skin cells of a frog [3]. Later, the same team found that it mediates nonvisual photoreceptive tasks [4]. In human eyes, melanopsin is found in intrinsically photosensitive retinal ganglion cells (iPRGCs), which are particularly sensitive to bluish light. These third class photoreceptors, next to the well-known cones and rods, are connected to the suprachiasmatic nucleus (SCN), which is also known as the “body clock.” Therefore, melanopsin has been considered to serve an important role in the photo entrainment of circadian rhythms [5]. Many studies on the treatment of seasonal affective disorder also unveil the impact of light on humans’ mental health [6,7].
As outlined earlier, light not only stimulates vision but also is of critical importance for our psychological and physiological behaviors [8]. In modern society, the majority of people spend more time indoor and are exposed to the artificial electric lighting during most of the time. Attention should be paid to visual, emotional, and biological impacts (cf. Figure 2.1) of the electric lighting. Recently, research on these impacts has intensified under the catchphrase of human-centric lighting, which is promoted by some lighting institutions, for example, LightingEurope [9].
Compared to the rapid progress in efficiency improvements of LED sources in the last decades, the progress in specifying the multidimensional light-quality properties is, despite significant research efforts, very limited. In this chapter, we mainly focus on the visual aspects of light, particularly those aspects that are determined by the light spectrum. Several measures have been reviewed. Recent activities to propose alternatives to better specify the light quality are also outlined.
Visual Emotional
15Spectrum-Related Quality of White Light Sources
2.2 Methods of Measuring and Specifying Light Quality
To some extent, the assessment of quality of light is a subjective pathway to perceive the “real” world. However, it is often desired to be objective in describing the relevant properties in a common language. How to specify and quantify these properties and their impact is crucial for the lighting industry and designers, in order to create a specific light effect and to achieve the design targets. On the other hand, end users also require understandable and simple terms or icons to make their purchasing and application choices. Some methods to quantify light properties have been developed particularly by the International Commission on Illumination (CIE) and have been widely adopted in the lighting industry for decades. However, LED lighting, as a rapidly evolving technology, creates light and applications in a completely new way, which in turn requires a studiously look on these characterization methods and CIE e-ILV 17-550 possible alternatives.
Humans have quite complex and diverse tastes in lighting. As a result, the definition of quality of light is multidimensional and cannot be described by a single figure, that is, there is no “one-size-fits-all” solution. Conventionally, we refer to the quality of light as how good or bad the perceived light in a given scene is. The nonvisual influence is typically neglected, and only the short-term visual sense is covered, although even not completely. Table 2.1 lists the most common methods, which measure various aspects of light quality.
2.3 White Light
In most cases, light used for general lighting refers to the white light source. An equal energy spectrum (EES) has a relative spectral power distribution (SPD) equal to one for all wavelengths in the visible s pectrum range (380–780 nm). Accordingly, the theoretical definition of a white object is that it diffuses
TABLE 2.1 Selected Methods for Characterizing the Quality of Light
Effect Quantity
StandardName Symbol
Visual Brightness, lightness Luminous flux Lumen (=cd·sr) lm CIE e-ILV 17-738 Illuminance Lux (=lm/m2) lx CIE e-ILV 17-550 Luminous intensity Candela (=lm/sr) cd CIE e-ILV 17-739 Luminance Nit cd/m2 CIE e-ILV 17-711
Comfort Glare Unified glare rating UGR CIE e-ILV: 17-330 Fidelity General Color rendering
index CRI or Ra CIE13.3:1995 & CIE
e-ILV 17-154 Temporal light
flicker indicator Pst or Plt IEC61547-1
Stroboscopic effect Stroboscopic effect visibility measure
SVM CIE TC 1-83
Color appearance and consistency
Emotional Feeling MacAdam ellipse Standard deviations of color matching
SDCM CIE-1932
safety Retinal blue light hazard Exposure time limit tmax IEC62471
Circadian and neurobehavioral regulation
Cyanopic/chloropic/ erythropic/melanopic/ rhodopic illuminance
tbd tbd CIE JTC9
light perfectly and has a 100% spectral reflectance across the same spectrum range. CIE 1964 defines the formula of whiteness (W) with a 10° field of view and using a CIE Standard Illuminant D65:
W Y x y= + -( ) + -( )800 0 3138 1700 0 3310. . (2.1)
where Y is the tristimulus value or relative luminance x and y are the chromaticity coordinates in the CIE 1931 color space
Apparently, if x = 0.3138 and y = 0.3310, the object has neutral color, with a whiteness that is equal to its luminance.
One can subtly, but easily, “feel” the quality of white light surrounding us. In many decades, the quality of light has been typically expressed with two characteristics, namely, correlated color temperature (CCT), combined with the distance to the blackbody locus, and color rendering index (CRI). They are well defined in international standards and are most familiar in the lighting industry and adopted by regulations across the regions. CCT and CRI together give a numerical estimate of the light appearance in comparison to a reference light source (a Planckian, or also called blackbody, radiator, or CIE illuminants representing natural daylight). As both CCT and CRI are based on colorimetry, not color appearance, they cannot reliably predict human’s color perception. In particular for narrowband light sources, with “gaps” in the emission spectrum, the correspondence between colorimetric numbers and perception becomes low. Nevertheless, a quick look of these well-established methods helps us to understand where we stand now and what their intrinsic problems are and how we can cope with these issues.
2.3.1 Chromatic Aspects of White Light
All colors can roughly be divided into two parts, namely, brightness (or whiteness) and chromaticity. In color science, the chromaticity characterizes the specification of the color, regardless of its luminance. It consists of two independent parameters, namely, hue and chroma (often confused with saturation). In practice, the CIE 1931 xyY chromaticity diagram is widely used to specify colors.
CCT is the temperature of a blackbody radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions [10]. This chromatic quantity, measured in kelvin (K), is used for indicating the general appearance of a white light source. For example, a typical incandescent lamp with a CCT of 2800 K looks warm and yellowish white, while a fluorescent tube with a CCT of 6500 K appears cool and blueish. But this single number does not communicate the chromaticity when the color point is outside of the blackbody locus, or the tint of the source. This problem is addressed by Δuv, or Duv, the distance from the blackbody locus measured in the CIE u,v 1960 chromaticity diagram. The combination of CCT and Duv, including its sign, provides color point information intuitively. A positive Duv (above the blackbody locus) indicates a more greenish tint, while a negative one (below the blackbody locus) means a more purplish appearance. The CCT only gives the white tone of the light source in the chromatic space, while a given CCT value can be achieved with, in principle, countless variations in the emission spectrum. That means that light sources may show differ- ences in color rendition, which is not disclosed by the CCT–Duv combination.
Table 2.2 lists the chromaticity coordinates and corresponding CCTs for most widely used CIE Standard Illuminants. These Standard Illuminants provide a basis for comparing colors recorded under different lighting sources.
To ensure a high-quality white light, manufacturers need to specify the chromaticities and tolerances of their lighting products. These categorized figures thus enable a common language across their various products and customers. For standardized CCTs, the rated values of the chromaticity coordinates are well defined in standards of IEC 60081 or ANSI C78.376 (cf. Table 2.3). It should be noted that both standards
are developed for traditional fluorescent lamps. In order to increase the overall yield of solid-state lighting products and take more applications into account, ANSI C78.377—Specifications for the Chromaticity of Solid-State Lighting Products—defines quadrangles for the tolerances of nominal CCTs, instead of MacAdam ellipses used in the conventional lighting. Figure 2.2 shows the color point of standardized 4000 K in the CIE 1931 chromaticity diagram. Note the difference of the center point and the tolerance between IEC F4000 and ANSI 4000 K.
Some applications require low-CCT light, which was addressed in the addition of the specifications for nominal CCTs of 2200 and 2500 K in ANSI C78.377 2014 revision. Although the latest Energy Star V2.0 points out that 2200 and 2500 K nominal CCTs are only applicable to filament-style lamps, it would be not surprising that these colors will become more common in “normally configured” LED bulbs in the near future, as they create a warmer and cozier atmosphere. Besides, a more dynamic scene can be achieved with a dual CCT LED configuration, in order to have the effect of sunset-like drop in terms of the color temperature when dimming down, for example, Philips Warm-Glow dimmable LED lamps from a yellowish 2700 K down to a more reddish-orange tone 2200 K. On the other hand, the lower CCT light source has, typically, less radiant energy in the short-wavelength region, especially in the spectral melanopic sensitivity range [11], which is believed to lead to a lower suppression of melatonin compared to that of the same intensity but higher CCT light. This finding enables configuring emission spectra of white-light sources that selectively enhance or reduce melatonin secretion and consequently impact our circadian rhythm and with that our sleep quality.
TABLE 2.2 Chromaticity Coordinates and CCTs for CIE Standard Illuminants
Source
Illuminant A 0.45117 0.40594 2856 Incandescent Illuminant D55 0.33412 0.34877 5503 Midmorning/midafternoon daylight Illuminant D65 0.31381 0.33098 6504 Noon daylight Illuminant F4 0.44925 0.39061 2940 Warm white fluorescent Illuminant F10 0.35061 0.35430 5000 Philips TL850 Illuminant F11 0.38544 0.37109 4000 Philips TL840 Illuminant F12 0.44265 0.39706 3000 Philips TL830
TABLE 2.3 Standardized CCTs and Nominal Chromaticity Coordinates for Different Lamp “Colors”
IEC 60081 ANSI Energy Star
C78.376 C78.377 V2.0
“Color” CCT (K) x y CCT (K) x y x Y CCT Descriptor
F6500 6400 0.313 0.337 6500 0.313 0.337 0.3123 0.3283 Daylight F5000 5000 0.346 0.359 5000 0.346 0.359 0.3446 0.3551 Daylight F4000 4040 0.380 0.380 4000 0.380 0.380 0.3818 0.3797 Cool white F3500 3450 0.409 0.394 3500 0.409 0.394 0.4078 0.3930 Neutral white F3000 2940 0.440 0.403 3000 0.440 0.403 0.4339 0.4033 Warm white F2700 2720 0.463 0.420 2700 0.459 0.412 0.4578 0.4101 Soft white
2500 0.4806 0.4141 Sunrise/sunset white
2200 0.5018 0.4153 Amber white
Note: The corresponding nomenclature to describe the color term is recommended by Energy Protection Agency (EPA), as outlined in the last column.
2.3.2 Color Rendition of Light
Traditionally, natural daylight is believed to offer the perfect color rendition, thanks to its continuous SPD that has a big overlap with the photopic sensitivity curve of the human visual system. In other words, we have virtually evolved and adapted to it. To assess the color rendering property of a light source, a standardized method is needed. The CIE defines the color rendering as the effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant [10].
The method of measuring and specifying color rendering properties of light sources is well described in CIE13.3-1995. The color appearance of 14 reflective test color samples (TCS) is simulated when illuminated by the test source and a CCT matched reference source. A blackbody radiator is used as the reference source if CCT of the test source is below 5000 K, otherwise a phase of daylight. With the chromatic adaptation of each sample by applying a von Kries transform, the difference in color appearance, namely, Euclidean distance ΔEi, for each sample between the test and reference light sources is calculated in CIE 1964 (U*V*W*) color space. The special CRI (Ri) is calculated for each reflective sample by
R Ei i= -100 4 6. D (2.2)
The most well-known term of CRI in the lighting industry actually refers to Ra, the CIE general CRI. It is the average of Ri for the first eight TCS. A high Ra is generally considered to represent high color fidelity, not to be confused with high preference, of the light source, and vice versa. It should be noted that the abbreviations Ra and CRI are normally used interchangeably, unless it is specified.
For decades, the CRI has been criticized for its limitations. The CIE U*V*W* color space is far from being perceptually equidistant and is considered to be outdated. The von Kries chromatic adaptation is also considered obsolete, but it is still adequate when the color point of the light source is close to the blackbody locus, which holds true when CRI computations are involved. The direction of color shifts is not taken into account.
Furthermore, all eight TCS used in the computation of Ra are pastels with relatively low color saturation. Ra alone is therefore not a good indicator for the (color rendering) quality of light in many applications
0.408
0.398
0.388
0.378
0.368
0.358
X
Y
IEC F4000
ANSI 4000 K
FIGURE 2.2 Standardized 4000 K in the CIE 1931 chromaticity diagram. Note the difference of the center point and the tolerance between IEC F4000 (triangle) and ANSI 4000 K (circle). 7 SDCM is used in the plot for the corresponding IEC MacAdam ellipse.
where more saturated color objects are involved. As already mentioned earlier, the predictive value for the color rendition is limited when light sources are composed of narrowband spectra, for example, white LED sources composed of individual narrowband LEDs. On the other hand, by optimization of LEDs’ spectra to those eight TCS, the Ra value can be optimized to meet a particular minimum threshold value, while the actual color appearance in the application could be disappointing.
Red colors are important in many applications, where Ra only represents the average color difference for the eight pastel colors. To address this issue, special CRI R9 has been added to LED lighting specifications to supplement Ra. The Energy Star program (V2.0) in the United States indeed requests that all LED lamps should have R9 larger than zero. It has to be noted that R9 can be less than 100 due to an increase in saturation (more energy in the long wavelength range) or due to a decrease in saturation (less energy in the long wavelength range). This information on saturation changes is not disclosed with any of the special CRI values.
Based on all issues with CRI [12], CIE Technical Committee TC1-62, Color Rendering of White LED Light Sources, concluded that the CIE CRI is generally not applicable to predict the color rendering rank order of a set of light sources when white LED light sources are involved in this set [13]. The committee, however, did not recommend an alternative measure.
CIE TC1-69, Color Rendition by White Light Sources, was established with the goal of recommending new assessment procedures. Till now, no solution has been identified and agreed upon. Two new technical committees were then formed. TC1-90, Color Fidelity Index, focuses on evaluating available fidelity indices to assess the color quality of white light source with a goal of recommending a single index for industrial use. The other one, TC1-91, New Method for Evaluating the Color Quality of White Light Sources, is looking for other available methods while explicitly excluding color fidelity indices.
Color fidelity is only one aspect of color rendition, while it is often mistakenly interpreted as the only measure for light quality. As a single one-dimensional index value, the CRI provides no information on hue discrimination, hue-angle changes, color saturation (or vividness in a more plain language), color preference, and object attractiveness. Perceived color quality is often subjective, emotional, application, and environmental dependent, by the assessment of an individual or a group who prefers the color appearance of a given light source or the illuminated objects. In general, color preference can be gender, age, region, culture, application, and event dependent. It depends not only on the SPD of the light source but also on the reflectance property of illuminated objects. In addition to the objective fidelity index, such as CRI, a related set of index values that describes more subjective aspects of the light source is apparently needed to cope with this dilemma (see, e.g., [14]).
As a consequence, many attempts have been made to develop and standardize alternative characterization methods, such as color quality scale (CQS) [15,16], gamut area index (GAI) [17], color discrimination index [18], color rendering capacity [19], flattery index [20], color preference index [21], and color saturation index (Ga) [28]. Recently, Houser et al. [22] reviewed 22 color rendition measures and concluded that those newer measures are not essentially different from the older ones, including the CRI. The fundamental problem of the color rendition is its multidimensional aspects, which have conflicting optimization criteria. Due to the intrinsic trade-off between fidelity, discrimination, and preference, it is not realistic to have a light source simultaneously performing the highest for all aspects. In other words, no single method can fully describe these aspects with one single index value as an output. Despite significant research efforts, it is unlikely that a new fidelity measure alone will show a significant and meaningful improvement in specifying color quality over the current CRI.
Based on analyzing color rendition vectors of 1269 spectrophotometrically characterized test samples, namely, the entire Munsell palette, Zukauskas et al. [23] proposed a four-dimensional approach to assess the color quality, namely, CCT, color fidelity index, color saturation index, and hue distortion index. This four-dimensional metric is illustrated by a notation with the corresponding letter of each dimension, namely, T (CCT), F (fidelity), S (saturation), and D (distortion). For example, T30-F60R-S25-D20YP stands for a light source with a nominal CCT of 3000 K. Illuminated by this light source, 60% of the color palettes are rendered with a high fidelity and with the dominance of a red hue, 25% of the color palettes
are rendered with increased saturation, and 20% of colors have distorted hues prominently for yellow and purple. With regard to the studied light sources, the hue distortion index is negatively correlated with the color fidelity index. So the hue distortion index might be an optional metric. There is no obvious correla- tion between the color fidelity index and the color saturation index. It indicates that no single figure of merit can combine the fidelity and the saturation. On the other hand, at least two uncorrelated metrics, say, one representing “fidelity” and the other one representing “saturation,” are believed to be able to characterize the color rendering properties of the light source. The alphanumeric approach proposed by Zukauskas et al. is relatively comprehensive to better reveal the color rendition properties, as it, to some extent, quantifies the multidimensional aspects embedded in the color quality charts. However, all of their color fidelity index, color saturation index, and hue distortion index refer to the average figure of merit. Unsurprisingly, it does not tell the color preference. Moreover, it is not practical to communicate to customers of the lighting industry, especially in a manner understandable to nonexperts in the field of color science and consumers.
Researchers at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute developed a “Class A color” specification based on the study in simulated residential applications [24,25]. Fresh fruits and vegetables in the boxes illuminated under various light sources were viewed by selected subjects. The subjects were asked to rate the naturalness (fidelity) and vividness (saturation) of the display and additionally the overall acceptability of the light source. The CRI is taken as the fidelity measure, while the GAI using the same TCS as those for CRI calculation is applied for the saturation measure. The CIE 1964 W*U*V* color space is used for the calculation. They concluded that light sources with CRI equal or greater than 80 and GAI between 80 and 100 have good color rendering (cf. Figure 2.3). Besides these criteria, to meet the “Class A color” designation, the light source should have the minimal tint (i.e., be “white”) and have a small chromaticity tolerance. Although these researchers made a good attempt to offer the lighting industry a “seal of approval” for color quality of light sources used in general lighting applications, the selected metrics to represent the color rendering properties may be not appropriate, for example, the nonuniformities of the outdated CIE 1964 U*V*W* color space, no chromatic adaptation is applied, and a fixed reference illuminant (an EES) is used. On the other hand, the defined values for good color rendering are to be further investigated, as the limited scenes and objectives have been studied, even in a simulated viewing environment. They may not solve the main issues from professionals like lighting designers and architects, neither the consumers facing various applications and configurations in the real world.
The latest and probably most influential campaign on promoting a new proposal for color rendition was unveiled in 2015. The Illuminating Engineering Society (IES) of North America published a technical
120
100
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60
40
20
60 80 100
FIGURE 2.3 The area of “Class A color” in a Cartesian coordinate plot of CRI and GAI, prescribed by LRC, namely, CRI ≥ 80 and 80 ≤ GAI ≤ 100.
memorandum (IES TM-30-15), IES Method for Evaluating Light Source Color Rendition. Similar to other proposals mentioned earlier, it proposed a method to evaluate the color rendition of the light source by quantifying the fidelity and gamut (chroma or saturation) in a two-dimensional system. The color preference is not in the scope of that work. Unlike the LRC, this IES method takes different approaches on cal- culating the fidelity index and the gamut index. Instead of using 8 CIE TCS, 99 color evaluation samples (CES) are utilized to determine the color fidelity. These color samples are statistically down-selected from more than 100,000 real object colors, which colors are believed to well represent our actual world. CIE 10° standard observer color matching functions (CMFs) are applied, instead of 2° CMFs used in the CRI calculation. Another noticeable feature of this IES method is that the chromatic adaptation transform is based on CAM02UCS [26].
The details about the development of the IES metrics can be found in the publication from David et al. [27]. The IES fidelity index Rf is, similar to Ra, a numerical measure to determine the difference between the chromaticity coordinates under the test source and the reference source, but based on the new set of color samples and the most recent color space. It is an average figure of merit, not attempting to reveal the perceived fidelity. The index values for typical phosphor-converted LED light sources, obtained with Rf, are typically 3–5 unit points lower than with Ra. Although computed with 99 CES, the average fidelity index value Rf is not necessarily more representative for the actually perceived fidelity of the environment than the CIE Ra value. Also the use of the 10° CMFs for the computations is arguable, because the CAT02 and HPE matrices were defined for the 2° CMFs. The IES gamut index Rg is the measure of the average area spanned by the (aM′, bM′) coordinates of the CES in CAM02UCS. The aM′–bM′ colorfulness space is evenly divided into 16 bins in a radial pattern. Within each bin, the arithmetic mean of the aM’ and bM’ coordinates for each CES within the bin is calculated under the test source and the reference source. The area of th

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Handbook of Solid-State Lighting and LEDs

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