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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
Recent titles in the series
Nanophotonics and Plasmonics: An Integrated View Dr. Ching Eng
(Jason) Png and Dr. Yuriy Akimov
Handbook of Solid-State Lighting and LEDs Zhe Chuan Feng
(Ed.)
Optical Microring Resonators: Theory, Techniques, and Applications
V. Van
Optical Compressive Imaging Adrian Stern
Singular Optics Gregory J. Gbur
The Limits of Resolution Geoffrey de Villiers and E. Roy Pike
Polarized Light and the Mueller Matrix Approach José J Gil and
Razvigor Ossikovski
Light—The Physics of the Photon Ole Keller
Advanced Biophotonics: Tissue Optical Sectioning Ruikang K Wang and
Valery V Tuchin (Eds.)
Handbook of Silicon Photonics Laurent Vivien and Lorenzo Pavesi
(Eds.)
Microlenses: Properties, Fabrication and Liquid Lenses Hongrui
Jiang and Xuefeng Zeng
Laser-Based Measurements for Time and Frequency Domain
Applications: A Handbook Pasquale Maddaloni, Marco Bellini, and
Paolo De Natale
Handbook of 3D Machine Vision: Optical Metrology and Imaging Song
Zhang (Ed.)
Handbook of Optical Dimensional Metrology Kevin Harding (Ed.)
Biomimetics in Photonics Olaf Karthaus (Ed.)
Handbook of Solid-State Lighting and LEDs
Edited by Zhe Chuan Feng
CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW,
Suite 300 Boca Raton, FL 33487-2742
© 2017 Taylor & Francis Group, LLC CRC Press is an imprint of
Taylor & Francis Group, an Informa business
No claim to original U.S. Government works
Printed on acid-free paper
International Standard Book Number-13: 978-1-4987-4141-5
(Hardback)
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regarded sources. Reasonable efforts have been made to publish
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assume responsibility for the validity of all materials or the con-
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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
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
and the CRC Press Web site at http://www.crcpress.com
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 hand- book 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 con- tents 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 confer- ences 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
Applications Changchun Institute of Optics, Fine Mechanics
and Physics Chinese Academy of Sciences Changchun, People’s
Republic of China
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
Technology Singapore, Singapore
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
Applications Changchun Institute of Optics, Fine Mechanics
and Physics Chinese Academy of Sciences Changchun, People’s
Republic of China
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
Technology Singapore, Singapore
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
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
Zi-Hui Zhang 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
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 cen- tury, 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 combina- tion 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 incan- descent 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 com- bination 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 ammo- nia 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, respec- tively,
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
6 Handbook of Solid-State Lighting and LEDs
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 commer- cially 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 sap- phire 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
7From the Dawn of GaN-Based Light-Emitting Devices to the Present
Day
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 interfer- ence 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
8 Handbook of Solid-State Lighting and LEDs
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 doc-
toral 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.
9From the Dawn of GaN-Based Light-Emitting Devices to the Present
Day
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 mecha- nism 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
technol- ogy, 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.
10 Handbook of Solid-State Lighting and LEDs
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683. 70. T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H.
Takeuchi, H. Amano, and I. Akasaki, Jpn. J.
Appl. Phys., 36 (1997) L382. 71.
http://www.enecho.meti.go.jp/category/electricity_and_gas/nuclear/001/pdf/001_02_001.pdf
[in Japanese]. 72. U.S. DOE Energy Savings Potential of Solid-State
Lighting in General Illumination Applications,
(January 2012)
(http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_energy-savings-
report_jan-2012.pdf).
73. Fuji Chimera Research Institute, Inc., 2016 LED Related Market
Survey (2016) p. 29 [in Japanese].
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 psychologi- cal 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 appli-
cation 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
16 Handbook of Solid-State Lighting and LEDs
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 qual- ity 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 com-
municate 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
17Spectrum-Related Quality of White Light Sources
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.
18 Handbook of Solid-State Lighting and LEDs
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 illu- minated 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 appear- ance, 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 correspond- ing IEC MacAdam ellipse.
19Spectrum-Related Quality of White Light Sources
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 specifica- tions 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, applica- tion, 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 characteriza- tion methods, such as color
quality scale (CQS) [15,16], gamut area index (GAI) [17], color
discrimina- tion 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 con- cluded 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 signifi-
cant 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
20 Handbook of Solid-State Lighting and LEDs
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 cus- tomers 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
80
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.
21Spectrum-Related Quality of White Light Sources
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 prefer- ence 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