focus issue: optics in leds for lighting
TRANSCRIPT
Focus Issue: Optics in LEDs for Lighting
Introduction
Since the invention of visible light-emitting diodes (LEDs) based on III-V semiconductor p-n junction materials by
Nick Holonyak, Jr., in 1962, LEDs have been developed extensively and are now challenging Edison-style
incandescent lamps and are being used in more and more applications in lighting. Their operating wavelengths have
decreased over time: first, red LEDs were developed, followed by amber and green LEDs. LEDs entered the market
in indicator and signal applications, replacing small incandescent bulbs, thanks to their high efficiency, and hence
lower power consumption, and their exceptional reliability. However, extending their operation to even shorter
wavelengths was limited by the widest direct-bandgap energy of III-AsP materials, roughly corresponding to a
yellow-green color. It took another 30 years for LEDs to cover the whole visible spectrum, when, in 1991, Shuji
Nakamura demonstrated commercially viable blue LEDs, based on new wide-bandgap III-N materials. This
milestone positioned LEDs as a serious competitor in general lighting, rather than mainly a technology for small
indicator lights. The ability of LEDs to emit photons in all three primary colors and to pump phosphors to produce
white light marked the beginning of the era of solid-state lighting (SSL).
The levels of efficiency and reliability achieved by LEDs today are by far superior to those of traditional light
sources used for lighting. For instance, the peak efficiency of white LEDs exceeds 260 lumens per watt (lm/W),
compared with ~16 lm/W for incandescent lamps and <100 lm/W for fluorescent lamps. This level of performance
is still growing. SSL based on LED technology will be an important technology for generating significant energy
savings and consequently for environmental benefits. However, in order for LEDs to become a main player in
lighting, several technical and nontechnical challenges still need to be overcome. The major challenges include
further improvements in luminous efficiency while simultaneously delivering superb color quality at reasonable
cost, especially for the development of high-power and large-area LEDs with improved light output power and
luminous efficiency under high-current injection conditions. While the peak efficiency of the LEDs is exceptionally
high, it does not remain high under high-current conditions. This is a well-recognized but not completely understood
phenomenon commonly referred to as an ―efficiency droop,‖ which is observed as a reduction in emission efficiency
with increasing injection current density. This illustrates the typical concurrent nature of work on visible LEDs,
which combines fundamental research, product development, and commercialization of the products—researchers
are still exploring the most critical and fundamental device physics issues of the LEDs, while consumers can acquire
LED light bulbs at local retail stores. Obviously, ownership cost is the most critical determining factor for successful
market penetration of SSL. This cost, however, is not purely an independent economical variable mainly governed
by capital cost of the SSL paid by consumers for the adoption of new lighting technology, but rather total cost,
heavily dependent on technology advances, consisting of not only capital and operating costs paid by each
individual but also environmental cost paid by the whole society. Further improvements in luminous efficiency and
reliability as well as reduction in manufacturing cost are required to lower the operating, capital, and social costs of
lighting.
The total external quantum efficiency, ηex, of an LED depends on the internal quantum efficiency, ηint (a product
of current injection efficiency, ηinj, and radiative recombination efficiency, ηrad), and the extraction efficiency, ηext:
so ηex = ηint × ηext = ηinj × ηrad × ηext. In order to improve extraction efficiency of LED devices, photons generated in
the active region should escape out of the naturally formed slab waveguide structure formed by the LEDs’ epitaxial
layers. In this Focus Issue, several novel approaches for enhancing extraction efficiency are presented. Seong-Ju
Park and his colleagues from the Gwangju Institute of Science and Technology and Samsung LED Co. in Korea
demonstrate that tungsten metal can be used not only as a mask for epitaxial lateral overgrowth, but also for the
formation of an air void underneath it, to improve both the internal quantum efficiency and the extraction efficiency.
Huai-Bing Wang and his colleagues from the Suzhou Institute of Nano-Tech and Nano-Bionics of the Chinese
Academy of Science take a similar approach: they form pyramidal patterns on sapphire substrates for the
improvement of both materials quality of the LED epitaxial structures and of the extraction efficiency. While similar
approaches using patterned sapphire substrates have been demonstrated, in this study the resistance to electrostatic
discharge effects, which impacts LED reliability, was improved simultaneously.
While internal quantum efficiency has been dramatically improved for blue- and red-emitting LEDs, the
efficiency of LEDs emitting in the green at λ = 540–550 nm is still substantially lower: e.g., according to the latest
reports: ηint > 60% for blue InAlGaN LEDs emitting at λ~460 nm; ηint > 90% for red InAlGaP LEDs (λ~650 nm);
while ηint < 20% for green InAlGaN LEDs (λ~550 nm). This performance deficiency is often referred to as a ―green
gap‖ and is associated with a number of fundamental scientific challenges that need to be resolved. For the
improvement of the internal quantum efficiency, improvements in material quality and epitaxial layer designs are
(C) 2011 OSA 4 July 2011 / Vol. 19, No. S4 / OPTICS EXPRESS A897#150112 - $15.00 USD Received 28 Jun 2011; published 1 Jul 2011
required to further reduce the density of nonradiative recombination centers and to mitigate the quantum-confined
Stark effect (QCSE) in the quantum-well active region, respectively, both of which adversely affect the radiative
recombination rate. The epitaxial structures based on III-N materials grown in polar directions on (0001) substrates
possess a built-in electrostatic field near the interfaces due to spontaneous and piezoelectric polarization effects.
This field is responsible for a reduced overlap of the carriers’ wavefunctions. A paper by Nelson Tansu and his team
from Lehigh University describe several strategies and propose optimized epitaxial structures for the mitigation of
the QCSE. They tailor the electronic band structure of the layers that form the active region of green-emitting LEDs
to improve the electron-hole wavefunction overlap, thereby improving the internal quantum efficiency and the
carrier injection efficiency. While Tansu et al. focus on the mitigation of the QCSE in polar III-N structures,
Christian Wetzel and Theeradetch Detchprohm from the Rensselaer Polytechnic Institute report on material and
device characteristics of QCSE-free nonpolar (10-10) and (11-20) structures, and they demonstrate wavelength-
stable green LEDs in order to address technical challenges associated with the polar structures. C. C. Yang and
Yean-Woei Kiang’s team from the National Taiwan University investigate surface plasmon coupling with radiating
dipoles (electron-hole pairs) experimentally and theoretically. The team demonstrates improvements in the
efficiency droop as well as in the internal quantum efficiency. They also numerically study the effects of coupling
between a radiating dipole and the localized surface plasmons induced by Ag nanoparticles.
Typical III-N-based visible LED structures are grown on sapphire substrates. While the capability to grow
device-quality materials on such substrates with a large lattice and thermal mismatch was a key breakthrough in the
development of visible LEDs, sapphire substrates are not an ideal choice. Their low thermal and electrical
conductivity are major limiting factors for high-power operation of visible LEDs, which is critically important in
SSL applications. Ideally, the sapphire substrate is separated from the epitaxial layer structure after growing the
LED material. Hyunsoo Kim and his colleagues from Chonbuk National University, Korea Polytechnic University,
and Korea Electronics Technology Institute describe a fabrication process and a LED design with a vertical
geometry and without a sapphire substrate. The paper reports in detail on the etching, ohmic metallization, laser lift-
off, light extraction, current spreading, and the stability and reliability of the devices. Sapphire substrates are also
smaller and more expensive than silicon substrates. Hence, a possible way to further lower the capital cost of SSL
technologies based on LEDs is to fabricate the devices on silicon substrates. Kei May Lau and her team from the
Hong Kong University of Science and Technology report on blue-emitting LEDs on silicon substrates. The paper
addresses many important technical issues associated with LEDs on silicon substrates, such as strain management
and crack-formation in the epitaxial structure, thermal management of the chips, and the external quantum efficiency
of the devices, including the light extraction.
In order to generate white light from LEDs, several approaches have been explored. The most common approach
at present is using blue-emitting LEDs to optically pump (and also to color-mix with) longer-wavelength broadband
emitting phosphors. Hao-Chung Kuo and Chien-Chung Lin and their colleagues from the National Chiao Tung
University, Tsing Hung University, and the Research Center for Applied Sciences in Taiwan propose a rather simple
but effective technique for the improvement of color and temperature stability of such ―phosphor-converted‖ LEDs.
The technique is based on the patterning of a remote phosphor packaging using a pulsed spray deposition. Even with
their current dominance in phosphor-converted LEDs, YAG:Ce3+
phosphors limit the color rending index of white
LEDs to a value of ~80. The color quality has to be further improved to compete with traditional lighting
technology. Christopher Summers and Hisham Menkara and their colleagues from PhosphorTech Corp. in USA
discuss nano-phosphors based on Mn-doped ZnSeS to enhance the color properties, luminosity, and efficiency of
phosphor-converted white LEDs. Another, potentially better, approach for producing white light is based on the
mixing of color elements red-green-blue (RGB) or red-yellow-green-blue (RYGB). Improvements in color quality
and luminous efficiency are obtained at the expense of the increased cost associated with the mixing of different
color elements from individually optimized active devices. Hence, this approach may fit well in high-end lighting
systems. Wen-Shing Sun and his team from the National Central University in Taiwan propose an interesting
scheme of hybrid light mixing by combining visible daylight and RGB LEDs. This paper also describes a method of
color mixing by RGB LEDs with and without sunlight. It is believed that current and future SSL is based on
individual primary color LEDs and/or LEDs combined with phosphors. Jeff Tsao and Jonathan Wierer and their
colleagues from Sandia National Laboratories, University of New Mexico, and the National Institute of Standards
and Technology challenge this common belief that the narrow spectral linewidth and the high capital cost of lasers
makes them unsuited for general illumination purposes. They discuss the use of lasers for higher power and
efficiency at high current densities for SSL and experimentally demonstrate that four-color (RYGB) laser white
illuminant is virtually indistinguishable from high-quality state-of-the-art white reference illuminants. This result
suggests that lasers can also be a serious contender for solid-state lighting in some applications.
(C) 2011 OSA 4 July 2011 / Vol. 19, No. S4 / OPTICS EXPRESS A898#150112 - $15.00 USD Received 28 Jun 2011; published 1 Jul 2011
Ultraviolet (UV) LEDs are also an important light source not only for pumping visible phosphors to produce
white light but also for replacing mercury lamps with new applications of air, water, and surface sterilization and
bioagent detection. For UV LEDs, AlGaN materials, as opposed to InGaN materials for visible LEDs, are used in
active region and the efficiency of UV LEDs are significantly lower than that of InGaN-based LEDs. In order to
address one of technical challenges associated with UV LEDs, i.e., low internal quantum efficiency, Kouji Hazu and
Shigefusa Chichibu from Tohoku University in Japan experimentally and theoretically investigate the optical
characteristics of QCSE-free but anisotropically strained AlGaN materials grown on non-polar (10-10) GaN
substrates.
Emerging nanotechnology can also impact the performance of LEDs. JianJang Huang and his team from the
National Taiwan University report on nanorod array structures in LEDs and demonstrate that strain and the optical
transition energy can be controlled in such structures. Nanorod-LED arrays may play an important role in next-
generation visible LEDs as the control of strain can impact the QCSE, which in turn affects the efficiency droop and
spectral stability.
The papers in this Focus Issue provide an overview of the current trends and the state-of-the-art in research and
development activities in the field of LEDs for SSL. We hope that the readers of Optics Express and its Energy
Express supplement will enjoy learning about the latest advances described in this Focus Issue and that it will
motivate them to publish their own latest discoveries in the area of optics for energy in Energy Express. We also
want to express our sincere gratitude to the contributors who accepted our invitation to contribute an article and who
worked to stringent publication deadlines. This Focus Issue, Optics in LEDs for Lighting, would not have been
possible without the efforts of Martijn de Sterke (University of Sydney), Editor-in-Chief of Optics Express; Bernard
Kippelen, Editor of Energy Express; and the work of the Associate Editors, reviewers, and the staff coordinating
OSA’s publications. We want to express our gratitude to all of them.
Atlanta, June 30, 2011
Jae-Hyun Ryou and Russell D. Dupuis
Guest Editors of Energy Express, Focus Issue: Optics in LEDs for Lighting
Center for Compound Semiconductors and School of Electrical and Computer Engineering Georgia Institute of Technology
(C) 2011 OSA 4 July 2011 / Vol. 19, No. S4 / OPTICS EXPRESS A899#150112 - $15.00 USD Received 28 Jun 2011; published 1 Jul 2011