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).
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: η
> 60% for blue InAlGaN LEDs emitting at λ~460 nm; η
> 90% for red InAlGaP LEDs (λ~650 nm); while η
< 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 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.
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.
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