Doctoral candidate Yik-Khoon Ee (right) is advised by Nelson Tansu, the P.C. Rossin Assistant Professor of electrical and computer engineering.
Yik-Khoon Ee, a doctoral candidate in electrical engineering and the Center for Optical Technologies
, has received an international honor for research that could reduce energy consumption, utility bills and pollution by improving the efficiency of light-emitting diodes (LEDs).
Ee, who is advised by Nelson Tansu, the P. C. Rossin Assistant Professor of electrical and computer engineering, won a Best Student Paper Award at the IEEE Photonics Global Conference 2008. IEEE (the Institute for Electrical and Electronic Engineers
) is the world’s largest technical organization, with more than 375,000 members, including 80,000 student members.
A total of 275 papers were presented at the IEEE conference, which was held in Singapore. Best Student Papers were awarded in four categories – nanophotonics, optical communications and networks, biophotonics, and high-power lasers and applications.
Ee won his award in the nanophotonics category, in which 96 students competed. His paper, “Optimization and Fabrication of III-Nitride Light-Emitting Diodes with Self-Assembled Colloidal-Based Convex Microlens Array,” expands on work he has published in Applied Physics Letters
. That work resulted from a collaboration between Tansu’s group and James F. Gilchrist’s group. Gilchrist, the P. C. Rossin Assistant Professor of chemical engineering, and Pisist Kumnorkaew, a doctoral candidate advised by Gilchrist, are coauthors on the published papers.
The need for a better light source
LEDs are gaining in importance, says Ee, because of the inefficiency of the incandescent lamp, or lightbulb, and the potential hazards posed by fluorescent lighting.
In the U.S. and in much of the rest of the world, according to the Optoelectronics Industry Development Association, about 20 percent of all generated electricity is used for lighting.
Lightbulbs account for most of this lighting, followed by fluorescent lighting. But the lightbulb, says the U.S. Department of Energy, dissipates 95 percent or more of the energy it consumes as infrared light or as heat.
“There is a joke in optical technologies circles that the lightbulb works better at generating heat than at generating light,” says Ee. “That is why it is too hot to touch!”
Fluorescent lamps are more energy-efficient and longer-lasting than lightbulbs, says Ee. But they contain mercury, which is toxic to humans and to the environment.
“If you accidentally break a fluorescent light tube, it’s best to evacuate the room and put on a protective face mask before you return to clean it up.”
LEDs are used widely as indicator lights – in the dashboards, headlights and taillights of cars, and in cell phones, traffic lights, billboards, laptops and TVs. They are more efficient than lightbulbs and have the potential to exceed fluorescent lighting as well, but their own light-emitting efficiency must first be improved.
LEDs emit light from the “active region” of a semiconducting material such as gallium phosphide (GaP) or indium gallium nitride (InGaN), the material used by Tansu and Ee. This region, called a quantum well, is only several nanometers thick (1 nm is one-billionth of a meter).
The relatively large difference between the refractive indices of GaN and air causes a phenomenon called total internal reflection, which traps approximately 80 percent of the emitted light inside the nanostructures of the semiconductor.
“Without further engineering, only about 20 percent of the light generated by a semiconductor can get out into free space,” says Ee. “This is an extremely low light-extraction efficiency.”
To break the total internal reflection and scatter more light from the semiconductor quantum wells, says Ee, engineers typically use a process called etching to make the semiconductor’s surface rough. However, this method, which is employed by several manufacturers of LEDs, has a drawback. “GaN is a very hard material,” says Ee, “which makes it difficult to produce the uniform roughness that is desired.”
A beautiful, simple inexpensive solution
In the past two years, Ee and Tansu have succeeded in improving the light-extraction efficiency of InGaN quantum wells LEDs by more than 2.5 times. Working with Gilchrist and Kumnorkaew, they have reduced total internal reflection by forming self-assembled monolayer silicon-dioxide (SiO2) and polystyrene based microlens arrays on the GaN surface.
“We deposit the microspheres of polystyrene first, then the microspheres of SiO2,” says Ee. “Next, we heat the polystyrene, which has a lower melting temperature than SiO2. The polystyrene melts and planarizes, semi-burying the SiO2 microspheres to form microlens arrays on top of the GaN.” The Lehigh researchers discussed this method in Applied Physics Letters
in 2007. They discussed the optimization of the microsphere deposition in the journal Langmuir
The group has been able to achieve a controlled surface roughness ranging in size from 300 nm to 1 micron. This increases the photon escape cone of the LED, causing more light to be scattered to free space, says Ee.
Using the microlens arrays, the group has achieved a light-extraction efficiency of 250 percent over LEDs with no surface improvement, says Ee.
“This is very significant,” he says. “Other groups have recorded only a 40- to 60-percent improvement. In addition, our method is very inexpensive.”
The four researchers published their results in Applied Physics Letters
in November 2007 in an article titled “Enhancement of light-extraction efficiency of InGaN quantum-well LEDs using SiO2/polystyrene microlens arrays.” The article was coauthored with Ronald A. Arif, a member of Tansu’s group who earned his Ph.D. in electrical engineering in 2008. The five researchers have filed for a U.S. patent on the innovation.
In January 2008, the group’s work was featured in Laser Focus World
, which wrote, “Numerous attempts to improve light-extraction efficiency of nitride quantum-well LEDs have not been successful in terms of cost, process control or scalable production. [This] inexpensive new approach using microlens arrays on top of the quantum-well structure has led to a significant increase in extraction efficiency, and is simple to control and is scalable.”
“The beauty of this technology,” says Tansu, “is that, although it is so simple and inexpensive, it leads to significant improvement in device application. We literally came up with the concept and principle of this technology over a weekend in our laboratory during summer session.”
The most recent paper by Tansu, Ee, Gilchrist and Kumnorkaew, titled “Optimization of Light-Extraction Efficiency of III-Nitride LEDs with Self-Assembled Colloidal-based Microlenses,” will appear this summer in the IEEE Journal of Selected Topics in Quantum Electronics
in a special July/August issue on solid state lighting. Hongping Zhao and Hua Tong, Ph.D. candidates in electrical engineering, are coauthors of the article.