The Photonic Squeeze
Whichever methods are chosen to make white light, the LEDs involved have to put out more light and become more energy efficient if they’re to replace Edison’s bulbs. White LEDs produce about 10 lumens of illumination per watt of electricity consumed, which is comparable to the performance of incandescent bulbs (a lumen is a measure of how brightly the eye perceives light). Roughly 10 percent of the electricity that they consume gets turned into light-marginally better than the 7 to 8 percent figure for incandescent bulbs. But LEDs are still too expensive to challenge your average GE Soft White. On sale at the local discount store, 100-watt incandescent light bulbs run about a dollar for a package of four and deliver 1,500 lumens of illumination apiece. “I cannot make an LED that gives you 1,500 lumens for 25 cents,” says Roland Haitz, research and development manager of the semiconductor product group at Agilent. He predicts that in “a few years,” his group will be able to make a 1,500-lumen LED that could sell for $150. He doubts the average homeowner will be rushing out to buy such a product.
LEDs made from AlInGaP are pretty efficient at turning electricity into light. About 90 percent of the electrons that enter the diode generate photons. Unfortunately, this semiconductor alloy also has a high index of refraction (a measure of how much a material bends light rays). Instead of shining out for all to see, therefore, most of the photons bounce around the interior of the diode and turn to waste heat; only 30 percent of them emerge as visible light. GaN has a lower index of refraction, so more light escapes from LEDs made from that material. Only 30 percent of the electricity fed into a GaN device is converted to light in the first place, though, so the final efficiency is still only about 10 percent. That’s plenty bright for something like a traffic light, but not for general illumination.
This is not an insurmountable problem, says George Crawford, chief technical officer at LumiLeds. Researchers there have been experimenting with new structures of the diodes to let more photons escape. Conventional LEDs consist of cube-shaped crystals. But by arranging the layers of the semiconductor differently and cutting the wafers to create sloping sides, LumiLeds has created an inverted pyramid that results in a shorter optical path for the photons to traverse. In the lab, LumiLeds has managed to get half the photons out of an inverted pyramid LED made of AlInGaP, and they hope to have such LEDs in commercial production this year. “Fifty percent is plausible, but hard,” Crawford says, but adds: “It’s hard for me to envision doing much better than that.”
Getting half the photons from AlInGaP may be enough to compete with fluorescent lights, but not by itself. Devices made of that material provide only the red and yellow light. The complementary blue photons needed to produce white light must come from gallium nitride, and there the technology is still embryonic. “We really don’t understand the fundamentals of how to build a better crystal in gallium nitride,” says Steve Johnson, leader of the lighting research group at Lawrence Berkeley National Laboratory.
Researchers are looking for a shot of government funding to help their quest. Arpad Bergh, president of the Optoelectronics Industry Development Association, wants a major R&D effort to bring LEDs to the point where they can compete with traditional light sources. His group is working with Johnson at Lawrence Berkeley to develop a research plan for more efficient white-light LEDs and intends to ask Congress for a five-year, $50 million per year funding program that would start as soon as 2002. Meanwhile, Haitz has written a white paper with Sandia National Laboratories calling for the government to pour $500 million into research over 10 years. Such an expenditure is needed, Haitz argues, to lift LED lighting over the hurdles that are now impeding progress. Left on its own, he argues, the lighting industry will advance LEDs only enough to take about one-tenth of the lighting market by 2025. But with government help, he says, the devices could by that year account for half the market. Because lighting accounts for about 20 percent of the electricity consumed in the United States, a shift to the more efficient LED technology could render significant energy savings.