Ultraviolet LEDs put efficiency first
A near-ultraviolet/violet light-emitting diode (LED) has demonstrated a quantum efficiency of 32%, a record result, say researchers from Cree, Inc. (Durham, NC), subsidiary Cree Lighting Co. (Goleta, CA). The indium-gallium-nitride (InGaN) device produces 21 mW of output at 390 nm, based on a 20-mA-drive current. The work is an improvement over July 2000 results that yielded 28% efficiency from a device generating 17 mW at 400 nm. According to Cree's vice president, Mike Dunn, the enhanced efficiency is rooted in a threefold approach. "There's improvement in the quality of the substrate and improvement in the quality of the filmsLED structure and quantum well," he says. "There are also certain chemical and mechanical technologies you can apply to the chip to improve the light extraction."
Increased efficiency is not the only challenge. "Beyond that there's the development of an optical encapsulant that can withstand a direct exposure to UVA light," says Dunn, noting that many materials become opaque at UV wavelengths. "It's very possible that the final LED product may not look anything at all like the traditional LED looks today."
The primary application for such devices is in solid-state lighting applications. By exciting a phosphor with the UV emission, it is possible to produce white light (see oemagazine, February 2001). "People are trying to make a high-efficiency white-light source," says Robert Steele of Strategies Unlimited (Mountain View, CA). "Getting the UV efficiency as high as possible is a major step on the road to doing that. It's a laboratory result, though. This isn't like a production device."
Dunn agrees. "We think we've achieved levels of brightness that are commercially viable," he says. "The issue now is repeatability." Cree's strategy is to market the chip rather than an integrated device. "The companies with the phosphor capability are close to commercializing a tricolor phosphor that's tuned to this wavelength," Dunn says. "At this point, we're going to supply the filament." The group now is performing lifetime testing.
In a related story, Toshiba America Electronic Components, Inc. (Irvine, CA) and Toyoda Gosei (Aisai, Japan) have jointly developed a white LED capable of producing 4.5 to 5 lm/W at 20 mA, with a correlated color temperature of 6500 to 9000 K. Toyoda manufactures the 380-nm-emitting LED, which Toshiba encapsulates in a transparent resin package mixed with a tricolor phosphor. Engineering samples are available this month, with volume production slated for November. "Right now the only people in the world producing white in significant volumes are Nichia and Osram," Steele says. "I think the product will be an entrée into this market for [Toshiba/Toyoda Gosei]."
Devices from Nichia Corp. (Tokushima, Japan) excite yellow phosphors with a blue gallium-nitride-based LED. "Nichia has a pretty strong patent position, so it's kind of a problem," Steele says. In part, patent issues may serve to drive companies to alternative technology. Another argument for the tricolor phosphor is that the two-color Nichia device does not generate a color-balanced white light.
Some claim that the tricolor approach provides better color balancing and improved color stability over time. "We see [the Toshiba/Toyoda release] as a good sign," Dunn says. "It means the industry is converging on a de facto standard."
