20 Nov 2003
Scientists engineer a device that emits 1.3 mW at 290 nm, and another producing 0.4 mW at 275 nm.
Researchers at Sandia National Laboratories in the US say that they have demonstrated two record-breaking deep-UV semiconductor optical devices. The first produces 1.3 mW of output power at a wavelength of 290 nm, and the second produces 0.4 mW at 275 nm.
“Emission wavelengths of 275 to 290 nm with such high power outputs is a major breakthrough in UV LED development,” said Bob Biefeld, manager of Sandia's Chemical Processing Science department. “Only a handful of research groups around the world have come anywhere close.” Both devices emit light in continuous-wave mode under DC operation.
Sandia’s LEDs are based on a sapphire substrate with conductive layers of AlGaN. The more aluminum that is added to the semiconductor material, the shorter the output wavelength becomes, but with increasing aluminum content the material becomes much harder to grow and drive electrical current through. The mix that reached the 275 nm wavelength is around 50 percent aluminum.
Also contributing to the advance, says Sandia, is a smart packaging technology that uses flip-chip geometry. Instead of the standard top-emitting LED, the LED die is flipped upside down and bonded onto a thermally conducting sub-mount. The finished LED is a bottom-emitting device that uses a transparent buffer layer and substrate.
Having the device emit light from the bottom serves two purposes, says Kate Bogart who together with Art Fischer developed the advanced packaging at Sandia.
“First, the light is two times brighter when the LEDs are in the flip-chip geometry, primarily because the light is not physically blocked by the opaque metal contacts on the top of the LED,” Bogart said.
“In addition, the flip-chip sub-mount pulls heat away from the device because we make it out of materials with high thermal conductivity. This improves efficiency levels with less energy getting converted to heat and more to light.”
According to Biefeld, another unique aspect of the device is that the high power output of 1.3 mW at 290 nm is obtained in a continuous-wave mode.
“That was a continuous wave power measurement under direct current operation, not a pulsed current measurement like other UV LED groups have reported,” he said.
Operating at the shorter UV wavelengths could lead to miniaturized devices that can detect biological agents, perform non-line-of-sight (NLOS) covert communications, purify water, cure polymers and other chemicals, and decontaminate equipment.
The LED devices are already being supplied to DARPA program participants making both non-line-of-sight communications and bio-sensor test beds.
Michael Hatcher is editor of Compound Semiconductor magazine.