Portland, Ore. - Sandia National Laboratories is reporting new levels of wavelength/power output for deep-ultraviolet LEDs, which are considered critical to future semiconductors, biosensors and communications.
Two deep-UV semiconductor LEDs registered output power of 1.3 and 0.4 milliwatts at wavelengths of 290 nanometers and 275 nm, respectively. Both were described as record-setting marks.
"You need certain power levels to achieve the sensitivity necessary for bioagent detection and optical communications," said Bob Biefeld, manager of Sandia's Chemical Processing Science Department. "Everybody is competing to make high-power UV LEDs like ours. But even though we have broken the world's record, we are not standing still. We are pushing for even higher powers and to make even better materials. We plan to investigate all the various aspects of the materials science of growing these devices." Also key to the project were researchers Jerry Simmons, Mary Crawford, Andy Allerman and Art Fischer.
"From decontamination to white-light generation, semiconductor-based ultraviolet (UV) light sources will have many applications, including an important role in America's security," said Jerry Simmons, manager of the Semiconductor Material and Device Science Department at the laboratories.
For instance, he said, to detect weaponized anthrax instead of naturally occurring organisms in the environment, a UV source needs to focus on the sample to excite fluorescence from special tagging. With LEDs as the UV source, the detectors can now become disposable and small enough to be worn as tags.
Non-line-of-sight communications gear for the military can also use UV LEDs, to bounce short-wavelength communication bursts off an aerosol so that UV transceivers can communicate with zero probability of interception and near-zero probability of detection.
Also, the conventional fluorescent bulb has a UV source inside and a phosphor coating that converts that to longer visible wavelengths. UV LEDs can be similarly outfitted with phosphor or more-modern coatings to create solid-state light bulbs. Similar benefits of miniaturization can be reaped by water and air purifiers that switch to UV LED light sources, as well as by polymer-curing and chemical-processing equipment.
Visible light spans from extreme red (infrared above 700 nm) to the extreme blue (UV below 400 nm). Deep UV is even shorter than UV-less than 300 nm. Traditionally, high-power and deep UV have not mixed, but Sandia has managed it through a novel mix of technologies.
The devices have a sapphire substrate with conductive layers of aluminum gallium nitride, which emit at lower frequencies in inverse relationship to how much aluminum is in the mix.
To shorten the wavelength to below 300 nm, as much as 50 percent aluminum must be used, making the material more brittle and resistive. The biggest processing problem with mixing so much aluminum, Biefeld said, is that excessive defects degrade the efficiency of the device.
"One of the major problems with these materials is that they are full of defects," he said. "We plan on reducing defects even more, and we are working on a bunch of different approaches to do that."
The group also used a flip-chip geometry to increase both output and thermal efficiency. Instead of the standard top-emitting LED, the die is flipped upside down and bonded onto a thermally conducting submount. The finished LED is a bottom-emitting device that uses a transparent buffer layer and substrate.
"We make contacts on it from the opposite side from where it emits, so you don't have to have transparent contacts," said Biefeld. "The light from an LED comes out in all directions, so when you bring it out through the surface, some of it gets reflected back, but our sapphire substrates are clear so we can shine it through the bottom of the chip."
Researchers Kate Bogart and Art Fischer developed the flip-chip packaging for the bottom-emitting UV LED. Bogart said the thermal efficiency boost from emitting through the sapphire substrate, which acts as a heat sink, is as important as the upside-down geometry. "The light is two times brighter when the LEDs are in a 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 submount 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."
The researchers worked for more than three years on the projects, the last one-and-a-half with funding from the Defense Advanced Research Projects Agency's project Suvos (for semiconductor ultraviolet optical source). Suvos funds U.S. research groups to develop compact deep-UV semiconductor optical sources for biological-agent detection systems such as the UV fluorescence-based Lidar (light detection and ranging).
"One of our long-term Darpa program goals was to reach 1-mW powers," said Crawford. "Our team has gone beyond that."
Crawford and Fischer will continue to characterize the new UV LEDs and determine exactly how best to put them to use. Other Darpa programs are integrating the devices into both non-line-of-sight communications and biosensor test beds.
The semiconductor process scientists will also look to improve the device's characteristics by refining its parameters and trying new materials. "The contacts can be improved, the active layers can be improved and the nucleation can be improved as well," Biefeld said.