Seeing into the ultraviolet (UV) would be useful for a variety of purposes, from tracking missile launches to studying distant stars. Silicon-based detectors, however, are not the best means of capturing UV light. To improve the technology, researchers at North Carolina State University (NCSU; Raleigh, NC), working with the US Army Night Vision Laboratory (Ft. Belvoir, VA), have developed a visible-blind UV digital camera based on gallium nitride.
The camera contains a 32 x 32 array of gallium nitride/aluminum gallium nitride (GaN/AlGaN) heterostructure PIN photodiodes. A base layer of n-doped AlGaN with approximately 20% aluminum is topped by an undoped GaN layer and a p-doped GaN layer. The entire structure is built on a polished sapphire substrate through which light can shine. Each photodiode is sensitive to light from approximately 320 to 365 nm. Wavelengths shorter than 320 nm are absorbed by the AlGaN base layer, and those longer than 365 nm pass through the GaN. Increasing the aluminum content in either the base or the top layers can change the photodiode's bandwidth.
"This camera doesn't sense any visible light at all," said Jan F. Schetzina, professor of physics at NCSU. "A visible camera might sense the ultraviolet, but if there were an object that were emitting light or reflecting light, it might all be mixed together."
Although it is visible-blind, the camera is not solar-blind, he stressed. Researchers took their camera outside and pointed it at the sun and had to use neutral-density filters to prevent pixel saturation.
Work, which also included researchers from the Honeywell Technology Center (Minneapolis, MN), was funded by the Army Research Office and the Defense Advanced Research Projects Agency because of the potential defense applications of a UV camera. The military would like a solar-blind UV detector, imaging in the 250-280-nm region, that would be able to track missile plumes without being confused by sunlight. "We haven't been able to do that yet," Schetzina said. "It would be the next step."
A UV camera could also be useful for industrial applications, studying flames or monitoring welding, for instance. And it could help combat the threat of biological weapons. A number of biological agents, if illuminated with a laser at approximately 270 nm, fluoresce in the 300 to 380-nm region, with the specific fluorescence dependent on the particular agent. For example, the nerve gas sarin, released on a subway by a Japanese cult, could be spotted by this method, Schetzina said.
Because the ozone layer absorbs UV radiation from the Sun, the camera could be used to study ozone depletion. NASA is also very interested in the camera, Schetzina said. Ultraviolet astronomy is "another potentially huge applications area," he said. Although photomultiplier tubes can detect UV radiation, they require very high voltage.
His next aim is to build a 128 x 128 array. This work is made possible by the same advances in GaN that have led to growth in the making of laser diodes and high-brightness light-emitting diodes that emit at shorter wavelengths. "What we have demonstrated is that this new material can make a detector that is much more sensitive than silicon," Schetzina said.