Researchers at Sandia National Laboratories developing ultraviolet (UV) light-emitting diodes (LEDs) recently demonstrated two deep UV semiconductor optical devices that set records for wavelength/power output.
One emits at a wavelength of 290 nanometers (nm) and produces 1.3 milliwatts of output power, and the other emits at a wavelength of 275 nm and produces 0.4 milliwatts of power.
‘Emission wavelengths of 275 to 290 nm with such high power outputs is a major breakthrough in UV LEDs 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.’
Operating at the shorter UV wavelengths makes it possible to build miniaturised 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 Sandia team is part of the US Defense Advanced Research Projects Agency (DARPA) project SUVOS (semiconductor ultraviolet optical source), which funds various research groups around the US to develop deep UV compact semiconductor optical sources.
The team reached the 275-290 nm breakthrough a two months ago after working on similar research for more than three years.
‘When we started the project, one group had been clearly leading the field for several years without any other group making much progress,’ said Andy Allerman, the Sandia scientist who leads the growth of the LED semiconductor material. ‘Our LED performance increased rapidly the past year and recently we reached record power at 290 nm and 275 nm.’
The device has a sapphire substrate with conductive layers of aluminium gallium nitride. Adding aluminium to the semiconductor material shortens the output wavelength. But with increasing aluminium content the material becomes much harder to grow and harder to flow electrical current through it. The mix that reached the 275 nm is around 50 percent aluminium. A key step in achieving the high powers was getting high-quality material growth at these high aluminium percentages.
Also contributing to the advance is a smart packaging technology that has a flip-chip geometry. Instead of the standard top-emitting LED, the LED 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.
Having the device emit light from the bottom serves two purposes, said 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 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 result is a device that is low-weight, small, and resistant to vibrations and shock.
Conventional UV sources are mercury vapour and other types of discharge lamps which are bulky, heavy, and power hungry, which is completely different from the new LEDs developed at Sandia that are no bigger than one square millimetre.
Biefeld said that another aspect of the device that makes it unique is that the high power output of 1.3 milliwatts at 290 nm is obtained in a continuous wave (CW) mode.
‘That was a continuous wave power measurement under direct current (DC) operation, not a pulsed current measurement like other UV LED groups have reported,’ Biefeld said. ‘We were able to control the heat issue to reach these powers in CW mode.’
While Crawford and Fischer continue to characterise the new UV LEDs and determine exactly how they can be used, LED devices are already being supplied to DARPA program participants making both non-line-of-sight communications and biosensor test beds.