Development breakthrough for UV LEDs

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 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.

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 device has a sapphire substrate with conductive layers of aluminium gallium nitride. It is well know that the more aluminium added to the semiconductor material, the shorter 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% aluminium. A key step in achieving the high powers was getting high-quality material growth at these high aluminium percentages, considered to be very difficult.

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, says Kate Bogart who together with Art Fischer developed the packaging.

"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 contact on the top of the LED," Bogart says. "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 - the new LEDs are no bigger than one square millimetre.

Bob Biefeld, manager of Sandia's Chemical Processing Science department says 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 says. "We were able to control the heat issue to reach these powers in CW mode."