Short-wavelength LEDs at high-power output
Researchers at Sandia National Laboratories developing ultraviolet light-emitting diodes 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 mW of output, and the other emits at a wavelength of 275 nm and produces 0.4 mW.
Operating at the shorter UV wavelengths makes it possible to build miniaturised devices that can detect biological agents, perform non-line-of-sight 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 known 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.
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.
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