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RIKEN Makes Record Bug-busting UV LED

Teaming with Matsushita Electric Works, the Japanese research institute has revolutionized InAlGaN LED design to make the highest output power device in the 280 nm region.

RIKEN has made a 282 nm LED that operates with 10.6 mW output power, which it says is the highest value ever recorded at this wavelength for a continuous wave UV-LED.

The breakthrough was reported by Hideki Hirayama at the International Symposium on Growth of III-Nitrides in Izu, Japan, on July 7. The Japanese research center has collaborated with Matsushita Electric Works to further develop its quaternary InAlGaN quantum wells and improve the output of its LEDs.

"We obtained remarkable emission enhancement due to reducing the internal piezoelectric field in the quantum well, and [cutting] oxygen impurities by silicon doping," Hirayama told compoundsemiconductor.net. The presence of oxygen in the InAlGaN layers can act as centers for non-radiative recombination, Hirayama said.

The approach used to silicon-dope the LED's InAlGaN barrier layers also improved the crystal quality in the quantum well (QW) they surround, by providing it with an atomically-flat surface to grow on.

The QW itself was grown at 0.03 µm/h - one quarter of the standard rate - which Hirayama said improved the crystal quality so much that its emission intensity was boosted six-fold.

The final device's output power at 100 mA and room temperature was 5.5 mW. It reached its output peak at around 275 mA, and its maximum external quantum efficiency was 1.2 percent.

260-280 nm light is used to sterilize water and surfaces, and a 10 mW 280 nm source can kill a colon bacillus from a 20  cm distance within a minute. Hirayama says that the record-setting LED could be useful for portable pen-shaped sterilization lights.

The previous record-holding device was an 8 mW, 280 nm packaged flip-chip LED produced by Asif Khan's group at the University of South Carolina.

"Our LED was measured as a bare wafer and [with] no heat sink," said Hirayama, "so much higher output is expected if we use flip-chip LED geometry."

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