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Research Review: SET Makes Brighter Ultraviolet Chips

A US collaboration led by Sensors Electronic Technology (SET) claims to have raised the bar for power output from a single ultraviolet LED chip. The team, which includes researchers from Rensselaer Polytechnic Institute and the US Army Research Laboratory, has produced a 273 nm chip emitting 30 mW, and a 247 nm version delivering 6 mW.


 




SET’s president, Remis Gaska, says that the longer wavelength chip could be used for water sterilization and disinfection. Higher output powers enable an increase in water flow rate, and could lead to a cut in the cost of treatment per liter.



“Although DNA absorption peaks at 265- 268 nm, the combination of DNA absorption and wavelength-dependent UV light attenuation in the water shifts the maximum germicidal efficiency to 270-275 nm," says Gaska. His 273 nm chip is in the center of this range.



The shorter wavelength, 247 nm LED is a promising source for water treatment and it could be combined with hydrogen peroxide for chemical decontamination.



The US team realized record output powers from its UV LED chips by reducing built in strain, improving n-type and p-type doping, and minimizing dislocations and point defects. Thanks to these advancements, the researchers could increase the area of their chips without sacrificing quantum efficiency at low current densities.



Epitaxial structures were produced on sapphire via a combination of MOCVD and migration-enhanced MOCVD, and processed into devices with junction areas of 0.5 mm2 and 1 mm2.



LED chips emitting at 273 nm that have a junction area of 1 mm2 showed an increase in external quantum efficiency as the drive current ramped to several hundred milliamps. At 700 mA, the CW output was limited by the heat dissipation from the junction of the TO-3 package. 247 nm LEDs with a chip area of 0.5 mm2 produced a peak CW output of 6 mW at 275 mA.



The researchers have also studied the reliability of their LEDs. The output power of a 273 nm , 0.5 mm2 chip driven at 400 mA (400 μs pulses, 10 percent duty cycle) halved after about 5000 hours of operation. Device degradation was caused by a change in the electrical properties of the ptype layers, which hampered electrical injection, leading to a cut in output power.



M. Shatalov et al. Appl. Phys. Express 3 062101 (2010)

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