23 April 2012

SETi reaches UV-C LED external quantum efficiency of 11%

Ultraviolet light-emitting diode maker Sensor Electronic Technology Inc (SETi) of Columbia, SC, USA has achieved what it reckons are record efficiencies for UV LEDs operating in the germicidal UV-C range of 11% external quantum efficiency (EQE) with a corresponding wall-plug efficiency (WPE) of 8%. The result was achieved under the US Defense Advanced Research Projects Agency (DARPA) Compact Mid-Ultraviolet Technology (CMUVT) program and in collaboration with the US Army Research Laboratories (ARL).

SETi says that its latest development represents more than a five-fold improvement in performance, due mainly to improved light extraction from encapsulated LED chips with a novel transparent p-region and a reflective contact and further reductions in defect density in the LED structure, grown on sapphire substrates.

Traditionally, UV LEDs have been manufactured with gallium nitride (GaN) p-type layers, due to the difficulties of p-doping AlGaN materials. However, GaN absorbs light with wavelengths shorter than 365nm, and hence reduces the extraction efficiency of UV LEDs operating at short wavelengths. So, instead, SETi has developed a completely new p-type region using doped AlGaN, which is transparent, even in the UVC range. This coupled, with a transparent p-contact, significantly increases extraction efficiencies.

SETi has also further developed its migration-enhanced metal-organic chemical vapor deposition (MEMOCVD)  growth process to reduce dislocation densities in the quantum well structure of UV LEDs grown on sapphire substrates, demonstrating threading dislocation densities (TDD) of less than 2x108, as measured by transmission electron microscopy (TEM). This improvement in TDD has led to a high internal quantum efficiency (IQE) of 60%. 

The milestone represents "a major step forward in reaching efficiencies of incumbent technologies, such as medium-pressure mercury vapor lamps, which typically operate at efficiencies of 15% or less,” says SETi's president & CEO Dr Remis Gaska.

The 350µm x 350µm encapsulated LEDs were designed for emission at 278nm and measured at ARL, where they were reported to emit 9.8mW at 20mA (the highest output power ever reported for an LED with a wavelength shorter than 365nm at this forward current) and 30mW at 100mA. Details will be presented at the IEEE Photonics Society's Conference on Lasers and Electro Optics (CLEO 2012) in San Jose, CA, USA (6-11 May).

SETi has previously reported LED-based water treatment systems that disinfect (with a more than 4 log reduction) drinking water flowing at 0.5 liters per minute with less than 35mW of optical power at 275nm. SETi reckons that the results from its latest development put UV LED disinfection systems within reach of consumer markets for applications such as point-of-use water purification, reckons SETi.

Tags: Sensor Electronic Technology UV LEDs

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