We discuss factors affecting the external quantum efficiency, droop and reliability of AlGaN deep ultraviolet (DUV) light emitting diodes (LED) grown on sapphire substrates. Improvement of LED performance is achieved by suppression of the nonradiative recombination in epitaxial structures with dislocation density reduced to below 5x10(8) cm(-2), transparent LED structure design and optimized UV encapsulation for enhanced light extraction. Relatively low light extraction efficiency remains to be a key factor limiting LED output power and quantum efficiency.

In 2011, According to the World Health Organization, roughly 768 million people did not use any improved sources for drinking water while 185 million people relied on only regular surface water in order to meet their drinking water needs. The number of people living in slums with no clean water supply is expected to double by 2030 to 2 billion. Each year in U.S. hospitals there are an estimated 1.7 million healthcare acquired infections (HAI) resulting in approximately 99,000 deaths (2007 data). Deep Ultraviolet Light Emitting Diodes (DUV LEDs) technology can help solve these and a variety of other problems related to public health and wellbeing. In contrast to more conventional UV sources, such as mercury lamps, DUV LEDs do not require any warm up time, are toxic chemical free, and possess the capability to be molded into compact systems. The DUV LED diodes fabricated by SET, Inc. are based on III-Nitride Semiconductors (AlGaN). These devices are capable of providing spectral power distribution with the peak emission wavelengths from 227 nm to 340 nm. A novel compact low flow water purification unit using DUV LEDs demonstrated a 4.15 LOG reduction of viral MS2 bacteriophage and > 6 LOG (99.9999%) reduction of E.coli at 100mL/min flow and 40mW of optical output power. At higher optical output powers, the LOG reduction begins to saturate and taper off. The water disinfection unit incorporated 20 TO-39 packages and was controlled by a single custom power supply. Arrangement of the LEDs insured minimal to no shadowing of influent water. The germicidal efficacy of the system was further enhanced by photon recycling using UV-reflecting chamber walls and multiple passes through exposed water stream. The water disinfection units are geared towards affordable and durable Point-of-Use (POU) drinking water systems. Current modifications are being made in order to further increase the efficacy of the water disinfection units at higher flow rates and lower power requirements. DUV LED technology was also used to design efficient hard surface microbial disinfection systems. Many microbes have become more resistant to everyday chemical disinfectants. Higher dose and more concentrated chemical solutions are needed to the point that they themselves become toxic for humans. Initial testing has produced visual evidence of microbial inactivation on hard surfaces contaminated with E.coli. Testing involved a single SETi TO-39 package with a wavelength of approximately 275nm. A higher than the 2 LOG reduction at approximately 1 mW optical output power was achieved. Based on these results, a 2nd generation system was designed for portable DUV LED cell phone disinfection containing few separate LED chips at 270-275 nm. Initial testing revealed an almost 2 LOG (99%) reduction of E.coli after 30 minutes of exposure at low microbial influent levels.

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

Low-irradiance, long-term UV treatment using DUV-LEDs extended the cold storage shelf life of strawberries up to 2-fold as based on weight, moisture content, anthocyanin concentration, soluble solids, titratable acidity, visible damage, and mold growth.

Deep UV LED structures with UV transparent design, reflective p-electrodes and die encapsulation exhibited external quantum efficiency above 11%. Progress in material growth and device fabrication will be discussed along with issues further limiting efficiency.

Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates. (C) 2012 The Japan Society of Applied Physics

Novel Deep UV-LEDs enabled a portable sensor device for high resolution detection of Ozone in the ppm and ppb range.