Army Advances in UV, LED Technology Lead to Lower Costs, Higher Capabilities for Soldiers
June 24, 2010
Improvised buildings in remote areas can give the U.S. military indispensable combat advantage, but cost millions of dollars to operate. But going 'green' in constructing forward operating bases in desert locations just may be the right cost-saving solution.
The U.S. Army Research Laboratory's advancements in III-Nitride sources and detectors have enabled the creation of novel solid state lighting (SSL) technology, which could offer potential cost savings for theater operations. The energy-saving technology has provided better performance than typical incandescent lighting, which costs more and requires more energy to run. SSL technology also generates even more heat within forward operating bases' typical airfield, hospital and administrative offices.
"LEDs are more compact, weigh less, have longer lifetimes, are more rugged, and are already more efficient than current lighting technology, which makes them ideal for insertion into U.S. Army Natick Soldier RD&E Center and U.S. Army Tank Automotive Research, Development and Engineering Center applications" said Dr. Meredith L. Reed, lead researcher for SSL efforts on the Nitride Semiconductor Optoelectronics Team within ARL,s Sensors and Electronic Devices Directorate. Reed's research efforts have focused on novel device structures and device physics of III-Nitrides for improved performance of detectors and sources for biodetection, communication, water monitoring and purification, and energy efficient green photonics.
"LEDs are also more efficient in military vehicles, offering high-efficiency cabin and bumper lighting " especially when the engine is off - and conserve battery energy," she added.
In 2009, Reed was awarded ARL Director's Research Initiative funding to exploit the negative polarization properties of InGaN/GaN heterostructures to achieve frequency doubled blue-green lasers with deep UV (< 250 nm) emission. Her expertise in the design, growth, characterization, fabrication, and device physics of III-Nitride optoelectronic devices resulted in the invention of a novel LED, as reflected in a May 2009 provisional patent. The effort also led to Reed's participation in the National Academy of Engineering's 2009 U.S. Frontiers of Engineering Symposium.
Reed teamed with Dr. Paul Hongen Shen, a senior scientist and ARL Fellow, and Dr. Michael Wraback, a senior researcher and ARL Fellow. Shen brought expertise in device physics and modeling to further develop and better control the electronic properties at the heterointerfaces within these devices, while Wraback provided institutional knowledge in III-nitride and U/V LEDs to further advance the fundamental understanding of the polarization charge at the heterointerface, and develop the technology into other military applications.
Reed is leveraging expertise and technology as the primary investigator and technical leader on a three year, $1.8 million Department of Energy project to address the efficiency droop at high current densities, which currently limits the luminous efficacy of solid state lighting.
More specifically, she said the project will address the detrimental effects of the positive spontaneous and piezoelectric polarization charge within these materials to develop SSL technology to meet DOE luminous efficacy requirements. Existing lighting technologies consume about 22 percent of U.S. electricity per year, and it is expected that solid-state lighting will reduce energy consumption for lighting by a factor of three to six times.
The trouble with LEDs is twofold. First, as current density climbs higher, its light input peaks out, meaning that it can only get so bright before it starts to self dim. Further, as its current density climbs, so too does the heat emitting from it, which Reed said is a problem for Soldiers because it means introducing more heat into austere combat locations like forward operating bases or vehicles.
"Currently all nitride LEDs suffer from efficiency droop at the high current densities required for lamps used in general illumination for solid-state lighting," explained Reed. "It is known that conventional LEDs suffer from carrier leakage and poor hole injection as a result of positive polarization charges at the heterointerfaces within these devices."
She added that industry researchers have attempted to address the problems associated with the polarization charge by adding a heteroepitaxial electron-blocking layer which, although moderately successful for some commercial applications, still results in LEDs that suffer from efficiency droop.
Reed's novel alternative LED design bridges the technological gap and solves the efficiency droop problem. Her device reaches peak efficiency at a current density five times larger than conventional devices and experiences only one-eighth the efficiency droop.
"This device design represents a significant discovery in the field of InGaN based lighting sources that promises to revolutionize this field by enabling previously unattainable high current density LED operation," Reed noted.