TECHNOLOGY AREA(S): Human Systems
OBJECTIVE: To demonstrate high efficiency deep ultraviolet (UV-C) light sources at 219 nm and 265 nm spectral peak wavelengths for use in instrument sterilization and water purification systems. Reliable and efficient LEDs are desired that exceed 15% wall-plug efficiency.
DESCRIPTION: An efficient light source emitting at ~265 nm has been identified to be the most efficient wavelength for disinfection application . To date, mercury vapor lamps have been predominantly used for these applications, which, however, are inefficient and produce mercury emissions to air, soil and water. In 2005 alone, over 2,000 tons of mercury was emitted to the environment, more than 10% of which was from mercury containing products such as mercury lamps. The need of a high efficiency, mercury-free solid-state deep ultraviolet lamp is therefore clear and urgent. AlGaN semiconductors have direct energy bandgap in the range of 6.2 eV to 3.4 eV, which have emerged as the material of choice for light emitters in the deep ultraviolet spectral range. To date, however, the highest external quantum efficiency for light emitting diodes (LEDs) operating at ~265 nm is limited to ~2%, or less . Moreover, the wall-plug efficiency is generally below 1% for LEDs operating at wavelengths ~ 265 nm. Many factors contribute to the poor efficiency, including the presence of defects and dislocations in the device active region, the inefficient current conduction in wide bandgap AlGaN, and the unique TM polarization of light emission in Al-rich AlGaN. Recently, significant progress has been made to address these challenges. For example, AlGaN nanostructures can exhibit significantly reduced dislocation densities and enhanced current conduction. Deep ultraviolet light sources have been demonstrated with the use of these nanostructures [3-5]. This call seeks innovative proposals to develop efficient and compact deep ultraviolet solid-state lamps operating at ~265 nm. The devices should operate at a wall-plug efficiency level >15%, i.e. ~5-10 times better than the current state-of-the-art. The devices should exhibit long-term stable operation and deliver output power exceeding 100 mW required for effective and rapid disinfection. Moreover, the device size and weight should be considerably smaller than the conventional mercury lamps and should not contain any significant toxic materials. The final deliverable should include a fully packaged device with detailed testing results, including efficiency, output power, and estimated lifetime. Significant improvements over state-of-the-art efficiencies are sought. Nanostructured materials and light emitting active regions will be given strong consideration over standard approaches. However, while nanostructures are highlighted within this topic as a desirable method to improve material quality and light extraction efficiency, other approaches will be considered if effective at improving the current state-of-the-art in LED performance.
PHASE I: To demonstrate light emitting diodes operating at ~265 nm with efficiency >5%, and to further determine the technical feasibility for achieving a wall-plug efficiency >15% (for 219 nm the efficiency can be much less, approximately an order of magnitude). With the efficiencies mentioned, power output goals (minimum) of 20 mW and 2 mW at 265 nm and 219 nm, respectively should be achieved at 500 mA drive current. The size of the LEDs should be 0.5x0.5 mm. Detailed analysis of the predicted performance needs to be developed. A particular interest for improvement of wall-plug efficiency will be light extraction efficiency improvements. Assessment of LED possibilities at shorter wavelengths around 219 nm should also be made to include wall-plug efficiency estimates by the end of phase II work. The goal of the shorter wavelength LEDs is aimed at enhanced water purification and sterilization capabilities.
PHASE II: To develop, test, and demonstrate a prototype LED lamp operating at ~265 nm with efficiency >15%, and > and to further perform preliminary lifetime analysis. Power output goals (minimum) that should be achieved, with the efficiency goals mentioned for 265 nm (and much less for 219 nm), are 120 mW and 20 mW at 265 nm and 219 nm, respectively for 500 mA drive current (again, 0.5x0.5 mm dimensions). The LED lamp should be fully packaged and ready for field testing. Reliability and assessment of further efficiency improvements possible should be made to develop further phase III plans. Alternative wavelengths such as 219 nm should be pursued secondarily at some level for use in more thorough water purification scenarios (to remove other toxins for certain types of ground water). Design consideration of LEDs for uses for instrument sterilization and water purification systems should be made. In particular, light extraction illumination patterns of water or instruments to be sterilized. The need for reflectors or dispersers for uniform omnidirectional illumination should be assessed from a packaging and system perspective. Other wavelengths in the solar-blind spectral region such as 275 nm would be a third spectral band of interest for sensor systems. Goals for the spectral band should meet or exceed the 265 nm band regime.
PHASE III: Continue UV LED development with long-term reliability testing and efficiency improvements. Manufacturable epitaxial crystal growth and fabrication processed should be refined and developed for useful military water purification and surface sterilization products for scalable levels of throughput (both portable and larger systems are of interest). Requirements from USAMMA PMO-MD for health support roles of care 1-3 will be brought to play for relevant systems to replace current sterilization or water purification systems that suffer from large size, weight and cost issues. Commercial uses in water purification should also follow suit. Studies on the exact requirements of UV wavelengths and power levels can be made in accordance with Army and other medical purification and sterilization requirements. Follow-on uses for the LEDs in biomolecule sensors should be possible.
1: S. Vilhunen, H. Sarkka, and M. Sillanpaa, "Ultraviolet light-emitting diodes in water disinfection," Environ. Sci. Pollut. Res. Int., vol. 16, pp. 439-42, 200
2: H. Hirayama, N. Maeda, S. Fujikawa, S. Toyoda, and N. Kamata, "Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes," Jap. J. Appl. Phys., vol. 53, p. 100209, 201
3: S. Zhao, S. Y. Woo, S. M. Sadaf, Y. Wu, A. Pofelski, D. A. Laleyan, et al., "Molecular beam epitaxy growth of Al-rich AlGaN nanowires for deep ultraviolet optoelectronics," APL Mater., vol. 4, p. 086115, 201
4: S. Zhao, A. T. Connie, M. H. Dastjerdi, X. H. Kong, Q. Wang, M. Djavid, et al., "Aluminum nitride nanowire light emitting diodes: Breaking the fundamental bottleneck of deep ultraviolet light sources," Sci. Rep., vol. 5, p. 8332, 201
5: Q. Wang, A. T. Connie, H. P. Nguyen, M. G. Kibria, S. Zhao, S. Sharif, et al., "Highly efficient, spectrally pure 340 nm ultraviolet emission from AlxGa1-xN nanowire based light emitting diodes," Nanotechnology, vol. 24, p. 345201, 201
KEYWORDS: Light Emitting Diode, LED, Deep Ultraviolet, AlGaN, Water Purification, Disinfection, Semiconductor, Solid-state Lamp, Nanowire, Nanostructures, Light Extraction Efficiency