industry UV LEDs
Six challenges to making UV LEDs
There are many challenges to overcome when fabricating UV LEDs. Six of the key ones are:
Managing the strain to enable growth of crack-free, thick, doped AlGaN epitaxial layers. Strain is much higher in DUV LEDs than in their visible counterparts due to larger lattice mismatch between AlN/AlGaN and sapphire.
Figure 3 (a) optical power of single-chip DUV LEDs and DUV LED lamps for CW and pulsed modes (b) UVClean high power LED lamp from SET (c) SET’s UVClean multi-chip lamp with over 100 mW of CW optical power at 275 nm at room temperature (d) SET’s large-area single-chip LED with over 100 mW CW optical output power at 275 nm
Realizing n-type doping in AlGaN with high aluminum composition. AlGaN with more than 50 percent aluminum is required for fabrication of sub-300 nm DUV LEDs. Due to a larger donor activation energy in higher aluminum-content AlGaN, room-temperature electron concentration decreases and sheet resistance increases, leading to severe current crowding effects in the devices.
Reducing concentration of non-radiative recombination centers in AlGaN that are responsible for low internal quantum efficiency.
Catering for the strong polarization effects in the AlGaN-based quantum well active region of DUV LEDs. Fields resulting from polarization can pull apart injected electrons and holes, ultimately reducing the radiative recombination rate.
Realizing a sufficiently high p-type doping of the AlGaN electron blocking or cladding layers, which is much more difficult than it is in p-GaN. Low p-doping efficiency of AlGaN is the main reason for high forward voltages and poor p-contact. Switching to lower aluminum content p-AlGaN or even a p-GaN contact layer is not necessarily beneficial, because it reduces light extraction due to strong UV light absorption in the p-contact layer.
Figure 4 (a) Water shifts the most effective wavelength for water treatement (b) Optimim wavelength for disinfection of potable water has been demonstrated to be 275 nm
Obtaining reasonable light extraction efficiencies. In DUV LEDs, extraction efficiencies tend to be lower than those in InGaN-based devices, due in the main to a combination of strong absorption in p-contact layers and larger internal reflection at the AlN/sapphire interface.
The Department of Defense has a significant interest in DUV LEDs to detect and identify biological and chemical agents that may be in use by enemy forces. Current detect methods require large, heavy equipment and a great deal of power. To address these deficiencies, DARPA’s (Defense Advanced Research Projects Agency) CMUVT program is targeting high performance UV semiconductor devices; 100mW LEDs operating at 250 – 275 nm with 20 percent wall-plug efficiency and 10 mW, 220 – 250 nm laser diodes. Today DUV LEDs address markets such as life sciences and scientific analysis where they are ideally suited for fluorescence and fluorescence lifetime measurements. The characteristics of DUV LEDs, including wavelength
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www.compoundsemiconductor.net January / February 2011
selection between 240 nm and 355 nm, switching speeds in the range of a few nanoseconds, small physical size with high power density and simplified optics and electronics present very large benefits over traditional UV light sources in the same wavelength range. This set of desirable characteristics has driven adoption of LEDs in systems for medical analysis, gas detection and monitoring and medical disinfection. However, with recent advancements in high-power LED lamp technology and the development of high-power, single-chip LEDs,
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