Fabrication of high performance UVC LEDs on aluminum-nitride semiconductor substrates and theirpotential application in point-of-use water disinfection systems

Abstract

Water disinfection has always been a focal point of public discourse due to both the shortage of water and the abundance of water-transmitted diseases. According to the World Health Organization (WHO), there are over 3.4 million reported deaths annually due to water, sanitation, and hygiene-related issues [1]. There are a number of technologies being developed to fight against issues related to waterborne diseases. The use of UV light is gaining popularity over chlorine disinfection due to a lack of aftertaste and harmful by-products after treatment. Specifically, UV radiation in the wavelength range of 250-280 nm (UVC) has been shown to effectively disinfect water. Current UVC technology, which uses low and medium pressure mercury lamps, is hampered by the use of fragile quartz housings, long warm up times, and the toxicity of mercury [2]. There is tremendous development happening in the field of semiconductor-based UVC LED technology. This technology can be efficient, cost-effective, and an environmentally friendlier alternative to traditional UVC technology. Emerging AlxGa1−xN and AlN-based UVC light-emitting diodes (LEDs) provide many advantages over mercury lamps— including design flexibility, low power consumption, and environmentally friendly construction [3]. Unlike low-pressure mercury lamp technology, which is limited to an emission wavelength near 254 nm, LEDs can be tailored to specific wavelengths throughout the UVC range. UVC LEDs are being developed for disinfection in the 265 nm wavelength range and are showing tremendous progress in power output and device lifetime [4]. This progress has been driven by the relatively recent development of high quality, single-crystal AlN substrates. These AlN substrates allow the growth of pseudomorphic AlxGa1−xN device layers with very low defect densities. These low defect densities have resulted in improvements in efficiency and power as discussed in this chapter. In addition, UVC LEDs emit radiation in a very different pattern than mercury lamps or other UVC sources that they are replacing. For instance, the LEDs can be designed to emit in a “Lambertian” pattern, allowing the LED to be imaged as a near point source. Using UVC LEDs in water disinfection requires substantial rethinking in the arrangement of UVC radiation sources to achieve an efficient system. In this chapter, we will discuss some of the important parameters necessary for successful flow cell design. The chapterwill show the design flexibility offered by UVC LEDs, with some examples of potential designs examined through optical modeling. We will also review some of the recent progress in improving UVC LEDs through pseudomorphic growth of AlxGa1−xN on single crystal AlN substrates.