industry UV LEDs
A very small proportion of the world’s water is suitable for drinking
environmental impact (although the mercury based UV lamps do need to be disposed of frequently).
Using UV light to disinfect drinking water is well established. It’s an approach that has been used for nearly one hundred years, and is recognized by many organizations around the world including the US Environmental Protection Agency (EPA). Wavelengthsranging from 240 nm to 280 nm can attack the DNA of micro-organisms, destroying their genetic information and preventing their reproductive capability. Without the ability to reproduce, these micro-organisms are rendered harmless when consumed by humans.
estimated the two out of every three people do not have access to toilets or latrines and over 90 percent of wastewater is discharged back into the water system without any treatment. This practice spreads fatal, water- related diseases such as cholera, hepatitis, dengue fever and other parasitic diseases that can be attributed to many of the four million deaths and many more disabilities. Reducing these deaths by producing potable water from contaminated water is not cheap: Today a typical small community treatment system costs upwards of $5 million. So a dire need exists for a low cost-of-operation, easy-to- use, sustainable water-cleaning system.
One of the most effective methods for treating unsafe water is UV disinfection. As a physical, chemical-free disinfection process, it has much to recommend it: It is easy to use, with no danger of over dosing; unlike chemical disinfection, it requires very little contact time; it does not require storage of hazardous materials; there are no toxic by-products; and the process itself has little or no
The major drawback of current UV systems is associated with their mercury lamps. These are bulky, fragile, require regular maintenance, have limited lifetime and present a disposal issue.
Recent developments in III-Nitride wide bandgap semiconductor technology demonstrate promising results to overcome these shortages of conventional UV light sources. A very attractive alternative is the AlGaN-based LED, which has peak emission wavelength shorter than 365 nm. These deep UV LEDs, or DUV LEDs, promise to enable the production of UV disinfection systems that will equip families and individuals with water purification systems that can provide a sustainable source of safe drinking water.
The UV spectrum can be sub-divided into four ranges: UV-A (320 nm – 400 nm); UV-B (290 nm – 320 nm); UV-C (200 nm – 290 nm); and vacuum UV (40 nm – 200 nm). The very longest of these wavelengths can be reached with active regions that pair GaN with InGaN, but for wavelengths of 365 nm or below, a combination of AlGaN and GaN must be employed.
Figure 1. Normalized room- temperature electro-
luminescence (EL) spectra of DUV LEDs with peak emissions at 235 nm, 250 nm, 255 nm, 265 nm, 280 nm, 295 nm, 305 nm and 320 nm
38
www.compoundsemiconductor.net January / February 2011
At Sensor Electronic Technology, Inc. (SET), a company based in Columbia, SC, we have been pioneering the development of these UV sources. Thanks to these efforts, we were the first company to the market with a product line spanning the spectral range 240 - 355 nm (see Figure 1).
DUV LEDs are far more difficult to make than their blue and white cousins, due to issues related to strain, doping, efficiency and polarization (see “Six barriers to making UV LEDs”, p.40). To overcome these challenges, we have developed two new epitaxial growth techniques: migration enhanced (ME) MOCVD and migration enhanced lateral epitaxial overgrowth (MELEO). These deposition technologies can decrease dislocation densities by orders of magnitude, which in turn enables the growth of thick, high-quality AlGaN, AlInGaN, AlInN and AlN epitaxial
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