Light Work

Hikers know that taking a drink from even a pristine mountain stream can make them sick from bacteria and parasites. Chemicals (such as chlorine, bromine, or iodine) used in municipal water supplies and hiking kits to combat water-born microbes make water taste bad.

New, deep-ultraviolet light-emitting diodes (UV LEDs) produced by Sensor Electronic Technology (SET; Columbia, SC), using a manufacturing process developed with MDA SBIR funding, may allow the use of UV light to sterilize water in commercial applications. In fact, today one SET customer has already patented a UV LED-based portable sterilization device for hikers and soldiers.

Using UV light to kill bacteria is more efficient and potentially healthier than using chemicals. However, the adoption of UV light systems has been limited because most current systems rely on high-voltage mercury discharge lamps that guzzle energy, need high-voltage electronics, and contain hazardous mercury. In addition, the lamps burn out after about 8,000 hours and only emit certain fixed-UV wavelengths.

BRIDGING THE GAP
SET did not start out with the goal to make UV LEDs, according to Dr. Remis Gaska, president and CEO. “We were focused on developing material and devices for high-power RF applications, such as next-generation MDA radars.” To make the necessary electronics, SET developed a new epitaxial growth technique they call migration enhanced metal-organic chemical vapor deposition (MEMOCVD™, pronounced memo-CVD).

“We believe MEMOCVD bridges the gap between MOCVD [metal-organic chemical vapor deposition] and MBE [molecular beam epitaxy] because it allows you to grow layers at much lower temperatures than conventional MOCVD… temperatures comparable to what is used in MBE,” Gaska said. The new technique improves the resulting material’s quality allowing the formation of abrupt, high-quality heterostructures (regions of non-uniform composition), accurate control of thickness and composition, and higher wafer uniformity. These add up to a dramatic improvement in the performance and reliability of the electronics that are produced. In addition, the process works with 4-inch diameter wafers, reducing manufacturing costs and increasing manufacturing throughput.

SET researchers realized that this technique could be applied to making blue and UV LEDs. The blue LED market was already competitive, but there were no UV LED suppliers because they require the addition of aluminum gallium nitride (AlGaN). According to Gaska, “It’s a real technological challenge to grow high-quality, high-aluminum-content AlGaN materials.”

BIO-AGENT DETECTION
Although SET continues to conduct research for MDA, DARPA began funding SET’s UV LED development for possible use in handheld bio-agent detectors. Not only would UV LEDs not need high-voltage power supplies or equipment designs to handle excess heat, but LEDs could be made to emit the exact wavelength that best excites the fluorescent markers that bind to biological molecules. “We made a breakthrough last summer; our group improved the device performance by a factor of 15 or 20,” Gaska recalled. “We became the first and sole supplier of these deep-UV LEDs with peak emission below 365 nm.”

SET is working with many new customers to explore the commercial potential of its UV LED technology. Applications include municipal water and sewage, analytical equipment, sensor calibration, and even secure wireless communication. One day, UV LEDs might even replace the mercury tubes in fluorescent lights for general lighting. Gaska said, “There is some design cycle and adjustment cycle needed, but we clearly see it’s a really huge market.”