At the 11th International Symposium on Semiconductor Light Emitting Devices (ISSLED 2017), PhD student David Laleyan and visiting scholar Xianhe Liu both won best student poster awards for their work showcasing new techniques for creating deep ultraviolet (UV) LEDs. Zetian Mi, professor of electrical and computer engineering and co-chair of ISSLED 2017, advises both students.
Deep UV LEDs have many current and potential applications, including air and water sterilization, industrial curing, printing, counterfeit detection, hazardous material decomposition, and many medical uses. Today’s deep UV LEDs, however, lack the efficiency and power of the visible wavelength LEDs that consumers buy as light bulbs and mobile phone screens.
The material quality is near-perfect by doing it in this manner.Xianhe Liu
Liu’s work, titled “Deep ultraviolet AlGaN Nanowire Light Emitting Diodes by Selective Area Epitaxial Growth,” can help increase the performance of deep UV LEDs by using a titanium mask as they are built.
To create an LED, tiny towers of atoms called nanowires are grown on a substrate using molecular beam epitaxy (MBE). Heat is applied to atoms in a vacuum, and the atoms are then shot like bullets and accumulate on the substrate. The typical process, however, can develop irregularities in the structure of the nanowires. To solve this, Liu applied a titanium mask to the substrate, allowing the atoms to accumulate only in certain places.
“The material quality is near-perfect by doing it in this manner,” says Liu. Compared to the typical nanowire LED structure, in which the nanowires can be tilted and randomly misshapen, Liu’s nanowires are like a perfect beehive.
“This can have all kinds of positive effects because the light won’t be trapped in the irregularities, and it can all be emitted,” Laleyan adds. “As more research develops around deep UV LEDs, the efficiencies will multiply because the starting point is better.”
Laleyan’s poster, titled “AlN/h-BN Nanowire Heterostructures for Deep Ultraviolet Photonics,” describes his work on using boron nitride to create more powerful deep UV LEDs.
“LEDs are built as a sandwich,” explains Laleyan. “The first layer provides the negative charge carriers, the electrons, the third layer provides the positive charge carriers, called holes, and the center is the active region where light is emitted.”
For the third layer, deep UV LEDs normally use aluminum gallium nitride (AlGaN) with small amounts of magnesium to provide the positive charge. “But magnesium is not ideal, in fact it’s only borderline functional,” says Laleyan.
In place of AlGaN with magnesium, Laleyan experimented with a layer of boron nitride on top of aluminum nitride. He found the boron nitride LED required less voltage, was able to withstand twice the current, and had ten times the light output as similar aluminum nitride nanowire devices.
“With boron you could get completely rid of magnesium and its problems,” says Laleyan.
Earlier this year, Prof. Mi was elected a Fellow of both SPIE, the international society for optics and photonics, and OSA, the Optical Society, in part for contributions to the development of high performance nitride nanowire photonic devices and deep UV lasers.