ASU Bisgrove Scholar illuminates the future of LED lighting

We encounter light-emitting diodes on a daily basis — the indicator lights on our smartphones, on the screens of our flat-panel TVs and in the latest energy-efficient light bulbs. Yuji Zhao thought he could make them work better and do more than simply shine a light.

The electrical engineering professor at Arizona State University proposed that by developing “smart” LEDs, the devices could heal wounds and even replace existing Wi-Fi technology with light-based “Li-Fi” wireless communication capable of 10,000 times higher capacity bandwidth.

In 2015, this was a risky proposal involving a new and relatively untested area of research in photonics and LEDs. Luckily for Zhao, his first proposal as an ASU faculty member was accepted by the Science Foundation Arizona Bisgrove Scholar program, which funded his research.

The Bisgrove Scholar program aims to attract and retain notable, early career faculty members in Arizona who have “the potential to transform ideas into great value for society.”

“This program recognized the high scientific merits and potential impact of our proposal and made this exciting research a reality,” said Zhao, a faculty member in the Ira A. Fulton Schools of Engineering.

In the years since, Zhao has made great strides on the frontier of LEDs in three areas.

First, Zhao and his team investigated the theoretical limits of LED efficiency and device structure bottlenecks in order to create a new and better structure that is less prone to efficiency “droop” and the limitations inherent to green LED light. Zhao says the solution lies in producing high-quality indium gallium nitride, or InGaN, with a low amount of defects and impurities. Green LED light is especially problematic because it requires InGaN with more indium, which is difficult to synthesize and is typically low quality with many defects. 

Next, they looked into using light rather than wireless radio frequency to send signals — Li-Fi versus Wi-Fi.

“What we propose is using LED light to transmit a signal by modulating a LED light at a very high speed to send binary signals,” Zhao explained.

Finally, they started to develop a LED or photonic structure in the ultraviolet light range that would be useful for a number of medical applications. Zhao and his research team are looking into creating integrated chips — devices that include circuits beyond just the LEDs — for new biomedical applications.

“The breakthrough we’re achieving now is we’re producing a particular photonic integrated chip that can work at the UV wavelengths, which can be used for a lot of medical applications,” Zhao said, which include biosensing and wound treatment.

The results of Zhao’s Bisgrove Scholar project have yielded 20 papers published in leading research journals and in the publications of international conferences.

Zhao’s overarching summary paper of smart LED research and related work also led to “one of the first comprehensive papers on the new frontier of smart LEDs, and it provides important information and valuable insights into the future of LED research,” the researcher said.

The summary paper, “Toward ultimate efficiency: progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes,” was published in the journal Advances in Optics and Photonics, which is ranked second out of 92 journals for impact in optics and photonics research.

The paper will likely serve as a source of seminal LED research and be referenced for years to come because it highlights the fundamental efficiency limits and possible future directions for LED technology.

The smart LED research builds on Zhao’s work in gallium nitride, or GaN, wide-bandgap semiconductors, and also expands his work on GaN to new projects supported by the Advanced Research Projects Agency-Energy, the Defense Threat Reduction Agency and NASA.

“The success of our Bisgrove Scholar program LED research enabled us to leverage our results on LEDs and expand the research of GaN to other important fields such as power electronics and space technologies,” Zhao said.

This compelling research was also made possible by Zhao’s mentors — David J. Smith, an ASU Regents’ Professor of physics, electrical engineering Professor Yong-Hang Zhang and Professor and Director of the School of Electrical, Computer and Energy Engineering Stephen Phillips, who provided advice, leadership, ideas, resources, teaching and laboratories including the ASU NanoFab facility directed by Zhang.

Nine ASU faculty members and postdoctoral scholars have been named Bisgrove Scholars by Science Foundation Arizona since 2011.