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Optics graduate student repeats as international award winner

Hongping Zhao, left, Ph.D. candidate in the Center for Optical Technologies, and her adviser, Nelson Tansu.

Hongping Zhao, a graduate student who works on many levels to make the generation of light more economical and energy-efficient, has won a premier international scholarship for the second year in a row.

Zhao, a Ph.D. candidate in electrical engineering, is receiving the Scholarship in Optical Science and Engineering from SPIE, the International Society for Optical Engineering. SPIE is the world’s largest nonprofit professional society for optics, photonics and imaging.

The award recognizes Zhao’s efforts to grow semiconductor materials, and to model, design and fabricate LEDs (light-emitting diodes) with nanostructures that are engineered to generate visible light more efficiently for energy-efficient solid state lighting technology.

LEDs are used in the dashboards, headlights and tail lights of cars, and in cell phones, traffic lights, billboards, laptops and TVs. They consume less energy and last longer than incandescent lightbulbs, and they have the potential to exceed the efficiency and reliability of fluorescent lighting as well.

Lighting accounts for almost 22 percent of electricity usage in the U.S. It is estimated that high-efficiency LED solid-state lighting can cut the nation’s total electricity consumption by more than 15 percent.

Zhao, a native of Suzhou in China’s Jiangsu Province, has published 45 refereed journal and conference articles, including 35 since January 2007, when she enrolled at Lehigh. She holds an M.S. in electrical engineering from Southeast University in Nanjing, China, where she was ranked first in her graduating class of 97 students, and a B.S. in physics from Nanjing Normal University, where she finished second out of 245 students.

At Lehigh, Zhao works in the Center for Optical Technologies (COT) and is advised by Nelson Tansu, associate professor of electrical and computer engineering.

A green Achilles’ heel

LEDs emit light from the “active region” of a semiconducting nanomaterial such as indium gallium nitride (InGaN) or gallium phosphide (GaP). This region, called a quantum well, is only several nanometers thick (1 nm is one billionth of a meter).

InGaN emits light in the blue and green portions of the spectrum, while InAlGaP (indium aluminum gallium phosphide) emits light in the red spectrum. A white LED must mix red, blue and green in the correct proportion to produce white light. Green LED light, however, is inefficient relative to blue LED light emission, and it thus limits the overall “radiative efficiency” of white LED light.

Zhao hopes to improve the efficiency of green LED light by optimizing the structure of the quantum wells that generate the light. The inefficiency of green light emission from the InGaN quantum well, she says, is caused when electron and hole carriers inside the quantum wells become spatially separated, causing a lower probability that electrons and holes will recombine and generate photons.

By nano-engineering the shape of the quantum wells with staggered InGaN quantum wells, Zhao is attempting to align the electron and hole carriers more precisely and thus reduce the spatial separation and improve the efficiency of light generation.

The idea of staggered quantum wells and other nano-engineering steps was originally proposed by Tansu’s group at Lehigh. The approaches have been patented by Tansu and his graduate students. Ronald A. Arif ’08 Ph.D., a member of Tansu’s group who worked on blue LED light, grew the staggered nanostructure by varying the flux rate of the reactant species flowing to the epitaxy reactors in the COT’s Smith Family Laboratory. Inside the reactor, the gases deposit the semiconducting materials, which are grown on sapphire substrates.

Zhao utilizes a different approach, engineering the growth temperatures at which deposition and growth of the nanostructure layers occur. She has also developed accurate numerical simulation models to help determine the optimized nanostructures that would result in the best performance for LEDs and lasers. This modeling requires significant knowledge in quantum mechanics and semiconductor physics. Zhao also works on other novel approaches to achieve enhancement in spontaneous emission rate in nanostructures for LED applications.

“I had to read a lot of papers and ask a lot of questions when I first came to Lehigh,” she says. “I had to understand the problems and gain the necessary knowledge before I could work on new ideas or approaches.”

“Our graduate students are expected to excel in both theory and experiment skills,” Tansu says. “Hongping has progressed very significantly during her years at Lehigh.”

A full-service lab

Nanostructure and photonics research require knowledge and expertise from multiple disciplines in science and engineering. In addition to theoretical works, Zhao and her colleagues in Tansu’s group optimize the experimental growth of nanostructures with a great degree of precision. Experiments are carried out in state-of-the-art growth reactors, and group members are trained to maintain the reactors and supporting equipment.

That equipment gives Lehigh’s COT a critical advantage over most other university labs: the ability to accomplish in one building all the steps—simulation, growing materials, and fabrication, device testing and material characterization—required to make a semiconductor photonics device from concept to prototype.

The COT’s facilities include two MOCVD (Metalorganic Chemical Vapor Deposition) epitaxy reactors, a cleanroom, etching tools, dielectric and metal evaporators, light-resistant lithography equipment, and optoelectronics device testing. A variety of optical and electron microscopy techniques are available on campus to characterize, or measure the properties of, the materials and devices.

With these facilities, says Zhao, Lehigh’s optics researchers can perform all their experiments at Lehigh without having to interrupt tests to send specimens to other labs for other fabrication or characterization steps.

“The availability of our research facilities at Lehigh,” she says, “enables us to be much more efficient.”

After she completes her Ph.D. at Lehigh, Zhao hopes to pursue a research and teaching career.

“I want to continue doing research in photonics and nanotechnology,” she says. “Hopefully I will be able to guide and inspire graduate students to work at the frontiers of these fields.

“I believe if one works hard enough and is persistent, nothing is impossible.”

--Kurt Pfitzer

Posted on Tuesday, June 23, 2009

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