First quantum oscillations observed in gallium nitride holes

Gallium nitride, a semiconductor that can operate at high voltages, temperatures and frequencies, has enabled technologies from LED lighting to high-power electronics. Now Cornell researchers have observed a quantum property of the material for the first time, an advance that could expand its technological reach.

Much of gallium nitride's value as a semiconductor lies in how quickly negatively charged electrons move through the material. But the material could become even more useful if scientists better understood its positively charged "holes," which behave like mobile pockets of missing electrons but have been difficult to study. Understanding how to control the flow of the holes - as engineers have achieved in silicon semiconductors - would allow gallium nitride to reach its full potential.

In a new study published March 23 in Nature Electronics, researchers report the first observation of quantum oscillations of holes confined in a sheet - called two-dimensional hole gas - at the interface of gallium nitride and aluminum nitride. These oscillations act as a probe of electronic structure, revealing crucial material properties such as effective mass.

"An important enabler of our studies is our ability to grow high-quality crystals with almost perfect lattices and very few defects," said Chuan Chang, lead author and doctoral student in the Jena-Xing Labat the Cornell Duffield College of Engineering. "That level of quality produced record-high hole mobilities, allowing the oscillations to emerge."

The researchers also relied on extremely high, pulsed magnetic fields available at the National High Magnetic Field Laboratory's Pulsed Field Facility at Los Alamos, and the development of electrical contacts that could operate reliably in cryogenic environments down to 2 kelvin.

Using those tools, the researchers were able to make measurements that provided a direct view of the valence band structure of gallium nitride, revealing key details about lighter holes that move relatively quickly and heavier holes that move more slowly, among other findings.

"Despite a half-century of gallium nitride research, no one until now had been able to observe quantum oscillations of holes in gallium nitride," said Huili Grace Xing, the William L. Quackenbush Professor who directs the research lab with Debdeep Jena, the David E. Burr Professor. Both are professors in the Department of Materials Science and Engineering and in the School of Electrical and Computer Engineering. "It's allowed us to understand a lot of the transport phenomena, mass, band structure - all very useful for engineering device design."

A blend of foundational science and engineering

The observation of quantum oscillations builds on several recent research papers from the Jena-Xing Lab that gradually mapped out the behavior of holes in gallium nitride, including the first discovery of the two-dimensional hole gasand light holes, and measurements of how fast the holes move. The line of research reflects the lab's broader philosophy of pursuing both fundamental physics and device engineering simultaneously.

"It is not very common that within a relatively small research group you can have this cycle of fundamental research as well as technological development," Jena said. "That's one of our strengths and a distinguishing feature of our work."

The group now hopes to use the new insights to work toward the design of semiconductor devices that combine the advantages of wide-bandgap materials with the charge-transport capabilities of silicon.

"We want to see if we can push the mobility of holes in gallium nitride even higher," said Joseph Dill, doctoral student in applied and engineering physics and co-author of the study. "That's the next direction we're moving toward now that we're equipped with this knowledge of what these effective masses are and what the band structure looks like."

Beyond improving transistor design, the research is enabling new opportunities for exploring quantum phenomena in wide-bandgap semiconductors.

"This is the first time this type of experiment has been done with holes in gallium nitride," Chang said. "We've introduced the material into this playground of quantum oscillation studies that wasn't possible before."

The research was supported by the Superior Energy-Efficient Materials and Devices Center, the Semiconductor Research Corporation sponsored by the Defense Advanced Research Projects Agency, the U.S. National Science Foundation, the U.S. Department of Energy and the U.S. Army Research Office.

Syl Kacapyr is associate director of marketing and communications for Duffield Engineering.