Proximity effect: Method allows advanced materials to gain new property

(Press-News.org) UNIVERSITY PARK, Pa. — Ferroelectrics are special materials with polarized positive and negative charges — like a magnet has north and south poles — that can be reversed when external electricity is applied. The materials will remain in these reversed states until more power is applied, making them useful for data storage and wireless communication applications.

Now, turning a non-ferroelectric material into one may be possible simply by stacking it with another ferroelectric material, according to a team led by scientists from Penn State who demonstrated the phenomenon, called proximity ferroelectricity.

The discovery offers a new way to make ferroelectric materials without modifying their chemical formulation, which commonly degrades several useful properties. This has implications for next-generation processors, optoelectronics and quantum computing, the scientists said. The researchers published their findings today (Jan 8) in the journal Nature.

“This work shows we can generate ferroelectricity in a material that does not have those properties just by stacking it with a material that is ferroelectric,” said Jon-Paul Maria, professor of materials science and engineering at Penn State and lead author of the study. “And, so, it has to be that the two materials are talking to each other. We call it proximity ferroelectricity because it is an effect of being in contact.”

In recent years, scientists at the University of Kiel in Germany and at Penn State have developed new families of nitride and oxide ferroelectric materials — respectively — with comparable properties but with much simpler structures and preparation methods that can be integrated directly into mainstream semiconductors, like silicon, thus maximizing the technology impact, the scientists said.

The new work builds on those discoveries, demonstrating a method to create similar materials but without needing the chemical modifications previously required for fabrication, the scientists said.

“The community got very excited in the last few years about two new emergent families of ferroelectrics that show very promising future impacts on electronic devices,” Maria said. “This is now another step in that process. It’s a second time that we’ve been stunned about what we did not know about ferroelectricity after 100 years of research.”

Maria and his team previously developed one such ferroelectric material that offers enticing performance but requires trade-offs: magnesium-substituted zinc oxide thin films. The zinc oxide has desirable properties, but it is not ferroelectric by itself. Adding magnesium allows scientists to make the material ferroelectric but degrades important properties such as heat dissipation during device operation and the ability to transmit light over very long distances.

Using proximity ferroelectricity, the researchers found they could now turn pure zinc oxide ferroelectric by stacking it with a ferroelectric material like the magnesium-substituted zinc oxide thin films.

“Imagine that I have the ability to stack these layers on top of each other, where one is ferroelectric and the other is normally not, but through proximity ferroelectricity,” Maria said. “It can exhibit the polarization reversal in its pure state. That’s the real appeal.”

In addition, the ferroelectric layer can represent as little as 3% of the total volume of the stack, meaning the vast majority is material with the most-desired properties. The ferroelectric, or switching layer, can be placed on the top or bottom or as an isolated internal layer, the scientists said.

The researchers observed proximity ferroelectricity in oxide, nitride and combined nitride-oxide systems, suggesting that there is a generic mechanism and that the technique could provide new avenues for ferroelectric property engineering and material discovery.

Maria said the work only scratches the surface of what’s possible with the technique and that future research should explore other possible compositions.

The technology could be especially useful for developing next-generation optics applications for electronics. A major challenge in computing involves finding ways to use less energy — and one option is changing the way processors talk to each other using light instead of electronics, Maria said.

“And a big part of that may be this next generation of opto-electronic materials,” Maria said. “Our findings could be one candidate. Alternatively, this could mean that other enabling materials are already known, and exciting functional properties like ferroelectric switching just need unlocking using this proximity effect.”

Also contributing from Penn State were: Chloe Skidmore, a graduate student and lead author; Devin Goodling, John Hayden, R. Jackson Spurling, Steven Baksa, Albert Suceava, and Joshua Nordlander, who contributed as graduate students; and Joseph Casamento, a post-doctoral scholar. Contributing faculty members include Susan Trolier-McKinstry, Evan Pugh University Professor and Steward S. Flaschen Professor of Ceramic Science and Engineering; Venkatraman Gopalan, professor of materials science and engineering and physics; and Betul Akkopru-Akgun, assistant research professor in the Materials Research Institute;

Also contributing were: Elizabeth Dickey, distinguished professor, Ismaila Dabo, professor, and Sebastian Calderson, special faculty, at Carnegie Mellon University; Andrew Rappe, professor, and Drew Behrendt, doctoral student, at the University of Pennsylvania; and Kyle Kelley, research and development associate at Oak Ridge National Laboratory.

The U.S. Department of Energy, the U.S. National Science Foundation and the Department of Defense provided support to researchers on this project.

END