Researchers detail new methods for exploiting hafnia’s ferroelectric characteristics to improve high-performance computing.
A group of scientists and engineers has spent the past ten years working to use an elusive ferroelectric material known as hafnium oxide, or hafnia, to introduce the next age of computing memory. A published study in the Proceedings of the National Academy of Sciences discussed progress toward making bulk ferroelectric and antiferroelectric hafnia accessible for use in various applications, spearheaded by the University of Rochester’s Sobhit Singh.
Hafnia displays ferroelectric properties in a specific crystal phase, meaning it has electric polarization that can be altered in one direction or the other by applying an external electric field. This attribute can be utilized in data storage technology. When employed in computing, ferroelectric memory has the advantage of non-volatility, meaning it maintains its values even when powered off, one of several benefits over most types of memory used today.
The Promise of Ferroelectric Memory
“Hafnia is a very promising material due to its practical applications in computer technology, particularly for data storage,” says Singh, an assistant professor in the Department of Mechanical Engineering. “Currently, to store data we use magnetic forms of memory that are slow, require a lot of energy to operate, and are not very efficient. Ferroelectric forms of memory are robust, ultra-fast, cheaper to produce, and more energy-efficient.”
But Singh, who conducts theoretical calculations to predict material properties at the quantum level, says that bulk hafnia is not ferroelectric at its ground state. Until recently, scientists could only get hafnia to its metastable ferroelectric state when straining it as a thin, two-dimensional film of nanometer thickness.
Progress in Material Science
In 2021, Singh was part of a team of scientists at Rutgers University that kept hafnia at its metastable ferroelectric state by alloying the material with yttrium and rapidly cooling it. However, this approach had some disadvantages. “It required a lot of yttrium to reach that desired metastable phase,” he says. “So, while we achieved what we were aiming for, at the same time we were hindering many of the material’s key features because we were introducing a lot of impurities and disorder in the crystal. The question became, how can we reach that metastable state with as little yttrium as possible to enhance the resulting material’s properties?”
In the new study, Singh computed that by applying significant pressure, one could stabilize bulk hafnia in its metastable ferroelectric and antiferroelectric forms—both of which are appealing for practical applications in next-generation data and energy storage technologies. A team led by Professor Janice Musfeldt at the University of Tennessee, Knoxville, conducted the high-pressure experiments and showed that, at the predicted pressure, the material converted into the metastable phase and remained there even when pressure was removed.
“This is as an excellent example of experimental-theoretical collaboration,” says Musfeldt.
The new approach required only about half as much yttrium as a stabilizer, thereby considerably improving the quality and purity of the grown hafnia crystals. Now, Singh says that he and the other scientists will push to use less and less yttrium until they figure out a way for producing ferroelectric hafnia in bulk for widespread use.
And as hafnia continues to draw increasing attention due to its intriguing ferroelectricity, Singh is organizing an invited focus session on the material at the upcoming American Physical Society’s March Meeting 2024.
Reference: “Structural phase purification of bulk HfO2:Y through pressure cycling” by J. L. Musfeldt, Sobhit Singh, Shiyu Fan, Yanhong Gu, Xianghan Xu, S.-W. Cheong, Z. Liu, David Vanderbilt and Karin M. Rabe, 24 January 2024, Proceedings of the National Academy of Sciences.
Funding: DOE/US Department of Energy, Gordon and Betty Moore Foundation, Office of Naval Research, National Science Foundation