New material approach could lead to lower-power devices

Scientists at the Department of Energy's Oak Ridge National Laboratory have shown for the first time that ferroelectricity can be directly written into aluminum nitride using a tightly focused helium ion beam at the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science user facility at ORNL. Ferroelectric devices don't need constant power to store data, which allows for devices that are more reliable and less power consuming than what's currently available.

The study, published in Advanced Materials, represents a new processing approach for wurtzite III-V nitrides, a class of semiconductors already widely used in microelectronics but whose ferroelectric potential has only been recognized since 2019.

"Today, both the material and the processing method are already employed in chip manufacturing: aluminum nitride is widely used in many 5G and Wi-Fi devices, and helium ion beams are common tools to make tiny changes to circuits," said Bogdan Dryzhakov, an ORNL postdoctoral research associate at CNMS. "What's new is putting them together to 'write' ferroelectric regions where we want them. That means chip makers wouldn't need to adopt a new material or a new manufacturing step - we're using what they already have in a new way."

Ferroelectric materials possess a built-in electric polarization that can be reversed by applying an external voltage. This reversible switching action between two polarization states can be used to store information, much like a digital switch represents binary data.

Traditional ferroelectric research and theory use the "softening" of a material's crystal structure to reduce the energy required to switch. In these ferroelectrics, defects are atomic disruptions in the crystal lattice, the regular repeating arrangement of atoms. Defects are considered undesirable because they raise the energetic cost of switching and increase electrical leakage.

But aluminum nitride belongs to a new family of ferroelectrics called wurtzite nitrides, and it behaves differently from traditional ferroelectric crystals. "Much of ferroelectric theory has been built around soft-mode materials, where the entire crystal lattice participates in the switching process," Dryzhakov said. "In aluminum nitride and other wurtzite III-nitrides, defects allow one-dimensional channels to switch independently of each other. It's a different way of thinking about how ferroelectric switching occurs in materials."

To test that idea, the researchers used a helium ion beam about 1 nanometer wide, small enough to target features with near-atomic precision. The beam creates carefully placed defects without breaking the overall crystal.

"We were able to expose aluminum nitride to profoundly large doses of helium ions, and it still maintained its crystalline structure," said ORNL R&D Associate Kyle Kelley, who also works at ORNL's CNMS. "That tells you something fundamental about the radiation tolerance of these wurtzite materials."

The team found that the treated aluminum nitride needed about 40 percent less energy to switch its polarization. They also saw a stronger piezoresponse, meaning the material mechanically deformed more when an electric field was applied. That electromechanical effect is useful for devices that convert electrical signals into motion or sound and vice versa, such as radio-frequency filters and resonators widely used in wireless communication hardware. Because polarization switching is the basis of ferroelectric memory, enabling it at accessible voltages in aluminum nitride using manufacturing compatible with today's mainstream silicon chipmaking could help scale more robust memory for demanding applications while strengthening U.S. leadership in advanced microelectronics.

Computer simulations and lab measurements suggest the key is how the defects affect switching. Instead of forcing the entire crystal to flip at once, defects allow narrow columns of atoms, like tiny vertical "threads" running through the material, to reverse their electrical direction on their own. The bulk crystal stays intact, while these thread-like regions do the switching work.

Because aluminum nitride is already common in standard chip manufacturing, the result may be easier to translate into real devices than many lab-only materials. The researchers also say the finding could widen the search for ferroelectrics, since it hints that other materials might switch in unexpected ways when defects are controlled rather than avoided.

A provisional patent has been filed for the ion-irradiation method, which selectively introduces defects while preserving the material's overall structure. The work was supported by DOE programs including the Center for 3D Ferroelectric Microelectronics and used capabilities at CNMS.

UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science works to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Scott Gibson