Hafnia-based ferroelectrics could help make future memory devices smaller, faster and more energy efficient. Unlike many conventional ferroelectric materials, they can retain ferroelectric properties when scaled down to only a few nanometers and are compatible with standard CMOS technology. But there is a problem: the thinner the ferroelectric film becomes, the harder it is to maintain strong and stable polarization — particularly when the device gets hot.
Researchers at Fudan University have now shown that the solution may lie in the grains that make up the material. By controlling how these grains form and grow, the team created ~5-nanometer-thick hafnia-based ferroelectric films that combine strong polarization with remarkable stability at temperatures as high as 225 °C.
“Making the ferroelectric layer thinner is only part of the challenge,” says Yingfen Wei, a corresponding author of the study at Fudan University. “At this scale, how the crystal phase and grains develop becomes increasingly important. Our results show that grain structure is a key factor in maintaining stable ferroelectricity at high temperatures.”
Going thinner
Hafnia-based materials are particularly interesting because their ferroelectricity survives at thicknesses where many conventional ferroelectrics lose this property. However, ferroelectricity in hafnia is associated mainly with a particular polar crystal phase. As the films become thinner, this phase competes with non-ferroelectric phases during crystallization.
At the same time, thinner films usually contain smaller grains and therefore more grain boundaries. These boundaries can become sites where defects accumulate. At elevated temperatures, the defects become more mobile and can interfere with polarization switching and gradually reduce memory stability.
Growing better grains
The Fudan team approached the problem from the interface beneath the ferroelectric film. The researchers combined an ultrathin TiN bottom electrode with a titanium oxygen-scavenging interfacial layer. Together, they changed the environment in which the hafnia-based film crystallizes and guided the formation of the ferroelectric phase. The resulting 5.5-nanometer-thick hafnium zirconium oxide (HZO), films contained unusually large and oriented ferroelectric grains. Although the film itself was only a few nanometers thick, the grains extended laterally to approximately 80 nanometers.
First-principles calculations helped explain the effect. The titanium layer changes how oxygen vacancies form and move near the interface, making the transformation from the nonpolar tetragonal phase to the polar orthorhombic phase more favorable. In this way, the interface does more than simply support the film: it helps determine which crystal phase forms and how the ferroelectric grains grow.
Why grains matter
Large grains mean fewer grain boundaries. That matters because grain boundaries can act both as pathways for defect movement and as sites where defects accumulate. Reducing their density therefore helps suppress defect-related degradation. Grain orientation is also important. Three-dimensional multidomain simulations showed that larger and better-oriented grains produce a more uniform local electric field and reduce defect accumulation at grain boundaries. As a result, ferroelectric domains are less likely to become pinned during repeated switching or prolonged exposure to high temperatures.
Taking the heat
The improved grain structure translates directly into device performance. The HZO films show strong ferroelectricity, reaching a remanent polarization, 2P r , of approximately 62 μC/cm 2 without the electrical “wake-up” cycling commonly required in hafnia-based ferroelectrics. More importantly, the devices remain stable at high temperatures. At 225 °C, they withstand more than 100 million polarization-switching cycles and retain their memory state for at least 1000 hours, with negligible polarization degradation. The devices also show strong resistance to partial-voltage disturb pulses, which can interfere with neighboring cells in high-density selector-less memory arrays.
The findings suggest that controlling its crystal phase, grain size and grain orientation is equally important for maintaining reliable operation. By linking interface engineering, grain structure and defect behavior, the study provides a strategy for combining ultrathin dimensions, strong polarization and high-temperature reliability in hafnia-based ferroelectric memories. The approach could ultimately help enable high-density, low-voltage memory systems for electronics operating in demanding thermal environments.
Science Bulletin
Experimental study