The thinnest ferroelectric material ever paved the way for new energy-efficient devices

The discovery of fascinating small-scale materials behavior could reduce the power requirements for computers.

As electronic devices continue to get smaller, the materials that power them must become thinner and thinner. For this reason, one of the key challenges facing scientists in the development of next-generation, energy-efficient electronics is to discover materials that can retain specific electronic properties at an ultra-thin size.

Advanced materials known as ferroelectrics represent a promising solution to reduce the power consumption of ultra-small electronic devices in mobile phones and computers. Ferroelectrics—the electrical analogue of ferromagnets—are a class of materials in which some of the atoms are arranged off-center, resulting in a spontaneous internal electrical charge, or polarization. This internal polarization can reverse direction when scientists subject the material to external stress. This offers great prospects for ultra-low-power microelectronics.

Unfortunately, conventional ferroelectric materials lose their internal polarization below about a few nanometers in thickness. This means they are not compatible with today’s silicon technology. This problem has so far prevented the integration of ferroelectrics into microelectronics.

But now a team of researchers from the University of California, Berkeley, conducting experiments at the US Department of Energy’s (DOE) Argonne National Laboratory, has found a solution that solves both problems simultaneously, by using the thinnest ferroelectric ever reported and has created the thinnest demonstration of a working memory on silicon.

In a study published in the journal Sciencethe research team discovered stable ferroelectricity in an ultra-thin zirconium dioxide layer just half a nanometer thick. That’s the size of a single atomic building block, about 200,000 times thinner than a human hair. The team grew this material directly on silicon. They found that ferroelectricity arises in zirconia – normally a non-ferroelectric material – when grown extremely thin, about 1-2 nanometers thick.

Remarkably, the ferroelectric behavior continues up to its near-atomic thickness limit of about half a nanometer. This fundamental breakthrough marks the world’s thinnest ferroelectric. This is surprising for a material that is not typically ferroelectric even in its bulk form.

The researchers were also able to switch polarization back and forth in this ultrathin material with a small voltage, enabling the thinnest demonstration of random access memory ever reported on silicon. It also offers significant prospects for energy-efficient electronics, especially considering that conventional zirconia is already present in today’s cutting-edge silicon chips.

“This work is an important step towards integrating ferroelectrics into large-scale microelectronics,” said Suraj Cheema, a postdoctoral researcher at UC Berkeley and the study’s first author.

Visualizing the ferroelectric behavior of such ultrathin systems required the use of Argonne’s Advanced Photon Source, a DOE Office of Science user facility. “X-ray diffraction gives the needed insight into how this ferroelectricity is formed,” said Argonne physicist John Freeland, another author of the study.

Aside from the immediate technological implications, this work also has significant implications for the design of new two-dimensional materials.

“Simply squeezing 3D materials to their 2D thickness limit provides a straightforward yet effective way to unlock hidden phenomena in a variety of simple materials,” said Cheema. “This greatly expands the materials design space for next-generation electronics to include materials that are already compatible with silicon technologies.”

As Cheema noted, simply growing just a few atomic layers of a 3D material can offer the potential for a new class of 2D materials — atomically thin 3D materials — that go beyond traditional layers of 2D materials like graphene. Researchers hope this work will stimulate further research on 3D two-dimensional materials that demonstrate emerging electronic phenomena relevant to energy-efficient electronics.

This work was led by Cheema and Sayeef Salahuddin of UC Berkeley, along with first co-authors Nirmaan Shanker and Shang-Lin Hsu. At Argonne’s Advanced Photon Source beamline 33-BM-C, researchers, in collaboration with Argonne physicists Freeland and Zhan Zhang, used synchrotron X-ray absorption spectroscopy and X-ray diffraction to probe the structural evolution of ferroelectricity at the atomic level and explore electronic origins.

At the DOE’s Lawrence Berkeley National Laboratory’s Advanced Light Source and Molecular Foundry, the ferroelectric crystal structure of the material was studied using soft X-rays and transmission electron microscopy in collaboration with scientists Padraic Shafer and Jim Ciston.