Key Takeaways
- Researchers discovered that an ultrathin version of titanium oxide displays spontaneous electrical alignment known as ferroelectricity.
- Compared to standard materials used in logic and memory devices, ferroelectric materials can operate at a much lower voltage.
- Finding ferroelectricity in ultrathin common materials like titanium oxide could offer an energy-efficient way to power microelectronic devices.
Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered that an ultra-thin version of a common material displays an electrical property that could make it a game-changer in the push toward energy-efficient microelectronic devices.
The few-nanometers-thick oxide could be used in microchips while consuming far less energy than conventional materials. Its superpower is its ferroelectricity. That means it not only exhibits a spontaneous internal electrical alignment, but the direction of that alignment can be reversed by exposure to an external electric field. Compared to standard computing hardware, ferroelectric materials can operate or "switch" at a much lower voltage. Finding versions that perform well in ultrathin sizes makes it possible to build more power into a tiny chip without overheating - offering a valuable route toward reducing the energy consumption of next-generation memory and logic devices.
"The fact that a dielectric material can go through a ferroelectric phase transition when thinned down to atomic scales opens up exciting opportunities for both phase transition physics and polar distortion in a completely new class of materials," said Sayeef Salahuddin, who led the study published in Science reporting the new results. Salahuddin is a faculty scientist in Berkeley Lab's Materials Sciences Division and professor of materials science and engineering at UC Berkeley.
In pursuit of ultrathin performance
Ferroelectric materials attain their internal alignment, or polarization, thanks to distortion in their crystal structure. But most materials that are ferroelectric in their bulk form lose that polarization as they get thinner. An additional, practical hurdle to designing ultrathin ferroelectric materials is making them compatible with the standard semiconductor technology used in silicon electronics - a major manufacturing challenge.
"It's all about making devices more efficient and reducing power consumption at ever smaller scales."
– Sayeef Salahuddin
In 2020, Salahuddin's team made a breakthrough demonstration of ferroelectric switching in a thin-film material. In collaboration with researchers at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab, they grew a one-nanometer-thick layer of hafnium oxide on a silicon substrate, at achievably low temperatures. Building on that success, they began to consider other materials that might have similar properties. That led them to titanium dioxide (TiO2), a simple dielectric commonly used as a pigment and UV-filter in paint and plastics.
Theoretical work had predicted that titanium dioxide would become ferroelectric under extremely high pressures that distorted its centrosymmetric crystal structure into an anisotropic form. "In bulk, titanium dioxide is a ubiquitous dielectric with interesting photophysics that we leverage in everything from sunscreens to catalysts. The unusual science happens at the nanoscale, where it becomes ferroelectric," said Archana Raja, a staff scientist at the Molecular Foundry who led optical measurements of the material's structure.
Now it's symmetric; now it's not
Using a technique called atomic layer deposition, Salahuddin and his colleagues synthesized titanium dioxide samples, from one to ten nanometers in thickness, on different substrates. The approach offers exquisite control over atomic positioning and does not require extreme temperatures. The researchers then probed each sample for signatures of electrical polarization: signs of anisotropy in the otherwise isotropic electronic structure.
One form of probing used X-rays at the Advanced Light Source (ALS), a DOE Office of Science User Facility at Berkeley Lab. Examining how the material absorbs linearly polarized X-rays for different polarization directions allowed Salahuddin's team together with Christoph Klewe, a research scientist at the ALS, to tease out anisotropies in its electronic structure. For the thicker samples, the absorption was identical in each orientation, indicating an isotropic electronic environment. But for samples thinner than 3 nm, the absorbed spectra showed clear differences, indicating anisotropy consistent with a polar distortion. "Titanium dioxide acts differently than most other ferroelectrics, exhibiting this distortion at smaller thicknesses. Usually it's the other way around," said Klewe.
The other form of probing used visible light at the Molecular Foundry. In second-harmonic generation (SHG), two same-frequency photons interacting within the material combine to generate a new photon with twice the energy, twice the frequency, and half the wavelength of the original photons, if the material has a particular symmetry associated with a ferroelectric phase. Measurements of the signal intensity - a benchmark for symmetry breaking - revealed a sudden jump for samples thinner than 3 nm. "We're directly measuring the crystal distortion since the atoms are literally displaced during the structural transition," said Raja.
Together the X-ray and optical measurements showed that titanium dioxide lost its centrosymmetric structure and stabilized into a distorted "orthorhombic" structure only in its thinnest versions. "Something happens around 3 nm and you see the new crystal structure," said Raja. Measuring the symmetry-breaking via complementary techniques that are sensitive to different length scales and polarization directions not only confirmed the ferroelectric behavior but led to a multifaceted picture of its emergence.
"We observe increasingly augmented distortion in the lattice as the material is thinned down, which beyond a certain point, induces the ferroelectric phase transition," said Koushik Das, a UC Berkeley graduate student who also led the study.
The work sets the stage for discovering more materials that become ferroelectric in their reduced dimensional forms. Ultimately, researchers hope to build a library of such materials and identify the ones best suited for specific applications. "If you want to create a device, you could pick and choose different materials depending on the device architecture," said Das.
Such materials could one day find a home in memory and logic chips for microelectronics. In the age of AI and power-hungry data centers, that matters more than ever. "It's all about making devices more efficient and reducing power consumption at ever smaller scales," said Salahuddin.
This work was supported by the DOE Office of Science.
