At 3 nanometers, ordinary titanium dioxide breaks its symmetry and turns ferroelectric

What happens when an ordinary material is pushed down to atomic-scale thickness? A team of researchers tested this idea using titanium dioxide (TiO₂), and what they found next was totally unexpected. The normally non-ferroelectric material became ferroelectric when its thickness fell below 3 nanometers. This transformation is tied to a structural change inside the material. As the TiO₂ films became thinner, their crystal lattice became increasingly distorted, eventually breaking the symmetry of the structure and producing a switchable electrical polarization. The ferroelectric phase persisted down to just one nanometer, roughly twice the size of a TiO₂ unit cell. This matters because ferroelectric materials can reverse their polarization with an external electric field and can operate at relatively low voltages, making them attractive for energy-efficient memory and logic. “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,” Sayeef Salahuddin, one of the study authors and a professor at the University of California (UC) Berkeley, said. Squeezing TiO₂ down to the atomic scale Ferroelectricity often disappears as materials become thinner, while integrating ultrathin ferroelectrics with silicon electronics remains challenging. To explore a different solution, the researchers took a different route. Instead of shrinking an existing ferroelectric, they asked whether a normally non-ferroelectric material could become ferroelectric when made sufficiently thin. The approach builds on the team’s 2020 demonstration of ferroelectric switching in a 1-nanometer-thick hafnium oxide film grown directly on silicon. That result prompted the researchers to look for other materials that might show useful ferroelectric behavior at atomic-scale thicknesses. They chose TiO₂, a widely used dielectric in semiconductor technologies and a material found in products such as pigments and UV filters. “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,” Archana Raja, one of the study authors and a staff scientist at Lawrence Berkeley National Laboratory, said. The researchers wondered whether reducing its thickness could trigger a similar distortion without applying extreme pressure. Using atomic layer deposition, they produced TiO₂ films from 1 to 10 nanometers thick at temperatures below 400°C. The films could be formed on silicon as well as amorphous surfaces including silicon dioxide and carbon. Such ultrathin dimensions are increasingly important as electronics approach the nanoscale, where researchers are exploring transistor designs below 4 nanometers. The 3-nanometer tipping point The decisive change appeared below about three nanometers. At Berkeley Lab’s Advanced Light Source, the researchers used linearly polarized X-rays to probe the films’ electronic structure. Thicker samples absorbed the X-rays similarly in different directions, consistent with their symmetric structure. Below 3 nanometers, however, the absorption became direction-dependent, indicating anisotropy associated with a polar distortion. The researchers confirmed the symmetry breaking using second-harmonic generation (SHG) at the Molecular Foundry. The SHG signal suddenly increased below three nanometers, providing another signature of the structural change. “We’re directly measuring the crystal distortion since the atoms are literally displaced during the structural transition,” Raja added. They also tested the ferroelectric response using piezoresponse force microscopy (PFM), polarization-electric-field measurements, and positive-up-negative-down (PUND) measurements. These electrical tests supported the conclusion that the structural distortion was accompanied by genuine ferroelectric behavior. Together, the measurements showed that ultrathin TiO₂ changed from its normal centrosymmetric structure into a distorted orthorhombic structure. The lattice distortion became stronger as the films grew thinner, and the ferroelectric phase survived at just one nanometer. A new way to find ferroelectrics The finding could provide another route toward ultrathin materials for energy-efficient memory and logic. Since TiO₂ is already familiar to semiconductor manufacturing, the discovery could eventually help enable denser, faster, and lower-power devices without requiring an entirely new fabrication approach. Researchers are also investigating other materials for low-power computing. However, this is not yet a finished electronic technology. The study demonstrates the ferroelectric phase and its formation on silicon and amorphous surfaces, but practical devices will require further testing of switching reliability, endurance, and performance. The researchers now want to find other materials that might develop useful properties when reduced to very small dimensions. Their broader goal is to build a library of such materials and match them to different device architectures. The study is published in the journal Science. 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