Ultrathin titanium dioxide film exhibits surprising properties that could advance semiconductor technology
Close-up view of a semiconductor wafer. (Image by iStock)
The growing popularity of electronic devices — from fitness trackers and laptops to smartphones — is driving demand for more energy-efficient computing chips. Now, researchers have found a way to change the electronic properties of a common semiconductor material, potentially laying the foundation for faster, lower-power data storage and processing.
In a study published in Science, a UC Berkeley-led team of researchers discovered they can transform titanium dioxide (TiO₂) into a ferroelectric material by reducing its thickness to less than 3 nanometers (nm), roughly the diameter of a single strand of human DNA. These findings, according to the researchers, could open a pathway toward ultra-scaled, energy-efficient electronic devices.
Ferroelectric materials, with their ability to switch electric polarizations, have a long history in the semiconductor industry. Today, many researchers believe that they may hold the key to enabling next-generation, energy-efficient nanoelectronics, including non-volatile memory, logic devices and emerging computing technologies.
But achieving robust ferroelectric behavior in an ultrathin material — an important factor in the miniaturized world of semiconductors — has posed a major obstacle. Another hurdle has been finding a ferroelectric material that can integrate well with existing silicon-based technologies.
To address these challenges, researchers from UC Berkeley, Lawrence Berkeley National Laboratory and SLAC National Accelerator Laboratory took a closer look at TiO₂. This material is widely used in modern computing chips as a dielectric, meaning it stores electrical charge but doesn’t demonstrate any electric polarization. By simply making TiO₂ ultrathin, the researchers found that they could change its electronic properties.
According to Sayeef Salahuddin, professor of electrical engineering and computer sciences and of materials science and engineering, these findings demonstrate how engineering materials at an atomic-scale thickness can unlock unexpected physical phenomena.
“We were quite surprised when we discovered that as the thickness of TiO₂ films dropped below 3 nm, the material becomes ferroelectric, a phase in which it shows spontaneous electric polarization that can be switched by applying an electric field,” said Salahuddin, the study’s principal investigator and a senior faculty scientist at Lawrence Berkeley National Laboratory. “More importantly, this new ferroelectric behavior remains stable in films as thin as about 1 nm, approximately two unit-cells thick.”
Based on this study, reducing a material’s thickness might provide a new way to control ferroelectricity and phase transition. Their findings also suggest, said Salahuddin, that other common dielectric materials of this class, commonly known as binary oxides or fluorite structure oxides, might develop new electronic behaviors in atomic-scale dimensions.
Koushik Das, a graduate researcher in the College of Chemistry and the Department of Electrical Engineering and Computer Sciences, explained how making a material thinner can induce ferroelectricity. Reducing the thickness of the TiO₂ film, said Das, changes its crystal structure, creating a “built-in electric polarization” that can be reversed with an electric field.
The study also showed that these ultrathin TiO₂ films retain their ferroelectric properties when deposited on different substrates. “Our work demonstrates that this ferroelectricity is stable on both crystalline, or silicon, and amorphous carbon film substrates,” said Das, the study’s lead author. “This indicates the feasibility of integration with silicon-based technologies and beyond.”
Das pointed out another advantage of TiO₂: its compatibility with existing semiconductor manufacturing processes. “The ultrathin TiO₂ films can be grown at a low temperature, less than 400°C, using atomic layer deposition [ALD], a technique already used in state-of the-art chip fabrication,” he said. “Furthermore, we can produce thin films with uniform thickness across all surfaces and polarization properties conducive to enabling new functionality for 3D integrated electronics.”
Beyond its immediate technological applications, this work also reveals a broader scientific insight, according to Salahuddin. “We’ve shown that simply reducing a material’s thickness can fundamentally change its properties as well as unlock functionalities with many exciting, new applications,” he said.
In addition to Salahuddin and Das, other co-authors from UC Berkeley include Kate Reidy, Sajid Husain, Jong Ho Park, Ramamoorthy Ramesh, Andrew Minor and Archana Raja. Please view the study for a complete list of authors and their affiliations.