A team of scientists at the U.S. Department of Energy's Oak Ridge National Laboratory studied the behavior of hafnium oxide, or hafnium, for its potential for use in new semiconductor applications. Scientists studying hafnium's potential for use in semiconductor applications found that its behavior can be affected by the surrounding atmosphere. Their findings provide good implications for future memory technology.

Using ultrahigh vacuum atomic force microscopy at the DOE Nanomaterials Science Center at ORNL, researchers have discovered a unique environment-induced ferroelectric phase transition in zirconium-hafnium oxide, a material important for the development of advanced semiconductors. Source: Arthur Baddorf/ORNL, Department of Energy

Materials like hafnium are ferroelectric, which means they can store data for long periods of time even without power. These properties suggest that these materials may be key to the development of new non-volatile memory technologies. Innovative non-volatile memory applications will alleviate the heat generated by the continuous transfer of data to short-term memory, paving the way for the creation of larger, faster computer systems.

Understanding the electrical behavior of hafnia

The scientists explored whether the atmosphere affects hafnia's ability to change its internal charge arrangement in response to external electric fields. The purpose is to explain a series of unusual phenomena discovered in Xiafu research. The research team's findings were recently published in the journal Nature Materials.

"We finally show that the ferroelectric behavior in these systems is coupled to the surface and can be tuned by changing the surrounding atmospheric environment. Until now, how these systems work has been a matter of speculation, a hypothesis based on numerous observations from our group and multiple groups around the world," said Kyle Kelley, a researcher at ORNL's Center for Nanomaterials Science. CNMS is a user facility of the Department of Energy's Office of Science. Kelly collaborated with Sergey Kalinin of the University of Tennessee, Knoxville, who conducted the experiments and conceived the project.

Surface layer and memory applications

Often materials used in memory applications have a surface or dead layer that affects the material's ability to store information. When a material is shrunk to just a few nanometers thick, the effects of the dead layer become severe enough to completely prevent its functional properties. By adjusting the behavior of the surface layer, in hafnia, this allows the material to transition from the antiferroelectric to the ferroelectric state.

"Ultimately, these findings provide an avenue for predictive modeling and device engineering of hafnium, which is urgently needed given the importance of this material in the semiconductor industry," Kelley said.

Predictive modeling enables scientists to use previous research to estimate the properties and behavior of unknown systems. The research led by Kelley and Kalinin focused on hafnia alloys mixed with zirconia, a ceramic material. However, future research could use these findings to predict how hafnium dioxide behaves when alloyed with other elements.

Research methods and collaboration

This research relied on atomic force microscopy inside a glove box and under ambient conditions, as well as ultrahigh vacuum atomic force microscopy, methods that CNMS can provide.

"Using the unique capabilities of CNMS, we are able to do this type of work," Kelly said. "We basically change the environment from ambient atmosphere all the way to ultra-high vacuum. In other words, we remove all the gases in the atmosphere to a negligible degree and then measure those reactions, which is very difficult to do."

Team members at Carnegie Mellon University's Materials Characterization Facility played a key role in the research by providing electron microscopy characterization, and collaborators at the University of Virginia led materials development and optimization efforts.

ORNL's Liu Yongtao (CNMS researcher) performed the environmental piezoelectric response force microscopy measurements. The model theory supporting this research project is the result of long-term collaborative research between Kalinin and Anna Morozovska from the Institute of Physics of the National Academy of Sciences of Ukraine.

Team insights

"I have been collaborating with colleagues in Kiev in ferroelectric physics and chemistry for almost 20 years," Kalinin said. "They did a lot of the work on this paper almost on the front lines of the war in that country. These people have been doing scientific research in conditions that most of us can't even imagine."

The team hopes their findings will inspire new research specifically exploring the role of controlled surface and interfacial electrochemistry - the relationship between electrical and chemical reactions.