A team of scientists hailing from the Department of Energy’s Oak Ridge National Laboratory (ORNL) has embarked on a comprehensive exploration of hafnium oxide, commonly known as hafnia, due to its potential in groundbreaking semiconductor applications. Materials like hafnia exhibit ferroelectric properties, enabling them to retain data over extended periods without the need for constant power. These characteristics hold promise for the development of innovative nonvolatile memory technologies, which could revolutionize the computing landscape by mitigating the heat generated during data transfers to short-term memory.
The researchers delved into whether the surrounding atmosphere had an impact on hafnia’s ability to alter its internal electric charge arrangement when subjected to an external electric field. Their aim was to shed light on the assortment of unusual phenomena observed in hafnia research. Their findings, recently published in the journal Nature Materials, conclusively establish that the ferroelectric behavior of these systems is interconnected with the surface and can be adjusted by altering the surrounding atmosphere. This discovery overturns previous speculations and hypotheses, providing solid evidence based on extensive observations by multiple research groups worldwide.
Kyle Kelley, a scientist affiliated with ORNL’s Center for Nanophase Materials Sciences, stated, “We have conclusively proven that the ferroelectric behavior in these systems is coupled to the surface and is tunable by changing the surrounding atmosphere. Previously, the workings of these systems were speculation, a hypothesis based on a large number of observations both by our group and by multiple groups worldwide.”
Traditionally, memory materials incorporate a surface layer, or a dead layer, that interferes with their data storage capabilities. As materials are scaled down to mere nanometer thicknesses, the impact of this dead layer becomes so significant that it entirely impedes their functionality. By manipulating the atmosphere, the scientists were able to modulate the behavior of the surface layer in hafnia, transitioning the material from an antiferroelectric to a ferroelectric state.
Kelley emphasized the importance of these findings, stating, “Ultimately, these findings provide a pathway for predictive modeling and device engineering of hafnia, which is urgently needed, given the importance of this material in the semiconductor industry.”
Predictive modeling empowers scientists to estimate the properties and behavior of unknown systems based on previous research. While this study focused on hafnia alloyed with zirconia, a ceramic material, its implications extend to anticipating how hafnia might behave when combined with other elements in future research endeavors.
The research employed atomic force microscopy under both controlled and ambient conditions, as well as ultrahigh-vacuum atomic force microscopy, leveraging the capabilities available at the Center for Nanophase Materials Sciences. Collaborators from Carnegie Mellon University’s Materials Characterization Facility contributed crucial electron microscopy characterization, while researchers from the University of Virginia led the materials development and optimization efforts. ORNL’s Yongtao Liu conducted ambient piezoresponse force microscopy measurements.
The theoretical foundation of this research was the result of a longstanding research partnership between Sergei Kalinin and Anna Morozovska at the Institute of Physics, National Academy of Sciences of Ukraine.
Sergei Kalinin expressed his admiration for his colleagues in Kyiv, who contributed to the research even while enduring challenging circumstances, stating, “I have worked with my colleagues in Kyiv on physics and chemistry of ferroelectrics for almost 20 years now. They did a lot for this paper while almost on the front line of the war in that country. These people keep doing science in conditions that most of us cannot imagine.”
The research team envisions that their discoveries will stimulate further investigations focused on the role of controlled surface and interface electrochemistries in influencing the performance of computing devices. By extending this knowledge to other systems, they hope to better understand how interfaces can positively affect device properties. Traditionally, surface exploration was limited to understanding chemical reactivity and catalysis or keeping semiconductor surfaces free of contaminants. However, this study demonstrates the intricate connection between surface properties and electrochemistry, allowing for the tuning of bulk functional properties through surface manipulation.
The paper, titled “Ferroelectricity in hafnia controlled via surface electrochemical state,” is available in the journal Nature Materials. This research was supported by the Center for 3D Ferroelectric Microelectronics, funded by the DOE’s Office of Science, Basic Energy Sciences program, and was conducted as a user proposal at the Center for Nanophase Materials Sciences.