Deciphering Hafnium Oxide: The Future of Semiconductor Technology

Employing the state-of-the-art ultrahigh-vacuum atomic force microscope situated at the Department of Energy’s Center for Nanophase Materials Sciences at Oak Ridge National Laboratory (ORNL), scientists have uncovered specific ferroelectric phase transitions in hafnium zirconium oxide. This material holds significant promise in the advancement of next-generation semiconductors. (Credit: Arthur Baddorf/ORNL, Department of Energy)

Researchers from the Oak Ridge National Laboratory have conducted in-depth studies on the potential of hafnium oxide, commonly known as hafnia, to be utilized in cutting-edge semiconductor applications. They discovered that the material’s behavior is considerably impacted by the ambient atmosphere, a revelation that holds encouraging prospects for the evolution of future memory technologies.

An interdisciplinary group of researchers from the Department of Energy’s Oak Ridge National Laboratory delved into the properties of hafnium oxide due to its promising application in innovative semiconductor technologies. Hafnia demonstrates ferroelectric traits, meaning it can retain data for extensive durations even in the absence of electrical power. Such attributes indicate that hafnia-based materials could be instrumental in the development of novel nonvolatile memory technologies, potentially revolutionizing computer systems by mitigating heat production during continuous data transfers to short-term memory.

Understanding Electrical Properties of Hafnia

The scientists investigated the role of the surrounding atmosphere in affecting hafnia’s capability to alter its internal electric charge orientation in response to an applied external electric field. The objective was to clarify a spectrum of uncommon phenomena previously observed in research on hafnia. These insights were recently documented in the scientific journal, Nature Materials.

According to Kyle Kelley of ORNL, who works with the Center for Nanophase Materials Sciences (CNMS), “It is now definitively established that the ferroelectric characteristics of these systems are connected to their surface layer and can be modulated by varying the ambient atmosphere. Prior understanding of these systems was largely conjectural, based on numerous observations by our research group as well as other international research teams.”

The research was conceived and executed by Kelley in collaboration with Sergei Kalinin from the University of Tennessee, Knoxville.

Surface Layer and Its Relevance to Memory Applications

Conventional materials used in memory technologies often have a surface layer, sometimes termed the ‘dead layer,’ which inhibits their data storage ability. As these materials are miniaturized to a few nanometers in thickness, this dead layer’s impact becomes pronounced enough to entirely negate their functional capabilities. By altering the environmental conditions, the research team succeeded in fine-tuning the behavior of hafnia’s surface layer, thereby transitioning the material from an antiferroelectric to a ferroelectric state.

Kelley notes, “This research paves the way for predictive modeling and device engineering focused on hafnia, an imperative given its relevance in the semiconductor sector.”

Predictive modeling allows researchers to extrapolate from existing studies to estimate the properties of as-yet-unstudied systems. Although this study was centered on hafnia alloyed with zirconia, its findings may be extended to anticipate the behavior of hafnia when combined with other elements.

Research Techniques and Collaborative Efforts

The research utilized atomic force microscopy in both glovebox and ambient settings, in addition to ultrahigh-vacuum atomic force microscopy techniques available at CNMS.

“Utilizing the specialized resources at CNMS enabled this rigorous investigation,” said Kelley. “We managed to create environments ranging from ambient atmosphere to ultrahigh vacuum, thereby eliminating nearly all atmospheric gases and measuring the resultant responses—an extremely challenging task.”

Partners from Carnegie Mellon University’s Materials Characterization Facility contributed by offering electron microscopy characterization, while collaborators from the University of Virginia led efforts in materials development and optimization. Yongtao Liu of ORNL’s CNMS executed ambient piezoresponse force microscopy measurements.

The theoretical framework that supported this research emerged from a long-standing academic partnership between Kalinin and Anna Morozovska at the Institute of Physics, National Academy of Sciences of Ukraine.

Reflections and Future Directions

“I have had ongoing collaborations with my peers in Kyiv for nearly two decades on the physics and chemistry of ferroelectrics,” said Kalinin. “Despite challenging circumstances, they made substantial contributions to this research.”

The team is optimistic that their discoveries will inspire additional research targeted at understanding how controlled surface and interface electrochemistries impact a computing device’s performance.

Kelley added, “Future investigations should broaden this understanding to other material systems to discern how interface characteristics impact device functionalities, which we anticipate will be favorable.”

Kalinin concluded, “Traditionally, the scientific focus has been on understanding surface properties at the atomic level for purposes like chemical reactivity and catalysis. In the realm of traditional semiconductor technology, the objective was merely to keep surfaces free from contaminants. Our work illustrates that these two spheres—the surface layer and electrochemistry—are intrinsically linked. Surface properties can be harnessed to modulate the bulk functional attributes of these materials.”

The published research paper is entitled “Ferroelectricity in hafnia controlled via surface electrochemical state.”

Reference: The paper was published on 14 August 2023 in Nature Materials with DOI 10.1038/s41563-023-01619-9.

Financial support for this research came from the Center for 3D Ferroelectric Microelectronics, an Energy Frontier Research Center funded by the Department of Energy’s Office of Science, Basic Energy Sciences program. Part of the research was also conducted as a user proposal at the CNMS.

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More about Hafnium Oxide in Semiconductor Technology

• Oak Ridge National Laboratory
• Department of Energy’s Office of Science
• Center for Nanophase Materials Sciences
• Nature Materials Journal
• Energy Frontier Research Centers
• Atomic Force Microscopy
• Nonvolatile Memory Technologies
• Ferroelectric Materials in Semiconductors