Harnessing atomic structure to advance semiconductor technology
Rohan Mishra and team are researching how manipulating material behavior at the atomic level can shape electronic devices of tomorrow
The growth of AI has increased demand for more efficient data storage. Instead of adding bulk to satiate this appetite however, what if we could increase memory capacity by manipulating materials’ characteristics at the atomic level?
Rohan Mishra, associate professor of mechanical engineering & materials science in the McKelvey School of Engineering at Washington University in St. Louis, and his team of researchers, have been pursuing this question, with the aid of funding from the National Science Foundation. Through computational modeling, advanced microscopy and experimental collaboration with researchers at the University of Nebraska-Lincoln and the Georgia Institute of Technology, and other institutions, Mishra and team have arrived at some exciting insights into material behavior.
Making materials used for silicon-based transistors work
One of Mishra’s research areas focuses on hafnia (HfO2), which makes up the thin insulating layer in silicon-based transistors. If the material were ferroelectric, it would maintain an electrical polarization that can reverse direction when an electric field is applied. More important, the material would remember which direction its polarization points, even after the electric field is removed. The feature would make hafnia a good candidate for memory storage.
The problem is most materials lose their ferroelectric behavior when extremely thin (less than 10 nanometers; for reference, a human hair is about 80,000 nanometers wide). But a group of researchers in Germany found that hafnia does not. They showed that extremely thin hafnia does demonstrate ferroelectric behavior.
It was head-scratching behavior: Most materials that are ferroelectric in bulk lose that property when extremely thin. But here was hafnia, a material otherwise not ferroelectric, suddenly becoming ferroelectric when extremely thin.
“It was a puzzle we are trying to understand,” Mishra says. His solution to that puzzle was published in Science. As part of the investigation, Mishra and team collaborated with the team of Xiaoshan Xu at the University of Nebraska-Lincoln to grow well-ordered, single-crystal films of lanthanum-doped hafnia on a substrate of yttria-stabilized zirconia. To test potential ferroelectric and antiferroelectric properties of hafnia, researchers had to grow a single crystalline structure so they could attribute whatever features they saw to the hafnia itself and not to any material defects such as grain boundaries.
Guodong Ren, who completed his doctoral degree under Mishra’s supervision and now works as a postdoctoral researcher at the University of Washington, Seattle, was able to confirm the single crystalline structure of the thin hafnia films using advanced microscopy, capable of resolving individual atomic columns in materials.
The researchers then determined the phase and behavior of the single crystals using electron microscopy. As predicted by the computational modeling, they found that the strained lanthanum-doped hafnia thin films stabilized in an antiferroelectric phase, where neighboring electric dipoles point in opposite (antiparallel) directions and their overall polarization cancels out. But applying a large “bias,” an electric voltage, aligns the opposing dipoles and makes the material behave as a ferroelectric, capable of memory storage.
The takeaway, according to Mishra, is that instead of struggling to force hafnia into a conventional ferroelectric phase, engineers might stabilize its more favorable antiferroelectric structure in thin films and control the transition electrically. “Instead of fighting nature, we can grow high-quality antiferroelectric films and we can then switch them during operation,” Mishra says.
Another advantage of antiferroelectrics is that they store a lot of energy and can be used as high energy density capacitors.
The intersection between theoretical modeling, microscopy and experimentation
The researchers in Mishra’s group complement atomic scale modeling — studying the stability and properties of an ensemble of atoms — with microscopy observations of synthesized materials.
“When you synthesize real materials, they always have defects, so our work was to understand how to minimize these defects so we can grow a high-quality film,” Mishra says.
The group used quantum mechanics to model and predict material behavior from the atomic-scale up, and collaborated with external groups to synthesize materials that closely align with the models. Once synthesized, Ren would evaluate what the real materials looked like at the atomic level.
“Guodong is an expert in combining theory and microscopy, one of the very few people who can do it at that level,” Mishra says, “Microscopy is a two-dimensional projection of a three-dimensional structure, so it helps us retrieve the real structure of the materials we study using a combination of microscopy and quantum mechanical calculations.”
Ren is also a co-first author of a separate study, co-led by Mishra with researchers at Georgia Tech and UNIST, South Korea, and published in Advanced Materials, which observed extremely small ferroelectric domains in zirconia (ZrO2, a close analogue of hafnia) with footprints of less than one square nanometer. Within each domain, electrical polarization points in a particular direction. Extra oxygen atoms helped balance electrical charge on the boundaries between these domains. Here too, calculations predict that the domain walls could conduct electricity, and move in response to an electric bias, which suggests that such atomically thin walls could one day serve as electronic components for memory storage.
Twisting manipulation
Yet another way of manipulating the ferroelectric behavior might be to stack two atomically thin hafnia layers and rotate the top one slightly. For work in Nano Letters, Jian Huang, a fourth-year doctoral student in Mishra’s group, modeled the effects of such twisting to show atoms line up and create a larger repeating pattern called a moiré superlattice.
The calculations predict that twisting the two hafnia layers creates a net electrical polarization perpendicular to the sheets, which could be reversed by an applied electric field.
“An electric field could drive one layer to slide relative to the other, flipping the polarization,” Huang says.
This alternative route to switchable polarization could again help with memory storage.
A special learning environment
Both Ren and Huang appreciate the holistic approach to exploring ferroelectric materials in the Mishra group.
“We have a very unique chance to combine theoretical studies and also microscopy experiments at the atomic level so we can gain a better understanding about different material systems,” Ren says.
Huang says he appreciates seeing the practical implications of the theoretical work he has conducted, which gives a holistic picture.
“You can really get a taste of the future, to see what’s next and understand material behavior more deeply,” he says.
The studies together present the idea of engineers controlling electronic behavior by manipulating materials at the atomic level. Even better, some of these materials are already in the silicon chips we use so manufacturers need not add new components.
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