In these electron microscope images, the left panel shows a sample with a lot of oxygen atoms (some indicated with arrows), while the right panel shows a sample with many oxygen vacancies (some indicated with arrows). The schematic illustration shows the movement of oxygen atoms and oxygen vacancies for two settings of the electric field over the capacitor. Image: Nukala et al./University of Groningen.Hafnium-based thin films with a thickness of only a few nanometers display an unconventional form of ferroelectricity, allowing the construction of nanometer-sized memories and logic devices. Until now, however, it has not been clear how ferroelectricity could occur at this scale.
A study led by scientists at the University of Groningen in the Netherlands has shown how atoms move in a hafnium-based capacitor, revealing that migrating oxygen atoms (or vacancies) are responsible for the observed switching and storage of charge. These findings, reported in a paper in Science, could point the way to new ferroelectric materials.
Ferroelectric materials exhibit spontaneous polarization, which can be reversed or switched using an electric field, and are used in non-volatile memories or the construction of logic devices. One drawback of these materials is that they lose their ferroelectric properties when the size of the crystals is reduced below a certain limit. However, some years ago, it was suggested that hafnium-based oxides could exhibit ferroelectricity at nanoscale dimensions.
In 2018, a team led by Beatriz Noheda, professor of functional nanomaterials at the University of Groningen, confirmed these special properties of hafnium oxides. "However, we didn't know exactly how this ferroelectricity occurred," she says. "We knew that the mechanism in these hafnium-based thin membranes is different. As ferroelectric switching is something that occurs at an atomic scale, we decided to study how the atomic structure of this material responds to an electric field, both using the powerful X-ray source at the MAX-IV synchrotron in Lund and our formidable electron microscope in Groningen."
The University of Groningen houses a state-of-the-art electron microscope at the Zernike Institute for Advanced Materials (ZIAM). In 2020, the group of Bart Kooi, co-author of the Science paper, used this electron microscope to image the lightest atom in the periodic table – hydrogen – for the first time.
"All of us were quite convinced that if there was one place where switching of hafnium could be visualized in situ at an atomic scale, it would be here at the ZIAM electron microscopy centre," Noheda says. "It benefits from a unique combination of the right expertise in materials science, microscopy and infrastructure."
But if preparing a sample for the imaging of atoms is tricky, then the need to apply an electric field across a device in situ increases the difficulty by several orders of magnitude.
"We imaged the atomic lattice of hafnium-zirconium oxide between two electrodes, including the light oxygen atoms," says first author Pavan Nukala, a former research fellow at the University of Groningen. "People believed that oxygen atom displacement in hafnium gives rise to polarization. So, any microscopy would only make sense if oxygen could be imaged and we had the exact tool for that. Then we applied an external voltage to the capacitor and watched the atomic changes in real time." Such an in situ experiment with direct imaging of oxygen atoms inside an electron microscope had never been done before.
"A significant feature that we observed is that the oxygen atoms move," explains Nukala. "They are charged and migrate following the electric field between the electrodes through the hafnium layer. Such a reversible charge transport enables ferroelectricity. This was a big surprise."
The researchers also detected a small shift in atomic positions at the picometer scale inside the unit cells, but the overall effect on the device response of the oxygen migration from one side to the other side is much larger. This discovery paves the way for new materials that could be used for nanometer-sized storage and logic devices.
"Hafnium-based ferroelectric memories are already in production, even though the mechanism behind their behaviour was unknown," says Nukala. "We have now opened up the road towards a new generation of oxygen-conducting, silicon-compatible ferroelectric materials."
Noheda, who is the director of CogniGron, the Groningen Cognitive Systems and Materials Center, which develops new materials for cognitive computing, can see interesting applications for the new type of ferroelectric materials. "Oxygen migration is much slower than dipole switching. In memory systems that could emulate the short-term and the long-term memory of brain cells, material scientists currently try to make hybrid systems from different materials to combine these two mechanisms. We can now do it in the same material. And by controlling oxygen movement, we could create intermediate states, again like you find in neurons."
Nukala, who is now an assistant professor at the Indian Institute of Science, is also interested in exploring the piezoelectric or electromechanical properties of the material. "All conventional ferroelectrics are also piezoelectric. What about these new non-toxic, silicon-friendly ferroelectrics? There is an opportunity here to explore their potential in microelectromechanical systems."
Ultimately, the properties of this new material stem from imperfections. "The oxygen can only travel because there are oxygen vacancies inside the crystal structure," says Nukala. "In fact, you could also describe what happens as a migration of these vacancies. These structural defects are the key to the ferroelectric behaviour and, in general, give materials novel properties."
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.