In a recent study, an international team of researchers, including those from the Indian Institute of Science (IISc), discovered how a rare form of ferroelectricity occurs in some nanoscale materials. The team included Pavan Nukala, Assistant Professor at IISc’s Centre for Nano Science and Engineering (CeNSE) and former Marie Curie Research Fellow at the University of Groningen in the Netherlands, where a large part of the work was conducted, and Tuhin Chakrabortty, a Ph.D. student at CeNSE and the results, which were published online by the journal Science on 15 April 2021.
They demonstrated experimentally for the first time that when an electric field is applied, ferroelectricity occurs in materials called hafnia-based oxides due to the displacement and reversible movement of negatively charged oxygen atoms, according to a statement released by Bengaluru-based IISc.According to Nukala, who is one of the corresponding authors of the study published in ‘Science,’ such materials are useful for low-power memory applications.
Thin films made of hafnium display an unusual type of ferroelectricity, with a thickness of just a few nanometres. This allows nanometer-sized memories or logic devices to be constructed. It was unclear, however, how ferroelectricity could occur at this scale. Migrating oxygen atoms (or vacancies) are responsible for the observed switching and storage of charge in a hafnium-based capacitor, according to a study led by scientists from the University of Groningen. The findings, which were published online on April 15, 2021, in the journal Science, point to new ferroelectric materials.
An electric field may be used to reverse or alter the spontaneous polarization of ferroelectric materials. It’s used to make non-volatile memory and logic devices. One disadvantage of these materials is that when the crystal size is decreased below a certain threshold, the ferroelectric properties are lost. However, it was suggested a few years back that hafnium-based oxides may exhibit ferroelectricity on the nanoscale.
These unique properties of hafnium oxides were confirmed in 2018 by a team led by Beatriz Noheda, Professor of Functional Nanomaterials at the University of Groningen. ‘However, we didn’t know how this ferroelectricity happened,’ she explains. They knew the process in these thin membranes made of hafnium was different. They decided to research how the atomic structure of this material reacts to an electric field, both using the strong x-ray source at the MAX-IV synchrotron in Lund and our formidable electron microscope in Groningen since ferroelectric switching occurs at an atomic scale.
The Zernike Institute for Advanced Materials has a state-of-the-art electron microscope, which the group Bart Kooi, co-author of the Science paper, used to image the lightest atoms in the periodic table, hydrogen, for the first time in 2020. This is where Pavan Nukala, the first author, comes in. He had a background in electron microscopy and materials science, particularly in these ferroelectric hafnium systems, and worked as a Marie Curie Research Fellow at the University of Groningen.
If preparing a sample for atom imaging is challenging, the need to apply an electric field through a device in situ adds many orders of magnitude to the complexity. Fortunately, Majid Ahmadi (a master of in situ experiments) entered Kooi’s group at around the same time. They were both convinced that the ZIAM electron microscopy center would be the best place to see hafnium switching in situ at an atomic scale. ‘It benefits from a one-of-a-kind mix of materials science, microscopy, and infrastructure expertise,’ Noheda explains.
Ahmadi and Nukala devised the proper protocols for constructing hafnium-based electron-transparent capacitors using a directed ion beam facility. Nukala said they used two electrodes to picture the atomic lattice of hafnium-zirconium oxide, including the light oxygen atoms. Polarization is thought to be caused by the displacement of oxygen atoms in hafnium. So any microscopy would only be useful if oxygen could be imaged, and we had the right tool for the job. Then they applied an external voltage to the capacitor and observed the atomic changes live. There has never been an in situ experiment with direct imaging of oxygen atoms within an electron microscope before.
The oxygen atoms move, according to Nukala, which is a significant feature that he observed. He also mentioned that they are charged and migrate through the hafnium layer in response to the electric field between the electrodes. Ferroelectricity is made possible by reversible charge transport. Noheda adds that it came as a complete shock to them. Inside the unit cells, there is a slight change in atomic positions at the picometre scale, but the overall impact of oxygen migration from one side to the other on the device response is much greater. This breakthrough opens the door to new materials that could be used in nanoscale storage and logic devices. Nukala also stated that Hafnium-based ferroelectric memories are already being produced, despite the fact that the mechanism underlying their operation is unknown. We’ve paved the way for a new generation of silicon-compatible oxygen-conducting ferroelectric materials he said.
Noheda, the director of CogniGron, the Groningen Cognitive Systems and Materials Center, which develops new materials for cognitive computing, sees potential for the new ferroelectric materials. He said that Oxygen migration takes a long time compared to dipole switching. Material scientists are currently attempting to create hybrid systems from various materials to combine these two mechanisms in-memory systems that could emulate the short-term and long-term memory of brain cells. ‘Now we can do it with the same materials.’ We could also create intermediate states, similar to those found in neurons, by regulating oxygen movement.’
Pavan Nukala, who became an Assistant Professor at the Indian Institute of Science, is also interested in learning more about the material’s piezoelectric and electromechanical properties. Piezoelectricity exists in all conventional ferroelectrics.’ How about these new ferroelectrics that are non-toxic and silicon-friendly? It’s a good time to look at their ability in microelectromechanical systems.
Finally, the properties of this new material are a product of imperfections. Oxygen can only travel because oxygen vacancies exist within the crystal structure, Nukala says. ‘In fact, you might call what happens a migration of these openings. These structural defects are the key to ferroelectric behavior and offer materials novel properties in general.