Visualising oxygen movement in ferroelectric materials

Using state-of-the-art atomic resolution microscopy, scientists in Bengaluru have shown experimentally, for the first time, how an unusual form of ferroelectricity arises in certain nano-sized materials.

The insights offered by the study open up new avenues for designing oxygen-conducting ferroelectric materials that could be used for miniature memory and logic devices.

The researchers from the Indian Institute of Science (IISc) and international institutes said that when an electric field is applied, ferroelectricity arises in materials called hafnia-based oxides, due to the displacement and reversible movement of negatively charged oxygen atoms.

Such materials are useful for low-power memory applications and microelectronics, explained Assistant Professor Pavan Nukala of IISc's Centre for Nano Science and Engineering (CeNSE) who is corresponding author for the study. He pursued this research while he was a Marie Curie Research Fellow at the University of Groningen, Netherlands.

"Hafnia-based ferroelectric memory devices are already in production, even though the mechanism behind their behaviour is not known,” he added.

Ferroelectric materials are like magnets in which they show spontaneous polarisation, or the separation of positive and negative charges – which can be reversed or switched using an electric field. However, they are generally not suitable for miniaturisation because they lose their ferroelectric properties when the crystal is made smaller than a particular size.

Although, scientists in 2011 showed hafnia-based oxides could exhibit ferroelectricity even when they are nano-sized, the scientific community did not clearly understand how ferroelectricity happens in such nano-sized materials.

Using an advanced electron microscopy technique that had recently been developed and earlier used by a research team at the University of Groningen, Nukala and colleagues visualised a hydrogen atom, the lightest chemical element. In the new study, they imaged thin films of hafnium-zirconium oxide sandwiched between two electrodes.

Tracking the movement

They were also able to track the movement of atoms, including oxygen, in real time when an electric field was applied.

The researchers found that charged oxygen atoms move from one electrode to another with the hafnia layer acting as a conduit. When the electric field was reversed, the direction of migration was also reversed. It was this migration that contributed significantly to the material's ferroelectricity, they found.

When the conduit size was reduced (as the device is made smaller), oxygen conduction became more robust. These findings were also confirmed by X-ray diffraction studies carried out in Sweden.

Oxygen migration occurs due to imperfections or "vacancies" in the crystal structure, explains Nukala. "These structural defects are the key to the ferroelectric behaviour, and in general give novel functions to materials.”

The study has been published in the journal Science.