Oxygen movement in ferroelectric materials

A new study unveils how an unusual form of ferroelectricity arises in certain nano-sized materials

June 21, 2021
The Scitech

Left: oxygen rich phase (arrows pointing to oxygen atoms). Right: oxygen deficient phase (arrows showing the oxygen vacancies). These states are reversible through the application of voltage which causes motion of oxygen (vacancies). Credit: Pavan Nukala

A new study by an international team of researchers, including those from the Indian Institute of Science (IISc), unveils how an unusual form of ferroelectricity arises in certain nano-sized materials. The team includes Pavan Nukala, Assistant Professor at the Centre for Nano Science and Engineering (CeNSE), IISc, and former Marie Curie Research Fellow at the University of Groningen, Netherlands as well as Tuhin Chakrabortty, PhD student at CeNSE. 

Using atomic resolution microscopy, they show experimentally for the first time 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, according to Nukala, who is one of the corresponding authors of the study published in Science.

In 2011, scientists showed that hafnia-based oxides could exhibit ferroelectricity even when they are nano-sized, and that this ferroelectricity in fact grows stronger as the material size gets smaller, opening up numerous opportunities for microelectronics. However, there was no clear understanding of how ferroelectricity happens in these nano-sized materials. Nukala and colleagues used an advanced electron microscopy technique that had recently been developed and earlier used by a research team at the University of Groningen to visualise a hydrogen atom, the lightest chemical element. In the new study, they imaged thin films of hafnium-zirconium oxide sandwiched between two electrodes. 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. 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.

Source: Edited from IISc news release