Examples of ferroelectric materials include the multiferroic semiconductor BiFeO3, ferroelectrics such as PbZr0.2Ti0.8O3, LiNbO3 and BaTiO3, and improper ferroelectrics such as hexagonal RMnO3 (where R = Sc, Y, In, Dy to Lu) and (Ca, Sr)3Ti2O7. All these compounds have a permanent dipole moment, or polarization, like their ferromagnetic counterparts. However, in ferroelectrics the dipole moment is electric rather than magnetic and so can be oriented using electric fields and not magnetic ones. The domain walls in these materials can have a significantly higher conductivity than the surrounding bulk, and what is more, these walls can be moved and erased on demand. This means they can be used as flexible interfaces with novel functional properties.

A team led by Dennis Meier from the Norwegian University of Science and Technology has now succeeded in reversibly controlling the electronic transport at a ferroelectric domain wall in the narrow bandgap p-type semiconductor ErMnO3. This material naturally develops all fundamental types of domain wall at room temperature, including neutral (side-by-side) as well as negatively (tail-to-tail) and positively charged (head-to-head) wall configurations.

A domain-wall-based binary switch

Conductance is higher in tail-to-tail walls in ErMnO3 because mobile holes (the majority charge carriers) accumulate at them. These holes come from interstitial oxygen anions and they screen the local electric field. In the same way, head-to-head walls usually have lower conductance because the majority carriers are fewer in number here.

The researchers were able to switch charged head-to-head walls in the material from being resistive to conductive using an electric field – something that corresponds to a domain-wall-based binary switch, they say.

Majority or minority carriers dominate the transport behaviour

“Previous studies focused on domain walls with enhanced conductance, enabled by the accumulation of mobile majority charge carriers,” explains Meier. “We have now involved walls at which both majority and minority carriers play a role. Depending on the applied electric field, either the majority or the minority carriers dominate the transport behaviour, leading to conducting and insulating states, respectively.

“For example, at low applied fields, there are less mobile holes at head-to-head walls than in the bulk and electrons are in a localized polaronic state, which makes the walls insulating,” he tells nanotechweb.org. “Likewise, at high fields, there are still less holes, but the electrons become itinerant and dominate the transport, which makes the wall conducting.”

Towards domain-wall-based electronic components

Being able to control the charge-carrier transport qualitatively and demonstrating the behaviour of a digital switch represents a first breakthrough towards making domain-wall-based electronic components, such as logical gates and transistors, adds Meier. “Our data shows that such devices, can, in principle, be realized using ferroelectric domain walls. This adds an important new dimension to the idea of using such walls as rewritable nanoscale wires.”

The team, which includes researchers from the US, Switzerland, France, Spain and Germany, says that it is now busy designing more sophisticated, device-oriented structures from ferroelectric materials. “Functional domain walls need to be integrated into nanoscale circuits to make all-domain circuitry,” explains Meier. “The dream is to develop nanoscale circuits that can be reconfigured and adapted throughout the lifetime of a device. In this way, nanocircuits would no longer need to be replaced when outdated – they could be upgraded, changed and improved at will.”

The research is detailed in Nature Materials doi:10.1038/nmat4878.