The hysteresis loop is the most important feature of ferroelectric thin films for device applications. It describes the strength of the electric polarization (remnant polarization), the magnitude of the electric field needed to switch it (coercive field) and the energy dissipation associated with the switching process.
Polarization switching does not occur homogeneously. Instead, domains of the reversed polarization nucleate and spread across the film. Nucleation commonly occurs heterogeneously at naturally occurring defects within the material.
Simulated switching
Using time-dependent Ginzburg-Landau theory-based computer simulations of the dynamics of the switching process, scientists at the Institute of High Performance Computing, Singapore, have shown how to control the nucleation of reversed polarization domains using nanoscale surface patterning or by manipulating surface roughness.
The patterned surface introduces electric field concentrators that serve as preferential sites for reversed polarization domain nucleation. This is demonstrated in the series of figures (see figure, right) where reversed polarization domains nucleate in the valleys of the patterned surface.
Remarkably, high aspect ratio surface profiles can also be used to trap reversed domains and engineer the asymmetry of the hysteresis loop. The graph (see figure, right) shows the variation of the coercive field with surface pattern wavelength and amplitude in a 100 nm thick film. Increasing surface profile amplitude can lead to very large reductions in the coercive field.
These new results suggest that surface patterning can be used to control the coercive field and the location of reversed polarization domain nucleation, which opens up important opportunities for the engineering of ferroelectric devices.
The researchers presented their results in the journal Nanotechnology.