Surface morphology effects on polarization switching in nanoscale ferroelectrics

The effects of surface morphology on polarization switching in thin ferroelectric films are investigated using a real-space, time-dependent Ginzburg-Landau model that incorporates electrostatic interactions. We consider a two-dimensional uni-axial ferroelectric film with a thickness that varies sinusoidally. Polarization switching, starting from a single domain remnant state, is simulated for several surface modulation amplitudes and wavelengths. We demonstrate that surface heterogeneities produce inhomogeneities in the electric field within the film. These inhomogeneities become preferential sites for easy nucleation of reverse domains. This has a profound effect on the external field necessary to switch the polarization. Increasing the surface undulation amplitude significantly reduces the coercive field compared to the ideal flat film, even for very small amplitude modulations in the thickness. Although surface roughness decreases the field required to form reverse domains, it also hinders subsequent domain wall migration. In fact, for very high amplitude and small wavelength surface morphologies, complete switching to a single domain state becomes impossible. This is because the domain walls become trapped near the peaks in the modulated surface. The technological implications of the present results for utilization of surface roughness and for surface morphology design are discussed.