Nanoscale behavior of 90 degree domains in ferroelectric films

Abstract

ABSTRACT

Investigation of ferroelectric and piezoelectric properties of ferroelectric films at nanoscale is not only of fundamental interest, but also critical to their applications as non-volatile ferroelectric memories and as microsensors and microactuators for microelectromechanical systems. Both intrinsic and extrinsic effects play a role in determining film properties. Although the extrinsic contribution from non-180 degree domain wall motion can be comparable to the intrinsic lattice contribution in bulk materials, this effect is largely suppressed in polycrystalline films because of film clamping by a substrate and pinning of 90 degree domain walls by defects.

Taking into account that integration of high quality ferroelectric films onto silicon based substrate is very important for the microelectronic industry, we choose the Pb(Zr0.2Ti0.8)O3 film epitaxially grown on Si substrate by pulsed laser deposition to study the 90 degree domain behavior in the continuous films as well as in patterned islands, which can be used as nanodevices. The goal is to investigate the mobility of 90 degree domain walls and to determine their extrinsic contribution to the piezoelectric response of the epitaxial islands with reduced substrate clamping depending on island geometry.

The cubic and the strip-like islands have been fabricated from sub-micron films by focused ion beam milling. Ferroelectric and piezoelectric measurements on patterned structures with top electrodes have demonstrated the dramatic enhancement of their piezoresponse in comparison with a continuous film. Piezoelectric force microscopy has revealed highly mobile two-domain structure in the strip-like islands which show the maximum piezoresponse. The intrinsic piezoelectric response of the patterned structure with an immobile polydomain structure is modeled using finite element analysis. The difference between the experimental measured and calculated piezoeffect has allowed us to estimate a large extrinsic contribution from the domain walls movement in the patterned structures. The study of patterned structures together with additional observations of the domain wall movement under local electric field in continuous films allow us to conclude that a relative small extrinsic effect in the continuous films is a result of the elastic substrate clamping rather than the domain wall pinning. Thus, the contribution of domain wall movement can be increased by engineering special domain architectures, for example, the plane-parallel domains in a film, which are patterned into the thin strips.