Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films

In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol-gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 mum thick. It was estimated that about 25%-50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180 degrees domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180 degrees domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were small in fine grain PZT films (especially those under 1.5 mum in thickness). However, as the films became thicker (>5 mum), nonlinear behavior between the converse piezoelectric coefficient and the electric driving field was observed. This indicated that there was significant ferroelectric non-180 degrees domain wall motion under high external excitation in thicker films. The activity of the non-180 degrees domain walls was studied through non-180 degrees domain switching. For fine grain films with film thicknesses less than 2 mum, non-180 degrees switching was negligible. Transmission electron microscopy plan-view micrographs evidenced non-180 degrees domain fringes in these films, where the vast majority of grains were 50-100 nm in diameter and showed a single set of domain fringes. Taken together, these measurements suggest that the pinning of non-180 degrees domain walls is very strong in films with thickness less than 2 mum. In thicker films, non-180 degrees domain switching was evidenced when the poling field exceeded a threshold field. The threshold field decreased with an increase in film thickness, suggesting more non-180 degrees domain wall mobility in thicker films. Non-180 degrees domain switching in large grained PZT films was found to be much easier and more significant than in the fine grained PZT films. (C) 2001 American Institute of Physics.