Characterization of the effective electrostriction coefficients in ferroelectric thin films

Electromechanical properties of a number of ferroelectric films including P bZrxTi1-xO3(PZT), 0.9PbMg(1/3)Nb(2/3)O(3)-0.1PbTiO(3)(PMN-PT), and SrBi2Ta2 O9(SBT) are investigated using laser interferometry combined with conventio nal dielectric measurements. Effective electrostriction coefficients of the films, Q(eff), are determined using a linearized electrostriction equation that couples longitudinal piezoelectric coefficient, d(33), with the polar ization and dielectric constant. It is shown that, in PZT films, electrostr iction coefficients slightly increase with applied electric field, reflecti ng the weak contribution of non-180 degrees domains to piezoelectric proper ties. In contrast, in PMN-PT and SBT films electrostriction coefficients ar e field independent, indicating the intrinsic nature of the piezoelectric r esponse. The experimental values of Q(eff) are significantly smaller than t hose of corresponding bulk materials due to substrate clamping and possible size effects. Electrostriction coefficients of PZT layers are shown to dep end strongly on the composition and preferred orientation of the grains. In particular, Q(eff) of (100) textured rhombohedral films (x = 0.7) is signi ficantly greater than that of (111) layers. Thus large anisotropy of the el ectrostrictive coefficients is responsible for recently observed large piez oelectric coefficients of (100) textured PZT films. Effective electrostrict ion coefficients obtained by laser interferometry allow evaluation of the e lectromechanical properties of ferroelectric films based solely on the diel ectric parameters and thus are very useful in the design and fabrication of microsensors and microactuators. (C) 2001 American Institute of Physics.