Stress effects in ferroelectric thin films

PZT (52/48) film response is measured as a function of externally applied mechanical stress using a double-beam laser-Doppler heterodyne interferometer. One laser beam is focused on the front film surface and a second is focused on the back of the substrate. This arrangement successfully eliminates any displacement contribution from flexural vibration of the substrate and enables measurement of the strain-electric field hysteresis loops as a function of applied stress. The field-induced strain in the PZT film increases as the applied uniaxial compressive stress increases, and the opposite trend is observed for applied tensile stress. The dependence of electromechanical response on the external stress is attributed to the initial tensile residual stress state in the film. Tensile stress creates an in-plane clamping effect on the domains in the film, hindering polarization switching. The application of a compressive stress reduces the tensile residual stress hi the film and the constraint on the domains, hence more 90° polarization switches are allowed to occur under applied electric field, leading to higher field-induced strains. Applied tensile stress amplifies the clamping effect, leading to lower field-induced strains.