Effect of stresses on the dielectric and piezoelectric properties of Pb(Zr_0.52Ti_0.48)O₃ thin films

Flexible piezoelectric microelectromechanical systems can experience a wide range of stress conditions. In order to explore the functional properties over this range, the dielectric and piezoelectric properties of 0.6 μm thick {001} sol-gel Pb_0.99⧠_0.01(Zr_0.52Ti_0.48)_0.98Nb_0.02O₃ (PZT) films on Si substrates and thin Ni foils were measured as a function of stress arising from thermal expansion mismatch during fabrication or applied by bending of a cantilever. Due to the differences in residual thermal stress, the remanent polarization, P_r, was approximately 21 ± 0.2 μC/cm² and 39.5 ± 2.3 μC/cm² for PZT films on Si and Ni, respectively, with the higher P_r on Ni originating from more "c" domains (out-of-plane polarization) due to the compressive stresses. The link between stress and domain orientation was further explored by bending films on Ni around mandrels with known radii of curvature to apply uniaxial strains of -0.2% to 0.5%. Films on Si were only exposed to strains between -0.06% and 0.06%, because of substrate failure. For films on 50 μm thick Ni foil, under a 0.5% tensile strain, the P_r decreased by 7%-10% and the permittivity increased up to 23% relative to zero applied stress samples. This trend reversed upon compressive strain. In addition, the piezoelectric coefficient, e_31,f, is reported to be -9.0 ± 0.45 μC/cm² and -7.1 ± 0.35 μC/cm² on Ni and Si, respectively, and increased in magnitude with applied uniaxial compressive strain. These changes are consistent with substantial levels of ferroelastic reorientation.