Lanthanum-doped lead zirconate titanate (PLZT) films, with thickness up to 10 μm, are fabricated on platinized silicon substrates through a modified sol-gel technique. Thicknessdependent piezoelectric properties measured with a double-beam laser interferometer show piezoelectric relaxation in field-induced strain as the ac driving field exceeds 10 kV/cm. In addition, the strain levels of PLZT thick films are approximately one third of those of undoped PZT films under the same fabrication and measurement conditions. For 1 μm PZT(55/45) films doped with 0, 2, and 4 mole% La, the P-E hysteresis exhibits decreasing squareness with increasing lanthanum content while the piezoelectric d33
coefficient reduces from 130 to 52 pC/N. Residual (tensile) stress in these films and resulted depoling effect may be responsible for this phenomenon.