High- T_c/high-coupling relaxed PZT-based single crystal thin films

Pb(Zr,Ti)O₃ (PZT)-based ferroelectric ceramics exhibit high piezoelectricity, however, their Curie temperature (T_c) values are not so high, i.e., T_c < 400 °C. PZT-based piezoelectric thin films with higher T_c would be beneficial for improved micro actuators, sensors, memories, and piezoelectric micro-electro mechanical systems. In-plane biaxial strained PZT thin films in a laminated composite structure are known to exhibit enhanced T_c; however, the thickness of PZT-based thin films is limited to below a critical thickness typically <50 nm. The T_c of relaxed PZT-based thin films with thicknesses greater than the critical thickness is the same as bulk T_c. However, a sort of relaxed PZT-based single-crystal thin films exhibit extraordinary high T_c, T_c = ∼600 °C. In addition, the films show extremely low dielectric constant, ε/ε_o ∼ 100 with high coupling factor, k_t ∼ 0.7, and large remnant polarization, P_r ∼ 100 μC/cm². These exotic properties would result from the single-domain/single-crystal structure. The enhanced T_c is possibly caused by the highly stable interface between the PZT-based thin films and substrates. Their ferroelectric performances are beyond those of conventional PZT. The high-T_c/high-coupling performances are demonstrated, and the possible mechanisms of the high T_c behavior in relaxed PZT-based single-crystal thin films are discussed.