Heterolayered Ferroelectric Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 Thin Films

Heterolayered ferroelectric thin films consisting of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) have been successfully developed via a combined sol-gel and RF-sputtering route. PZT possesses all the excellent electrical properties that are best possible for non-volatile ferroelectric random access memory (NVFRAM) application; however its poor fatigue endurance has severely upset the application. On the other hand, BNT is well-known for high fatigue resistance with moderate ferroelectrical behavior. Bilayered ferroelectric thin films with two different stacking sequences, namely BNT/PZT and PZT/BNT, exhibit rather different ferroelectric and dielectric properties. The BNT/PZT bilayered film shows much improvement in fatigue resistance as compared to that of the single layered PZT film. The much enhanced fatigue resistance with increasing BNT layer thickness suggests an effective coupling between the PZT and BNT layers, gives rise to an almost fatigue-free behavior. In addition, a fatigue anomaly has been observed with the PZT/BNT bilayered films, whereby a switchable polarization peak, which is more than 5 times higher than that of the initial state, occurs upon polarization switching for 108-109 cycles. Upon repeated loading with external electric field, the switchable polarization of the heterostructured film is significantly enhanced, instead of being degraded. A thorough understanding has been made into the key aspects of this new interesting phenomenon, whereby a model is proposed to explain such unique observation.