Temperature-modulated crystallographic orientation and electrical properties of BiFeO₃ thick films sputtered on LaNiO₃/Pt/Ti/SiO₂/Si for piezo-MEMS applications

In this work, thick BiFeO₃ films (~1 μm) were prepared on LaNiO₃-buffered (111)Pt/Ti/SiO₂/(100)Si substrates via radio-frequency magnetron sputtering without post-growth annealing. The effects of the substrate temperature on the film’s crystallinity, defect chemistry, and associated electrical properties were investigated. In contrast to the poorly crystallized BiFeO₃ film deposited at 300 °C and the randomly-oriented and (111)-textured films deposited at 500 and 650 °C, respectively, a (001)-preferred orientation was achieved in the BiFeO₃ film deposited at 350 °C. This film not only showed a dense, fine-grained morphology but also displayed enhanced electrical properties due to the (001) texture and improved defect chemistry. These properties include a reduced leakage current (J ≈ 2.4×10⁻⁵ A/cm²@200 kV/cm), a small dielectric constant (ε_r ≈ 243–217) with a low loss (tanδ ≤ 0.086) measured from 100 Hz to 1 MHz, and a nearly intrinsic remnant polarization (P_r) of ~60 μC/cm². A detailed TEM analysis confirmed the R3c symmetry of the BFO films and hence ensured good stability of their electrical properties. In particular, single-beam cantilevers fabricated from BiFeO₃/LaNiO₃/Pt/Ti/SiO₂/Si heterostructures showed excellent electromechanical performance, including a large transverse piezoelectric coefficient (e_31,f) of ~−2.8 C/m², a high figure of merit (FOM) parameter of ~4.0 GPa, and a large signal-to-noise ratio of ~1.5 C/m². An in-depth analysis revealed the intrinsic nature of the e_31,f piezoelectric coefficient, which is well fitted along a straight line of e_31,f ratio = (ε_rP_r) ratio with the reported representative results. These high-quality lead-free piezoelectric films processed with a reduced thermal budget can open many possibilities for the integration of piezoelectricity into Si-based micro-electro–mechanical systems (MEMSs).