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. Effects of the substrate temperature on the film’s crystalline quality, defect chemistry, as well as the associated electrical properties were investigated. In contrast to the poorly crystallized BiFeO₃ film deposited at 300 °C and the randomly-oriented & (111)-textured films deposited at 500 °C & 650 °C, respectively, a (001)-preferred orientation was achieved in the BiFeO₃ film deposited at 350 °C. Such a film not only showed a dense, fine-grained morphology, but also displayed enhanced electrical properties due to the (001) texture and an 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 (tand ≤ 0.086) measured from 100 Hz to 1 MHz, and an nearly-intrinsic remnant polarization P_r ~ 60 μC/cm². A detailed TEM analysis confirmed the R3c symmetry of the BFO films and hence ensured a good stability of their electrical properties. Particularly, single-beam cantilevers fabricated from the BiFeO₃/LaNiO₃/Pt/Ti/SiO₂/Si heterostructures showed an excellent electromechanical performance, including a large transverse piezoelectric coefficient e_31,f ~ ‒2.8 C/m², a high figure of merit parameter ~4.0 GPa, and a large signal-to-noise ratio ~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 = (P_r×e_r)_ratio with the representative results in the literature. These high-quality lead-free piezoelectric films processed with a reduced thermal budget can open up many possibilities for the integration of piezoelectricity into Si-based micro-electromechanical systems (MEMS).