Integration-Friendly, Chemically Stoichiometric BiFeO₃ Films with a Piezoelectric Performance Challenging that of PZT

As a prototype single-phase multiferroic, BiFeO₃ exhibits excellent electrical, magnetic, and magnetoelectric properties, appealing to many modern technological applications. One of its overlooked merits is a high piezoelectric performance originating from its large remnant polarization (P_r) and low dielectric constant (ε_r). Furthermore, its high Curie temperature and large coercive field ensure good stabilities in device applications. However, to achieve close-to-intrinsic properties, a high processing temperature is usually used for the preparation of highly crystalline (epitaxial or highly oriented) BiFeO₃ films. Proliferation of defects due to loss of volatile Bi₂O₃ in the high-temperature process and its incompatibility with CMOS-Si technologies have hindered the development of BiFeO₃ film-based piezoelectric micro-electro-mechanical systems (piezo-MEMS) devices. In this work, we successfully sputter-deposited highly (100) oriented BiFeO₃ thick films (∼1 μm) on Si at 350 °C through the use of a conductive perovskite buffer layer of LaNiO₃. Formation of bulk and interfacial defects is suppressed by the combination of a low deposition temperature and an oxygen-rich processing atmosphere, resulting in chemically stoichiometric BiFeO₃ films. These films displayed a high P_r (∼60 μC·cm^-2), a low ε_r (∼200), and a small dielectric loss (<0.02), as well as large coercive and self-bias voltages in their as-grown and aged states. Together with a large transverse piezoelectric coefficient (e_31, ∼ -2.8 C·m^-2), excellent electromechanical performances with outstanding fatigue and aging resistances are demonstrated in patterned BiFeO₃-Si cantilever devices. These integration-friendly BiFeO₃ films are ideal replacements of PZT films in piezo-MEMS applications.