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 (Pr) and low dielectric constant (εr). 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 Pr (∼60 μC·cm–2), a low εr (∼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 (e31,f ∼ −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.