Structure, properties and applications of HfO₂-based piezoelectric films

Recently, metal element doped HfO₂ ultrathin films, i.e. Hf_1-xA_xO₂ (A = Zr, Si, La, Y, Sr, Al, Ta, etc.), have garnered significant attention in the pursuit of developing ferroelectric memory. Hf_1-xA_xO₂ ferroelectric films also exhibit strong piezoelectricity, with direct piezoelectric coefficients (d₃₃) exceeding 37 pC N^-1. Moreover, epitaxial strain and polarization switching can modulate the converse piezoelectric coefficient () from positive to negative values. By utilizing Hf_1-xA_xO₂ piezoelectric films, a wide variety of actuators, resonators, sensors, and other energy conversion devices can be manufactured. Importantly, high-quality, large-size Hf_1-xA_xO₂ films can be grown using atomic layer deposition (ALD) with an annealing temperature of ≤500 °C, which aligns with MEMS fabrication technology at the level of the CMOS backend-of-line (BEOL). Integrating MEMS/NEMS devices with CMOS chips can significantly improve the speed of operation and data collection, opening new possibilities for advanced electronic systems in future.