Rhombohedral BiFeO3 thick films integrated on Si with a giant electric polarization and prominent piezoelectricity

Strikingly challenging the widely accepted opinion that a giant spontaneous polarization (Ps) of ~150 μC/cm2 in pure BiFeO3 can only be achieved in strain-induced tetragonal(-like) thin films up to a few hundred nm thick, rhombohedral-like, (110)-oriented BiFeO3 thick films (~2 μm) sputter-deposited at 500°C and 450°C exhibit giant Ps values of 152 μC/cm2 and 126 μC/cm2, respectively. Using a thermodynamic computation based on the Landau-Ginzburg-Devonshire potential (LGD) and a density functional theory (DFT) calculation, the enhanced ferroelectric polarization can be qualitatively explained by a (110) growth-orientation and a moderate compressive strain (~ -1.3%). While the (110)-orientation gives a boost in electric polarization by a ratio of ~2as compared to that of (100), and an enhanced piezoelectric response with respect to that of (111), the compressive strain, which is due to a low deposition temperature on Si, further improves the electric polarization and the related piezoelectric response. The resulting large piezoelectric d33 coefficient of ~120 pm/V in the 500°C-deposited film is well correlated with its giant Ps. This work demonstrates how to achieve a large ferroelectric polarization and a high piezoelectric coefficient in bulk-like BiFeO3 films on Si, implying a great potential of this lead-free multiferroic for applications in Si-based integrated devices.