Constructing Morphotropic Phase Boundary in Epitaxial BiFeO₃ on SrTiO₃ by Suppression of Strain Relaxation

The strain-driven morphotropic boundary in BiFeO₃ can enhance the piezoelectric properties. However, the tetragonal phase has generally been observed in BiFeO₃ films grown on substrates with intense compressive strain (more than −4.5%) within a limited thickness range (<300 nm) due to significant thickness-dependent strain relaxation during film growth at high deposition temperatures. This work proposes suppressing thickness-dependent strain relaxation by decreasing growth temperature. Utilizing a hydrothermal method, the growth temperature of epitaxial BiFeO₃ films decreases to 200 °C. As a result, the tetragonal phase is observed in 600-nm-thick BiFeO₃ film on (001) SrTiO₃ substrates (strain equals only −1.5%), accompanied by the monoclinic phase. This SrTiO₃-available morphotropic phase boundary significantly enhances the piezoelectric response (Formula presented.) in epitaxial BiFeO₃ film. Ex situ and in situ measurements, theoretical calculations, and simulation confirm that the SrTiO₃-available morphotropic phase boundary originates from the suppressed strain relaxation. Furthermore, a critical temperature (400 °C), below which the tetragonal phase can be maintained, is identified to offer an applicable strategy for extending strain-driven morphotropic phase boundary for high-performance piezoelectric films.