Designer defect stabilization of the super tetragonal phase in >70-nm-thick BiFeO₃ films on LaAlO₃ substrates

The stability of the “T-like” (T$) phase in BiFeO₃ films grown on LaAlO₃(001) is investigated. We show that the T$ phase can be stabilized for thicknesses >70nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600°C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of >1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T$ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.