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Multiferroic BiFeO$_3$ (BFO) possesses a rich phase diagram that allowsstrain tuning of its properties in thin-film form. In particular, at largecompressive strain, a supertetragonal (T) phase with giant polarization isstabilized over the more common rhombohedral (R) structure. To utilize thefunctionality of such metastable BFO phases in device applications, it isessential to understand the ferroelectric phase evolution upon insertion innanoscale heterostructures. Here, we explore the emergence of ferroelectricphases close to the morphotropic phase boundary in compressively strained BFOduring thin-film growth using in-situ optical second harmonic generation. Wefind that the epitaxial films form at the growth temperature in the ideal Tphase without critical thickness for the polarization. Signatures of T-like andR-like monoclinically distorted phases only appear upon sample cooling. Wefurthermore demonstrate a robustness of single-domain polarization in thehigh-temperature T phase during the growth of capacitor-likemetal|ferroelectric|metal heterostructures. Here, a reduction in tetragonalityof the T phase, rather than domain formation, lowers the electrostatic energy.At this lower tetragonality, density-functional calculations and scanningtransmission electron microscopy point to the stabilization of a new metastableR-like monoclinic structure upon cooling the heterostructure down to roomtemperature. Our results thus show that the combination of strain andelectrostatic phase stabilization in BFO heterostructures yields a prominentplatform for exploring ferroelectric phases and realizing ultrathinferroelectric devices.

Combined electrostatic and strain engineering of BiFeO$_3$ thin films at the morphotropic phase boundary