We demonstrate the robustness of polarization in ultrathin compressive
strained BiFeO3β single layers and heterostructures during epitaxial
thin-film growth. Using in-situ optical second harmonic generation (ISHG), we
explore the emergence of ferroelectric phases at the strain-driven morphotropic
phase boundary in the ultrathin regime. We find that the epitaxial films grow
in the ferroelectric tetragonal (T-) phase without exhibition of a critical
thickness. The robustness of this high-temperature T-phase against
depolarizing-field effects is further demonstrated during the growth of
capacitor-like (metal|ferroelectric|metal) heterostructures. Using
temperature-dependent ISHG post-deposition, we identify the thickness-dependent
onset of the monoclinic distortion in the T-matrix and trace the signature of
the subsequent emergence of the strain-relaxed rhombohedral-like monoclinic
phase. Our results show that strain-driven T-phase stabilization in BiFeO3β
yields a prominent candidate material for realizing ultrathin ferroelectric
devices.Comment: 5 pages, 3 figure