Strain evolution and in situ phase transitions in freestanding BaTiO₃ epitaxial membranes via a La_2/3Sr_1/3MnO₃...

Freestanding epitaxial thin films offer huge potential for flexible electronics and heterogeneous integration with next-generation devices. Herein, we successfully demonstrate the support-free lift-off of a single-crystalline BaTiO₃ (BTO) epilayer using a La_2/3Sr_1/3MnO₃ (LSMO) sacrificial layer deposited on a SrTiO₃ (001) single-crystalline substrate. The freestanding BTO membrane was heterogeneously integrated onto glass, enabling the study of the etched surface. With a c/a ratio of 1.005, the epitaxial BTO exhibited an out-of-plane compression with an in-plane tensile strain compared to the bulk counterparts, with an out-of-plane strain relaxation observed in the freestanding membrane compared to the epitaxial stacks. Surface characterization via X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and optical profilometry confirms the structural and surface integrity. Temperature-dependent confocal Raman spectroscopy demonstrates in situ phase transitions in the freestanding BTO layer across the low- and high-temperature regimes (-170-200 °C). Overall, this work advances the heterogeneous integration of freestanding single-crystalline BTO membranes, providing critical insights into their strain-state evolution and temperature-dependent phase stability.