Epitaxial growth of pure Sr₃Al₂O₆ sacrificial layer for high quality freestanding single-crystalline oxide membranes

The water-soluble cubic Sr₃Al₂O₆ has recently boosted the study of freestanding single-crystalline oxide membranes with extraordinary mechanical, ferroelectric, piezoelectric, or ferromagnetic properties for flexible electronics. Secondary phases usually exist in ceramic target due to complexity of SrO-Al₂O₃ binary solution. Therefore, pure Sr₃Al₂O₆ epitaxial thin film is critical for obtaining high-quality freestanding single-crystalline oxide membranes. In this study, we prepare phase-pure Sr₃Al₂O₆ thin films from both phase-pure and phase-impure targets as pulsed laser deposition targets and fabricate SrTiO₃(001)/Sr₃Al₂O₆/SrRuO₃ heterostructures to obtain freestanding single-crystalline SrRuO₃ membranes. The phase of the sacrificial layer is very sensitive to the laser energy density, and a secondary phase appears when it is below 1.5 J/cm² and disappears under a higher laser energy density. Similar behavior is observed again when using a stoichiometric but phase-impure Sr₃Al₂O₆ ceramic target. Such a secondary phase is identified to be SrAl₂O₄, and its appearance accompanies a Sr/Al gradient in the sacrificial layer, possibly due to the film growth kinetics. The freestanding SrRuO₃ membranes crack easily during transferring when such secondary phase appears in the heterostructure while eliminating it facilitates crack-free membranes. This study brings insight into the growth of phase-pure Sr₃Al₂O₆ films to prepare high-quality freestanding single-crystalline oxide membranes.