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.