Epitaxial Strain Engineering of High-Quality Freestanding Single-Crystalline Complex Oxides

Freestanding strongly correlated complex oxide thin films hold great potential in next-generation electronic and energy applications. Their high integrability and tunable properties may surpass those in bulk or epitaxial films. Water-assisted lift-off method is widely used to produce single-crystalline complex oxide membranes. It involves utilizing water-soluble sacrificial oxides such as Sr₃Al₂O₆ (SAO), incorporated during multilayer growth. A key challenge is the incompatibility of direct growth between certain functional oxides (e.g., ferroics) and SAO, along with limited understanding of how film growth impacts both SAO dissolution and subsequent membrane quality. Here, we demonstrate how epitaxial strain engineering of SAO-based heterostructures, combined with overlying oxide layers (SrTiO₃ and La_0.7Sr_0.3MnO₃) grown via pulsed laser deposition, yields scalable, high-quality single-crystalline membranes through rapid, reproducible fabrication. Our findings reveal that the epitaxial strain of SAO layer is strongly affected by the subsequent growth condition (e.g., oxygen partial pressure) of upper oxide layers. Minimizing the strain relaxation of the SAO layer is crucial for both the layer-by-layer growth of upper oxide films and efficient dissolution dynamics of SAO. The use of a tetragonally strained SAO layer, remaining under oxygen-reduced conditions throughout the multilayer growth, allows for uniform and rapid (≤10 min) dissolution in water, producing millimeter-scale, microcrack-free, functional single-crystalline oxide membranes. This work provides key insights into how epitaxial growth affects membrane release and quality, providing a robust pathway for reliable, high-performance integration of freestanding oxide membranes using water-soluble layers.