Non-epitaxial Heterostructures and Devices of Ferroelectric BaTiO₃ Membranes

Perovskite oxide membranes have recently emerged as a promising materials platform for stabilizing novel phases and functionalities unattainable through traditional epitaxial growth. For instance, these freestanding membranes enable flexible devices and offer the ability to finely tune strain through external stress. A crucial step in designing flexible oxide devices is the fabrication of large-area, crack-free membrane heterostructures. However, the lift-off of epitaxial membrane heterostructures often leads to crack formation after exfoliation, due to the intrinsic lattice mismatch between epitaxially grown layers. In this work, we demonstrate the non-epitaxial integration of freestanding, single-crystalline oxide membranes into ferroelectric device heterostructures by assembling individual freestanding membranes. Ferroelectric BaTiO₃ (BTO) membranes were lifted from their growth substrates by etching a water-soluble sacrificial layer, followed by the integration with electrode layers of metal (Au) or SrRuO₃ (SRO) membranes. We also successfully fabricate such device heterostructures on flexible substrates and characterize their ferroelectric properties, which can be further tunable by strain engineering. This approach provides a general framework for designing flexible, strain-tunable oxide-based devices.