Ultrathin Freestanding BiFeO₃ Membranes Grown by Pulsed Laser Deposition

Recent advances in the fabrication of single-crystalline oxide membranes have opened a promising avenue for the realization of freestanding ferroelectric films with potential applications in the field of flexible electronics, such as sensors, actuators, and energy harvesting devices. A widely used fabrication method is the integration of water-soluble Sr₃Al₂O₆ (SAO) as a sacrificial layer between the perovskite-type substrate and the ferroelectric film. Here we report the design of few-unit-cell-thick ferroelectric membranes using pulsed laser deposition. This work reveals the tendency for strontium diffusion from the SAO layer into the BiFeO₃ (BFO) films during the pulsed deposition, leading to the spontaneous formation of an Aurivillius-like layered phase. Mitigation of such undesired effects is achieved here by introducing a SrRuO₃ (SRO) buffer layer between the SAO and BFO layers. This prevents strontium diffusion and enables the fabrication of BFO membranes down to 5-unit cells with high structural integrity and a consistent polar tetragonal distortion. Our results demonstrate that freestanding ferroelectric films can be fabricated with a degree of precision comparable to that of molecular beam epitaxy. Our strategy can be further extended to other membrane compositions.