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.