Freestanding Oxide Membranes for Epitaxial Ferroelectric Heterojunctions.

Since facile routes to fabricate freestanding oxide membranes were previously established, tremendous efforts have been made to further improve their crystallinity, and fascinating physical properties have been also reported in heterointegrated freestanding membranes. Here, we demonstrate our synthetic recipe to manufacture highly crystalline perovskite SrRuO freestanding membranes using new infinite-layer perovskite SrCuO sacrificial layers. To accomplish this, SrRuO/SrCuO bilayer thin films are epitaxially grown on SrTiO (001) substrates, and the topmost SrRuO layer is chemically exfoliated by etching the SrCuO template layer. The as-exfoliated SrRuO membranes are mechanically transferred to various nonoxide substrates for the subsequent BaTiO film growth. Finally, freestanding heteroepitaxial junctions of ferroelectric BaTiO and metallic SrRuO are realized, exhibiting robust ferroelectricity. Intriguingly, the enhancement of piezoelectric responses is identified in freestanding BaTiO/SrRuO heterojunctions with mixed ferroelectric domain states. Our approaches will offer more opportunities to develop heteroepitaxial freestanding oxide membranes with high crystallinity and enhanced functionality.