Remote epitaxy of K_0.5Na_0.5NbO₃ films on SrTiO₃

Remote epitaxy represents a novel method for the preparation of high-quality epitaxial single crystalline films capable of being transferred onto arbitrary substrates for electronic and flexible functional devices. It was initially applied for semiconductors and later for functional complex perovskite oxides. However, the understanding of the impact of substrate orientation, growth mode, crystallinity and strain relaxation on oxide remote epitaxial systems is still lacking. Lead-free potassium sodium niobate (KNN) thin films have attracted intense interests owing to their superior piezoelectric properties and environment-friendly features. However, the remote epitaxy of KNN has rarely been reported. Therefore, in this study, we present the remote epitaxy of KNN on single-layer graphene-covered SrTiO₃ (STO) substrates with different orientations. All STO substrates with three orientations, i.e. (001), (011) and (111), permitted the remote epitaxy of KNN, with STO (001) leading to superior crystallinity. The growth mode of KNN on graphene/STO (G-STO) was found to be Volmer-Weber, with initial island nucleation on the wrinkles of graphene and a subsequent coalescence to complete the growth and obtain flat films. Furthermore, remote epitaxial KNN on G-STO (001) displayed an abrupt interface without pinholes in graphene or layer interdiffusion. The strain relaxation of remote epitaxial KNN films was explored as a function of film thickness, which already underwent partial relaxation due to the weaker substrate clamping effect through graphene but experienced a slight strain increase after island coalescence. These results not only show that the applied material systems for remote epitaxy can be expanded to more complex oxides, but also enrich the understanding of the oxide heteroepitaxy mechanism involving a graphene monolayer, particularly nucleation and strain relaxation.