Supporting Information

XRD of BSAO (16 nm)/GdScO3 (110) and SrNbO3 (10 nm)/BSAO (16 nm)/GdScO3 (110); Optical microscopic images of transferred membranes using different buffer layers; XPS of SrNbO3 film; Transport properties of SrNbO3 with and without capping layers; RHEED intensity during the BSAO growth; RHEED images of the GdScO3 (110) substrate and the heterostructure; AFM images of BaTiO3 (4 nm)/SrNbO3 (10 nm)/BaTiO3 (12 nm) membrane; Optical spectrum of BaTiO3 film; Methods