Characterization of the insulator barrier and the superconducting transition temperature in GdBa₂Cu₃O_7−δ/BaTiO₃ bilayers for application in tunnel junctions

The optimization of the superconducting properties in a bottom electrode and the quality of an insulator barrier are the first steps in the development of superconductor/insulator/superconductor tunnel junctions. Here, we study the quality of a BaTiO₃ tunnel barrier deposited on a 16 nm thick GdBa₂Cu₃O_7−δ thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO₃ barrier thickness between 1.6 and 4 nm. The BaTiO₃ layers present an energy barrier of ≈1.2 eV and an attenuation length of 0.35–0.5 nm (depending on the applied voltage). The GdBa₂Cu₃O_7−δ electrode is totally covered by a BaTiO₃ thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO₃ top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO₃ thickness. This fact can be associated with stress at the interface and a reduction of the orthorhombicity of the GdBa₂Cu₃O_7−δ. The reduction in the orthorhombicity is expected by considering the interface mismatch and it can also be affected by reduced oxygen stoichiometry (poor oxygen diffusion across the BaTiO₃ barrier)