Characterization of the insulator barrier and the superconducting transition temperature in GdBa_2Cu_3O_(7-δ)/BaTiO_3 bilayers for application in tunnel junctions

摘要

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 BaTiO_3 tunnel barrier deposited on a 16 nm thick GdBa_2Cu_3O_(7-δ)5 thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO_3 barrier thickness between 1.6 and 4nm. The BaTiO_3 layers present an energy barrier of ≈1.2eV and an attenuation length of 0.35-0.5 nm (depending on the applied voltage). The GdBa_2Cu_3O_(7-δ) electrode is totally covered by a BaTiO_3 thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO_3 top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO_3 thickness. This fact can be associated with stress at the interface and a reduction of the orthorhombicity of the GdBa_2Cu_3O_(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 BaTiO_3 barrier).