Structure and formation mechanism of interface-modified layer in ramp-edge Josephson junctions with La-doped 123-type superconducting electrodes

Ramp-edge-type Josephson junctions (JJ) with interface-modified barriers were fabricated. Yb_0.9La_0.2Ba_1.9Cu₃O_y and Y_0.9La_0.2Ba_1.9Cu₃O_y were used for the counter- and base-electrodes, respectively. Reliable junctions with I_c≃1 mA and 1σ=6.5-8.0% for 25-100 JJ arrays were reproducibly obtained. For the junctions, the structure and composition of the barrier were investigated using a transmission electron microscope. A rather homogeneous layer with a thickness of a few nm was observed in the barrier region. The layer had a cubic lattice with a≃0.39×0.40 nm² and its cationic composition was Ba:La:(Y+Yb):Cu=38:10:20:32. It implies that the barrier consists of a thin layer of a cubic perovskite (Ba,La)(Y,Yb,Cu)O₃. The existence of La seems to stabilize the cubic perovskite and improve homogeneity of the barrier layer.