Interface microstructure in epitaxial YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox ramp-type Josephson junctions

The interfaces in ramp-type YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson junctions with different ramp slopes were characterized electrically and structurally. From the cross-sectional TEM images, it has been found that the epitaxy remains through all layers even on the ramp surface, providing the ramp angle is less than 45°. No big defects such as grain boundaries and secondary phases occurred at the interface. The major defects which appeared at the interface are antiphase boundaries which were formed by lattice shifts with c/3 unit between PrBa2Cu3Ox and YBa2Cu3Ox. The interface between YBa2Cu3Ox base-layer and PrBa2Cu3Ox barrier is more defective in comparison with that between PrBa2Cu3Ox barrier and top-layer. Alteration of the ramp slope from 10° to 45° leads to a significant increase in the density of defects near the base-YBa2Cu3Ox/PrBa2Cu3Ox interface. The average roughness of the ramp surface created by ion milling is about 10 nm, which seems insensitive to the ramp slope. The damaged ramp surface could be repaired during the subsequent deposition process, leading to a negligible interface resistance.