Identification of different phases in barriers of interface-engineered ramp-edge Josephson junctions: Formation mechanisms and influences on electrical properties

Interface-engineered ramp-edge Josephson junctions with different critical current values and different electrode materials are investigated by transmission electron microscopy. High- resolution transmission electron microscopic observations show the coexistence of Y2O3, Y2Ba2O5, and a Ba-based pseudocubic phase in the barrier. The formation mechanism is related to the etching time and the composition of electrode materials. It is found that Y2O3 domains with typical sizes between 4 and 25 nm2 frequently exist in the thick regions of the barrier and the more the base electrode is fully covered by the domains of these phases, the lower the critical current value; the higher the fraction of Y2O3 and Y2Ba2O5 to the Ba-based pseudocubic phase in the barrier, the larger the Ic spread. Moreover, the atomic structure and the composition of the barrier in the junctions with La-doped electrodes are examined. La doping seems to suppress the formation of Y2O3 and reduce the lattice mismatch between the barrier and the electrodes. In the junction with the highest critical current value, there are many Y extra planes, i.e., stacking faults rather than cubic or pseudocubic phases along the barrier, therefore, it seems that the barrier is composed of many minute grain boundaries between the 123 and 223 phases.