Characterization of different types of Nb-AlOx based Josephson tunnel junctions

Three types of Josephson tunnel junctions, standard Nb/Al,AlO_x/Nb, symmetric Nb/Al,AlO_x/Al/Nb, and Nb/Al,AlO_x/AlO_x/Nb containing a double-oxide layer were investigated by means of temperature-dependent I-V measurements, conductance-voltage measurements, noise analysis, and Auger electron spectroscopy scanning across the edge of a sputtered crater profile. In standard junctions, frequently small leakage currents have been observed as well as resistance fluctuations, leading to telegraph noise. Both effects can be related to the direct contact between the AlO_x and the Nb counter electrode. Leakage currents larger than 0.01% of the theoretical maximum critical current have not been observed in any of the symmetric junctions. The sub-gap current of these junctions is dominated by single- and two-particle tunneling. The SNAP process that was used to define the junction areas affects the tunnelling mechanisms below the sum-gap voltage, probably by the introduction of barrier inhomogeneities at the edges of the junctions. The AlO_x barrier in symmetric and asymmetric junctions cannot completely be represented by a trapezoidal barrier shape. The metal-insulator interface between Al and AlO_x in both junction types is probably not very sharp, which might be due to oxygen diffusion. The metal-insulator interface between AlO_x and Nb in standard junctions can be represented by a step-wise increase of the potential barrier, indicating that this interface is very distinct. The AlO_x barrier in double-oxide layer junctions is not homogeneous and probably contains low barrier channels