Measurements of tunneling barrier thicknesses for Nb/Al-AlOx/Nb tunnel junctions

The tunnel barrier thicknesses of Nb/Al-AlOx/Nb tunnel junctions were measured using transmission electron microscopy (TEM) and X-ray Reflection (XRR). By investigating the barrier thickness dependence of current density Jc, the barrier height for Nb/Al-AlOx/Nb junctions was calculated. Nb/Al-AlOx/Nb junctions with different Jc were fabricated by controlling the O2 exposure in Al oxidation. The junctions show good tunneling properties with subgap leakage factor Vm larger than 30 mV in the range of Jc from tens of A/cm2 to several kA/cm2. TEM images showed clear interface and indicated the AlOx thicknesses ranging from 0.8 nm to 1.9 nm, and the average barrier height was estimated to be 0.17 eV for Nb/Al-AlOx/Nb tunnel junctions.