Aluminum thickness influence on tunnel junction conductance in Nb/Al based N/I/S junctions

The tunneling density of states spectra from superconducting Nb/Al bilayer based normal-insulator-superconductor (N/I/S) junctions are presented as a function of the Al thickness, revealing a larger increase in the zero-bias conductance than can be explained by the change in the superconducting transition temperature alone. Nb/Al bilayers are widely used in fields such as quantum information, thermometry, and bolometry. We fabricated Nb/Al/AlOx/normal metal tunnel junctions with different Al thin film thickness and measured the tunnel junction properties. The shape of the tunneling density of states and the magnitude of the sub-gap conductance from the tunneling spectra is found to change significantly at different Al thicknesses and as a function of temperature. We accurately model the data using the Blonder-Tinkham-Klapwijk (BTK) theory and discuss trends in BTK barrier strength, the superconducting energy gap, and broadening as the Al film thickness and temperature are changed.