Observation of secondary gap-like structure in Nb/Al bilayers with thick Al

A second, gap-like structure emerges in the sub-gap region of the tunneling density of states (TDOS) above ~60 nm Al thickness in Nb/Al bilayers with 40 nm Nb. Additionally, with increasing Al thickness, the primary gap decreases linearly in energy and the normalized zero-bias conductance increases linearly. Nb and Al are principal materials for superconducting electronics, as Nb provides the highest Tc of the elemental superconductors and Al enables the growth of high quality AlOx tunnel barriers. To investigate the evolution of the superconducting state with Al thickness, we fabricated superconducting Nb/Al-based normal-insulator-superconductor (NIS) tunnel junctions with different Al thicknesses and measured the tunnel junction conductance spectra. We model the data using the Blonders-Tinkham-Klapwijk theory using barrier strength (Z), superconducting energy gap (Δ), and Dynes broadening (Γ) and interpret the implications of the best fit values.