Ultra-thin planar tunnel junctions grown on Nb thin films by atomic layer deposition

Atomic layer deposition (ALD) is used to grow ultra-thin and low-defect Al2O3 tunnel junction barriers on sputter-deposited Nb thin films. Junctions of sizes on the order of 100 × 100 μm2 and barrier thicknesses of 4.4 to 11 Å exhibit over 90% reproducibility. The conductance characteristics at low temperature show the clear density of states signature of superconducting Nb. The junction resistance times area product increases exponentially with barrier thickness, further supporting the high quality of the junctions, in which single-step elastic tunneling predominates. The background conductance at low temperature could not be fit with the Brinkman-Dynes-Rowell model, indicating the barriers are not likely to act as a trapezoidal potential. Our work shows that ALD is an effective method in preparing planar tunnel junctions with ultra-thin barriers.