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 x 100 mu m(2) and barrier thicknesses of 4.4 to 11 angstrom 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. (C) 2016 Elsevier B.V. All rights reserved.