Atomic Layer Deposition (ALD) is a promising technique for the production of ultrathin, low-defect-density tunnel barriers for metal-insulator-metal tunnel junctions (MIMTJs). A fundamental challenge lies with the atomic scale control of the M-I interface. Using an Al wetting layer with a controlled H2O surface activation, Nb/Al/Al2O3/Nb Josephson junctions (JJs) were fabricated in situ with ALD-Al2O3 tunnel barriers ranging from 0.1 – 1.2 nm in thickness. In situ Scanning Tunneling Spectroscopy was utilized to compare the ALD-Al2O3 tunnel barriers against traditional thermal AlOx tunnel barriers formed via oxygen diffusion. Fundamental differences in tunnel spectra characteristics, barrier height, and breakdown behavior were observed. Remarkably, the tunnel barrier height of the former was found to be significantly higher than the latter. This property was corroborated by the JJ’s I-V characteristics at cryogenic temperatures. Together, these measurements suggest that ALD-Al2O3 is a substantially denser, less defective, and leak-free ultrathin tunnel barrier for high performance MIMTJs.