Modification of tunneling barriers on Nb by a few monolayers of Al

We report that the normal-state tunneling behavior of Nb---oxide---Ag junctions is radically changed when very thin Al is deposited over the Nb before oxidation. From measurements of the oxidation rate, the conductance at high bias, the zero-bias anomaly, and observations of molecular excitations, we show that behavior generally associated with tunneling through niobium oxide disappears when the Al thickness is about two monolayers.