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.

• Received 10 June 1981

DOI:https://doi.org/10.1103/PhysRevB.24.2278