Metal-oxide-metal tunneling junctions on Ta and Nb: Background conductivity resulting from different oxide barriers

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Abstract
Two different ways of preparing the oxide barrier in tunneling junctions on Ta and Nb yield different conductance‐voltage characteristics. While a ’’dirty’’ preparation method with atmospheric air as an oxidation medium leads to strongly polarity‐dependent but temperature‐independent characteristics, we find rather symmetric but strongly voltage‐dependent and temperature‐dependent (hence ’’low‐quality’’) characteristics when oxidation is performed in a pure oxygen dc plasma discharge. High or low quality of the junctions in the former and latter case, respectively, are also reflected in the superconducting characteristics. The results are tentatively interpreted in terms of band bending in the oxide on account of ionic surface charges in the dirty case.