Partially oxidized amorphous silicon as the tunneling barrier for Josephson devices

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Abstract
Superconducting Nb(polycrystalline)/Pb tunnel junctions were fabricated by electron‐beam deposition. Partially oxidized amorphous silicon (a‐Si) is used as the tunneling barrier. A thin layer of Al (10 Å) passivates the Nb surface against the formation of conductive Nb suboxides. A detailed study of the behavior of oxidized a‐Si as the tunneling barrier is reported. The insulator is shaped as a double height barrier to account for the partially oxidized nature of the a‐Si deposited layer. Average barrier heights are inferred to be 15±5 meV and 1.2± 0.2 eV for the a‐Si and SiOX layers, respectively. Increasing the deposited a‐Si thickness, one observes a systematic degradation of the Nb/Pb superconducting tunnel characteristics, which correlates with deviations from the metal‐insulator‐metal tunneling theory. These anomalies are associated with the intrinsic properties of the unoxidized fraction of the a‐Si layer. From the technological point of view, our results indicate that such oxidized semiconducting barriers can yield high‐quality Josephson junctions. The high Vm products (30±10 mV) together with the low values of the specific capacitances (2.0±0.5 μF/cm2) guarantee favorable properties for fast device applications.

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