Improved characteristics of Nb3Ge tunnel junctions using sputter-deposited amorphous-silicon barrier

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Abstract
We have fabricated Nb3Ge/amorphous-silicon/Pb Josephson tunnel junctions using high Tc(≥20 K) and stoichiometric (25±1 at. %Ge) Nb3Ge films. Good I-V characteristic with large Josephson current Ic and high gap voltage is obtained when the amorphous-silicon barrier is sputter deposited at low rf voltage and high Ar pressure to reduce the damage to the Nb3Ge by the sputtered particles and to obtain good coverage over the Nb3Ge surface. Peculiar I-V characteristics showing the inverse ac Josephson effect and chaotic transition betwen current steps are observed in the response to microwave. The temperature dependence of Ic agrees well with the theoretical calculation when the Ge enrichment at the top surface of the Nb3Ge electrode is reduced.