High quality niobium nitride-niobium Josephson tunnel junctions

Niobium nitride-niobium Josephson tunnel junctions with sputtered amorphous silicon barriers (NbN-αSi-Nb) have been prepared using processing that is fully compatible with integrated circuit fabrication. These junctions are of suitable quality and uniformity for digital circuit and S-I-S detector applications. At 4.2K, the product of the critical current and normal resistance is 1.8 mV; the subgap resistance is ≥14 times the normal resistance; and the sum of the electrode gaps is ∼3.8 mV -- in good agreement with the expected value. The current density can be controlled from ∼10 amp/cm²to >500 amp/cm²by varying the thickness of the αSi barrier. The processing steps required to achieve these results will be described and their effect on the physics of the tunneling process will be discussed.