Microwave Characteristics of NbN/NbN x /NbN Josephson Junctions for Quantum Voltage Standards

All NbN Josephson junctions with polycrystalline NbN films for both electrodes and barriers have been fabricated and their microwave characteristics for a quantum voltage standard were investigated. A superconductor and a normal metal were prepared by changing the partial pressure of N 2 during the rf sputter deposition of NbN films. For the Josephson junctions normal resistance was controlled by changing the partial pressure of N 2 during the NbN x barrier deposition, and critical current was controlled by changing barrier thickness. As a result, it was found that the product of current and normal resistance can be optimized to maximize the current range of the constant-voltage step for quantum voltage standards.