Trilayer (MgB2/AlN/NbN) Josephson junctions were fabricated on c-plane sapphire substrates, and the dependence of the MgB2 surface morphology upon MgB2 growth temperature was investigated. AFM measurement showed that the grain size was about 150nm and the surface roughness was about 10nm when deposition was done at the highest critical temperature. The critical temperature of the MgB2 layer after SIS junction fabrication remained the same as that for bare MgB2 film. The current–voltage characteristics of the MgB2/AlN/NbN junctions exhibited a very clear Josephson current and a gap structure. The critical current density was above 1kA/cm2, and the ratio of the sub-gap resistance to the normal resistance was 16.6 when the AlN insulator thickness was 0.14nm. The critical current was ideally modulated by applying a magnetic field, indicating that showed that the Josephson current flowed uniformly in the junctions.

Characteristics of MgB2/AlN/NbN Josephson Junctions with Optimized Conditions. H.Shimakage, K.Tsujimoto, Z.Wang, M.Tonouchi: Superconductor Science and Technology, 2004, 17, 1376-80