MgB2 thin films and MgB2/AlN/MgB2 SIS junctions

As-grown MgB2 thin films were fabricated by a sputtering method and a co-evaporation method. We found that the critical temperatures strongly depended on the deposition rate. The MgB2 thin films showed critical temperatures of 29 K for the sputtering method and 35 K for the co-evaporation method with relatively low substrate temperatures. For the fabrication of all MgB2 SIS junctions, we selected AlN as the material for the barrier layer, which has hexagonal crystal structure. In fabricating MgB2/AlN/NbN junctions, we found that the AlN deposition with a higher substrate temperature formed insulator layers other than AlN. We also prepared MgB2/AlN/MgB2 trilayers for SIS junctions using low AlN deposition temperature condition without breaking the vacuum. The MgB2/AlN/MgB2 junctions showed a clear Josephson current and gap structures. The critical current density was 120 A/cm2 and the ratio of sub-gap resistance and the normal resistance was 3.3 when the AlN insulator thickness was 0.14 nm. We showed current–voltage characteristics of the MgB2/AlN/MgB2 junctions with varying AlN layer thicknesses.