Characteristics of MgB2/AlN/NbN Josephson junctions with optimized conditions

We fabricated MgB 2 /AlN/NbN trilayer Josephson junctions on c -plane sapphire substrates, and investigated the dependence of the MgB 2 surface morphology on MgB 2 growth temperature. AFM measurement showed that the grain size was about 150 nm and the surface roughness was about 10 nm when deposition was done at the highest critical temperature. The critical temperature of the MgB 2 layer after SIS junction fabrication remained the same as that for bare MgB 2 film. The current–voltage characteristics of the MgB 2 /AlN/NbN junctions showed a very clear Josephson current and a gap structure. The critical current density was over 1 kA cm −2 , and the ratio of the sub-gap resistance to the normal resistance was 16.6 when the AlN insulator thickness was 0.14 nm. The critical current was ideally modulated by applying a magnetic field, indicating that showed that the Josephson current flowed uniformly in the junctions.