All-MgB2 tunnel junctions were fabricated on a C-plane sapphire substrate. The current-voltage characteristics showed both quasiparticle and Josephson tunneling currents and a clear gap structure. The current density was 115A∕cm2 for a 20×20μm2 junction with a 0.14-nm-thick AlN layer, and the ratio of the subgap resistance and normal resistance was 3.3. The gap voltages of lower and upper MgB2 electrodes were estimated to be 2.2 and 1.5mV, respectively. The temperature dependence of the Josephson current indicated that a normal layer existed between the AlN and the MgB2 electrodes. The Josephson tunneling currents were clearly modulated by applying an external magnetic field.

We fabricated MgB2/AlN/NbN trilayer Josephson junctions on c-plane sapphire substrates, and investigated the dependence of the MgB2 surface morphology on MgB2 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 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 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.

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