All-MgB2 tunnel junctions with aluminum nitride barriers

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.