Josephson tunneling properties in MgB2/AlN/NbN tunnel junctions

We report on the fabrication of Josephson tunnel junctions with as-grown MgB2 thin films and on the Josephson tunneling properties of the junctions. MgB2/AlN/NbN trilayers were continuously deposited on sapphire (0001) substrates in the same vacuum run. The as-grown MgB2 thin films were fabricated using the conventional sputtering method at the low-substrate temperature of 250 degreesC, and the AlN tunnel barrier and the NbN counter electrode were deposited by reactive sputtering at the ambient substrate temperature. The T-c of the MgB2 thin films was 28 K. The junctions demonstrated clear Josephson tunneling characteristics with evident supercurrents, gap voltages, and small subgap leakage currents. The normal resistance of these junctions had an ideal dependence on the junction areas and on the AlN-barrier thickness. The temperature dependence of the gap voltages and the external dc magnetic-field dependence of the supercurrents were measured to investigate the Josephson tunneling behaviors in the MgB2/AlN/NbN junctions. The junctions showed BCS-like temperature dependence of the gap voltage and ideal Fraunhofer patterns. (C) 2002 American Institute of Physics.