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We report on the fabrication of Josephson tunnel junctions with as-grown MgB[sub 2] thin films and on the Josephson tunneling properties of the junctions. MgB[sub 2]/AlN/NbN trilayers were continuously deposited on sapphire (0001) substrates in the same vacuum run. The as-grown MgB[sub 2] thin films were fabricated using the conventional sputtering method at the low-substrate temperature of 250°C, and the AlN tunnel barrier and the NbN counter electrode were deposited by reactive sputtering at the ambient substrate temperature. The T[sub c] of the MgB[sub 2] 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 MgB[sub 2]/AlN/NbN junctions. The junctions showed BCS-like temperature dependence of the gap voltage and ideal Fraunhofer patterns. [ABSTRACT FROM AUTHOR]

We have developed high-quality epitaxial NbN/AlN/NbN Josephson tunnel junctions with a wide range of current density Jc. The junctions show excellent tunneling properties with a large gap voltage of 5.6 mV and a large IcRN product of 3.5 mV. The quality factor Rsg/RN is about 60 for the junctions with a Jc of 2.2 A/cm2, and above 10 for the junctions with a Jc of 25 kA/cm2. The crystal structures across the junction barrier are investigated using x-ray diffraction and cross-sectional scanning transmission electron microscopy, and demonstrate epitaxial growth of the NbN/AlN/NbN trilayers for the wide range of Jc. [ABSTRACT FROM AUTHOR]