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.
Ultrathin MgB2
films were fabricated by using a co-evaporation method. We increased the Mg evaporation rate up to
32 nm s−1
by adjusting the chamber configuration and succeeded in fabricating
MgB2
thin films with high quality at a high substrate temperature of
350 °C. The thickness dependence of the thin film characteristics showed that an ultrathin film with a
thickness of 4.2 nm had superconducting properties with a critical temperature of 8 K and a resistivity
of 1200 µΩ cm
at 40 K. The mean roughness of a 15 nm thick
MgB2
thin film was 0.18 nm, which indicated that our
MgB2
thin films have the potential to fabricate sub-micron size patterns.
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 °C, and the AlN tunnel barrier and the NbN counter electrode were deposited by reactive sputtering at the ambient substrate temperature. The Tc 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.
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