Fabrication of Josephson junctions with as-grown MgB2 thin films

We report on the fabrication of as-grown MgB₂ thin films and MgB₂/AlN/NbN thin film heterostructures having as-grown MgB₂ thin films. The MgB₂ thin films deposited by using a carousel-type sputtering system at substrate temperatures T_s of 252°C typically showed a critical temperature of 28 K. The results of XRD measurements of the thin films indicated a c-axis orientation. The surface morphology of the films fabricated at T_s=252°C was determined to be very flat and smooth from SEM images. The MgB₂/AlN/NbN trilayer was continuously deposited on sapphire [001] substrates in a single vacuum run. The AlN-barrier and NbN-counter layers were deposited by using DC-reactive magnetron sputtering at ambient substrate temperatures. The junctions were fabricated by using conventional photolithography, reactive ion etching, and electron cyclotron resonance etching techniques. The junctions demonstrated excellent quasiparticle tunneling characteristics having ideal dependence of the normal resistance on the junction area and on the AlN-barrier thickness. Also, the DC magnetic field and temperature dependences of the supercurrent I_c were measured to investigate the Josephson tunneling behavior in the MgB₂/AlN/NbN junctions. The junction I_c-H curve showed the ideal Fraunhofer pattern.