Properties of MgB2 Thin Films and SIS Junctions Fabricated Under Various Fabrication Conditions

We investigated the optimal conditions to fabricate MgB₂ superconductor-insulator-superconductor junctions. By investigating thin film qualities obtained under various deposition conditions, we found that the critical temperature depended strongly on the Mg sputtering power. Although a higher Mg sputtering power increased the MgB₂ thin film´s critical temperature, the dependence of the zero resistance transition width suggested that the higher Mg deposition power produced Mg-rich thin Alms. To evaluate the gap voltage of MgB₂ thin films on c-plane sapphire substrates, we fabricated superconductor- insulator-normal metal junctions and determined that the gap voltage of MgB₂ was 2.0 mV. Using these MgB₂ thin Alms, we produced an MgB₂/SiC/MgB₂ junction and an MgB₂/AlN/MgB₂ junction. For both of these SIS junctions, we obtained clear gap structures, but at this stage, the AlN insulator appeared to be better than the SiC insulator based on current-voltage characteristics. We found that the current-voltage characteristics depend on the MgB₂ thin film quality when using an AlN insulator. Our TEM measurements revealed epitaxial growth of the lower MgB₂ thin films and amorphous-like growth of the upper MgB₂ thin films. We obtained their depth profile with Auger electron spectroscopy, and the profile indicated that the trilayer has a nitrided and oxidized region at the insulator portion.