Properties of ${\hbox{MgB}}_{2}$ Thin Films and SIS Junctions Fabricated Under Various Fabrication Conditions

Abstract

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.