Characteristics of all-MgB2 superconductor–insulator–superconductor junctions obtained with as-grownMgB2 thin films

Abstract

All-MgB2 SIS junctionswere fabricated on c-plane sapphire substrates using AlN film for the insulator layer. The critical temperature of the lowerMgB2 layer after the SIS junction fabrication process was 29 K, which remained the same as that of bareMgB2 films. On the other hand, the critical temperature of the upperMgB2 layer was depressed to 17 K. The current–voltage characteristics showed typical SIS properties withvery clear tunnelling currents and gap voltages, demonstrating an excellent insulator covering of thelower MgB2 films. The Josephson current decreased exponentially withincreasing AlN insulator layer thickness, and a Josephson current of115 A cm−2 was obtained for the junction with a 0.14 nm thick AlN insulator. However, the Josephson currents ofMgB2/AlN/MgB2 junctions werelower than that of MgB2/AlN/NbN junctions with the same AlN thickness; this indicated the existence of other insulatorlayers. The ideal modulation of Josephson current was observed by applying anexternal magnetic field; this indicated a uniform tunnelling current flow in thejunctions. The gap voltages were investigated on the basis of the current–voltagecharacteristics.