NbN/AlN/NbN Josephson Junctions on Sapphire for SIS Receiver Applications

The most developed integrated receivers for THz radiation nowadays are based on Josephson junction superconductor-insulator-superconductor devices. In this kind of devices, the highest receivable frequency is determined by the energy gap of the superconducting electrodes and limited to approximately 700 GHz in case of Nb/AlOx/Nb multilayers. We have developed a technology for NbN/AlN/NbN Josephson junctions on sapphire substrates which allow operation at frequencies above 1 THz. The trilayers are deposited in situ in a three-chamber dc-magnetron sputtering system at temperatures as high as 775 degrees C. Each layer is reactively sputtered in an argon and nitrogen atmosphere. By variation of the partial gas pressures and discharge current of the plasma, optimum deposition conditions for the trilayers were found. So far, gap voltages as high as 5.1 mV at 4.2 K have been achieved which allows the operation frequency of the JJ devices to exceed 1 THz.