NbN/AlN/NbN Josephson junctions on sapphire for receiver applications

TL;DR

This paper reports the development of NbN/AlN/NbN Josephson junctions on sapphire that operate above 1 THz, surpassing the frequency limits of traditional Nb/AlOx/Nb devices, enabling higher frequency receiver applications.

Contribution

The authors developed a new fabrication process for NbN/AlN/NbN Josephson junctions on sapphire, allowing operation at frequencies above 1 THz, which is higher than existing Nb/AlOx/Nb devices.

Findings

Achieved gap voltages up to 5.1 mV at 4.2 K.

Demonstrated operation frequencies exceeding 1 THz.

Optimized deposition conditions for trilayers in-situ.

Abstract

The most developed integrated receivers for THz radiation nowadays are based on Josephson junction SIS 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 3-chamber DC-magnetron sputtering system at temperatures as high as 775 {\deg}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 tri-layers 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.