Fabrication and Properties of NbN/NbNx/NbN and Nb/NbNx/Nb Josephson Junctions

TL;DR

This paper reports on the fabrication and electrical characterization of NbN/NbNx/NbN and Nb/NbNx/Nb Josephson junctions, exploring their properties for superconductor electronics integration.

Contribution

It introduces a novel fabrication process for NbN-based Josephson junctions with detailed electrical properties and compares them to Nb-based junctions, highlighting potential for miniaturization.

Findings

NbN/NbNx/NbN JJs have lower Jc than Nb/NbNx/Nb JJs with similar barriers.

JJs exhibit resistively and capacitively shunted behavior without excess current.

IcRn products up to 0.5 mV achieved at Jc ~ 0.6 mA/um^2.

Abstract

Increasing integration scale of superconductor electronics (SCE) requires employing kinetic inductors and self-shunted Josephson junctions (JJs) for miniaturizing inductors and JJs. We have been developing a ten-superconductor-layer planarized fabrication process with NbN kinetic inductors and searching for suitable self-shunted JJs to potentially replace high Josephson critical current density, Jc, Nb/Al-AlOx/Nb junctions. We report on the fabrication and electrical properties of NbN/NbNx/NbN junctions produced by reactive sputtering in Ar+N2 mixture on 200-mm wafers at 200 oC and incorporated into a planarized process with two Nb ground planes and Nb wiring layer. Here NbN is a stoichiometric nitride with superconducting critical temperature Tc =15 K and NbNx is a high resistivity, nonsuperconducting nitride deposited using a higher nitrogen partial pressure than for the NbN electrodes. For comparison, we co-fabricated Nb/NbNx/Nb JJs using the same NbNx barriers deposited at 20 oC. We varied the NbNx barrier thickness from 5 nm to 20 nm, resulting in the range of Jc from about 1 mA/um^2 down to ~10 uA/um^2, and extracted coherence length of 3 nm and 4 nm in NbNx deposited, respectively at 20 oC and 200 oC. Both types of JJs are well described by resistively and capacitively shunted junction model without any excess current. We found the Jc of NbN/NbNx/NbN JJs to be somewhat lower than of Nb/NbNx/Nb JJs with the same barrier thickness, despite a much higher Tc and energy gap of NbN than of Nb electrodes. IcRn products up to ~ 0.5 mV were obtained for JJs with Jc~ 0.6 mA/um^2. Jc(T) dependences have been measured.