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Japanese Journal of Applied Physics

Fabrication of Magnetic Barrier NbN Tunnel Junctions with the Gap Voltage over 5 mV

Ryo Terajima, Joe Sakai, Akira Kawakami, Syozo Imai

Published Mar 1, 2008• 0 citations

Abstract

Superconductor–insulator–superconductor (SIS) tunnel junctions including magnetic barriers, with structures NbN/AlNiN/NbN or NbN/MgO/NiO/MgO/NbN, were fabricated. The junctions with upper NbN layers of good crystallinity showed a gap voltage ( V g ) higher than 5 mV. In an NbN/Al 0.942 Ni 0.058 N/NbN junction, its Josephson current I c was suppressed to approximately a third of a theoretical value under no magnetic field. Therefore, it would be possible to avoid Josephson noise in magnetic barrier junctions with a much smaller magnetic field than junctions with conventional barriers. The Ni ion area density dependence of I c suppression ratio was identical for both types of junction, suggesting that the area density of magnetic ions is the factor that determines the I c suppression ratio. On the other hand, the extents of degradation of V g and increase in leakage current caused by the increase in Ni ion area density were smaller in MgO/NiO/MgO barrier junctions than in AlNiN barrier junctions. This suggests that the proximity effect of magnetic ions was weakened owing to the separation of Ni ions and NbN layers by the inserted thin MgO layers.

DOI: 10.1143/jjap.47.1563

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Within the past few years all-refractory superconductor-insulator-superconductor (SIS) tunnel junctions based on Nb/Al/AlO/sub x//Nb technology have displaced lead alloy devices as the preferred junctions for SIS mixers. There is growing interest in materials with higher superconducting energy gaps. NbN has a high gap ( approximately 3 meV) and higher critical temperature than Nb. Its main drawback, apart from the greater complexity of the material itself, stems from its short superconducting coherence length which results in a decrease in the measured gap voltage in proximity barrier structures, as generally used for Nb-based junctions. For this reason, most recent work has concentrated on the direct deposition of MgO as a tunnel barrier. The authors summarise work on Nb devices for MM-wave mixers and then report recent work performed at Cambridge on the fabrication of high quality NbN device structures.

A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.