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Superconductor Science and Technology

Fabrication and characterization of Nb/Al-AlN/Nb superconducting tunnel junctions

Alexey Pavolotsky, François Joint, Udupa Sujit Manjunatha, Victor Y. Belitsky, Denis Meledin, Takafumi Kojima, Sho Masui, Narayanan Ravishankar, Vincent Desmaris

Published Dec 18, 2025• 3 citations
Publisher

Abstract

Abstract We report a Nb/Al-AlN/Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron cyclotron resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with R n A down to ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance C s versus R n A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted C s .

DOI
10.1088/1361-6668/ae2f33

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•53 citations
Abstract We report a Nb/Al-AlN/Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron cyclotron resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with R n A down to ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance C s versus R n A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted C s .
We report a Nb/Al-AlN /Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron-cyclotron-resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with RnAR_n ARn​A down to ≈3,Ω,μm2\approx 3,Ω,μ\mathrm{m}^2≈3,Ω,μm2. Junctions maintain excellent quality, exhibiting Rj/Rn≥25R_j/R_n \ge 25Rj​/Rn​≥25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room-temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance CsC_sCs​ versus RnAR_n ARn​A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted CsC_sCs​.
Modern radio astronomy demands for broadbandreceiver systems. For SIS mixers, this translates into objective toemploysuperconducting tunnel junctions with very low RnA andlow specific capacitance. The traditionally used Nb/AlOx/Nbjunctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it isAlN-barrier junctions, which are needed for further progressingof the broadband instrumentation for radio astronomy. Inmanuscript we present results of developing a fabrication processfor high quality Nb/Al-AlN/Nb junctions and characterization thejunctions’ DC electric properties and their aging and annealingstability, as well as the junctions’ specific capacitance.
We investigate the influence of substrate bias on the plasma nitridation process to form tunnel barriers in superconducting tunnel junctions, especially focusing on the bias effects on the barrier transparency. In addition, we assess the role of substrate bias in the nitridation process in the enhancement of the reproducibility of AlN-SIS fabrication process. We find that this parameter is critical to optimize the fabrication of high-performance tunnel-junctions for superconducting mixer technology.
Modern radio astronomy demands for broadband receiver systems. For SIS mixers, this translates into objective to employ superconducting tunnel junctions with a very low RnA and low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it is AlN-barrier junctions, which are needed for further progressing of the broadband SIS mixer instrumentation for radio astronomy. In this work, we present the progress in fabrication of high quality Nb/Al-AlN/Nb superconducting tunnel (SIS) junctions and their characterization in terms DC electric properties junctions’ specific capacitance.
High quality, low-leakage superconductor-insulator-superconductor (SIS) junctions with Nb electrodes and aluminum oxide barriers are widely reported in the literature, and have become integral in the design and fabrication of various superconducting circuits. However, as current densities are increased, aluminum oxide based tunnel barriers show excess leakage currents due to defects and pinholes in the barrier layer. First reported by our group in 2007, AlN tunnel barriers grown via inductively coupled plasma (ICP) nitridation of Al overlayers are a promising alternative, producing low-leakage Nb/Al-AlN/Nb SIS junctions, with current densities in excess of 30 kA/cm2. A correlation between junction quality and plasma dissociation has been reported for Nb/Al-AlN/Nb junctions produced via an electron cyclotron resonance (ECR) plasma. In this work, a quantitative measure of the relative dissociation (RD) of an ICP is determined through the use of a commercially available Ocean Optics USB4000 Optical Spectrometer. The effects of various ICP parameters on the RD, and a correlation between the RD and quality of the resulting Nb/Al-AlN/Nb junctions are reported.
We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.