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Versatile co-sputtered Nb/NbxSi1-x/Nb Josephson junctions for Josephson voltage standards and high-speed superconducting digital electronics | NIST

David I. Olaya, Paul D. Dresselhaus, Samuel P. Benz

Published Jun 16, 2009• 0 citations
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Arrays with double-stacked NbxSi1-x-Barrier Junctions for Use in Programmable Josephson Voltage Standards driven at 70 GHz | NIST

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300 GHz Operation of Divider Circuits using High-Jc Nb/NbxSi1-x/Nb Josephson Junctions

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We investigated high-current-density Josephson junctions with NbxSi1-xbarriers in single-flux-quantum (SFQ) digital circuits to evaluate their performance at clock frequencies of hundreds of gigahertz. We fabricated static divider SFQ circuits with a-NbxSi1-xand a-Si barriers for Jcranging from 3.5 to 85 kA/cm2. For junctions in the lower Jcrange (3.5-17 kA/cm2), we used optical lithography, and for the 85-kA/cm2junctions, we used electron-beam lithography. We measured SFQ circuits with a maximum operating speed of 300 GHz by use of intrinsically shunted junctions having submicrometer dimensions and an 85-kA/cm2critical current density. These results demonstrate that silicide-barrier junctions have the potential to achieve high-speed SFQ circuits and high-density circuits because the junctions are small and do not require external shunt resistors.

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Programmable Josephson voltage standards (PJVSs) in combination with fast switchable DC current sources have opened up new applications in the field of low-frequency AC metrology. The growing interest in output voltages of up to plusmn10 V initiated efforts by several National Metrological Institutes to realize 10 V PJVSs. Presently, only 10 V PJVSs from PTB based on SINIS junctions have been successfully incorporated into existing setups for AC metrology. However, the fabrication of 10 V SINIS arrays that are driven at 70 GHz suffers from very low yield. The recent technological progress made at NIST enabled the drop-in replacement of the low-yield SINIS arrays by more robust SNS arrays. The N-material is an amorphous NbxSi1-xalloy near the metal-insulator transition and is deposited by co-sputtering. For the first time, fully operational 1 V and 10 V PJVSs with SNS junctions that are suitable for a 70 GHz drive have been fabricated and tested. This work was done in close cooperation between NIST and PTB.

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Nb/a-Si/Nb Josephson junctions for high-density superconducting circuits

David I. Olaya, John Biesecker, Manuel A. Castellanos-Beltran +4 more

We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.

Applied Physics Letters•May 1, 2023•16 citations
We investigated high-current-density Josephson junctions with NbxSi1-xbarriers in single-flux-quantum (SFQ) digital circuits to evaluate their performance at clock frequencies of hundreds of gigahertz. We fabricated static divider SFQ circuits with a-NbxSi1-xand a-Si barriers for Jcranging from 3.5 to 85 kA/cm2. For junctions in the lower Jcrange (3.5-17 kA/cm2), we used optical lithography, and for the 85-kA/cm2junctions, we used electron-beam lithography. We measured SFQ circuits with a maximum operating speed of 300 GHz by use of intrinsically shunted junctions having submicrometer dimensions and an 85-kA/cm2critical current density. These results demonstrate that silicide-barrier junctions have the potential to achieve high-speed SFQ circuits and high-density circuits because the junctions are small and do not require external shunt resistors.
Programmable Josephson voltage standards (PJVSs) in combination with fast switchable DC current sources have opened up new applications in the field of low-frequency AC metrology. The growing interest in output voltages of up to plusmn10 V initiated efforts by several National Metrological Institutes to realize 10 V PJVSs. Presently, only 10 V PJVSs from PTB based on SINIS junctions have been successfully incorporated into existing setups for AC metrology. However, the fabrication of 10 V SINIS arrays that are driven at 70 GHz suffers from very low yield. The recent technological progress made at NIST enabled the drop-in replacement of the low-yield SINIS arrays by more robust SNS arrays. The N-material is an amorphous NbxSi1-xalloy near the metal-insulator transition and is deposited by co-sputtering. For the first time, fully operational 1 V and 10 V PJVSs with SNS junctions that are suitable for a 70 GHz drive have been fabricated and tested. This work was done in close cooperation between NIST and PTB.
We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.