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IEEE Transactions on Terahertz Science and Technology

Specific Capacitance Dependence on the Specific Resistance in Nb/Al–AlOx/Nb Tunnel Junctions

Parisa Yadranjee Aghdam, Hawal Rashid, Alexey Pavolotsky, Vincent Desmaris, Denis Meledin, Victor Y. Belitsky

Published Jul 25, 2017• 8 citations
Publisher

Abstract

The junction specific capacitance (Cs) is an essential parameter in designing tuning circuitry for superconductor- insulator-superconductor (SIS) mixers. However, our knowledge of the junction capacitance only relies on the few available empirically obtained Csversus specific normal resistance (RnA) relations, which are inconsistent especially at low RnA values, RnA2. In this paper, we report the Nb/Al-AlOx/Nb SIS junction capacitance data from our recently presented direct microwave (4 GHz) measurements at 4 K for junctions with various RnA values ranging from 8.8 to 68 Ω·μm2. New insight is provided into the extraction of the true geometrical specific capacitance of SIS junctions. We show that even at such low microwave frequencies, the so-far-neglected nonlinear susceptance is significant, especially for junctions with low RnA values. This susceptance originates from the real part of the response function, IKK, which can be calculated through the Kramers-Kronig transform of the dc tunnel current. The new specific capacitance, which accounts for this contribution, is presented as a function of RnA. We provide an improved and more accurate Cs(RnA) relation, which can be a reliable and useful tool for circuit designers. The obtained Cs(RnA) relation is compared with those available in the literature, and the possible reasons giving rise to the disparity among these relations are discussed. By comparing the modeled and the measured noise temperature of the APEX SHeFI band 3\(385-500 GHz) double sideband mixer, we show that the new Cs(RnA) relation offers a great potential for improving the performance of SIS mixers.

DOI
10.1109/tthz.2017.2723803

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Superconductor-Insulator-Superconductor (SIS) tunnel junctions are widely used as the mixing component in mm and sub-mm receivers for radio astronomy and environmental science. SIS mixers offer superior noise performance close to the quantum limit for frequencies up to 1 THz. An important property of an SIS junction is the intrinsic specific capacitance (Cs), which is defined by the geometrical capacitance (Cj) divided by the junction area (A). In order to prevent loss of the RF signal, a tuning circuitry is required to resonate out the Cj. The RF bandwidth of an SIS mixer is determined by the quality factor of the resulted resonator-like tuning circuitry, Q = ωRnCj, where Rn is the normal resistance of the junction and ω is the angular frequency. In order to increase the RF bandwidth of the mixer, junctions with lower specific resistance (RnA) and hence thinner insulator layer are required. For the practical SIS mixers, junctions with RnA 100 Ω.µm2 (often as low as 20 Ω.µm2) are typically used. In this RnA range, the published data on the Cs vs. RnA are often spread and not consistent between different sources. Therefore, at this limit, the design of an accurate tuning circuitry becomes challenging. Hence, there is a well-recognized demand for accurate measurement of Cs.\nSo far, all the developed characterisation methods measured the specific capacitance effect indirectly, e.g., in a resonant circuit. In this context, the specific capacitance was extracted by fitting the measurement data into a theoretical model. In this thesis, a new method was developed, which directly measures the junction capacitance at microwave frequencies (1 - 7 GHz) with estimated uncertainty varying from ±2% to ±11% depending on the junction area. In this measurement method, an original calibration approach was suggested and implemented.\nUsing this method, the capacitance of 34 junctions with various RnA values ranging from 8 to 68 Ω.µm2 was measured. The observed scatter in the specific capacitance is discussed. We argue that, it is the tunnel barrier (thickness) nonuniformity, which is responsible for the observed spread of the Cs data. The nonlinear capacitance as a result of the Kramers-Kronig transformation of the dc IV characteristic of the SIS junction was shown to be a significant contribution for RnA 40 Ω.µm2 even at frequencies as low as 1-7 GHz. The extracted specific capacitance from the measurements was corrected for the nonlinear capacitance. The obtained Cs(RnA) data agrees well with the model relation Cs = a/ln(RnA), where a =0.227 for the RnA 40 Ω.µm2. \n

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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.

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The junction specific capacitance (Cs) is an essential parameter in designing tuning circuitry for superconductor- insulator-superconductor (SIS) mixers. However, our knowledge of the junction capacitance only relies on the few available empirically obtained Csversus specific normal resistance (RnA) relations, which are inconsistent especially at low RnA values, RnA2. In this paper, we report the Nb/Al-AlOx/Nb SIS junction capacitance data from our recently presented direct microwave (4 GHz) measurements at 4 K for junctions with various RnA values ranging from 8.8 to 68 Ω·μm2. New insight is provided into the extraction of the true geometrical specific capacitance of SIS junctions. We show that even at such low microwave frequencies, the so-far-neglected nonlinear susceptance is significant, especially for junctions with low RnA values. This susceptance originates from the real part of the response function, IKK, which can be calculated through the Kramers-Kronig transform of the dc tunnel current. The new specific capacitance, which accounts for this contribution, is presented as a function of RnA. We provide an improved and more accurate Cs(RnA) relation, which can be a reliable and useful tool for circuit designers. The obtained Cs(RnA) relation is compared with those available in the literature, and the possible reasons giving rise to the disparity among these relations are discussed. By comparing the modeled and the measured noise temperature of the APEX SHeFI band 3(385-500 GHz) double sideband mixer, we show that the new Cs(RnA) relation offers a great potential for improving the performance of SIS mixers.
Superconductor-Insulator-Superconductor (SIS) tunnel junctions are widely used as the mixing component in mm and sub-mm receivers for radio astronomy and environmental science. SIS mixers offer superior noise performance close to the quantum limit for frequencies up to 1 THz. An important property of an SIS junction is the intrinsic specific capacitance (Cs), which is defined by the geometrical capacitance (Cj) divided by the junction area (A). In order to prevent loss of the RF signal, a tuning circuitry is required to resonate out the Cj. The RF bandwidth of an SIS mixer is determined by the quality factor of the resulted resonator-like tuning circuitry, Q = ωRnCj, where Rn is the normal resistance of the junction and ω is the angular frequency. In order to increase the RF bandwidth of the mixer, junctions with lower specific resistance (RnA) and hence thinner insulator layer are required. For the practical SIS mixers, junctions with RnA 100 Ω.µm2 (often as low as 20 Ω.µm2) are typically used. In this RnA range, the published data on the Cs vs. RnA are often spread and not consistent between different sources. Therefore, at this limit, the design of an accurate tuning circuitry becomes challenging. Hence, there is a well-recognized demand for accurate measurement of Cs.\nSo far, all the developed characterisation methods measured the specific capacitance effect indirectly, e.g., in a resonant circuit. In this context, the specific capacitance was extracted by fitting the measurement data into a theoretical model. In this thesis, a new method was developed, which directly measures the junction capacitance at microwave frequencies (1 - 7 GHz) with estimated uncertainty varying from ±2% to ±11% depending on the junction area. In this measurement method, an original calibration approach was suggested and implemented.\nUsing this method, the capacitance of 34 junctions with various RnA values ranging from 8 to 68 Ω.µm2 was measured. The observed scatter in the specific capacitance is discussed. We argue that, it is the tunnel barrier (thickness) nonuniformity, which is responsible for the observed spread of the Cs data. The nonlinear capacitance as a result of the Kramers-Kronig transformation of the dc IV characteristic of the SIS junction was shown to be a significant contribution for RnA 40 Ω.µm2 even at frequencies as low as 1-7 GHz. The extracted specific capacitance from the measurements was corrected for the nonlinear capacitance. The obtained Cs(RnA) data agrees well with the model relation Cs = a/ln(RnA), where a =0.227 for the RnA 40 Ω.µm2. \n
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
The need for operating frequencies well into THz region with higher sensitivity and wider bandwidth pushes superconductor-insulator-superconductor (SIS) technology towards junctions with more transparent barrier and higher current densities. Obtaining accurate knowledge of the specific capacitance, which is related to the transparency of the junctions, leads to a precise design of the tuning circuitry. Previously, characterization of the SIS junction’s specific capacitance involved complex measurements providing data relying on various model fitting. Herein, we present the characterization of the specific capacitance by directly measuring the impedance of the SIS tunnel junction at microwave frequencies (~3 GHz).
We have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.
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.
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.