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).
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).
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
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.
With a gap frequency of 1.4 THz, all-NbN Superconductor-Insulator-Superconductor (SIS) junctions are potentially applicable to sensitive heterodyne mixers for THz astronomy and atmospheric research. In this paper, we report the design, fabrication, and performance characterization of a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and an NbN/MgO/NbN tuning circuit with no impedance transformers. The noise performance of two NbN SIS junctions with different tuning inductances is measured, compared with the simulated results without and with the multiple Andreev reflection (MAR) effect taken into account, and also compared with the video detection response. In addition, the IF-output-power stability and aging effect of fabricated NbN SIS junctions are investigated.
A new element for superconducting electronic circuitry—a variable attenuator—has been proposed, designed, and successfully tested. The principle of operation is based on the change in the microwave impedance of a superconductor-insulator-superconductor (SIS) Josephson tunnel junction when dc biased at different points in the current-voltage characteristic. Both numerical calculations based on the Tien–Gordon theory and 70-GHz microwave experiments have confirmed the wide dynamic range (more than 15-dB attenuation for one stage) and the low insertion loss in the ‘‘open’’ state. The performance of a fully integrated submillimeter receiver circuit which comprises a flux–flow oscillator (FFO) as local oscillator, a superconducting variable attenuator, and a microwave SIS detector with tuned-out capacitance is also reported.
Superconducting SIS (Superconductor-Insulator-Superconductor) tunnel junctions are promising application prospect in the incoherent detection of astronomy observation. In this paper, the direct detection performances of Nb and NbN based superconducting SIS junctions, which cover about 250 GHz and 140 GHz centered at 600 GHz respectively, are characterized at a bath temperature of 4.2 K. The experiments show that Nb and NbN based SIS junctions have a current shot noise of 2.5×10-12and 3.1×10-12A/radicHz, a current responsivity of 303 and 187 A/W (corresponding to the quantum efficiency of 0.67 and 0.43) respectively. Thus their respective NEPs are 8.4×10-15and 1.7×10-14W/√Hz.
This paper presents a method for measuring the capacitance of superconductor-insulator-superconductor (SIS) junctions with a 4-K probe station. The probe station can be used to perform on-wafer characterization of sample lots at frequencies ranging from dc to microwave. In our study, we evaluated SIS junctions that had a variety of areas, and estimated the capacitance after subtracting the parasitic elements of the probe contact pad. The results show that the RNC products for the evaluated junctions were uniform within +/-5% across the wafer.