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Applied Physics Letters

High quality tantalum superconducting tunnel junctions for microwave mixing in the quantum limit

Dean W. Face, D. E. Prober, William R. McGrath, P. L. Richards

Published Apr 21, 1986• 44 citations
Publisher

Abstract

We report on a novel step-defined process used to produce small area (1–6 μm2), high critical current density (102–104 A/cm2) superconducting tunnel junctions with Ta base electrodes and Pb0.9Bi0.1 counterelectrodes. These junctions have extremely small subgap leakage currents, a ‘‘sharp’’ current rise of width ΔV∼30 μV at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. Accurate measurements at 34.5 GHz with a 1.5 GHz intermediate frequency (IF) give single sideband mixer gains up to G=1.1±0.1 and mixer noise temperatures as low as TM=3.8±1.0 K. This value of TM is close to the quantum limit TQ=ℏω/k ln 2=2.5 K. The lowest receiver noise temperature measured at the 1.3 K input to the mixer block, TR=24±1 K, is the best value reported for any heterodyne receiver. Infinite available gain is obtained for a low (25 MHz) IF and a coupled gain as large as 1.6±0.4 is observed.

DOI
10.1063/1.96610

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Journal of Applied Physics•Dec 1, 1987•35 citations
We report on a novel step-defined process used to produce small area (1–6 μm2), high critical current density (102–104 A/cm2) superconducting tunnel junctions with Ta base electrodes and Pb0.9Bi0.1 counterelectrodes. These junctions have extremely small subgap leakage currents, a ‘‘sharp’’ current rise of width ΔV∼30 μV at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. Accurate measurements at 34.5 GHz with a 1.5 GHz intermediate frequency (IF) give single sideband mixer gains up to G=1.1±0.1 and mixer noise temperatures as low as TM=3.8±1.0 K. This value of TM is close to the quantum limit TQ=ℏω/k ln 2=2.5 K. The lowest receiver noise temperature measured at the 1.3 K input to the mixer block, TR=24±1 K, is the best value reported for any heterodyne receiver. Infinite available gain is obtained for a low (25 MHz) IF and a coupled gain as large as 1.6±0.4 is observed.
We have made accurate measurements of the noise and gain of heterodyne mixers employing small-area (1 μm2) Ta/Ta2O5/Pb0.9Bi0.1 superconductor-insulator-superconductor tunnel junctions. These junctions have very low subgap leakage current and an extremely sharp current rise at the sum gap. We have measured an added mixer noise of 0.61±0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta for a single-sideband mixer. Values of the imbedding admittances are deduced from the shapes of I-V curves pumped at the upper and lower sideband frequencies. Using these admittances, the mixer performance calculated from the quantum theory is in good agreement with the experiment.
We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.
We report on the fabrication of small area (1-6 μm2), high critical current density (103-104A/cm2) Ta/Ta oxide/ Pb0.9Bi0.1SIS tunnel junctions and six junction series arrays designed for use as broadband SIS quasiparticle mixers as 85-110 GHz. These junctions have small subgap leakage currents (1 to 5%), and a "sharp" current rise of width ΔV ∼30-50 μV at the sum gap voltage. An RF filter is fabricated on the substrate along with the junctions. The impedance of this filter combined with the junction capacitance is designed to provide a broadband RF match to the mixer over the entire frequency range. A sliding backshort is the only mechanical tuning element. The single junctions as well as arrays are fabricated with a window geometry on fused quartz substrates. The fused quartz substrates require the deposition of a thin amorphous Ge layer which is conducting as room temperature to avoid charging effects during the ion-beam processing steps. Preliminary measurements at 85- 100 GHz show a relatively low gain (-6.9 dB) compared to the performance expected for junctions of this quality. Model calculations suggest that the low gain arises from errors in the implementation of the RF filter design; this can be improved in future work.
We report the observation of heterodyne mixing in superconductor-insulator-superconductor tunnel junctions with significant conversion gain and with a noise temperature comparable to the photon noise limit. (Double sideband L−1=1.400.14, TM≲1.5 K at 36 GHz.) The mixing arises from the strong nonlinearity in the quasiparticle tunneling currents at voltages comparable to the full energy gap. Gain is observed when the onset of quasiparticle current is sufficiently sharp that quantum effects are important. The observed gain is in good quantitative agreement with calculations based on the work of Tucker. It should make possible the construction of photon-noise-limited microwave heterodyne receivers.
Accurate measurements have been made of the noise and gain of superconducting-insulating- superconducting (SIS) mixers employing small-area (1 micrometers 2) Ta/Ta2O5/Pb0.9Bi0.1 tunnel junctions. The authors have measured an added mixer noise of 0.61 +/- 0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta. A detailed comparison is made between theoretical predictions of the quantum theory of mixing and experimentally measured noise and gain. The authors used the shapes of I-V curves pumped at the upper and lower sideband frequencies to deduce values of the embedding admittances at these frequencies. Using these admittances, the mixer noise and gain predicted by quantum theory are in excellent agreement with experiment.
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.