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IEEE Transactions on Applied Superconductivity

RF characterization of Josephson flux-flow transistors: design, modeling, and on-wafer measurement

Y.M. Zhang, E. Carlsson, Dag Winkler, G. Brorsson, Herbert Zirath, E. Wikborg

Published Jun 1, 1995• 7 citations
Publisher

Abstract

Josephson flux-flow transistors based on 0-32/spl deg/ [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal junctions have been fabricated with Au loops as the control current lines. In order to measure the S-parameters of the transistors, no matching elements are implemented and 50 Omega coplanar lines are used as the input and output ends of the transistors. The rf properties of the transistors have been modeled on an HP microwave CAD design system, with the junction parameters taken from our dc measurements. Simulations indicate that the transistors can have a maximum stable gain of 20 dB at 7 GHz and 6 dB around 35 GHz under matched conditions. An 'on-wafer' transmission-reflection-load (TRL) method is used for calibrating the S-parameters of the transistor. A low temperature microwave probe station has been built: a pair of microwave probes inside a cryostat are moveable in the xyz-directions. Together with a Wiltron 360B vector network analyzer, this setup provides a convenient way for doing on-wafer characterization of the transistors in the frequency range from 40 MHz to 60 GHz, and in the temperature range from 20 to 80 K.>

DOI
10.1109/77.403318

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IEEE Transactions on Applied Superconductivity•Dec 13, 2024•0 citations
Josephson flux-flow transistors based on 0-32/spl deg/ [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal junctions have been fabricated with Au loops as the control current lines. In order to measure the S-parameters of the transistors, no matching elements are implemented and 50 Omega coplanar lines are used as the input and output ends of the transistors. The rf properties of the transistors have been modeled on an HP microwave CAD design system, with the junction parameters taken from our dc measurements. Simulations indicate that the transistors can have a maximum stable gain of 20 dB at 7 GHz and 6 dB around 35 GHz under matched conditions. An 'on-wafer' transmission-reflection-load (TRL) method is used for calibrating the S-parameters of the transistor. A low temperature microwave probe station has been built: a pair of microwave probes inside a cryostat are moveable in the xyz-directions. Together with a Wiltron 360B vector network analyzer, this setup provides a convenient way for doing on-wafer characterization of the transistors in the frequency range from 40 MHz to 60 GHz, and in the temperature range from 20 to 80 K.>
By using a superconducting integrated sub-mm wave receiver, the linewidth of a flux-flow type Josephson oscillator (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 i2 microstripline was about 0.5 jiW, and could be adjusted by changing the current bias of the oscillator junction. The integrated receiver consists of two identical Josephson flux-flow oscillators and an SIS (superconductor-insulator-superconductor) mixer. We can determine the composite linewidth of the oscillators by mixing them down to an intermediate frequency in the 515 mixer. The FPOs are connected to the SIS mixer via centerfed interdigital capacitors. One FFO is well coupled to the SIS mixer by using a 3-step Chebychev rnicrostripline transformer and a fairly large interdigital capacitor, while the second H0 has about 15 20 dB weaker coupling. A strip inductor, rf-terminated by a radial stub resonates out the SIS junction capacitance at about 340 GHz. Radial stubs were also used as RF chokes for the if-line (4 GHz). The FFOs and the SIS element are made of Nb/Nb-oxide/PbBi tunnel junctions. The circuit was optimized by careful CAD modeling (HP Microwave Design System). Normal conductors were used with the resistivity set to zero. Microstripline widths and lengths were recalculated for the superconducting case with the London penetration depths of the electrodes (Nb and PbBi) taken into account. After adjusting the area of the SIS element and the length of the tuning strip inductor, a good response was observed from 280 to 330 GHz, with the LC resonance at 340 GHz. The power was high enough to pump the SIS mixer.
Microwave two-port S-parameter measurements and modeling were performed on superconducting flux flow transistors. These transistors, based on the magnetic control of flux flow in an array of high-temperature superconducting weak links, can exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive, while the output impedance is resistive and inductive. These devices are potentially useful in numerous applications, including matched amplifiers.>
The first object of this work is to theoretically study the effects of tunnel junction parameters on flux-flow Josephson oscillators (FFOs) by numerical simulation and to experimentally study the rf properties of FFOs by integrating them together with an SIS (superconductor-insulator-superconductor) mixer/detector on a chip. By using a CAD modeled integrated sub-mm wave receiver, the composite linewidth of two FFOs was measured to be less than 2.1 MHz in the band 280-330 GHz. The receiver consists of two identical FFOs and an SIS mixer. The linewidth was determined by mixing the two signals in the SIS mixer and analyzing the intermediate frequency. The output power coupled to a 10 .OMEGA. microstripline was about 0.5 µW, and could be adjusted by changing the current bias of the oscillator. The FFOs are connected to the SIS mixer via interdigital capacitors. The FFOs and the SIS element are made of Nb/NbOx/PbBi junctions by the "reactive ion beam oxidation" technique with the critical current density jc controlled by the oxidation time. Lift-off stencils for defining the junction sizes and the top-electrodes were made by using electron-beam writing on PMMA/Copolymer double layer resist. The jc of a long Josephson junction (LJJ) determines the fluxon motion. For LJJs which had jc > 1 kA/cm2, velocity-matching (VM, where the phase velocity of propagating fluxons equals that of the electromagnetic wave in the LJJ) steps could be tuned linearly from about 0.2 to 1.4 mV by an applied magnetic field. Coupled radiation at around 100 GHz (about 3 nW), 261 GHz (4.6 nW), and 330 GHz (0.43 µW), from flux-flow oscillations, was observed. From the step's steepness, the junction quality factor QJ is determined. QJ is between 4 to 7 in the voltage range of 0.4 to 1 mV. Coupled radiation (> 3 nW) at around 100 GHz corresponding to resonant soliton oscillation in low jc samples (250 to 1000 A/cm2) was detected. Simulations show that, for a uniform bias feed, the height of the VM step .gammam increases almost linearly with the external magnetic field .beta.e and the quasiparticle loss .alpha., but is less dependent on the surface loss .beta.. An approximate expression .gammam = a + .alpha(1+b).beta.e agrees with simulations. The maximum ac output voltage decreases with increasing dampings .alpha. and .beta.. It is important to include the .beta.-term in the mathematical model. The second part of this work is devoted to rf properties of long YBa2Cu3O7-.delta. (YBCO) grain boundary junctions made on 0- 32 °. [001]-tilt YSZ bicrystals. Josephson flux-flow resonances and Fiske resonances were observed. The first resonances could be tuned from about 0.5 to 2.0 mV in a 30 µm long junction at 16 K, and we determined the ratio between barrier thickness and relative dielectric constant td/.epsilon.r=2.9 Å and the loss .alpha.= 1.3 at 16 K. Simulations support the experimental observation that Josephson vortices can propagate unidirectionally in an overdamped, homogeneous junction up to .alpha.Ã? 2. From the Fiske resonances at 256-638 GHz, their Q-values, and the Steward-McCumber parameter, we find td/.epsilon.r=4 Å, the London penetration depth .gamma.L=145±5 nm and the surface resistance RS ~20 m.OMEGA. at 280 GHz. We conclude that the boundary is dielectric and the YBCO grains are well superconducting within 5-20 nm from the boundary. The dielectric between the grains could be part of a tunnel barrier and/or a matrix for filamentary conduction. Josephson flux-flow transitors based on long bicrystal junctions were made with an Au loop used as the control line. The line modulated the critical current Ic. Current gain larger than unity was obtained below 70 K. The gain increased with the junction length and varied roughly like the square root Ic when adjusting the temperature. A transresistance larger than 5 .OMEGA. was obtained at 15 K. Better performance is expected from junctions with high current densities.
Microwave two-port S-parameter measurements and modeling of superconducting flux flow transistors are presented. The transistors, based on the magnetic control of flux flow in any array of high temperature superconducting weak links, exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive while the output impedance is both resistive and inductive. It is shown that the characteristics of these devices are useful in numerous applications including matched amplifiers, phase shifters and active impedance convertors.>
By using an integrated superconducting sub-mm wave receiver, the composite linewidth of two Josephson flux- flow oscillators (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 R microstripline was about 0.5 pW, and could be adjusted by changing the current bias of the oscillator junction. The receiver consists of two identical FFO's and an SIS (superconductor- insulator-superconductor) mixer. The composite linewidth of the oscillators was determined by mixing the two signals in the SIS mixer to a lower intermediate frequency (if). The FFO's are connected to the SIS mixer via centerfed interdigital capacitors. One FFO is well coupled to the SIS mixer by using a 3-step Chebychev microstriplie transformer and a fairly large inter- digital capacitor, while the second FFO has about 15-20 dB weaker coupling. A strip inductor, rf-terminated by a radial stub, resonates out the SIS junction capacitance at about 340 GHz. Radial stubs were also used as rf chokes for the if-line (4 GHz). The FFO's and the SIS element are made of NbNb-oxidelPbBi tunnel junctions. The receiver circuit was optimized by careful CAD modeling (HP Microwave Design System). and gives rise to the emission of a pulse of electromagnetic energy when it hits the ends of the junction. If the arrival of solitons at one end of the junction is periodic, rf radiation is emitted. The propagation mode of the fluxon in a LJJ depends on the junction quality factor QJ, which is related to the junction losses as (3) QJ = ((wo/uJ)2P -t a)-'.
We present the RF characterization of 2-µm-wide superconducting coplanar waveguides made of 100-nm-thick niobium (Nb) or niobium nitride (NbN) films fabricated on 200 mm silicon wafers. Measurements of the two-ports S-parameters up to 25 GHz in a 4.7K–14 K temperature range with an added direct current up to 115 mA were done using a cryogenic probe station. With proper RF calibration and de-embedding procedure, both temperature and dc current dependence of the RF superconducting properties are analyzed. We report a kinetic inductance of 0.15 pH/□\square□, a critical current density of 50 MA/cm2^{2}2, and a critical temperature of 8.9 K for Nb as well as 4.27 pH/□\square□, 3 MA/cm2^{2}2, and 13.8 K for NbN. A telegraphist transmission line model taking into account the BCS theory has been developed and the comparison with the measurements is discussed.