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AIP conference proceedings

Millimeter wave response of Nb variable thickness bridges

D. W. Barr, Robert J. Mattauch, Li-Kong Wang, Bascom S. Deaver

Published Jan 1, 1978• 0 citations
Publisher

Abstract

Niobium variable thickness bridges are being fabricated for testing as millimeter wave mixers. Bridges have been fabricated on fused quartz, silicon and gallium arsenide substrates and characterized by measurements of the I−V curve, and dV/dI versus V and by mixing experiments at 80‐100 GHz. The bridges on Si and GaAs exhibit small Josephson steps but the response of all the bridges is limited by self‐heating, and mixing appears to be primarily bolometric. However the cooling of the bridges of Si is much better than the others.

DOI
10.1063/1.31362

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Japanese Journal of Applied Physics•Jun 1, 1982•5 citations
Niobium variable thickness bridges are being fabricated for testing as millimeter wave mixers. Bridges have been fabricated on fused quartz, silicon and gallium arsenide substrates and characterized by measurements of the I−V curve, and dV/dI versus V and by mixing experiments at 80‐100 GHz. The bridges on Si and GaAs exhibit small Josephson steps but the response of all the bridges is limited by self‐heating, and mixing appears to be primarily bolometric. However the cooling of the bridges of Si is much better than the others.
Using RIE (reactive in etching) and lift-off techniques, all-NbN nanobridges with gap structure in the I-V (current-voltage) curves have been reproducibly constructed for testing as millimeter-wave mixers. The nanobridges were constructed on NbN/MgO/NbN edge junctions. They were characterized by measurements of their I-V curve and dI/dV versus V and by mixing experiments at 105.9 GHz. These devices had high normal-state resistance, and showed almost ideal Josephson response to external magnetic flux and to millimeter-wave irradiation. The I-V curve remained nonhysteretic from T/sub c/ to 4.2 K, and millimeter-wave-induced steps in the I-V curve were observed up to approximately 3 mV. The cooling of the NbN nanobridges was quite good due to their small size and VTB geometry and the high thermal conductivity of the MgO films. Hence the Josephson response of these devices to millimeter waves was comparatively less limited by self-heating.>
Abstract To reduce the self‐heating effect and thermal noise in bridge‐type Josephson mixers, all‐NbN nanobridges with an Al “direct shunt” resistor were fabricated. This paper describes the millimeter‐wave response and mixing properties of the NbN(g)/Al nanobridge‐type Josephson mixers in the 100‐GHz frequency. The effective noise temperature TNeff of the nanobridges was decreased to the bath temperature (liquid helium temperature, 4.2 K) by the Al “direct shunt” resistor and the millimeter‐wave induced steps were clearly observed at higher voltage (∼4 mV) than in the nanobridge without the Al “direct shunt” resistor. The millimeter‐wave current dependence of the height of the millimeter‐wave induced steps was analyzed quantitatively, taking into account the thermal noise in the RSCJ model. The simulated result agreed with the experimental result very well. The systematic decrease of the induced step height with an increase of millimeter‐wave current was explained as the increase of effective noise temperature due to absorption of millimeter‐wave power. In the 100‐GHz Josephson mixing experiment, a large IF power output was obtained at a voltage up to about 1.6 mV. The effective noise power NEP and conversion efficiency were about 3 × 10−20 W/Hz and 0.2, respectively.
To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF 4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 – 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.
The testing of NbN(g) nanobridges with Al direct shunt resistors for use as millimeter-wave Josephson mixers is discussed. The NbN(g)/Al nanobridges have a nearly sinusoidal current-phase relation, high resistance, well-defined gap structure at approximately 4 mV, large I/sub J/ R/sub N/ products of approximately 3 mV, and nonhysteresis I-V curves. By means of the Al direct shunt, the effective noise temperature T/sub N//sup eff/ of the NbN(g) nanobridges, which was calculated by transition-state theory, was reduced to 4.2 K as a result of reducing the self-heating and noise rounding effects. The NbN(g)/Al nanobridges show almost ideal Josephson response to millimeter-wave radiation at 106 GHz. The well-pronounced Josephson steps were observed up to approximately 4 mV. The observed systematic decreasing of the step heights with increasing the RF current can be accounted for by the effect of the thermal noise if an effective noise temperature of T/sub N/=15 K (at i/sub RF/=3) is assumed. The IF peaks in the Josephson mixing were obtained up to the bias voltage of approximately 1.2 mV. The NEP was about 3*10/sup -20/ W/Hz at 4.2 K.
An mm-wave mixing integrated circuit with a Josephson junction has been fabricated. An Nb–Te–Nb sandwich-type Josephson junction with no hysteresis on its I-V curve was used as the active element. Both RF and IF circuits were constructed in the form of superconducting microstriplines using an Si wafer as the dielectric layer. The circuit was tested at a signal frequency of 50 GHz, and the conversion efficiency η was measured at -7.44 dB, for a junction with a 0.42 mV I C R N product. The measured efficiency showed good ageement with the theoretical one calculated using the RSJ model and taking the impedance of the RF source into account.