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Applied Physics Letters

ac Josephson effect in superconducting all-Nb thin-film nanobridges up to 693 GHz

Yoshinori Uzawa, Etsuo Kawate, Nobumitsu Hirose, Isao Motohashi, Y. Harada, Matsuo Sekine, Yukinobu Miki, Masahiro Okaji

Published Aug 24, 1992• 3 citations
Publisher

Abstract

All-niobium thin-film nanobridges were fabricated with bridge sizes of 40-nm length, 50-nm width, and 100-nm thickness. We investigated the performance of the nanobridges by monitoring Shapiro steps induced on the current-voltage curves by an optically pumped far-infrared laser. These steps have been observed up to 693 GHz (433 μm). This is a twofold increase over that previously reported on thin-film nanobridges.

DOI
10.1063/1.108472

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Terahertz Shapiro Steps in High Temperature SNS Josephson Junctions

Peter A. Rosenthal, Erich N. Grossman

We have studied the far infrared behavior of high -T/sub c/ superconductor-normal metal-superconductor (SNS) microbridges with T/sub c/>85 K and critical current-resistance products (I/sub c/R/sub N/) as high as 10 mV at 4 K. These are the highest I/sub c/R/sub N/ products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R/sub N/ between 6 and 38 /spl Omega/, reasonably well matched to the antenna's estimated RF impedance of 53 /spl Omega/. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T/sub c/ junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics. >

Softwaretechnik-Trends•Apr 1, 1993•0 citations
All-niobium thin-film nanobridges were fabricated with bridge sizes of 40-nm length, 50-nm width, and 100-nm thickness. We investigated the performance of the nanobridges by monitoring Shapiro steps induced on the current-voltage curves by an optically pumped far-infrared laser. These steps have been observed up to 693 GHz (433 μm). This is a twofold increase over that previously reported on thin-film nanobridges.
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 Shapiro steps in the submillimeter wave (SMMW) region were measured in point-contact Josephson junctions made of a Nb strip with a sharpened edge and a YBaCuO thin film. The thin film was prepared by the chemical vapor deposition (CVD) method. The dependence of the Shapiro step width on the SMMW power was measured for the wavelength of 570.6 µm (525.4 GHz). The dependence was similar to that found in microbridges made of low T c superconductors. The first-order Shapiro step was observed for the wavelength of 232.9 µm (1.287 THz).
DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.
All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness ofcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.
We have studied the far infrared behavior of high -T/sub c/ superconductor-normal metal-superconductor (SNS) microbridges with T/sub c/>85 K and critical current-resistance products (I/sub c/R/sub N/) as high as 10 mV at 4 K. These are the highest I/sub c/R/sub N/ products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R/sub N/ between 6 and 38 /spl Omega/, reasonably well matched to the antenna's estimated RF impedance of 53 /spl Omega/. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T/sub c/ junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics. >