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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Planar SNS Josephson junctions using multilayer Bi system

Kentaro Setsune, Koichi Mizuno, Hidetaka Higashino, Kiyotaka Wasa

Published Mar 1, 1991• 1 citation
Publisher

Abstract

Planar S/N/S Josephson junctions were successfully fabricated by multi-layered thin films of Bi-based high-Tc oxide superconductors. Two Bi2Sr2Ca1Cu2O(x) films and Bi2Sr2Cu1O(y) film were successively layered as superconducting electrodes and normal conducting buffer layer, respectively. These films were epitaxially deposited on (100) MgO substrate by in situ process of RF magnetron sputtering. Microjunction areas were defined by photolithography and Ar ion milling. These S/N/S type junctions clearly exhibited the AC Josephson effect under the irradiation of radio frequency waves of 12 GHz. More than 20th Shapiro steps were observed and those step heights were oscillated according to the irradiation power. These characteristics were stable and reproducible for different configuration of function area.

DOI
10.1117/12.25734

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Planar S/N/S Josephson junctions were successfully fabricated by multi-layered thin films of Bi-based high-Tc oxide superconductors. Two Bi2Sr2Ca1Cu2O(x) films and Bi2Sr2Cu1O(y) film were successively layered as superconducting electrodes and normal conducting buffer layer, respectively. These films were epitaxially deposited on (100) MgO substrate by in situ process of RF magnetron sputtering. Microjunction areas were defined by photolithography and Ar ion milling. These S/N/S type junctions clearly exhibited the AC Josephson effect under the irradiation of radio frequency waves of 12 GHz. More than 20th Shapiro steps were observed and those step heights were oscillated according to the irradiation power. These characteristics were stable and reproducible for different configuration of function area.
A sandwich-type high- Josephson junction coupled with a coplanar-type transmission line was fabricated and heterodyne mixing characteristics were investigated. The junction was fabricated from a stacked film structure of (BSCCO/BSNCO/BSCCO) and the transmission line was made of sputter-deposited Pt film. The junction had a rectangular shape of . Current - voltage (I - V) curves of the junction showed weak-link-type characteristics. Two microwave sources of a frequency synthesizer and a sweep oscillator were used as a local oscillator (LO: 20 GHz) and a radio frequency signal source (RF: 19 GHz) for heterodyne mixing experiments. Intermediate signal (IF: 1 GHz) was transmitted through the transmission line and detected by a power meter. The conversion efficiency of -44 dB was estimated for an LO oscillator level of -23 dB m at 5.7 K when the junction was biased at the point below the first Shapiro step.
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.
Superconducting step-edge junctions were fabricated on step-edges in (100) LaAlO/sub 3/ substrates by sputter-deposited epitaxial YBaCuO and TlBaCaCuO thin films. The substrate steps were patterned by ion beam etching using metal film as a mask. The transport properties of the junctions were characterized by four-probe electrical measurements. The critical current-normal resistance product (I/sub c/R/sub N/) was larger than 0.2 mV (77 K) for most of the junctions. Clear Shapiro steps were observed in the current-voltage characteristics of the junctions with 36 GHz or 95 GHz millimeter-wave irradiation. The high frequency properties of these high T/sub c/ Josephson junctions showed that they were quite suitable for the Josephson harmonic mixer used in the digital phase locked millimeter and sub-millimeter wave sources.>
We have fabricated both NbN/Ta x N/NbN SN*S-type, NbN/MgO/NbN SIS-type Josephson junctions and microwave band-pass filters on different substrates (Si, Sapphire, MgO). NbN films have been deposited on both sides of (100) oriented, 250 µm thick, MgO substrates with a high crystalline texture quality. The aim was to investigate the performances and the maximum achievable operating frequency in an NbN based RSFQ modulator front-end of an ADC in the 4 K-10 K temperature range. We observed that Ta x N thin films can be tuned from an insulating phase to a superconducting phase (Tc ∼4K) by varying the nitrogen content during sputter deposition while the barrier height of MgO can also be controlled by deposition conditions and by tri-layer postdeposition annealing. Junction properties (Jc∼10-25 kA/cm 2 ), Mac Cumber parameter and RnIc product measured up to 1 mV are shown to be controlled by the reactive sputtering conditions. We have designed three pole band-pass filters and resonators in a micro-strip configuration and studied the junction coupling with the filters. We will show that a sigma-delta NbN technology is a suitable solution for analogue-to-digital conversion in the future generations of telecommunication satellites to achieve high sampling frequency and large bandwidth at high carrier frequency signal.
Superconductor-Normal-Superconductor (SNS) Josephson junctions were fabricated by ion implantation. The base electrode, NbN was prepared by reactive sputtering. After the NbN film surface was cleaned by Ar^+ ion (5.4keV), Ni^+ ion (16.2keV) was implanted. The doses of ion bombard the junction region were in the range of 1×10^<16>∿1×10^<17> ions/cm^2 for Ar^+ ion and 0.5×10^<15>∿1×10^<16> ions/cm^2 for Ni^+ ion. The counterelectrode was Pb. Current-voltage characteristics of the junctions subjected to the microwave radiation were measured. The junctions exhibited the well-defined ac Josephson steps, the best of which can show microwave induced Josephson steps up to 0.6mV. The magnetic field dependence of the maximum dc Josephson current was measured and found to be simillar to that of dc SQUID.
We successfully fabricated and studied planar superconductor-normal-superconductor (SNS) Josephson junctions made of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) high-TC{\mathrm{T}}_{C}TC​ superconductor (S) and Au normal metal (N). The crucial step in fabrication was done by using a Pt/Au bilayer encapsulation of the patterned YBCO nanogap. This procedure preserves the YBCO nanogap from the degradation due to the contact with resists, water, and chemicals involved in further processing for hybrid devices. The junctions exhibit Fraunhofer-like modulation patterns and Shapiro steps when they are irradiated by a rf field. Furthermore, we analyzed the IC(T){I}_{C}(T)IC​(T) characteristics of our hybrid junctions employing a ${\mathrm{SINI}}^{\ensuremath{'}}\mathrm{S}$ (superconductor--insulator--normal conductor--insulator--superconductor) model.