Nb-based circuits have broad applications in quantum-limited photon detectors, low-noise parametric amplifiers, superconducting digital logic circuits, and low-loss circuits for quantum computing. The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction material is the Gurvitch trilayer process based on magnetron sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al overlayer [1]. However, a current limitation of elemental Nb-based circuits is the low-loss operation of THz circuits operating above the 670 GHz gap frequency of Nb and operation at higher temperatures for projects with a strict power budget, such as space-based applications. NbTiN is an alternative higher energy gap material and we have previously reported on the first NbTiN/AlN/NbTiN superconducting-insulating-superconducting (SIS) junctions with an epitaxially grown AlN tunnel barrier [2]. One drawback of a directly grown tunnel barrier compared to thermal oxidation is control of the barrier thickness and the uniformity of thickness across a substrate, leading to variations in current density (Jc). Semiconductor barriers with smaller barrier heights enable thicker tunnel barriers for a given Jc[3]. GaN is an alternative semiconductor material with a closed-packed Wurtzite crystal structure similar to AlN and it can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion Beam Deposition (RBTIBD) technique. This work presents the preliminary results of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with underdamped SIS I(V) characteristics
Nb-based circuits have broad applications in quantum-limited photon detectors, low-noise parametric amplifiers, superconducting digital logic circuits, and low-loss circuits for quantum computing. The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction material is the Gurvitch trilayer process based on magnetron sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al overlayer [1]. However, a current limitation of elemental Nb-based circuits is the low-loss operation of THz circuits operating above the 670 GHz gap frequency of Nb and operation at higher temperatures for projects with a strict power budget, such as space-based applications. NbTiN is an alternative higher energy gap material and we have previously reported on the first NbTiN/AlN/NbTiN superconducting-insulating-superconducting (SIS) junctions with an epitaxially grown AlN tunnel barrier [2]. One drawback of a directly grown tunnel barrier compared to thermal oxidation is control of the barrier thickness and the uniformity of thickness across a substrate, leading to variations in current density (Jc). Semiconductor barriers with smaller barrier heights enable thicker tunnel barriers for a given Jc[3]. GaN is an alternative semiconductor material with a closed-packed Wurtzite crystal structure similar to AlN and it can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion Beam Deposition (RBTIBD) technique. This work presents the preliminary results of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with underdamped SIS I(V) characteristics
High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology-reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.
Within the past few years all-refractory superconductor-insulator-superconductor (SIS) tunnel junctions based on Nb/Al/AlO/sub x//Nb technology have displaced lead alloy devices as the preferred junctions for SIS mixers. There is growing interest in materials with higher superconducting energy gaps. NbN has a high gap ( approximately 3 meV) and higher critical temperature than Nb. Its main drawback, apart from the greater complexity of the material itself, stems from its short superconducting coherence length which results in a decrease in the measured gap voltage in proximity barrier structures, as generally used for Nb-based junctions. For this reason, most recent work has concentrated on the direct deposition of MgO as a tunnel barrier. The authors summarise work on Nb devices for MM-wave mixers and then report recent work performed at Cambridge on the fabrication of high quality NbN device structures.
We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.
A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
ABSTRACT: Active superconducting circuits based on picoseconds switching, i.e. THz oscillating, Josephson tunnel junctions are expected to find a large domain of applications in spatial sub-millimeter wave and FIR detectors as well as in satellite based wideband signal and data processors. In the first case SIS (superconductor-insulator-superconductor) tunnel junctions are required, while in the second case SNS (superconductor-normal metal-superconductor) self-shunted junctions are preferred. We present the advantages of the nitride junction technology currently developed at CEA-Grenoble, based on high-performance MTS (medium temperature superconductor) NbN films in both SIS and SNS junctions. In the SIS case the device performances rely on a sputter deposited and post-annealed, only 0.5-0.7 nm thick, dielectric MgO barrier. This leads to junctions with a Josephson critical current in the range of 10-25 kA/cm, and a large superconducting energy gap (> 5 meV) associated with a low sub-gap quasi-particle leakage current. These parameters are suitable for low “noise temperature ” heterodyne mixers, local oscillators, and integrated receivers in the 0.8 to 1.4 THz frequency range. In the SNS case, a new