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IEEE Transactions on Applied Superconductivity

NbN-MgO-NbN junctions prepared on room-temperature quartz substrates for quasiparticle mixers

B. Plathner, Matthias Schicke, Thomas Lehnert, K. H. Gundlach, Hermann Rothermel, Masahiro Aoyagi, S. Takada

Published Jun 1, 1997• 5 citations
Publisher

Abstract

NbN tunnel junctions are of great interest for THz heterodyne receivers because their large gap voltage of 5 mV yields an upper frequency limit of 4/spl Delta//h=2.4 THz for quasiparticle mixing. AC losses in NbN films, however, imply that a NbN matching circuit can be used at most to the NbN gap frequency, 2/spl Delta//h=1.2 THz. Another issue is the poor thermal conductivity of NbN films which complicates heat extraction from the junction. One solution to these problems are hybride mixer chips in which NbN junctions are integrated into appropriate superconducting or normal metal matching circuits. Both for waveguide mixers and quasioptical mixers quartz substrates are the first choice. We report on a room-temperature process for NbN-MgO-NbN junction fabrication which permits the realization of hybride mixer chips on quartz substrates. Appropriate plasma conditions for NbN film deposition are created by employing a second Nb target as a selective nitrogen pump. NbN films on quartz substrates without and with intermediate layers of Nb, Al, Al/SiO/sub 2/, and Nb/SiO/sub 2/ have critical temperatures above 15 K and normal state resistivities from 100 to 130 /spl mu//spl Omega/cm. In the first step, NbN junctions with nominal areas of 0.36 /spl mu/m/sup 2/ and current densities of 10 kA/cm/sup 2/ at 5.5 mV were integrated into Nb films forming the tuning circuit and a dipole antenna. The lowest receiver noise temperature in the 350 GHz range was 245 K double side band.

DOI
10.1109/77.621772

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Superconductor Science and Technology•Jun 10, 2009•10 citations
NbN tunnel junctions are of great interest for THz heterodyne receivers because their large gap voltage of 5 mV yields an upper frequency limit of 4/spl Delta//h=2.4 THz for quasiparticle mixing. AC losses in NbN films, however, imply that a NbN matching circuit can be used at most to the NbN gap frequency, 2/spl Delta//h=1.2 THz. Another issue is the poor thermal conductivity of NbN films which complicates heat extraction from the junction. One solution to these problems are hybride mixer chips in which NbN junctions are integrated into appropriate superconducting or normal metal matching circuits. Both for waveguide mixers and quasioptical mixers quartz substrates are the first choice. We report on a room-temperature process for NbN-MgO-NbN junction fabrication which permits the realization of hybride mixer chips on quartz substrates. Appropriate plasma conditions for NbN film deposition are created by employing a second Nb target as a selective nitrogen pump. NbN films on quartz substrates without and with intermediate layers of Nb, Al, Al/SiO/sub 2/, and Nb/SiO/sub 2/ have critical temperatures above 15 K and normal state resistivities from 100 to 130 /spl mu//spl Omega/cm. In the first step, NbN junctions with nominal areas of 0.36 /spl mu/m/sup 2/ and current densities of 10 kA/cm/sup 2/ at 5.5 mV were integrated into Nb films forming the tuning circuit and a dipole antenna. The lowest receiver noise temperature in the 350 GHz range was 245 K double side band.
This letter reports a low noise submillimeter-wave mixer using NbN tunnel junctions integrated in Nb matching circuits. The double side band receiver noise temperature was 245 K at 345 GHz. Plasma conditions for NbN film deposition on quartz substrates at room temperature are created by using a second Nb target as a selective nitrogen pump. Electrodes for tunnel junctions with critical temperatures above 15 K and normal state resistivities in the range from 130 to 160 μΩ cm were obtained. This permits integrating NbN junctions into normal metal or non-NbN superconducting matching circuits, which is of great interest for THz mixers.
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.
We report on the mixing properties of waveguide SIS mixers with epitaxial NbN/AlN/NbN junctions and NbN/SiO2/NbTiN microstrips on MgO substrates. The superconducting transition temperature and 20-K resistivity of NbTiN film on the SiO2dielectric were 14.7 K and 175 muOmega cm, respectively. The junction tuning circuit was composed of an NbN/SiO2/NbTiN microstrip and two parallel-connected NbN/AlN/NbN junctions 0.9 mum in diameter. The critical current density and normal-state resistance of the fabricated NbN junction were 13 kA/cm2and 22 Omega, respectively. The heterodyne response was measured using 295- and 77-K blackbody sources, a local oscillator (LO) source, and a 25-mum-thick Kapton film as a beam splitter. An intermediate frequency (IF) amplifier with a bandwidth of 4-12 GHz was used in the measurements. The receiver noise temperature corrected for losses in the beam splitter and in the vacuum window of the cryocooler was 370 K at 860 GHz and was comparable to that of all-NbN mixes with NbN/MgO/NbN microstrips.
The need for extremely sensitive heterodyne receivers in the astronomical community has created strong efforts to develop appropriate frequency mixers. Nb/Al-Al x O y -Nb tunnel junctions with Nb matching circuits lead to best results up to approximately 700 GHz, the energy-gap frequency of Nb. For higher frequencies the Nb in the matching circuit becomes lossy and is replaced beyond 800 GHz by Al (which operates in the normal conducting state). The gap frequency of NbN is as high as 1.2 THz, but NbN technology is not yet mature. Practical films still have substantial radio frequency losses and the barrier of NbN tunnel junctions is too leaky. More recently, NbTiN, for which the gap frequency is also about 1.2 THz, was found to have very low losses, and is therefore a good choice for tuning circuits. New interesting tunnel junctions for mixers around 1 THz are NbTiN-MgO-NbTiN and Nb/Al-AlN x -NbTiN. Excellent performance from about 1 THz up to several terahertz can be expected from hot-electron transition-edge bolometer mixers. They consist of NbN or Nb microbridges, they do not need matching circuits and their frequency limit is not determined by the gap frequency.
A generic NbN Superconducting Tunnel Junctions (STJ) technology has been developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 /spl mu/m/sup 2/, Jc of 10 kA/cm/sup 2/ and low sub-gap leakage current (Vm>25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250/spl deg/C. Using a thin MgO buffer layer deposited underneath the NbN electrodes, ensures lower NbN surface resistance values (Rs=7 /spl mu//spl Omega/) at 10 GHz and 4 K. Epitaxial NbN [100] films on MgO [100] with high gap frequency (1.4 THz) have also been achieved under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We have developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, we have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 /spl mu/m/sup 2/ junction area, made on large area substrates (up to 8 inches).
The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctions and an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated at frequencies above the gap frequency of Nb. To design the mixer, the complex refractive index of the MgO substrate was measured by using a THz time-domain spectrometer. The superconductive properties of NbTiN films on MgO were examined in detail. An uncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IF bandwidth of 4-12 GHz at a bath temperature of 4.2 K. By comparing the measured and calculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimated to be 0.98 dB at 810 GHz.