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IEEE Transactions on Applied Superconductivity

Hot electron effect in terahertz hybrid devices

Bruno Leone, J. R. Gao, T. M. Klapwijk, Brian D. Jackson, W. M. Laauwen, Gert J. de Lange

Published Mar 1, 2001• 7 citations
Publisher

Abstract

We analyse both the direct current and submillimeter pumped current-voltage characteristics of a hybrid superconductor-insulator-superconductor terahertz mixer consisting of a Nb tunnel junction integrated with NbTiN tuning striplines. We find that the presence of the Nb/NbTiN interface gives rise to a hot electron effect. The assumption of an equilibrium electron temperature, as distinct from the phonon temperature, allows a complete understanding of the processes involved. The impact of the hot electron effect on superconducting terahertz detectors is discussed.

DOI
10.1109/77.919428

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IEEE Transactions on Terahertz Science and Technology•Sep 30, 2020•5 citations
We analyse both the direct current and submillimeter pumped current-voltage characteristics of a hybrid superconductor-insulator-superconductor terahertz mixer consisting of a Nb tunnel junction integrated with NbTiN tuning striplines. We find that the presence of the Nb/NbTiN interface gives rise to a hot electron effect. The assumption of an equilibrium electron temperature, as distinct from the phonon temperature, allows a complete understanding of the processes involved. The impact of the hot electron effect on superconducting terahertz detectors is discussed.
In this paper, we present a method to measure the effective physical temperature of the tunnel junction in a terahertz superconductor-insulator-superconductor (SIS) mixer when illuminated with a strong local oscillator (LO) power that results in local hot spot around the tunnel junction. We observed that the gap voltage of the pumped current-voltage (IV) curve is suppressed when the LO pumping level is increased, indicating that the junction physical temperature is increased beyond the mixer block temperature. We quantified this extra heating effect by recovering the effective junction temperature through comparing the gap voltage of the pumped IV curves measured at a fixed block temperature, with the unpumped IV curve measured at varying block temperatures. We found that the heat trapped in the tunnel junction can be as high as 1.7 K when the mixer stabilised at 3.3 K is pumped at only 21% of the gap current.
We have designed superconductor-insulator-superconductor (SIS) mixers using Nb/AlN/Nb tunnel junctions integrated with NbTiN/SiO2/NbTiN microstrip tuning circuits for the Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 (0.78-0.95 THz). In order to design the tuning circuits, we used terahertz time domain spectroscopy to experimentally derive the complex conductivity of relatively thick NbTiN films deposited on quartz substrate directly as a ground plane and on RF-sputtered SiO2as a microstrip. Results showed that the conductivities of both films can be described by a modified Mattis-Bardeen theory. We also investigated the junction heating effects originating from quasi-particle trapping in the Nb electrodes embedded in NbTiN circuits by varying the volume of the electrodes. It was confirmed that current-voltage curves of Nb/AlN/Nb tunnel junctions with thick (~200 nm) counter electrodes contacting NbTiN wirings showed higher gap voltages as well as reduced back-bending than those with thin (~50 nm) electrodes. Making use of these findings, our designed SIS mixers employing Nb/AlN/Nb junctions with a current density of 15 kA/cm2and all-NbTiN tuning circuits may cover the Band 10 with good sensitivity.
It is expected that the integration of NbTiN striplines with Nb-based SIS junctions will extend the range of operation of sensitive SIS mixers up to the energy gap of NbTiN (— 1.2-1.25 THz) to meet the requirements of the HIFI instrument for the Far-Infrared Space Telescope (FIRST). We have fabricated prototype RS waveguide mixers in which Nb/A1-A10,INb SIS junctions are integrated with NbTiN and Al wiring layers. DC current-voltage measurements of these devices show that the voltage gap is somewhat lower and the sub-gap current is somewhat higher for devices with two NbTiN wiring layers in comparison with those in which the top wiring layer is Al (Vgap = 2.7 mV vs. 2.8 mV, and R 2.0/RN — 32 vs. 35). Additionally, the gap voltage of the NbTiN stripline devices is further suppressed by the application of 890-1040 GHz local oscillator power during heterodyne measurements. These effects are attributed to the trapping of heat in the Nb junction electrodes due primarily to quasi-particle trapping at the Nb/NbTiN interface. FTS measurements result in what is believed to be the first reported observation of direct detection response in a NbTiN stripline device above 1.1 'THz. From an analysis of the resonance frequencies of several devices, it appears that the frequency response of the NbTiN striplines is predicted reasonably well by the MattisBardeen theory for the conductivity of a superconductor in the extreme anomalous limit. Furthermore, we present what is believed to be the first reported heterodyne measurements of NbTiN stripline devices in the 0.9-1 THz range. These measurements yield a receiver sensitivity of TN = 830 K at 930 GHz at a bath temperature of 3.3 K, after correction for beamsplitter losses.
The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.
We describe the performance of a superconductor- insulator-superconductor (SIS) mixer operating in the frequency range of 780-950 GHz. Unlike most SIS mixers, the tunnel junction employs two different superconductors, a niobium nitride top and a niobium bottom electrode sandwiching an aluminum nitride barrier layer, fabricated on a niobium titanium nitride ground plane. The mixer was tested in a pulse tube cryostat, with all the optical components, in the signal path, mounted inside the vacuum environment to avoid attenuation of the RF signal as it propagates from the hot/cold loads to the mixer. With this setup, we have measured an RF-corrected noise temperature of ~220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current-voltage (I-V ) curve as the LO pumping level is increased. Similar observation has already been reported, and attempts were made to estimate the effective temperature of the device using equations of heat transfer between the mixer chip layers. Here, we present an experimental method of quantifying this effect by recovering the effective temperature of the junction through comparing the pumped I-V curves at different pumping levels and fixed bath temperature, with the unpumped I-V curves obtained at varying bath temperatures. We also estimate, for the first time, the effect of heating on the noise temperature as a function of bath temperature and frequency. We show that for typical experimental parameters, the LO heating can increase the double-sideband receiver noise temperature by as much as 20%, and that in the frequency range of the measurements, the effective temperature of the junction at fixed LO power increases linearly with frequency at a rate of 0.5 K/100 GHz.