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Gao JR, Hajenius M, Tichelaar FD, Klapwijk TM, Voronov B, Grishin E, et al. Monocrystalline NbN nanofilms on a 3C-SiC∕Si substrate. Appl Phys Lett. 2007;91(6):062504 (1 to 3).
Abstract: The authors have realized NbN (100) nanofilms on a 3C-SiC (100)/Si(100) substrate by dc reactive magnetron sputtering at 800°C. High-resolution transmission electron microscopy (HRTEM) is used to characterize the films, showing a monocrystalline structure and confirming epitaxial growth on the 3C-SiC layer. A film ranging in thickness from 3.4to4.1nm shows a superconducting transition temperature of 11.8K, which is the highest reported for NbN films of comparable thickness. The NbN nano-films on 3C-SiC offer a promising alternative to improve terahertz detectors. For comparison, NbN nanofilms grown directly on Si substrates are also studied by HRTEM.
The authors acknowledge S. V. Svetchnikov at National Centre for HRTEM at Delft, who prepared the specimens for HRTEM inspections. This work was supported by the EU through RadioNet and INTAS.
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Baselmans JJA, Baryshev A, Reker SF, Hajenius M, Gao JR, Klapwijk TM, et al. Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers. J Appl Phys. 2006;100(8):084510 (1 to 7).
Abstract: We present a detailed experimental study of the direct detection effect in a small volume (0.15μm×1μm×3.5nm) quasioptical NbN phonon cooled hot electron bolometer mixer at 673GHz. We find that the small signal noise temperature, relevant for an astronomical observation, is 20% lower than the noise temperature obtained using 300 and 77K calibration loads. In a separate set of experiments we show that the direct detection effect is caused by a combination of bias current reduction when switching from the 77 to the 300K
load in combination with the bias current dependence of the receiver gain. The bias current dependence of the receiver gain is shown to be mainly caused by the current dependence of the mixer gain.
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Yang ZQ, Hajenius M, Baselmans JJA, Gao JR, Voronov B, Gol’tsman GN. Reduced noise in NbN hot-electron bolometer mixers by annealing. Supercond Sci Technol. 2006;19(4):L (9 to 12).
Abstract: We find that the sensitivity of heterodyne receivers based on superconducting hot-electron bolometers (HEBs) increases by 25–30% after annealing at 85 °C in vacuum. The devices studied are twin-slot antenna coupled mixers with a small NbN bridge of 1 × 0.15 µm2. We show that annealing changes the device properties as reflected in sharper resistive transitions of the complete device, apparently reducing the device-related noise. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and a bath temperature of 4.3 K.
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Baselmans J, Kooi J, Baryshev A, Yang ZQ, Hajenius M, Gao JR, et al. Full characterization of small volume NbN HEB mixers for space applications. In: Proc. 16th Int. Symp. Space Terahertz Technol. Göteborg, Sweden; 2005. p. 457–62.
Abstract: NbN phonon cooled HEB’s are one of the most promising bolometer mixer technologies for (near) future (space) applications. Their performance is usually quantified by mea- suring the receiver noise temperature at a given IF frequency, usually around 1 – 2 GHz. However, for any real applications it is vital that one fully knows all the relevant properties of the mixer, including LO power, stability, direct detection, gain bandwidth and noise bandwidth, not only the noise temperature at low IF frequencies. To this aim we have measured all these parameters at the optimal operating point of one single, small volume quasioptical NbN HEB mixer. We find a minimum noise temperature of 900 K at 1.46 THz. We observe a direct detection effect indicated by a change in bias current when changing from a 300 K hot load to a 77 K cold load. Due to this effect we overestimate the noise temperature by about 22% using a 300 K hot load and a 77 K cold load. The LO power needed to reach the optimal operating point is 80 nW at the receiver lens front, 59 nW inside the NbN bridge. However, using the isothermal technique we find a power absorbed in the NbN bridge of 25 nW, a difference of about a factor 2. We obtain a gain bandwidth of 2.3 GHz and a noise bandwidth of 4 GHz. The system Allan time is about 1 sec. in a 50 MHz spectral bandwidth and a deviation from white noise integration (governed by the radiometer equation) occurs at 0.2 sec., which implies a maximum integration time of a few seconds in a 1 MHz bandwidth spectrometer.
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Wild W, de Graauw T, Baryshev A, Bos A, Gao JR, Gunst A, et al. Terahertz technology for ESPRIT – a far-infrared space interferometer. In: Proc. 16th Int. Symp. Space Terahertz Technol. Göteborg, Sweden; 2005.
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Hajenius M, Baselmans JJA, Gao JR, Klapwijk TM, de Korte PAJ, Voronov B, et al. Improved NbN phonon cooled hot electron bolometer mixers. In: Proc. 14th Int. Symp. Space Terahertz Technol. Tucson, USA; 2003. p. 413–23.
Abstract: NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized with negligible contact resistance to Au pads. By adding either a 5 nm Nb or a 10 nm NbTiN layer between the Au and NbN, to preserve superconductivity in the NbN under the Au contact pad, superior noise temperatures have been obtained. Using DC I,V curves and resistive transitions in combination with process parameters we analyze the nature of these improved devices and determine interface transparencies.
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Klapwijk TM, Barends R, Gao JR, Hajenius M, Baselmans JJA. Improved superconducting hot-electron bolometer devices for the THz range. In: Proc. SPIE. Vol 5498.; 2004. p. 129–39.
Abstract: Improved and reproducible heterodyne mixing (noise temperatures of 950 K at 2.5 THz) has been realized with NbN based hot-electron superconducting devices with low contact resistances. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, has been used to understand the physical conditions during the mixing process. We find that the mixing is predominantly due to the exponential rise of the local resistivity as a function of electron temperature.
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Zhang W, Miao W, Zhong JQ, Shi SC, Hayton DJ, Vercruyssen N, et al. Temperature dependence of superconducting hot electron bolometers. In: Not published results: 24th international symposium on space terahertz technology.; 2013.
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Gao JR, Hajenius M, Tichelaar FD, Voronov B, Grishina E, Klapwijk TM, et al. Can NbN films on 3C-SiC/Si change the IF bandwidth of hot electron bolometer mixers? In: Proc. 17th Int. Symp. Space Terahertz Technol.; 2006. p. 187–9.
Abstract: We realized ultra thin NbN films sputtered grown on a 3C-SiC/Si substrate. The film with a thickness of 3.5-4.5 nm shows a 1', of 11.8 K, which is the highest I`, observed among ultra thin NbN films on different substrates. The high-resolution transmission electron microscopy (HRTEM) studies show that the film has a monocrystalline structure, confirming the epitaxial growth on the 3C-SiC. Based on a two-temperature model and input parameters from standard NbN films on Si, simulations predict that the new film can increase the IF bandwidth of a HEB mixer by about a factor of 2 in comparison to the standard films. In addition, we find standard NbN films on Si with a T c of 9.4 K have a thickness of around 5.5 nm, being thicker than expected (3.5 nm).
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Gao JR, Hajenius M, Baselmans JJA, Yang ZQ, Baryshev AM, Barends R, et al. Twin-slot antenna coupled NbN hot electron bolometer mixers for space applications. In: Proc. 9-th WMSCI. Vol 9. International Institute of Informatics and Systemics; 2005. p. 148–53.
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