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Kooi, J. W., Baselmans, J. J. A., Hajenius, M., Gao, J. R., Klapwijk, T. M., Dieleman, P., et al. (2007). IF impedance and mixer gain of NbN hot electron bolometers. J. Appl. Phys., 101(4), 044511.
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Gao, J. R., Hajenius, M., Tichelaar, F. D., Klapwijk, T. M., Voronov, B., Grishin, E., et al. (2007). Monocrystalline NbN nanofilms on a 3C-SiC∕Si substrate. Appl. Phys. Lett., 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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Kooi, J. W., Baselmans, J. J. A., Baryshev, A., Schieder, R., Hajenius, M., Gao, J. R., et al. (2006). Stability of heterodyne terahertz receivers. J. Appl. Phys., 100(6), 064904 (1 to 9).
Abstract: In this paper we discuss the stability of heterodyne terahertz receivers based on small volume NbN phonon cooled hot electron bolometers (HEBs). The stability of these receivers can be broken down in two parts: the intrinsic stability of the HEB mixer and the stability of the local oscillator (LO) signal injection scheme. Measurements show that the HEB mixer stability is limited by gain fluctuations with a 1∕f spectral distribution. In a 60MHz noise bandwidth this results in an Allan variance stability time of ∼0.3s. Measurement of the spectroscopic Allan variance between two intermediate frequency (IF) channels results in a much longer Allan variance stability time, i.e., 3s between a 2.5 and a 4.7GHz channel, and even longer for more closely spaced channels. This implies that the HEB mixer 1∕f noise is strongly correlated across the IF band and that the correlation gets stronger the closer the IF channels are spaced. In the second part of the paper we discuss atmospheric and mechanical system stability requirements on the LO-mixer cavity path length. We calculate the mixer output noise fluctuations as a result of small perturbations of the LO-mixer standing wave, and find very stringent mechanical and atmospheric tolerance requirements for receivers operating at terahertz frequencies.
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Hajenius, M., Baselmans, J. J. A., Baryshev, A., Gao, J. R., Klapwijk, T. M., Kooi, J. W., et al. (2006). Full characterization and analysis of a terahertz heterodyne receiver based on a NbN hot electron bolometer. J. Appl. Phys., 100(7), 074507.
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Baselmans, J. J. A., Baryshev, A., Reker, S. F., Hajenius, M., Gao, J. R., Klapwijk, T. M., et al. (2006). Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers. J. Appl. Phys., 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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Zhang, W., Miao, W., Zhong, J. Q., Shi, S. C., Hayton, D. J., Vercruyssen, N., et al. (2014). Temperature dependence of the receiver noise temperature and IF bandwidth of superconducting hot electron bolometer mixers. Supercond. Sci. Technol., 27(8), 085013 (1 to 5).
Abstract: In this paper we study the temperature dependence of the receiver noise temperature and IF noise bandwidth of superconducting hot electron bolometer (HEB) mixers. Three superconducting NbN HEB devices of different transition temperatures (Tc) are measured at 0.85 THz and 1.4 THz at different bath temperatures (Tbath) between 4 K and 9 K. Measurement results demonstrate that the receiver noise temperature of superconducting NbN HEB devices is nearly constant for Tbath/Tc, less than 0.8, which is consistent with the simulation based on a distributed hot-spot model. In addition, the IF noise bandwidth appears independent of Tbath/Tc, indicating the dominance of phonon cooling in the investigated HEB devices.
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Zhang, W., Khosropanah, P., Gao, J. R., Kollberg, E. L., Yngvesson, K. S., Bansal, T., et al. (2010). Quantum noise in a terahertz hot electron bolometer mixer. Appl. Phys. Lett., 96(11), 111113–(1–3).
Abstract: We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The beta-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.
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Baselmans, J. J. A., Baryshev, A., Reker, S. F., Hajenius, M., Gao, J. R., Klapwijk, T. M., et al. (2005). Direct detection effect in small volume hot electron bolometer mixers. Appl. Phys. Lett., 86(16), 163503 (1 to 3).
Abstract: We measure the direct detection effect in a small volume (0.15μm×1μm×3.5nm)(0.15μm×1μm×3.5nm) quasioptical NbN phonon cooled hot electronbolometermixer at 1.6THz1.6THz. We find that the small signal sensitivity of the receiver is underestimated by 35% due to the direct detection effect and that the optimal operating point is shifted to higher bias voltages when using calibration loads of 300K300K and 77K77K. Using a 200GHz200GHzbandpass filter at 4.2K4.2K the direct detection effect virtually disappears. This has important implications for the calibration procedure of these receivers in real telescope systems.
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Khosropanah, P., Gao, J. R., Laauwen, W. M., Hajenius, M., & Klapwijk, T. M. (2007). Low noise NbN hot electron bolometer mixer at 4.3 THz. Appl. Phys. Lett., 91, 221111 (1 to 3).
Abstract: We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300 K at the optimum local oscillator (LO) power of 330 nW, which is about 12 times the quantum noise (hnu/2kB). Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations.
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Gao, J. R., Hovenier, J. N., Yang, Z. Q., Baselmans, J. J. A., Baryshev, A., Hajenius, M., et al. (2005). Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer. Appl. Phys. Lett., 86, 244104 (1 to 3).
Abstract: We report the first demonstration of an all solid-stateheterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.
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