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Gao, J. R., Hajenius, M., Baselmans, J. J. A., Klapwijk, T. M., de Korte, P. A. J., Voronov, B., et al. (2004). NbN hot electron bolometer mixers with superior performance for space applications. In E. Armandillo, & B. Leone (Eds.), Proc. Int. workshop on low temp. electronics (pp. 11–17). Noordwijk.
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Baselmans, J. J. A., Hajenius, M., Gao, J. R., Klapwijk, T. M., de Korte, P. A. J., Voronov, B., et al. (2003). Noise performance of NbN hot electron bolometer mixers at 2.5 THz and its dependence on the contact resistance. In Proc. 14th Int. Symp. Space Terahertz Technol. (pp. 11–19).
Abstract: NbN hot electron bolometer mixers (HEBM) are at this moment the best heterodyne receivers for frequencies above 1 Thz. However, the fabrication procedure of these devices is such that the quality of the interface between the NbN superconducting film and the contact structure is not under good control. The result is a low transparency interface between the bolometer itself and the contact/antenna structure. In this paper we report a detailed experimental study on a novel idea to increase the transparency of this interface. This leads to a record sensitivity and more reproducible performance. We compare identical bolometers, coupled with a spiral antenna, with different NbN bolometer-contact pad interfaces. We find that cleaning the NbN interface alone results in an increase in the noise temperature. However, cleaning the NbN interface and adding a thin additional superconductor prior to the gold contact deposition improves the noise temperature of the HEBm with more than a factor of 2. A device with a contact pad on top of an in-situ cleaned NbN film consisting of 10 nm of NbTiN and 40 nm of gold has a DSB noise temperature of 1050 K at 2.5 THz.
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Hoevers, H. F. C., Bento, A. C., Bruijn, M. P., Gottardi, L., Korevaar, M. A. N., Mels, W. A., et al. (2000). Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer. Appl. Phys. Lett., 77(26), 4421–4424.
Abstract: The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.
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