Low-noise NbN Phonon-cooled Hot-Electron Bolometer Mixers for Terahertz Heterodyne Receivers

摘要

The mixers were fabricated from NbN film deposited on a high-resistivity Si substrate with a MgO buffer layer. The mixer element was integrated with a log-periodic spiral antenna. The noise temperature measurements were performed at 2.5 THz and at 3.8 THz local oscillator frequencies for the 3×0.2 μm{sup}2 active area devices. The best uncorrected receiver noise temperatures found for these frequencies are 1300 K and 3100 K, respectively. A water vapour discharge laser was used as the LO source. The gain bandwidth of the mixer based on 3.5 nm NbN film deposited on Si with MgO is 4.2 GHz and the noise band-width for the same device amounts to 5 GHz. The results of the research into decrease of the direct detection contribution to the measured Y-factor and a possible error of noise temperature calibration are presented. The use of a square nickel cell mesh as an IR-filter enabled us to avoid the effect of direct detection and measure apparent value of the noise temperature which was 16% less than that obtained using conventional black polyethylene IR-filter.