IF bandwidth and noise temperature measurements of NbN HEB mixers on MgO substrates

We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.