The 700-950 GHz Superconducting Receiving Structures for Radio Astronomy

We have designed, fabricated, and tested the waveguide receiving element, which is based on the tunnel superconductor-insulator-superconductor structures, for the frequency range 790-950 GHz. Two Nb/AlN/NbN tunnel junctions are incorporated in a microstrip line consisting of the bottom NbTiN-film electrode with a thickness of about 300 nm and the top 500 nm-thick aluminum electrode. The production-process optimization allows one to ensure the following characteristics of these junctions: submicron area (0.5 μm2 for each junction), current density about 30 kA/cm2, and band gap width 3.2 mV. Such tunnel junctions ensure the receiver operation in a wide frequency range (700-950 GHz), which was confirmed by the Fourier transform spectroscopy and the noise-temperature measurements. At a frequency of 725 Hz, the corrected noise temperature of the receiver amounts to 120 K, which is only threefold greater than the quantum limit hf/kB, where h is the Planck's constant, f is the frequency, and kB is Boltzmann's constant. In the upper part of this frequency range, the noise temperature increases up to 390 K.