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/k_B, where h is the Planck’s constant, f is the frequency, and k_B is Boltzmann’s constant. In the upper part of this frequency range, the noise temperature increases up to 390 K.