Development of Advanced Technologies for High Frequency Radio Astronomy Detectors

The terahertz (THz) region of electromagnetic spectrum has a vast amount of astronomical information which is significantly unexplored despite having been the subject of research over the past few decades. Approximately 98% of all the photons emitted since the big bang are observed in the millimeter/THz range. However, this spectrum is still greatly unexplored due to the absence of low-noise sensors, the presence of atmospheric interference, and the vastness of sky. THz receivers used in the radio telescopes are extremely important for astronomers to study the chemistry of sky. High frequency signals collected by the telescopes need to be down-converted to a lower frequency, so they can be further amplified and analyzed. The down conversion needs to be accomplished by retaining as much information as possible of the original signal in a low noise process. High quality Nb-based superconductor-insulator-superconductor (SIS) mixers with aluminum oxide tunnel barriers grown from Al overlayers exhibit low noise temperature for signals below 670 GHz that makes them ideal for the terahertz detectors and are broadly reported in the literature. Almost all of the 230 GHz detectors of the Atacama Large Millimeter/sub-millimeter Array (ALMA) radio telescope in Chile that was recently used to image the black hole at the center of Messier 87 galaxy are based on this stack of material and were made at the University of Virginia Microfabrication Laboratories during the past three decades.

Current state-of-the-art for above 1 THz SIS based heterodyne mixers utilize the prototypical Nb/Al-AlNx/Nb(Ti)(N) junctions, however when operated above the gap frequency of Nb the performance is greatly reduced. For operation at higher frequencies and to realize larger IF bandwidths (even at lower frequencies), SIS junctions with higher current densities (lower tunnel barrier thickness) are of interest. Performance of SIS mixers are limited, among other things, by the characteristics of the superconducting material and insulating tunnel barrier. A turning point in the high frequency SIS technology is utilizing alternative superconducting materials with larger energy gap than Nb, such as niobium-titanium-nitride (NbTiN) and alternative insulating layer barrier materials such as aluminum-nitride (AlN) that is capable of supporting higher current densities. Development and characterization of alternative superconducting and insulation barrier material and improvement of SIS fabrication techniques, possess significant engineering and scientific challenges. This study lays the framework of investigation and development of higher frequency superconducting junctions utilizing novel deposition technologies and optimized fabrication processes.