Investigation of Material Growth and Fabrication Processes for THz Frequency SIS Mixers

Research and development of THz electronics seeks to comprehend and utilize one of the last uncharted regions of the electromagnetic spectrum. Sandwiched directly between the microwave and far-infrared regions, THz devices often involve a hybrid fusion of optical and small-scaled RF technologies, frequently requiring novel design, materials, and fabrication techniques. Despite the increased complexity, when compared to many well-established RF and optical technologies, THz receivers used in telescope imaging applications are of great importance to radio astronomers, with current large scale multinational radio telescope projects, such as the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile designed to observe the universe from 31 to 950 GHz--- the field of radio astronomy will be one of the immediate benefactors from the ongoing research of higher frequency THz detection.

Heterodyne receivers, based on the Nb/Al-Al2O3/Nb SIS tunnel junction, have demonstrated sensitivities approaching the quantum limit, making them ideal for high-sensitivity THz detectors. However, as design frequencies approach 1 THz and beyond, both the Nb electrodes and Al2O3 tunnel barrier of the prototypical Nb/Al-Al2O3/Nb SIS junction begin to demonstrate poor performance. A turning point in SIS technology was through the development of ICP grown AlN as an alternative barrier material, first reported by our research group in 2007. However, due to run to run variation, our original ICP configuration made realization of AlN based SIS mixers impossible at the time.

In this work, we present the modifications of our trilayer deposition system that allowed our research group to realize AlN based SIS mixers meeting all ALMA Band-8 specifications. Additionally, we report on a novel fabrication process that significantly reduced fabrication time of high-quality SIS test devices. ICP nitridation of Al overlayers is inherently a complex process, and little is reported in the literature as to what affects AlN growth. With this in mind, we report on detailed investigations of ICP nitridation, using ellipsometry and spectroscopic analysis, and provide a better understanding of the processes involved in the realization of high-quality ICP grown AlN based SIS trilayer.