Thin Films and Component Technologies for Low-Noise Terahertz Electronics

Doktorsavhandling, 2010

This thesis deals with several distinct subjects such as thin film deposition, design, fabrication and characterization of components for low-noise THz electronics. High energy-gap superconducting materials are of specific importance for low-noise THz mixer applications. Extending the upper operation frequency of superconductor-insulator-superconductor (SIS) mixers above 1.5 THz is of particular interest. Here, a novel approach of producing Nb3Al thin film based on DC magnetron co-sputtering using heteroepitaxial, as well as epitaxial growth techniques, has been employed. As a result, a superconducting transition temperature, as high as 15.7 K has been observed. Structural and electrical characterization techniques have been utilized to evaluate the set of material and superconducting parameters of the deposited Nb3Al thin films. The superconducting properties of the obtained Nb3Al films have revealed strong potential for using them in practical applications. It has been also demonstrated that Nb3Al can be epitaxially grown on yttria-stabilized-zirconia substrates, thus opening a possibility for hot-electron bolometer (HEB) applications. Another substantial part of this work deals with the in-house development of phonon-cooled HEB mixers. A reliable deposition process for high-quality ultra-thin NbN films has been established. Epitaxial ultra-thin NbN films grown on 3C-SiC/Si substrate with state-of-the-art performance have shown a strong potential to be used in THz HEB mixers with a purpose for improving the intermediate frequency (IF) bandwidth. A novel membrane-like layout for 1.6-2.0 THz HEB mixer has been suggested, developed and implemented for use with micromachined all-metal waveguides. Being the most used in mm- and submillimeter mixer technology for radio astronomy, Nb/Al-AlOx/Nb tunnel junction stability properties are of great importance. In this thesis, the behaviour of a large number high-quality Nb/Al-AlOx/Nb tunnel junctions, subjected to long-term room temperature aging and thermal annealing is systematically studied and explanations of the observed phenomena are suggested. Finally, the work on development of a planar superconducting IF/biasing circuit for use with a SIS mixer is presented. The circuit was integrated into a 385-500 GHz DSB SIS mixer currently under operation at the APEX telescope. Circuit design methodology, measurement results and overall mixer performance with the integrated circuit are shown.