Growth and Characterization of Nb3Al Thin Films for Low-Noise Terahertz Electronics

Licentiate thesis, 2008

Demand on ultimately sensitive heterodyne receivers for, e.g., radio astronomy and environmental science instrumentation, has driven large research activities on novel materials and devices for extending the frequency range into the THz domain. The useful RF band of traditionally used niobium-based superconductor-insulator-superconductor (SIS) mixers is limited by the niobium superconducting gap frequency to 700 GHz. Therefore, for mixing applications at higher frequencies, high-energy gap superconductors are needed. As an alternative mixing component, the hot electron bolometer (HEB) device can also offer a compromise solution at THz frequencies and new materials could be an useful extension of this technology as well. This thesis presents the work on deposition and characterization of Nb3Al superconducting high-energy gap material thin films. The focus of the study aims at Nb3Al as a promising material for SIS tunnel junctions and HEB devices. A heteroepitaxial growth on a thin niobium underlayer has been applied to the preparation of the Nb3Al thin films by means of dc magnetron co-sputtering. Subsequent ex-situ rapid heat treatment of the deposited Nb3Al films has been proved to enhance the crystal ordering and thus, the superconducting properties of the film. The grown films have been studied by XRD, AFM, TEM, etc. techniques. The highest critical temperature observed in this work was 15.7 K and the values of all major superconducting parameters, such as penetration depth, coherence length etc. were consistent with previously reported results on A15 materials, hence, confirming the prospective to use such films in SIS tunnel junctions. In addition, an epitaxial growth of Nb3Al films on yttria stabilized zirconia substrate showed substantial possibility to grow monocrystalline Nb3Al ultra thin films. It was also found that the studied Nb3Al thin films possess the ability to carry substantially high current densities in the range of above 10E7 A/cm2 and can possibly find appropriate application where high magnetic fields are to be used.