Fabrication techniques of superconducting devices based on intrinsic Josephson effect

iv Introduction The present thesis is the result of the work performed during my stage at INRIM (Istituto Nazionale di Ricerca Metrologica) which is part of an extended project with the final scope of fabrication of superconducting devices, based on BSCCO whiskers, single crystalline micrometric samples of the Bi-based High Temperature Superconductor (HTS). Such task will be accomplished by exploiting the peculiar structural properties of these material, characterized by an alternated sequence of insulating and superconducting planes, allowing to have a great number of in series Intrinsic Josephson Junctions (IJJs) in a single crystal. Josephson junctions of low temperature superconducting materials are very important as fundamental building blocks for several applications already produced on large scale and for other still under study. Possible examples are: electrical digital circuitry for ultrafast computers using Josephson logic, or Superconducting Quantum Interference Devices (SQUIDs) already largely applied as extremely sensitive magnetometers, or high precision voltage standards based on microwave frequency irradiation of in series connected junctions. In this framework, the production of devices for high level metrological applications based on HTS Intrinsic Josephson junctions, which could also be converted in high frequency emitters (GHz-THz range), is presently underway of study by many research groups. Our attention was focused on the optimization of the first steps of fabrication for these devices, taking into account the thermodynamic instability of Bi-based whiskers, the problems with the contact resistance between whiskers surface and electrodes. In order to avoid these disadvantage different techniques were tried: the first one based on annealing procedures, already known by the group but not yet optimized, the second technique is called CLEAVING, meaning the separation of the whiskers in two parts, managing to obtain a very smooth surface. In both cases the purpose is to obtain very low contact resistances, since the metal electrodes are in contact with the inner layers of the material, not compromised by the atmosphere and ageing. The cleaving process was also modified by adding a graphene-like exfoliation method, which resulted to be very effective. This dissertation is divided in two parts taken place at University of Turin and INRIM (Istituto Nazionale di Ricerca Metrologica). The first part was carried out at University of Turin, where a preliminary work was made consisting in the synthesis of BSCCO whiskers, choice of the best whiskers from a morphological point of view, and characterization of our crystals, electrically and structurally. About this work, the procedure and the instruments used will be explained (furnaces, deposition