Applications of tunnel junctions in low-dimensional nanostructures

Abstract

This thesis concentrates on studies of AlOx based tunnel junctions and their feasibility for cooling, thermometry and strain sensing in suspended nanostructures. The main result of the thesis is cooling of one dimensional phonon modes of a suspended nanowire with normal metal insulator superconductor (NIS) tunnel junctions. Simultaneous cooling of both electrons and phonons was achieved, and the lowest phonon temperature reached in the system was 42 mK with an initial temperature of 100 mK. In addition, suspended devices show cooling still at a bath temperature of 600 mK. The observed thermal transport characteristics show, that the heat flow is limited by the scattering of phonons at the bulk nanowire interface. The properties of Al AlOx Al tunnel junctions can be improved by thermal annealing in vacuum at 350 450 C. Annealing treatment will lead to complete stabilization of the junctions, and on increase in both tunneling resistance and charging energy. In addition, the annealing process shows a marked improvement in the tunneling conductance spectrum, indicated by a disappearance of several resonances, which are a fingerprint of either resonant or inelastic tunneling processes caused typically by impurities located in the tunneling barrier. The superconducting gap of Al is not affected, but the supercurrent is reduced, consistent with the increase of tunneling resistance. Feasiblity of conventional, sub micron sized Al AlOx Al tunnel junctions in sensing strain, and therefore displacement, is demonstrated in the final chapter of this thesis. Tunnel junctions show a good response to applied strain (gauge factor), which is competitive with existing strain and displacement detectors.