A range of tunnelling phenomena were observed in clean Nb-Si interfaces. In the most non-transmissive Nb-Si interfaces, the Schottky barrier acted as an insulating tunnel barrier. Thus, electrical characteristics similar to those obtained from normal-insulator-superconductor junctions were observed with a conductance dip about the Nb superconducting energy gap. As the transmission through the interface was increased, analysis of differential resistance bias dependence measurements indicated that only a small percentage of the contact contributed to conduction. This was attributed to the formation of a highly non-uniform Schottky barrier when the depletion width is comparable with the inter-dopant spacing. Evidence of constructive quantum interference between electrons and Andreev reflected holes was observed and its insensitivity to an applied magnetic indicated that it was caused by the presence of a disordered elastic scattering region adjacent to the Nb-Si interface. Subsequent enhancement in the transmissivity of the junctions indicated that Andreev reflection induced weak localization was occurring as predicted by theory. The fundamental limit in Nb-Si barrier transmissivity was found to be caused by the Fermi velocity mismatch between the Nb and the Si. When Nb<SUB>3</SUB>Si particles were present at the Nb-Si interface, a dramatic enhancement in conductance was observed when the temperature was lowered past the Nb<SUB>3</SUB>Si superconducting critical temperature. The presence of Nb<SUB>3</SUB>Si particles reduced the tunnelling path length through the Schottky barrier and opened new conduction channels. As the temperature was lowered further, the normal coherence length exceeded the Nb-Nb<SUB>3</SUB>Si separation distance so that interfacial Josephson junctions were formed. The critical current of these Josephson junctions could be measured directly by applying a large d.c. voltage bias across the interface. The critical current of the Josephson junctions was quantized as a function of temperature because the junction cross-sectional area increased with decreasing temperature. This phenomenon was successfully modelled by assuming that superconductivity was suppressed at the edge of a Nb<SUB>3</SUB>Si particle over a temperature dependent coherence length and that the Josephson junction cross-sectional width is equivalent to the width of the Nb<SUB>3</SUB>Si particle. This provides a temperature dependent expression for the number of one-dimensional quantized modes passing through the Josephson junction which is in good agreement with the experimentally observed critical current dependence.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596675 |
| Date | January 1997 |
| Creators | Black, M. J. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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