The influence of contact transparency on the superconducting proximity effect in thin semiconducting films

The superconducting proximity effect allows for the introduction of pair correlations into otherwise normal metals provided that they are coupled through a sufficiently transparent junction. The influence of this proximity effect manifests itself by modifying both the normal layer sheet resistance in the proximity affected region, Rs, and the junction conductance across the N-S boundary, Gc. These two quantities are impossible to measure simultaneously with any single two terminal device even if it is a four point measurement. However, a new three terminal device structure allows us to make two independent four point voltage measurements, which permits the extraction of these two intrinsic aspects of the proximity effect when combined with simple Ohm's law modeling. Devices with completely in-situ junctions between niobium and heavily doped n-GaAs and n-InAs were fabricated via molecular beam epitaxy. In order to reduce the Schottky barrier, a graded and delta-doped InGaAs cap was inserted at the interface. Careful construction of the doping profile in the cap allows for extremely transparent junctions just prior to the onset of superconductivity, the most transparent Nb-GaAs junctions yet reported. The transparency of the junction can be evaluated by calculating the number of available quantum channels between the two different Fermi surfaces and using the Landauer formalism to determine the ideal junction conductance. Comparison to the experimental junction conductance permits the discovery of the fundamental transmission coefficient for transport across the N-S interface. If the semiconducting depth is small enough the presence of correlations in the semiconductor are observed. Samples with deeper depths exhibit no direct evidence of superconductivity inside the semiconductor. Samples consisting of doped InAs were also fabricated and measured. These samples exhibit almost perfect contact between the superconductor and the semiconductor and pair correlations are observed in the semiconductor despite their thickness. These observations confirm that the manifestation of the superconducting proximity effect is due to the competition between the normal and superconducting reservoirs. When the semiconducting layers are thick there exists a region that is unaffected by superconductivity. This region acts as an effective normal reservoir. The weak coupling at the Nb-InGaAs interface limits the strength of pairing in the semiconductor, and if a normal reservoir is present the superconductivity is completely suppressed. This effect is not seen in the more transparent InAs-Nb interfaces. This implies that the InAs is sufficiently transparent that the strong coupling to the superconductor across the N-S interface overcomes the negative effect on pairing due to the normal reservoir and a proximity affected region in the semiconductor near the interface is created. In summary, we are able to tune the strength of the induced pair correlations in the semiconductor by adjusting either the transmission coefficient of the N-S interface and by turning on or off the coupling to a normal reservoir.