Ballistic transport and Andreev resonances in Nb/In superconducting contacts to InAs and LTG-GaAs

We have formed superconducting contacts in which Cooper pairs incident from a thick In layer must move through a thin Nb layer to reach a semiconducto r, either InAs or low temperature grown (LTG) GaAs. The effect of pair tunn eling through the Nb layer can be seen by varying the temperature through t he critical temperature of In. Several of the In/Nb-InAs devices display a peak in the differential conductance near zero-bias voltage, which is stron g evidence of ballistic transport across the NS interface. The differential conductance of the In/Nb-LTG-GaAs materials system displays conductance re sonances of McMillan-Rowell type. These resonant levels exist within a band of conducting states inside the energy gap, formed from excess As incorpor ation into the LTG-GaAs during growth. Electrons propagating in this band o f states multiply reflect between the superconductor and a potential barrie r in the GaAs conduction band to form the conductance resonances. A scatter ing state theory of the differential conductance, including Andreev reflect ions from the composite In/Nb contact, accounts for most qualitative featur es in the data. (C) 1999 Academic Press.