Andreev reflection and enhanced subgap conductance in NbN/Au/InGaAs-InP junctions

We report on the fabrication of highly transparent superconductor/normal metal/two-dimensional electron gas junctions formed by a superconducting NbN electrode, a thin (10 nm) Au interlayer, and a two-dimensional electron gas in an InGaAs/InP heterostructure. High junction transparency has been achieved by exploiting developed process of Au/NbN evaporation and rapid annealing at 400 degreesC. This allowed us to observe a decrease in the differential resistance with pronounced double-dip structure within the superconducting energy gap in superconductor/two-dimensional electron gas (2DEG) proximity systems. The effect of a magnetic field perpendicular to the plane of the 2DEG on the differential resistance of the interface was studied. It has been found that the reduced subgap resistance remains in high magnetic fields. Zero-field data are analyzed within the previously established quasiclassical model for the proximity effect. (C) 2004 American Institute of Physics