Granular superconducting contacts to GaAs:AlGaAs semiconductor heterostructures

The differential resistance characteristics of junctions formed from annealed granular superconducting contacts and GaAs:AlGaAs heterostructures are reported. At low bias voltage there is a smooth resistance dip due to Andreev reflection at the interfaces and diffusive electron transport in the semiconductor. At higher bias there is a peak in the resistance corresponding to the onset of phonon-mediated processes, and further structure with an unusual temperature dependence. The width of the Andreev dip and its apparent insensitivity to temperature are due to a combination of the temperature dependence of the superconductor gap, the thermal carrier distribution in the normal regions, and the geometry of the contacts. Disorder near the interfaces is believed to be responsible for the high degree of Andreev reflection, and this increased probability persists in an applied magnetic field, as has been predicted.