Granular superconducting contacts to buried two-dimensional electron gases

Superconducting contacts to GaAs:AlGaAs two-dimensional electron gases have been characterised by scanning and transmission electron microscopy. The contacts were formed by the rapid thermal annealing of a tin:chrome:gold multilayer in an electron beam system, which results in junctions which can be very transmissive and show a large degree of Andreev reflection. They also have a relatively high critical temperature and critical magnetic field. The contacts were found to be granular to a depth well below the original position of the heterojunction, and to contain a variety of alloy phases and voiding.