SUPERCONDUCTORS COUPLED WITH A 2-DIMENSIONAL ELECTRON-GAS IN GAAS/ALGAAS AND INAS/ALGASB HETEROSTRUCTURES

We fabricated superconducting Sn/Pd contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures by an alloying technique. A dip around zero bias and peaks at finite voltages have been observed in the differential resistance of a 2DEG between two superconductors. The resistance dip may be a result of Andreev reflection at 2DEG-superconductor interfaces and the peaks are probably due to the Josephson coupling between two very close superconducting islands in a superconducting contact region, which may be formed by alloying. Alternatively, we made superconducting Nb contacts to a 2DEG in an InAs/AlGaSb quantum well structure. The resistance of Nb-2DEG-Nb devices shows a clear indication of Andreev reflection.