Is the 'Finite Bias Anomaly' in planar GaAs-superconductor junctions caused by point-contact-like structures?

We correlate transmission electron microscope (TEM) pictures of superconduc ting In contacts to an AlGaAs/GaAs heterojunction with differential conduct ance spectroscopy performed on the same heterojunction. Metals deposited on to a (100) AlGaAs/GaAs heterostructure do not form planar contacts, but, du ring thermal annealing, grow down into the heterostructure along crystallog raphic planes in pyramid-like 'point contacts'. Random surface nucleation a nd growth gives rise to a different interface transmission for each superco nducting point contact. Samples annealed for different times, and therefore having different contact geometry, show variations in dI/dV characteristic s of ballistic transport of Cooper pairs, wave interference between differe nt point emitters, and different types of weak localization corrections to Giaever tunneling. We give a possible mechanism whereby the 'finite bias an omaly' of Poirier et al. [Phys. Rev. Lett. 79, 2105 (1997)], also observed in these samples, can arise by adding the conductance of independent superc onducting point emitters in parallel. (C) 1999 Academic Press.