Is the `Finite Bias Anomaly' in planar GaAs-Superconductor junctons caused by point-contact like structures?

TL;DR

This study links the finite bias anomaly in GaAs-superconductor junctions to the formation of point-contact structures during annealing, showing how these microscopic features influence conductance and transport phenomena.

Contribution

It reveals that the finite bias anomaly arises from parallel conductance of multiple superconducting point contacts formed by annealing.

Findings

Point contacts form along crystallographic planes during annealing.

Variations in contact geometry affect conductance and transport properties.

The finite bias anomaly can be explained by summing conductance of independent point emitters.

Abstract

We correlate transmission electron microscope (TEM) pictures of superconducting In contacts to an AlGaAs/GaAs heterojunction with differential conductance spectroscopy performed on the same heterojunction. Metals deposited onto a (100) AlGaAs/GaAs heterostructure do not form planar contacts but, during thermal annealing, grow down into the heterostructure along crystallographic planes in pyramid-like `point contacts'. Random surface nucleation and growth gives rise to a different interface transmission for each superconducting point contact. Samples annealed for different times, and therefore having different contact geometry, show variations in $dI/dV$ characteristic of ballistic transport of Cooper pairs, wave interference between different point emitters, and different types of weak localization corrections to Giaever tunneling. We give a possible mechanism whereby the `finite bias anomaly' of Poirier et al. (Phys. Rev. Lett., {\bf 79}, 2105 (1997)), also observed in these samples, can arise by adding the conductance of independent superconducting point emitters in parallel.