Coherent transport in Nb/delta-doped-GaAs hybrid microstructures

TL;DR

This paper investigates coherent transport phenomena in Nb/GaAs superconductor-semiconductor microstructures, highlighting the effects of engineered barriers and the application of advanced theoretical models to understand their quantum transport properties.

Contribution

It introduces a fabrication method for Nb/GaAs structures with delta-doped layers and demonstrates how engineered barriers influence coherent transport and interference effects.

Findings

Reflectionless tunneling dominates transport in both structures.

Engineered junctions exhibit low-temperature resonance peaks.

Theoretical analysis aligns with experimental magnetotransport data.

Abstract

Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is presented. The structures fabrication procedure is based on delta-doped layers grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap layer to protect the active semiconductor layers during ex situ transfer. The superconductor is then sputter deposited in situ after thermal desorption of the protective layer. Two types of structures in particular will be discussed, i.e., a reference junction and the engineered one that contains an additional insulating AlGaAs barrier inserted during the growth in the semiconductor. This latter configuration may give rise to controlled interference effects and realizes the model introduced by de Gennes and Saint-James in 1963. While both structures show reflectionless tunneling-dominated transport, only the engineered junction shows additionally a low-temperature single marked resonance peaks superimposed to the characteristic Andreev-dominated subgap conductance. The analysis of coherent magnetotransport in both microstructures is successfully performed within the random matrix theory of Andreev transport and ballistic effects are included by directly solving the Bogoliubov-de Gennes equations. The impact of junction morphology on reflectionless tunneling and the application of the employed fabrication technique to the realization of complex semiconductor-superconductor systems are furthermore discussed.