Anodic Oxidation of Epitaxial Superconductor-Semiconductor Hybrids

TL;DR

This paper introduces a novel anodic oxidation process for fabricating and passivating epitaxial superconductor-semiconductor hybrids, enhancing superconducting properties and device performance while enabling precise patterning.

Contribution

The study presents a new AO-based fabrication method that improves superconducting characteristics, passivates heterostructures, and allows high-resolution patterning for quantum devices.

Findings

Enhanced critical magnetic fields up to 3.5 T in oxidized Al films.

Higher 2DEG mobilities with AO passivation compared to wet etching.

Successful local patterning of Josephson junctions with operation up to 0.3 T.

Abstract

We demonstrate a new fabrication process for hybrid semiconductor-superconductor heterostructures based on anodic oxidation (AO), allowing controlled thinning of epitaxial Al films. Structural and transport studies of oxidized epitaxial Al films grown on insulating GaAs substrates reveal spatial non-uniformity and enhanced critical temperature and magnetic fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a conducting quantum well show similarly enhanced superconducting properties transferred to the two-dimensional electron gas (2DEG) by proximity effect, with critical perpendicular magnetic fields up to 3.5 T. An insulating AlOx film, that passivates the heterostructure from exposure to air, is obtained by complete oxidation of the Al. It simultaneously removes the need to strip Al which damages the underlying semiconductor. AO passivation yielded 2DEG mobilities two times higher than similar devices with Al removed by wet etching. An AO-passivated Hall bar showed quantum Hall features emerging at a transverse field of 2.5 T, below the critical transverse field of thinned films, eventually allowing transparent coupling of quantum Hall effect and superconductivity. AO thinning and passivation are compatible with standard lithographic techniques, giving lateral resolution below <50 nm. We demonstrate local patterning of AO by realizing a semiconductor-based Josephson junction operating up to 0.3 T perpendicular.