Development of a Nb-based semiconductor-superconductor hybrid platform

TL;DR

This paper introduces a novel Nb-based semiconductor-superconductor hybrid platform with a large induced superconducting gap, achieved through an innovative Al interlayer, promising enhanced performance for quantum applications.

Contribution

The study develops a new hybrid material combining InAs 2DEG with Nb/NbTi superconductors using an Al interlayer to create highly-transparent interfaces with a large superconducting gap.

Findings

Induced superconducting gap of ~1 meV in the hybrid system.

Epitaxial metal-to-metal connection achieved via an Al interlayer.

Large superconducting gap indicates highly-transparent interfaces.

Abstract

Semiconductor-superconductor hybrid materials are used as a platform to realise Andreev bound states, which hold great promise for quantum applications. These states require transparent interfaces between the semiconductor and superconductor, which are typically realised by in-situ deposition of an Al superconducting layer. Here we present a hybrid material based on an InAs two-dimensional electron gas (2DEG) combined with in-situ deposited Nb and NbTi superconductors, which offer a larger operating range in temperature and magnetic field due to their larger superconducting gap. We overcome the inherent difficulty associated with the formation of an amorphous interface between III-V semiconductors and Nb-based superconductors by introducing a 7 nm Al interlayer. The Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs 2DEG. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor-semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms.