MBE Growth of Al/InAs and Nb/InAs Superconducting Hybrid Nanowire Structures

TL;DR

This study demonstrates the in situ growth of high-quality Al and Nb superconducting shells on InAs nanowires via molecular beam epitaxy, optimizing parameters for defect-free hybrid structures relevant to quantum computing.

Contribution

It provides systematic insights into growth parameters affecting the quality of Al/InAs and Nb/InAs nanowire hybrids, enabling improved superconductor/semiconductor interfaces for quantum devices.

Findings

Substrate temperature critically affects Al shell smoothness.

Nb shell quality depends on deposition angle, not temperature.

Successful growth of impurity-free, crystalline superconductor/InAs interfaces.

Abstract

We report on \textit{in situ} growth of crystalline Al and Nb shells on InAs nanowires. The nanowires are grown on Si(111) substrates by molecular beam epitaxy (MBE) without foreign catalysts in the vapor-solid mode. The metal shells are deposited by electron-beam evaporation in a metal MBE. High quality supercondonductor/semiconductor hybrid structures such as Al/InAs and Nb/InAs are of interest for ongoing research in the fields of gateable Josephson junctions and quantum information related research. Systematic investigations of the deposition parameters suitable for metal shell growth are conducted. In case of Al, the substrate temperature, the growth rate and the shell thickness are considered. The substrate temperature as well as the angle of the impinging deposition flux are explored for Nb shells. The core-shell hybrid structures are characterized by electron microscopy and x-ray spectroscopy. Our results show that the substrate temperature is a crucial parameter in order to enable the deposition of smooth Al layers. Contrary, Nb films are less dependent on substrate temperature but strongly affected by the deposition angle. At a temperature of 200{\deg}C Nb reacts with InAs, dissolving the nanowire crystal. Our investigations result in smooth metal shells exhibiting an impurity and defect free, crystalline superconductor/InAs interface. Additionally, we find that the superconductor crystal structure is not affected by stacking faults present in the InAs nanowires.