Transparent Gatable Superconducting Shadow Junctions

TL;DR

This paper presents a new in-situ shadowing technique for fabricating high-quality, gate-tunable superconducting junctions in semiconductor nanowires, achieving high transparency and supercurrent, advancing quantum device engineering.

Contribution

It introduces a single-step epitaxial shadowing process to create superior superconducting junctions with high transparency and critical supercurrent, improving fabrication reliability for quantum applications.

Findings

Shadowed junctions outperform etched ones in quality.

Sharper edges lead to higher junction transparency.

Achieved supercurrent close to the KO-2 limit.

Abstract

Gate tunable junctions are key elements in quantum devices based on hybrid semiconductor-superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single crystalline InAs, InSb and $\mathrm{InAs_{1-x}Sb_x}$ nanowires with epitaxial superconductors and in-situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in-situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high $I_\mathrm{C} R_\mathrm{N}$, close to the KO$-$2 limit. This study demonstrates a promising engineering path towards reliable gate-tunable superconducting qubits.