Shadow evaporation of epitaxial Al/Al2O3/Al tunnel junctions on sapphire utilizing an inorganic bilayer mask

TL;DR

This paper introduces an inorganic Ge/Nb bilayer shadow mask for fabricating epitaxial Al/Al2O3/Al superconducting tunnel junctions on sapphire, enabling high-quality growth in ultra-high vacuum conditions.

Contribution

It presents a novel inorganic mask fabrication method that overcomes limitations of organic resists, facilitating epitaxial growth of superconducting tunnel junctions using molecular beam epitaxy.

Findings

Successful growth of epitaxial Al tunnel junctions on sapphire.

High-quality tunnel junctions with measurable current-voltage characteristics.

Demonstration of inorganic mask's compatibility with ultra-high vacuum deposition.

Abstract

This letter describes a new inorganic shadow mask that has been employed for the evaporation of all-epitaxial Al/Al2O3/Al superconducting tunnel junctions. Organic resists that are commonly used for shadow masks and other lift-off processes are not compatible with ultra-high vacuum deposition systems, and they can break down at even moderately elevated temperatures. The inorganic mask described herein does not suffer these same shortcomings. It was fabricated from a Ge/Nb bilayer, comprising suspended Nb bridges supported by an undercut Ge sacrificial layer. Utilizing such a bilayer mask on C-plane sapphire, the growth of epitaxial Al tunnel junctions was achieved using molecular beam epitaxy. Crystalline Al2O3 was grown diffusively at 300 C in a molecular oxygen background of 2.0 utorr, while amorphous oxide was grown at room temperature and 25 mtorr. A variety of analysis techniques were employed to evaluate the materials, and tunnel junction current-voltage characteristics were measured at millikelvin temperatures.