NbN-based Josephson junctions grown by plasma-assisted molecular beam epitaxy

Abstract

Abstract Improved process for the growth of NbN by plasma-assisted molecular beam epitaxy (PAMBE) is needed for the fabrication of Josephson junctions, that are building blocks of qubits in quantum computers. Here we propose an approach to grow cubic δ-NbN on (0001) GaN substrates by PAMBE. We use metal-rich conditions with indium as a surfactant to facilitate the growth of NbN layers, InAlN/NbN, and InGaN/NbN superlattices. For In-rich conditions: (i) structural and electrical quality of NbN is improved as confirmed by larger grain size and higher transition temperature to superconducting phase, (ii) the growth of thin InAlN and InGaN layers on NbN is possible with flat interfaces. High critical current density up to 1 kA cm−2 was obtained for NbN/InAlN/NbN Josephson junctions with 3 nm tunneling barrier. Indium-rich conditions allow the integration of δ-NbN with other III-N compounds by PAMBE preserving the high structural quality of the grown heterostructures.