Imaging Defects in Epitaxial β-Nb₂N /AlN/ β-Nb₂N Josephson Junctions via Multislice Electron Ptychography

Coupling of superconducting qubits to the external environment through dielectric traps, oxidized surfaces, defects, or inhomogeneous phases can introduce undesired energy levels in form of host two-level systems leading to quantum decoherence. Identifying and eliminating the defects responsible for the decoherence will lead to scalable quantum computers. Recent developments in epitaxial growth of single phase, atomically smooth β-Nb₂N has confirmed superconductivity with a critical temperature 0.35&lt;T_c&lt;0.6K [1]. This opens the opportunity for large single-crystal layers to be used in Josephson junctions and enables easier identification of defects in comparison to amorphous and polycrystalline materials.<br/>Here, we investigate an epitaxial transition metal nitride (TMN) Josephson junction with β-Nb₂N /AlN/ β-Nb₂N trilayer structure grown by molecular beam epitaxy (MBE) on c-plane sapphire. We measure non-linear I-V characteristics showing superconducting current across the junction. Hexagonal (non-polar) β-Nb₂N and wurtzite (polar) AlN are symmetry-matched and have only a 1.8% lattice mismatch. Here, we use multislice electron ptychography on four-dimensional scanning transmission electron microscopy (4D-STEM) to obtain a three-dimensional reconstruction of the junction and its interfaces with atomic-scale imaging and depth sectioning to show the presence of polarity-inversion boundaries of AlN at β-Nb₂N step edges.<br/>[1] Wright, J. <i>et al</i>., <i>Phys. Rev. Mater.</i> <b>7</b>, 074803 (2023). DOI: 10.1103/PhysRevMaterials.7.074803