Fabrication of Co2FeSi Heusler-alloy epitaxial film on NbN epilayer with improved surface morphology

Abstract

We demonstrate the process to improve surface morphology of an epitaxial NbN layer and the epitaxial growth of a Co-based Heusler alloy, Co2FeSi (CFS), film on the surface-flattened NbN epilayer. The surface roughness of the epitaxial NbN layers was suppressed by employing Cr buffer layers and post-deposition annealing at 800 ˚C, resulting in a very small average surface roughness of ∼1.7 Å. The CFS layers were epitaxially formed on the surface-flattened NbN layers, and we confirm that the CFS/NbN and NbN/Cr interfaces were atomically flat using high-angle annular dark-field scanning transmission electron microscopy. Although energy-dispersive X-ray spectroscopy (EDS) elemental maps show almost no significant interdiffusion of constituent atoms in the CFS/NbN/Cr on the MgO substrate, EDS concentration profiles indicate minor diffusion of N atoms of the NbN layer into the Cr buffer layer. The diffusion of N atoms could be an origin of the degradation of crystallinity of the NbN layer.