Structure and Superconductivity of Epitaxial and Polycrystalline VN Thin Films

Abstract

In this work, we present a systematic study of superconducting vanadium mononitride (VN) thin films that were simultaneously grown on single crystalline MgO (100) and amorphous SiO₂ substrates. The structure of both samples was probed using X-ray diffraction measurements under different geometries, i.e., specular, off-specular, and ϕ-scan. It was found that the lattice parameter (LP) of VN films grown on SiO₂ was 4.098(7) Å, while on MgO, it was slightly larger at 4.124(2) Å. Though the LP of both samples is within the experimentally obtained and theoretically predicted values, the obtained difference suggests that the polycrystalline VN has a tensile strain as compared to the epitaxial film. The effect of such strain is more clearly reflected on the superconducting transition temperature (T_C) and normal state electrical resistivity (ρ_n). In case of the epitaxial VN sample, the T_C was at 8.1 K, while in the polycrystalline sample, it was at 5.2 K. The ρ_n was also found to be significantly smaller in epitaxial films, suggesting a better crystalline quality. We performed X-ray absorption and hard X-ray photoelectron spectroscopy measurements to probe the electronic structure. It was found that the surface region of polycrystalline or epitaxial samples remains unaffected by such strains. However, high magnetic field resistivity measurements exhibited a two-step T_C in the epitaxial film which originates due to formation of a thin strained region formed during the initial stages of growth. Our in situ reflection high energy electron diffraction measurements confirmed that the initial stages of growth is strained due to lattice mismatch of about +2.13% between MgO and VN. Overall, this comparative study of epitaxial and polycrystalline samples elucidates the role of stress and strain on the structural, electronic, and superconducting properties of VN thin films.