Superconductivity of disordered NbN films deposited on magnesium oxide and oxidized silicon substrates

Abstract

This study investigates the evolution of superconducting properties as a function of disorder in epitaxial and polycrystalline niobium nitride (NbN) thin films grown on MgO and SiO2/Si substrates, respectively. By reducing the film thickness from 100 to 4 nm, the effective disorder in these two sets of films encompasses a large range, with the Ioffe–Regel parameter in the range of 5.7–9.3 and 1.5–3.0, respectively. Moreover, the density of states at the Fermi level [N(0)] of epitaxial NbN films decreases from 1.91 × 1028 to 1.46 × 1028 states eV−1m−3 with the superconducting transition temperature (Tc ) ranging from 16.0 to 11.8 K. In contrast, the N(0) of polycrystalline NbN films decreases from 2.18 × 1028 to 1.80 × 1028 states eV−1m−3 with Tc ranging from 14.9 to 2.56 K. The experimental results reveal that the Tc of NbN films with different thicknesses is mainly controlled by N(0), and the greater Coulomb pseudopotential (μ*) in the polycrystalline films leads to lower Tc and larger resistivity. The changes in N(0) and μ* should collectively account for the reduction of Tc with disorder in NbN thin films.