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 SiO 2 /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 × 10 28 to 1.46 × 10 28 states eV −1 m −3 with the superconducting transition temperature ( T c ) ranging from 16.0 to 11.8 K. In contrast, the N (0) of polycrystalline NbN films decreases from 2.18 × 10 28 to 1.80 × 10 28 states eV −1 m −3 with T c ranging from 14.9 to 2.56 K. The experimental results reveal that the T c 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 T c and larger resistivity. The changes in N (0) and μ * should collectively account for the reduction of T c with disorder in NbN thin films.