Thickness-driven structural evolution and vacancy-linked superconductivity in epitaxial NbN thin films

Epitaxial -NbN thin films (9–205 nm) were grown on MgO(001) and MgO(110) to examine how thickness-driven structural evolution controls normal-state transport and superconducting performance, and how these changes are linked to vacancy-mediated lattice evolution. Structural characterization shows that increasing thickness generally improves the microstructural coherence length. The room-temperature resistivity generally decreases with thickness when a single epitaxial registry is preserved. However, the 205-nm NbN(110) film, which exhibits a weak secondary (111) component, shows a higher resistivity despite its larger microstructural coherence lengths, indicating that resistivity is not governed by structural coherence alone. All measured films exhibit a negative temperature coefficient of resistivity, and the broad minimum in d ρ x x / d T appears for all samples around 50–80 K, which correlates more closely with the disorder parameter k F l than with microstructural coherence lengths. Hall measurements indicate that the effective carrier density n eff increases with thickness and correlates with the evolution of unit-cell volume. Together with density functional theory calculations showing that percent-level vacancy disorder can produce comparable changes in lattice volume, this supports a thickness-driven vacancy-linked lattice evolution accompanied by strain relaxation, coinciding with an increase of T c (8.5–12.3 K). The largest estimated upper critical field is 44.9 T for the 205-nm NbN(110) film, corresponding to a superconducting coherence length of 2.7 nm.