Epitaxial δ-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 $\mathrm{d}{\rho }_{xx}/\mathrm{d}T$ appears for all samples around 50–80 K, which correlates more closely with the disorder parameter $k_F\mathcal{l}$ than with microstructural coherence lengths. Hall measurements indicate that the effective carrier density $n_{\mathrm{eff}}$ 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.

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• Superconductivity
• Nitrides
• Density functional calculations
• Epitaxy
• Sputtering