Dependence of the Superconducting Transition Temperature on the Electronic Density of States and Lattice Parameter in Pulsed Laser Deposited Niobium Nitride Thin Films

High quality epitaxial niobium nitride (NbN) thin films were grown on MgO (001) substrates by pulsed laser deposition (PLD) from a pure NbN target while varying the nitrogen partial pressure inside the chamber. This variation provides an additional degree of freedom in the film formation: by adjusting the nitrogen partial pressure, we modify the lattice parameter of the delta-NbN phase systematically, and tune the superconducting transition temperature ( TC). X-ray Diffraction and Hall conductivity measurements show that this change in TC arises from the combined influence of lattice strain, and carrier concentration. A phenomenological analysis based on the McMillan and Allen Dynes strong coupling models closely captures the observed trends. Our results demonstrate that rational design of epitaxial superconducting materials, essential for interface control of physical properties, can be achieved by tuning the lattice parameter and carrier concentration in delta-NbN. Furthermore, our results provide a pathway to separate intrinsic structural factors affecting the transition temperature from proximity-induced effects in superconducting hybrid structures.