Phase control and optical response in NbNx ultra-thin films by reactive PLD

Ultra-thin films of NbNx were synthesized by reactive PLD with the nitrogen partial pressure (pN₂) set to 8, 12, 16, and 20 Pa to elucidate the effects of nitridation on phase composition and functional properties. The film thickness decreased from 68 nm to 32 nm as pN₂ increased. Grazingincidence X-ray diffraction (GI-XRD) and Scherrer analysis revealed a gradual transformation from mixed tetragonal γ-NbN_0.64 and hexagonal ε-NbN phases to a predominantly hexagonal ε-NbN phase with increasing pN₂, accompanied by a linear reduction in crystallite size from ∼12 nm to ∼3.6 nm. X-ray photoelectron spectroscopy showed a very thin (∼5 nm) oxide surface layer and a slight decrease in the N 1s-Nb 3d splitting, indicating enhanced Nb–N charge transfer, with very small amounts of oxide within the ultra-thin films. Raman spectroscopy exhibited up-shifts in first-order phonon modes alongside pronounced acoustic bands, confirming the films’ polycrystalline nature. Finally, UV–Vis spectrophotometry demonstrated near-zero absorbance in the infrared region, suggesting strong IR reflectivity, which makes the films suitable for applications in coatings.