Optical and electronic properties of niobium oxynitrides with various N/O ratios: insights from first-principles calculations

The electronic and optical properties of niobium oxynitride (NbON) complexes as a function of the N/O ratio were examined using the Perdew–Burke–Ernzerhof exchange–correlation functional of density functional theory. The investigated NbON complexes (i.e., pristine NbON crystal and Nb16O16N16, Nb16O15N17, Nb16O14N18, Nb16O17N15, and Nb16O18N14 super crystals) were found to be indirect bandgap semiconductors. Varying the N/O ratio reallocates the band edges, giving rise to redshifts in the absorption spectra at higher N/O ratios. Partial density of states calculations showed valence band dependency on the N 2p orbital energy, with the conduction band predominantly composed of Nb 4d states. The dielectric constant and refractive index of pristine NbON are found to be comparable to those of isostructural TaON, and varying the N/O ratio leads to a significant improvement in the optical properties of the studied systems.