Stoichiometry-Led Electronic Structure Reconstruction Finetunes the Color Characteristics of Titanium Nitride Thin Films

Atomic-vapor-deposited nitride thin films attract considerable interest as decorative coatings in various industrial applications. Recent research efforts in the process-structure relationship for such films have converged on the need to develop electronic characteristics-led design principles. Here, we varied substrate-temperature (180, 450 °C) for deposition of titanium nitride (TiNx) thin films in a rocksalt structure with varying stoichiometry (x: 0.98–0.87). Reciprocity of the varying color characteristics of the deposited TiNx thin films was studied against their structural and electronic details by employing combined methods of experiments and simulations. The long-range and local structural order of the thin films were studied using grazing incidence X-ray diffraction analysis and Ti K-edge extended X-ray absorption fine structure spectroscopy (EXAFS), respectively. N K-edge X-ray absorption near-edge spectroscopy (XANES) and simulation of the spectrum using Green formalism (multiple scattering) delineated electronic reconstruction occurring during the atomic-level structural changes. In a key finding, no change in the density of states (DOS) was observed in the simulation carried out on a single NTi6 octahedra extracted from the long-range structure of TiNx thin films. The axial compression of the octahedra under varying stoichiometry did not cause significant change electronically and therefore did not contribute to the varying color characteristics. Long-range structural characteristics led by substoichiometry of the TiNx thin films induced electron localization near the conduction band edge. Small change produced in the band gap thus tuned the color characteristics of the thin films.