The optical functions (complex dielectric function, complex index of refraction, and complex conductivity) of sputtered zirconium nitride films are derived starting from optical reflectance measurements. Their evolution with the different bias voltages applied during the films growth is used to deduce information about the variations in the electronic structure influenced by a different oxygen and nitrogen content in the films. Improvement in the electrical conductivity is observed at increasing bias voltage due to a reduction in both oxygen contamination and nitrogen content. The separation of the different contributions (free conduction electrons and different electronic transitions) in the optical functions is achieved through the Drude–Lorentz model, allowing the detection of an unusual low-energy electronic transition in films grown at low bias voltages. Through considerations about the electronic structure and about the …