Nature of Tunable Optical Reflectivity of Rocksalt Hafnium Nitride Films

Although the phenomenon that optical reflectivity of hard group IVB transition metal nitrides depends on stoichiometry has been reported, the microscopic origin of this behavior has not been well explored yet. Here we find that optical reflectivity of rocksalt hafnium nitride films (δ-HfNx) can be effectively tuned by stoichiometry x, and the underlying mechanism can be well elucidated by Drude–Lorentz fitting and first-principles calculations. It is shown that the observed tunability of optical reflectivity arises from a transition from N vacancies (VN) to Hf vacancies (VHf) in the films because this evolution from VNto VHfhas important roles in changing electronic properties of the films in the following three aspects: (i) density of free electrons, wherein VNand VHfact as donor-like and acceptor-like defects, respectively; (ii) mean free path of free electrons, in which VNand VHfare the main electron scattering sites in sub- and overstoichiometric films, respectively; (iii) interband transition absorption of bound electrons, wherein three previously unreported absorption bands originating from VNand VHfare found to occur at ∼0.81, 2.27, and 3.75 eV. These point-defect-induced variations significantly affect the dielectric function of δ-HfNxfilms and thus drive the tailored evolution in reflectivity properties with x.