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 (delta-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 (V-N) to Hf vacancies (V-Hf) in the films because this evolution from V-N to V-Hf has important roles in changing electronic properties of the films in the following three aspects: (i) density of free electrons, wherein V-N and V-Hf act as donor-like and acceptor-like defects, respectively; (ii) mean free path of free electrons, in which V-N and V-Hf are 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 V-N and V-Hf are found to occur at similar to 0.81, 2.27, and 3.75 eV. These point-defect-induced variations significantly affect the dielectric function of delta-HfNx films and thus drive the tailored evolution in reflectivity properties with x.