Structural and optical properties of vanadium and hafnium nitride nanoscale films: effect of stoichiometry

Vanadium and hafnium nitride nanoscale films were synthesized onto (100)-oriented silicon wafers by reactive dc-magnetron sputtering. The temperature of the substrate was fixed at 150 °C. Several analysis techniques were used to characterize the resulting films: Wave Dispersion Spectroscopy (WDS) to determine nitrogen concentration, X-ray Diffraction (XRD) to study crystallographic structures, X-ray Reflectivity (XRR) to estimate their thicknesses and finally optical spectrophotometry (UV-Vis-IR) to measure the optical reflectance. The experimental conditions were selected in order to achieve two goals. The first one is to obtain ultra thin films (in the nanoscale range). The second one is to compare two symmetric systems, in terms of stoichiometry: over-stoichiometric hafnium nitrides and sub-stoichiometric vanadium nitrides. The XRR results showed that all synthesized films approximately presented the same thickness in the nanoscale range (less than 70 nm). The nitrogen concentration, measured by means of WDS, was in the sub-stoichiometric region for vanadium nitrides (N/V atomic ratio between 0.78 and 0.83) and over-stoichiometric region for hafnium nitrides (N/Hf atomic ratio between 1.18 and 1.25). From XRD patterns, the crystallographic film structure was found to be of NaCl type for the two systems, δ-VN and δ-HfN, with an amorphous part in the first system. Optical constants were derived from experimental reflectance spectra. Good agreement was found between experimental and calculated reflectance spectra using Drude and extended Drude models