The effect of crystal structure and morphology on the optical properties of chromium nitride thin films

We study the microstructure of various CrxNy (1<x<2, y∼1) coatings grown by unbalanced reactive magnetron sputtering (UBRMS), using X-ray diffraction (XRD) and reflectivity (XRR). The coatings consist of various CrN phases, depending on the growth conditions. XRD has shown that a Cr adhesion layer below CrxNy eliminates the stress and promotes the growth of bigger grains. XRR determined the film density, which can be used also for the phase identification. We found that the UBRMS can produce single-phase CrN and Cr2N coatings with density equivalent to the corresponding single-crystals. The optical properties of the coatings were studied by spectroscopic ellipsometry (SE). The variations of optical properties of CrxNy coatings have been evaluated from SE data using the combined Drude–Lorentz model, which describes the optical response of the conduction and valence electrons, respectively, and provides the conduction electron density and the energy positions of the interband transitions. Finally, the optical properties were used to quantify the volume fractions of each phase using effective medium theories.