Simulating the influence of stoichiometry on the spectral emissivity of Mo x Si y thin films

In this work, we simulate the spectral emissivity of various stoichiometric crystal phases of ${\mathrm{Mo}}_{x}{\mathrm{Si}}_{y}$ compounds using density functional perturbation theory. The dielectric function, including electronic and ionic contributions, is calculated for each phase. We use the bulk properties obtained to simulate the optical absorption spectrum originating from the compound in thin film ($\ensuremath{\sim}20\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$) form. We find that most thin films of ${\mathrm{Mo}}_{x}{\mathrm{Si}}_{y}$ are metallic, however, our results indicate that their emissivity is not simply correlated with the Mo content. For hot metallic films at around 900 K, we predict a maximal emissivity between 5--10 nm thickness. Our results are in good qualitative agreement with experiments, confirming that the emissivity of hexagonal ${\mathrm{MoSi}}_{2}$ is much lower than in the tetragonal phase. This is related to the small band gap (hexagonal ${\mathrm{MoSi}}_{2}$) and low density of states at the Fermi level (tetragonal ${\mathrm{MoSi}}_{2}$). Furthermore, test calculations on defected ${\mathrm{MoSi}}_{2}$ demonstrate that the infrared emissivity of ${\mathrm{MoSi}}_{2}$ thin films can be substantially increased by introducing defects.