Simulating the influence of stoichiometry on the spectral emissivity of Mo$_x$Si$_y$ thin films

TL;DR

This study uses density functional perturbation theory to simulate how different stoichiometries of Mo$_x$Si$_y$ thin films affect their spectral emissivity, revealing dependencies on phase, thickness, and defects.

Contribution

It provides the first detailed simulation of spectral emissivity in Mo$_x$Si$_y$ compounds considering electronic, ionic, and defect effects, aligning with experimental observations.

Findings

MoSi$_2$ emissivity varies significantly between phases.

Hexagonal MoSi$_2$ has lower emissivity than tetragonal phase.

Defects can substantially increase infrared emissivity.

Abstract

In this work, we simulate the spectral emissivity of various stoichiometric crystal phases of Mo$_x$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 ($\sim$20 nm) form. We find that most thin films of Mo$_x$Si$_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 MoSi$_2$ is much lower than in the tetragonal phase. This is related to the small band gap (hexagonal MoSi$_2$) and low density of states at the Fermi level (tetragonal MoSi$_2$). Furthermore, test calculations on defected MoSi$_2$ demonstrate that the infrared emissivity of MoSi$_2$ thin films can be substantially increased by introducing defects.