Relation between electronic structure and emissivity of MoSi₂-based thin membranes for radiative cooling

Passive radiative cooling is becoming an increasingly important topic in electronic and optical applications where dimensions keep shrinking. When devices operate in low ambient pressures and contain or comprise of ultrathin layers radiative cooling can contribute significantly to prevent overheating. Molybdenum silicides are promising candidates for high-performance radiative cooling materials due to their oxidation resistance, chemical stability and emissivity. This study investigates the dependence of emissivity on Mo fraction and annealing temperature in free-standing MoSi2-based ultrathin membranes. Distinct phases of the films (hexagonal + amorphous formed at 600 °C and tetragonal MoSi2 formed at 900 °C) influence electronic band structure and subsequently emissivity. Employing Fourier-transform infrared spectroscopy and simultaneously analyzing reflectance and transmittance via multilayer Drude-Lorentz oscillators model, we uncover how free-electron conductivity plays a role in membranes emissivity. Dielectric functions of tetragonal MoSi2 films exhibit combined free-electron and interband contributions, while for the films based on hexagonal and amorphous MoSi2 the dielectric functions are mainly shaped by interband contributions. For the latter films increasing Mo content enhances conductivity slightly and emissivity significantly. Conversely, for tetragonal MoSi2 increasing Mo content leads to strong increase in conductivity and small increase in emissivity. The revealed correlation between emissivity and electrical conductivity allows for emissivity tuning using methods that modulate electrical conductivity, such as doping, adjusting Mo content, leveraging photoconductivity effects, or manipulating film crystallinity. This work explores of MoSi2 emissivity, proposing a methodology for studying the emissivity of thin films, particularly in the context of radiative cooling applications.