Thermophysical properties and atomic structure of liquid Si-Ge alloys

The thermophysical properties and atomic structure of liquid Si-Ge alloys were investigated by the molecular dynamic simulations with Stillinger-Weber potential over a broad temperature range, including both superheating and undercooled regime. The maximum undercooling in this work is 611 K for pure Ge. For all the Si_100-xGe_x (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100) alloys, the densities exhibit quadratic relationship with temperature. The density of Si at the melting temperature is 2.59 g·cm⁻³, existing a slight deviation of 0.39% compared with the experiential data. For liquid Ge, the density at the melting temperature is 5.62 g·cm⁻³, nearly identical with the reported values. In all the cases, the densities of Si-Ge alloys with different compositions are larger than those of ideal solution. Therefore, liquid Si-Ge alloys display negative excess volume, which reaches the minimum at 40% Ge content. The specific heat was obtained by thermodynamic function and energy fluctuation approaches, respectively. Both of the results are in good agreement with experimental data around the liquidus temperatures. Furthermore, the diffusion coefficients were obtained by calculating the mean square displacement. The pair distribution function was applied to analyze the liquid structure. It is found that the peaks occur around the liquidus temperatures, which implies the structure change between normal and undercooled liquid alloys.