Structural vs. compositional disorder in thermal conductivity reduction of SiGe alloys

We use equilibrium molecular dynamics simulations to determine the relative role of compositional and structural disorder in a phononic thermal conductivity reduction by studying three 50-50 SiGe alloy structures: ordered alloys, disordered alloys, and amorphous alloys, as well as pure amorphous Si and Ge structures for reference. While both types of disorders significantly reduce thermal conductivity, structural disorder is much more effective to this aim. The examination of phonon lifetimes in disordered alloys shows high values in a low frequency regime governed by Umklapp scattering that are reduced rapidly with increasing frequency following Rayleigh scattering behavior. The local properties analysis reveals that the structural disorder leads to elastic heterogeneities that are significantly larger than density heterogeneities, which is likely the key reason for amorphous semiconductor alloys having lower thermal conductivity than disordered alloys. Temperature dependence of thermal conductivity indi...