Thermal conduction in disordered materials

Abstract

We investigate 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 disorder 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 indicates the importance of propagating phonons and associated Umklapp scattering in SiGe alloy structures. Interestingly, longitudinal modes in amorphous and disordered alloys exhibit similar lifetimes, while transverse modes lifetimes show significant differences and are more temperature dependent.