The role of straining and morphology in thermal conductivity of a set of Si-Ge superlattices and biomimetic Si-Ge nanocomposites

The ability to alter the thermal and mechanical properties of nanostructures by tailoring nanoscale morphology has led to vast activity in applications such as high figure of merit (ZT) thermoelectric, microelectronic and optoelectronic devices. Two types of nanostructures that have gained significant attention are Si-Ge superlattices and Si-Ge biomimetic nanocomposites, in which one phase is distributed in the other phase in a staggered biomimetic manner similar to biological materials. A systematic comparison of the atomistic factors that affect their thermal behaviour under different extents of straining at a range of temperatures remains to be performed. In this investigation, such analyses are performed for a set of Si-Ge superlattices and Si-Ge biomimetic nanocomposites using non-equilibrium molecular dynamics (NEMD) simulations at three different temperatures (400, 600 and 800 K) and at strain levels varying between -10% and 10%. Analyses indicate that the nanoscale morphology differences between the superlattices and the nanocomposites lead to a striking contrast in the phonon spectral density, interfacial thermal boundary resistance and thermal conductivity. In the case of the nanocomposites, morphology variation at the nanoscale and the tensile or compressive straining at temperatures from 400 to 800 K do not have a significant effect on the changes in thermal conductivity values. Such factors, however, strongly influence the thermal conductivity of superlattices. The thickness of the nanocomposites, however, is found to influence the thermal conductivity values significantly under straining, with the effect of straining increasing with increasing nanocomposite thickness. A relation based on the effective medium approach is shown to fit the NEMD calculated nanocomposite thermal conductivity values.