Thermal phonons and defects in semiconductors: The physical reason why defects reduce heat flow, and how to control it

It is generally accepted that heat-carrying phonons in materials scatter off each other (normal or Umklapp scattering) as well as off defects. This assumes static defects, implies quasi-instantaneous interactions and at least some momentum transfer. However, when defect dynamics are explicitly included, the nature of phonon-defect interactions becomes more subtle. Ab initio microcanonical molecular-dynamics simulations show that (1) spatially localized vibrational modes (SLMs), associated with all types of defects in semiconductors, can trap thermal phonons; (2) the vibrational lifetimes of excitations in SLMs are one to two orders of magnitude longer (dozens to hundreds of periods of oscillation) than those of bulk phonons of similar frequency; (3) it is phonon trapping by defects (in SLMs) rather than bulk phonon scattering, which reduces the flow of heat; and (4) the decay of trapped phonons and therefore heat flow can be predicted and controlled - at least to some extent - by the use of carefully selected interfaces and δ layers.