Heat flow in Si nanowires containing delta-layers

Abstract

The interactions between heat flow and defects is universally believed to involve the scattering of thermal phonons by the defect. This process is assumed to conserve momentum and sometimes to be elastic as well. But the atomistic nature of the processes involved is never described. This Thesis deals with the theoretical description of the interactions between a heat front and well-defined defects using entirely first-principles tools: density-functional theory for the electronic states and ab-initio molecular-dynamics (MD) simulations for the nuclear dynamics. No empirical parameters are used and no assumption about the nature of the interactions is made. The host material is a Si nanowire containing a thin layer of atoms X = Ge or C, and the Si|X interface is the defect. The theoretical developments include the construction of a strictly microcanonical periodic ‘cluster’ in which heat flow initially in just one direction, MD simulations performed without thermostat and with unprecedented temperature control, and the analysis of the energy distribution vs time which distinguishes between one- and two-phonon processes. The results of the MD simulations show that the dynamic properties of the defect play the central role in phonon-defect interactions and that no scattering process of any kind occurs. The interactions include only the coupling between delocalized (host-material) and localized (defect-related) oscillators. These interactions are temperature dependent: a given defect is predicted to behave differently in different temperature ranges. The consequences of these findings are discussed.