Thermal conductivity of small Si nanowires: An atomistic look at the impact of defects
Silicon nanowires (SiNWs) are studied experimentally and theoretically for their potential use in devices as well as for thermoelectric applications. The measured thermal conductivity of SiNWs is much lower than that of bulk Si and depends on their diameter d. However, very little is understood at the atomic level about the way impurities and/or surface conditions affect the thermal conductivity of SiNWs. I have extended the theoretical ‘laser-flash’ method, recently developed by Gibbons and Estreicher, to calculate the thermal conductivity of Si nanostructures entirely from first principles in SiNWs which contain impurities. This dissertation describes this method and my result, the thermal conductivity of the Si200X32 (X = H, D, or OH) and Si296X112 (X=H or D) nanowires. The main emphasis is on the role of the surface and of defects, such as Ge or C δ-layers and random distributions of these impurities. The localized vibrational modes of these defects are explicitly included in the calculations and no empirical defect-related parameter is introduced. I find that the surface Si-H wag modes couple resonantly to each other much faster than they decay into bulk modes, which leads to distinct surface and bulk contributions to the thermal conductivity. The spatially-localized vibrational modes associated with the Ge or C impurities as well as the δ-layers trap thermal phonons thus reducing the thermal conductivity.