Thermal conductivity of Si nanowires: A first-principles analysis of the role of defects

The theoretical laser-flash method is used to calculate 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 defects, which are described using first-principles methods. The defects considered are the surface of the nanowire, random distributions of substitutional C or Ge impurities, and monoatomic δ layers of C or Ge. The localized vibrational modes of these defects are explicitly included in the calculations and no empirical defect-related parameter is introduced. We 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.