Shape and orientation effects on the ballistic phonon thermal properties of ultra-scaled Si nanowires

Abstract

The effect of geometrical confinement, atomic position, and orientation of silicon nanowires (SiNWs) on their thermal properties are investigated using the phonon dispersion obtained using a Modified Valence Force Field (MVFF) model. The specific heat (Cν) and the ballistic thermal conductance (κlbal) shows anisotropic variation with changing cross-section shape and size of the SiNWs. The Cν increases with decreasing cross-section size for all the wires. The triangular wires show the largest Cν due to their highest surface-to-volume ratio. The square wires with [110] orientation show the maximum κlbal because they have the highest number of conducting phonon modes. At the nano-scale a universal scaling law for both Cν and κlbal are obtained with respect to the number of atoms in the unit cell. This scaling is independent of the shape, size, and orientation of the SiNWs, revealing a direct correlation of the lattice thermal properties to the atomistic properties of the nanowires. Thus, engineering the SiNW cross-section shape, size, and orientation open up new ways of tuning the thermal properties in the nanometer regime.