Theoretical phonon thermal conductivity of Si/Ge superlattice nanowires

An incoherent particle model has been developed to calculate the phonon thermal conductivity of superlattice nanowires. This is an extension of the photon net-radiation method and Schuster-Schwarzschild approximation to dispersive acoustic phonons in a gray medium. By comparing the roughness and geometric variations of typical nanowires to the characteristic phonon wavelength (similar to1 nm at 300 K), diffuse scattering and incoherent three-dimensional dispersion are justified. An isotropic sine-type (Born-von Karman) dispersion is used, which requires only the sound velocity, atomic number density, and bulk conductivity to fully describe a material. A simple picture is also given in terms of Matthiessen's rule and three effective mean free paths. Agreement with available experimental data is poor at the smallest diameters, but good above 30 nm diameter. Compared to a conventional superlattice, calculations show that the additional sidewall scattering in a superlattice nanowire can reduce the thermal conductivity by a factor of 2 or more. (C) 2004 American Institute of Physics.