The mechanism for phonon scattering by nanostructures and by point defects in nanostructured silicon and the silicon germanium alloy and their thermoelectric properties were investigated. It was found that the thermal conductivity was reduced by a factor of 10 in nanostructured Si in comparison with bulk crystalline Si. However, nanosize interfaces were not as effective as point defects in scattering phonons with wavelengths shorter than 1nm. It was further found that 5at%Ge replacing Si was very efficient in scattering phonons shorter than 1nm, resulting in a further thermal conductivity reduction by a factor of 2, thereby leading to a thermoelectric figure of merit 0.95 for Si95Ge5, similar to that of large grained Si80Ge20 alloys.

Increased Phonon Scattering by Nanograins and Point Defects in Nanostructured Silicon with a Low Concentration of Germanium. G.H.Zhu, H.Lee, Y.C.Lan, X.W.Wang, G.Joshi, D.Z.Wang, J.Yang, D.Vashaee, H.Guilbert, A.Pillitteri, M.S.Dresselhaus, G.Chen, Z.F.Ren: Physical Review Letters, 2009, 102[19], 196803