Impact of Isotope Doping on Phonon Thermal Transport in Silicon Nanowires

Phonon transport in silicon nanowires (Si NWs) doped with isotopes is investigated theoretically. The ballistic thermal conductance and diffusive thermal conductivity are calculated at room temperature using the phonon dispersion relations derived through a semiempirical atomistic approach. The thermal conductance and conductivity in 28 Si NWs randomly doped with 29 Si are smaller than those in the corresponding pure 28 Si NW, which can be fully explained by the effect of isotope impurities on the dispersion relations. In [001]-oriented 28 Si NWs having a square cross section with a side length of 1.086 nm and randomly doped with 29 Si, the maximum reduction in thermal conductivity reaches more than 20%. This reduction leads directly to an improvement in the thermoelectric figure of merit by more than 25%. It is also found that the impact of isotope impurities on phonon transport becomes large with increasing mass difference between the constituent and impurity isotopes or with increasing wire cross-sectional area. Phonon transport in isotopic core--shell Si NWs is also investigated. Some of these Si NWs show increased thermal conductance and conductivity although the increase is very small.