Reducing the thermal conductivity of silicon nanowires (SiNWs) can enhance the thermoelectric figure of merit. Applying non-equilibrium molecular dynamics (NEMD) simulation, it has been demonstrated that the thermal conductivity of SiNWs is approximately two orders of magnitude lower than bulk silicon crystal and that it can be reduced remarkably by including vacancy defects. It has been found that "surface vacancy defect" reduces thermal conductance much more than "center vacancy defect". The thermal conductivity reaches the minimum, which is about 17% of that of pristine SiNW, when 2% surface vacancy defect is introduced in the nanowire. In order to reveal the origin of this drastic reduction of thermal conductivity, the vibrational density of states (VDOS) analysis is performed and it has been found that due to the high surface to volume ratio, the various boundary inelastic scatterings of phonon reduce thermal conductivity significantly. Also, larger mass difference due to voids induces smaller thermal conductivity values. These results indicate that the inclusion of vacancy defects offers an available way for improving the thermoelectric performance of silicon nanowires.
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