Ultrascaled Silicon Nanowires as Efficient Thermoelectric Materials

The room-temperature thermoelectric figure of merit (ZT) of highly doped silicon nanowires (SiNWs) of square cross section was calculated by solving the electron and phonon Boltzmann transport equations with a proper account of the two dimensional confinement of both electrons and phonons. The ZT in SiNWs is almost two orders of magnitude larger than that of bulk silicon. The enhancement of ZT in SiNWs occurs primarily because of strong phonon-boundary scattering that degrades the lattice thermal conductivity by about two orders of magnitude from its value in bulk silicon. With decreasing wire cross section, the electrical conductivity (sigma) and thermal conductivity (k) decrease, whereas the Seebeck coefficient (S) increases. Therefore, the ZT variation with cross section is nonmonotonic, with ZT maximal for a wire of cross section 4 times 4 nm². Boundary roughness scattering indeed proves to have a significant effect on both electronic and thermal transport in SiNWs.