Thermoelectric properties of ultrascaled silicon nanowires

The efficiency of thermoelectric devices is quantified by the thermoelectric figure of merit, ZT = S 2sigmaT/kappa, where S, sigma, and kappa are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively, and T is the operating temperature. For a material to be considered for TE applications, ZT should be at least 1. Bulk silicon has a ZT of about 0.01 and is typically not a good candidate for TE conversion. ZT in silicon nanowires (SiNWs) has been experimentally shown to have ZT close to 1 owing to extremely low kappa in them. It is believed that the power factor (S2sigma) can be increased by decreasing the wire cross section.;In this work, the room-temperature ZT of highly doped ultrascaled silicon nanowires is calculated by solving the electron and phonon Boltzmann transport equations (BTE) with a proper account of the two dimensional confinement of both electrons and phonons using the ensemble Monte Carlo transport (EMC) kernel. Electron and phonon scattering with impurities, boundaries, and phonons are included in the EMC kernel. sigma decreases with deceasing wire cross section because of the strong increase in the electron surface roughness scattering and phonon scattering. With increasing confinement, the energy separation between the conduction band and the Fermi level increases, which results in an increase in the average energy carried by electrons, therefore S increases with decreasing wire cross section. The thermal conductivity decreases with decreasing wire cross section because of the increase in phonon-boundary scattering. The phonon drag component of S is negligibly small in SiNWs because of a strong reduction in the phonon mean free path.;The ZT in ultrascaled SiNWs calculated from the themoelectric simulator is about 30 times larger than that in bulk silicon: the enhancement in ZT occurs primarily because of the decrease in kappa due to strong phonon-boundary scattering and not from an enhancement in the power factor. ZT does not increase exponentially with decreasing wire cross as predicted by earlier studies, because in ultra-narrow wires, roughening (apart from degrading kappa) severely degrades electrical conductivity.