Nano-engineering thermoelectrics in silicon

Though silicon is a poor thermoelectric (TE) material, silicon nanowires have been demonstrated to achieve the TE figure-of-merit ZT=1, comparable to commercially applied materials. This enhancement comes predominantly from the nearly two orders of magnitude reduction in thermal conductivity possibly due to the dimensional crossover or/and the increased boundary scattering. Here we present the study of the TE properties of a novel hole-bar-like silicon thin film structure (25 nm thick). The hole-bar consists of a two-dimensional array of 14 nm-wide holes at a pitch of 34 nm, made by Superlattice Nanowire Pattern Transfer technique. This phonon-crystal material exhibits thermal conductivity as low as 2 W/mK at room temperature, about 3 times smaller than that of similarly dimensioned nanowires. This is the first such demonstration of the TE enhancement mediated by a nanoscale phonon crystal.