Geometry effects on thermoelectric properties of silicon nanowires based on electronic band structures

The thermoelectric properties of silicon nanowires with different shapes, sizes, and orientations are theoretically investigated using sp(3)d(5)s* tight-binding model coupled with ballistic transport approach. We found that the thermoelectric properties significantly depend on nanowire geometry. Compared to [111] and [100] nanowires, n-doped and p-doped [110] nanowires show the worst performance in terms of power factor per cross-section area and figure of merit (ZT). As nanowire size decreases, thermoelectric properties of nanowires can be enhanced. As a result, triangular nanowires with side length of 1 nm have the best results of ZT and it can be enhanced to 1.5 and 0.85 for an n-type nanowire along [111] orientation and a p-type nanowire along [100] orientation, respectively. For extremely narrow nanowires, thermoelectric properties are only dependent on the number of the transmission modes instead of material properties such as carrier effective mass. Moreover, cross-section shape and thermal conductance contributed by electrons play important roles in ZT while their influence can be ignored for large size nanowires. Even though smaller size nanowires have better performance with the consideration of the single nanowire thermoelectric properties, they might be less efficient than larger diameter nanowires, as packing space is not very dense. (C) 2010 American Institute of Physics. [doi:10.1063/1.3273485]