  As a result of suppressed phonon conduction, large improvements of the
thermoelectric figure of merit, ZT, have been recently reported for
nanostructures compared to the raw materials' ZT values. It has also been
suggested that low dimensionality can improve a device's power factor as well,
offering a further enhancement. In this work the atomistic
sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the
electronic structure of silicon nanowires (NWs). The linearized Boltzmann
transport theory is applied, including all relevant scattering mechanisms, to
calculate the electrical conductivity, the Seebeck coefficient, and the
thermoelectric power factor. We examine n-type nanowires of diameters of 3nm
and 12nm, in [100], [110], and [111] transport orientations at different
carrier concentrations. Using experimental values for the lattice thermal
conductivity in nanowires, the expected ZT value is computed. We find that at
room temperature, although scaling the diameter below 7nm can be beneficial to
the power factor due to banstructure changes alone, at those dimensions
enhanced phonon and surface roughness scattering degrades the conductivity and
reduces the power factor.
