|
This improvement in the power factor further proves the point that boundary
scattering is more effective in reducing the thermal conductivity than reducing the power
factor. As a result, phonon engineering techniques that cause additional reductions in the
thermal conductivity could provide improvements of 
, despite the consequent reduction in the electrical conductivity. For example, in the case of a fully
diffusive boundary for phonons, either by special engineering of the roughness [148,149,21], by decorating the surfaces with various species [150,59,151], or by modulating the nanowire's diameter [124], the
performance could be increased. This is illustrated in Fig. 6.5, showing
for the same nanowires as before in Fig. 6.3, but now we assume a fully diffusive boundary for phonons, e.g. the specularity parameter is set to 
for all wavevectors. In this case,
is increased to values close to
for both 
-type and 
-type nanowires (in the best cases). This is almost a factor of
improvement compared to the case we present in Fig.6.3 where we employ the 
-dependent specularity parameter 
, rather than fully diffusive boundaries for all phonons.
. Results for nanowires in the
(blue-circle),
(red-square), and
(green-triangle) orientations are shown. In the calculation of the power factor electron-phonon plus electron-boundary scattering is considered. In the calculation of the thermal conductivity phonon-phonon and phonon-boundary scattering are considered, but in this case the boundary is assumed to be fully diffusive. The dotted lines in (a) show for reference the