Silicon Ultra-Thin-Layers (UTLs) have attracted significant attention as efficient electronic and thermoelectric devices after the realization that length scale provides an additional degree of freedom in engineering their electronic and thermal transport properties. Thermoelectric devices traditionally suffer from low efficiency. Nanostructured thermoelectric devices with enhanced performance compared to their bulk counterparts, however, have recently been realized. Silicon UTLs, and low dimensional channels in general, with a thermoelectric performance two orders of magnitude higher than that of bulk silicon have been demonstrated. The thermoelectric performance of UTLs can be further optimized by using the most beneficial transport and confinement orientations. In this way, the desired improved properties of materials can be engineered to some degree.
We use the atomistic tight-binding sp3d5s*-SO model and the Boltzmann transport theory, with all relevant scattering mechanisms included, to investigate thermoelectric transport in silicon UTLs. We study the effect of quantum confinement on the electronic structure of UTL channels and identify the main electronic structure factors that influence their performance. It was found that structural quantization can severely affect the electronic properties of UTL channels by changing the effective masses, the curvature of the bands, and altering degeneracies through valley and subband splitting. Different UTL confinement and transport orientations result in different electronic properties. Specifically for the thermoelectric power factor of p-type UTLs, it was found that the (110)/[110] channel provides a two to three times larger power factor compared to all other channels, regardless of surface or transport orientations as shown in figure 1. Interestingly, the power factor increases as the layer thickness decreases. The rest of the channels exhibit similar thermoelectric power factors. This behavior originates from confinement-induced large curvature variations in the subbands of this particular channel. It demonstrates how the geometry and the length scale degree of freedom can be utilized to improve the electronic and thermoelectric properties of nanoscale devices. A similar behavior was observed for ultra-narrow nanowires, in which case the power factor of the p-type [110] and [111] channels largely increases as the nanowire diameter is scaled.