Silicon is a remarkably useful element. Its abundance, low cost and low toxicity, combined with vast practical know-how means it is a leading material on which to base technologies. However for good reason, certain applications have under-utilised Si, with thermoelectrics (TE) being one example. Compared to other materials, highly-doped Si has desirably large Seebeck coefficient and high electrical conductivity, but these are negated by high thermal conductivity, meaning that TE performance is relatively poor for bulk Si, about 100-fold worse than for popular TE materials.
This talk will review some of the methods being explored to reduce the Si thermal conductivity, highlighting the significant engineering challenges involved, particularly as solutions until now have involved substantial alteration of the Si crystal structure, such as the introduction of porosity or the formation of nanowires or nanostructures. The talk will describe a range of successful alternative processes – such as so-called vacancy-engineering – which, via ion implantation and rapid-thermal annealing, leaves the Si structure almost indistinguishable from bulk Si. This novel approach shows that the introduction of large concentrations of lattice vacancies in silicon creates more than a 25-fold reduction in thermal conductivity, while electrical conductivity and Seebeck coefficient are largely maintained. This results in thermoelectric performance comparable to silicon nanowires, but in a more robust material that is straight-forward to fabricate . Other novel strategies for Si thermoelectrics being explored by my group – such as dislocation-engineering  – will also be discussed, as will our recent work to overcome engineering challenges to apply these methods to first, thin-films , and ultimately to bulk materials.
 N. S. Bennett, N. M. Wight, S. R. Popuri and J. W. G Bos, “Efficient thermoelectric performance in silicon nano-films by vacancy-engineering”, Nano Energy 16, 350-356 (2015).
 N. S. Bennett, D. Byrne and A. Cowley, “Enhanced Seebeck coefficient in silicon nanowires containing dislocations” Appl. Phys. Lett. 107 (1), 013903 (2015).
 N. M. Wight and N. S. Bennett, “Reduced thermal conductivity in silicon thin-films via vacancies”, Solid State Phenomena 242, 344-349 (2016).