Paradoxical enhancement of the power factor in polycrystalline silicon due to the formation of nanovoids

N. Neophytou Institute for Microelectronics, Technical University of Vienna, Wien, Austria, and School of Engineering, University of Warwick, Coventry, UK Tel.: +43 58801 36030 Fax: +43 58801 36099 Abstract Holey silicon has been considered a viable candidate as a thermoelectric material in view of its low thermal conductivity. However, since voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common embodiment, namely porous silicon, to be of practical interest. In this communication we show that high power factors can indeed be achieved in nanoporous structures in highly doped silicon. High power factors, up to a huge 22 mW K−2m−1 (more than six times higher than bulk values), were observed in heavily boron doped nanocrystalline silicon films where nanovoids (NVs) were generated by He ion implantation. Differently than in single-crystalline silicon where He implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000 ◦C results in the formation of a homogeneous distribution of NVs with final diameters of about 2 nm and densities on the order of 10 cm−3 with an average spacing of 10 nm. Its morphology shows silicon nanograins of 50 nm in diameter decorated by SiBx 5-nm precipitates. We recently reported that PFs up to 15 mW K−2m−1 can be achieved in silicon-boron nanocomposites (without NVs) due to a simultaneous increase of the electrical conductivity and Seebeck coefficient. In that case, high Seebeck coefficient was achieved by potential barriers on the grain boundaries, and high electrical conductiv-