Silicon Reloaded - Novel Perspectives of Silicon as a Thermoelectric Material

Silicon is known to be a poor thermoelectric material. Despite its high power factor (PF), its large thermal conductivity (≈ 130 W/mK) makes its thermoelectric figure of merit zT as low as 0.01 around room temperature. This notwithstanding, silicon has found applications in integrated devices, where its compatibility with microelectronic technology has prevailed over meager conversion efficiencies. However, in 2008 it was shown how phonon scattering at nanowire walls may reduce Si thermal conductivity without affecting its PF, leading to zT of ≈ 0.6 at 300 K. We will show how selective charge carrier scattering may be used instead to enhance its PF. Energy-selective charge carrier scattering (also referred to as ‘energy filtering’) was demonstrated long ago to provide a route to enhance PF in nanocomposites. In the presence of potential barriers, charge carriers are scattered with an efficiency dependent upon their kinetic energy. Low-energy carriers are scattered more efficiently than high-energy ones, causing the average carrier mobility to increase – while mobile (non-localized) carrier density decreases. This makes the Seebeck coefficient increase while keeping the electrical conductivity about steady. To our knowledge, such an effect was never reported in silicon until 2010, when we observed that very heavily boron-doped nanocrystalline Si films display a remarkable increase of their PF when extensively annealed at temperatures above 800 °C. Transmission electron microscopy revealed that annealing promotes the precipitation of silicon boride around grain boundaries. Computational and theoretical analyses showed that the potential barriers generated at the interphases filter out low-energy holes, enhancing the PF that raises from fractions of mW/mK2 in as-implanted films to ≈ 20 mW/mK2 in fully annealed samples. More recently, we could further elucidate the physical chemistry of the phenomenon – and why such an effect was not previously reported. Boron precipitates only when hydrogen (embedded upon CVD deposition) diffuses out of the sample, since H forms complexes with boron, preventing its precipitation. In wafers this requires prohibitively long high-temperature processing while the process is viable in small chips. Procedures to promote H diffusion out of large-size silicon chips could then be devised, enabling energy filtering also in wafer-scale thin films. Availability of high-zT Si thin films paves then the way to relevant novel applications of thermoelectrics, impacting microharvesting for several miniaturized devices.