Enhancing the Figure of Merit, ZT, of Silicon Germanium Nanocrystal Films by Synthesizing Dense Films using Nonthermal Plasma and Post Processing

Abstract

A thermoelectric material’s dimensionless figure of merit (ZT) determines the efficiency of conversion of heat into electricity of a thermoelectric generator (TEG). Increase in ZT, increases the efficiency of a thermoelectric generator. The figure of merit increases with increase in material’s electrical conductivity and Seebeck coefficient while decreases with an increase in thermal conductivity. Recently, the ZT of silicon germanium alloys have been increased by nanostructuring the bulk which lead to decrease in thermal conductivity by increase in phonon scattering due to nanoscale crystal grain sizes. Plasma synthesised doped silicon germanium nanocrystals have a narrow size distribution and are promising candidates as opposed to ball-milled nanopowder. Nanocrystals produced by plasma synthesis needs to be fabricated as thin films for microelectronic applications. Nanocrystal films synthesised, by rastering of substrates, in the nonthermal plasma reactor are very porous and hence have low electrical conductivity. To produce denser nanocrystal films, the plasma reactor was modified and post processes were introduced. Mixed-phase silicon films were produced by dual-plasma setup and these films were annealed to form fully nanoscrystalline silicon films. The mixed-phase and fully nanocrystalline silicon films were determined to have low porosity. These films were characterized for their crystallinity, thickness and the average crystal grain size. After characterization of the mixed-phase and nanocrystalline silicon films, the thermoelectric properties (Seebeck coefficient, thermal conductivity and electrical conductivity) were determined and hence the ZT. This ZT was determined with just silicon and thus low. Future work can be undertaken with mixed-phase film composed of doped silicon germanium alloy and to determine the thermoelectric properties.