Physical and Chemical Aspects of Thermoelectric Phenomena in Nanostructured Materials

Thermoelectric efficiency of a material is described by the so-called thermoelectric figure of merit ZT = S2σκ-1T, where S is the Seebeck coefficient, σ and κ are correspondingly the electrical and the total thermal conductivities. Nanostructuring has opened new ways to improve thermoelectric performance of a material either by decreasing κ or by increasing S2σ. We have studied thermoelectric properties of two nanostructured systems: heavily-doped polycrystalline silicon films with embedded nanocavities and InGaAs films with high concentration rare-earth TbAs embedded nanoparticles. In the first system, we have analysed the effect of the formation of nanocavities, which were expected to act as efficient phonon scattering centres, thus reducing the thermal conductivity. The thermal conductivity was about half of the reported value in bulk polycrystalline silicon, however, the same low value was measured in the samples without nanocavities. This might suggest that the film microstructure dominates the thermal conductivity in all cases. The material also showed outstanding thermoelectric properties. Upon thermal treatments at temperatures above 800°C we measured higher Seebeck coefficients than those normally found in monocrystalline silicon at corresponding doping level. This increase was found to be connected to the electron energy filtering by the potential barriers at the grain boundaries which accumulated dopant precipitates during the thermal treatments. As the result we have obtained a maximum ZT of 0.18 at room temperature. In the second system, we studied the effect of embedded TbAs nanoparticles on the thermoelectric properties of InGaAs. In this group of materials, the nanoparticles serve to reduce thermal conductivity (through phonon scattering), increase Seebeck coefficient (through electron energy filtering), and increase of electrical conductivity (through nanoparticle donation of electrons). Both presence of the electron filtering and decrease of the thermal conductivity was experimentally observed. The electrical conductivity, however, drastically decreased and no enhancement of ZT was achieved.