The effect of self-organization during deposition on the segregation behaviour of Au in the Si-Ge-Au nano-multilayer thermoelectric generator system

Si-Ge-Au amorphous-nanocrystalline composites are important thermoelecric materials where the Seebeck coefficient and the electrical conductivity may be tuned independently. There are many attempts to improve the thermoelectric properties of Si/Ge systems by the addition of additives like Au. However, the effectiveness of doping and the role of Au is not clear. We report on structural transformations and Au segregation behaviour in post-annealed sputter deposited multilayer thin films using composition depth profiling, electron microscopy, x-ray diffraction and computer simulation. We show that depending on the nominal thickness of the initial Au layers the film volume becomes depleted of Au for thinner Au layers while for thicker Au layers the Au content is much higher in the form of large precipitates. Due to the self-organization and changing of microstructure during deposition the segregation of Au is very different for different Au individual layer thicknesses. In case of thinner Au layers it is enriched at the free surface, while for thicker initial Au layer thickness gold accumulated at the substrate. We have carried out computer simulations based on stochastic kinetic mean-field model. These calculations support the argument that initial Au island size can drastically affect the dissolution/growth and segregation behaviour of gold which can significantly influence the thermoelectric parameters. Based on these results we provide a possible explanation to the difference in thermoelectric properties found in the literature.