(Invited) The Use of Silicon-Germanium Superlattices in Thermoelectric Devices and Microfabricated Generators
Authors: Ferre Llin Lourdes , Mirando Francesco , Samarelli Antonio , Odia Ameze , Cecchi Stefano , Etzelstorfer Tanja , Müller Gubler Elisabeth , Chrastina Daniel , Isella Giovanni , Stangl Julian , Weaver John M R , Dobson Phil , Paul Douglas J 
Published: February 2020

This is significant interest in thermoelectrics for energy harvesting especially for the automotive industry where at present PbTe material demonstrates the best performance but cannot be used in many consummer products due to the toxicity of lead. SiGe thermoelectric devices have powered NASA spacecraft for many years but no civilian applications have used SiGe materials as the room temperature performance is low compared to BiTeSb alloys. Microfabricated thermoelectric generators allow low cost and high performance through the higher energy density and reduced material costs for the smaller devices compared to pick-and-place bulk generators. The BiTeSb alloys are expensive, use rare elements, are not easy to process and the efficiencies of the thermoelectric generation is not high enough for many applications therefore there is significant interest in developing higher performance thermoelectric materials that are cheaper, more practical to process and are sustainable.

In this paper we demonstrate extraction of the key properties required to optimise thermoelectric generators using a range of bulk SiGe and Ge/SiGe superlattice materials. The work will demonstrate how low dimensional structures can be used to increase the thermoelectric performance compared to bulk materials. In particular, Ge/SiGe superlattices have been used to engineer improved thermoelectric properties through the reduction of the thermal conductivity and the increase of the Seebeck coefficient. Novel microfabricated devices have been developed to allow the thermal conductivity to be extracted in a single device that also allows the Seebeck and electrical conductivity to be obtained. This is essential to enable the figure of merit ZT (= Seebeck coefficient^2 x electrical conductivity x T / thermal conductivity) and the power factor to be determined. The thermal conductivity results will also be compared to Raman and time domain techniques that allow the properties of unprocesses materials to be determined. By choosing barriers which scatter particular acoustic phonon wavelengths we demonstrate how the thermal conductivity can be reduced whilst improving the electrical transport in the material. Thermal conductivities down to 4.4 +/- 0.5 W/mK with Seebeck coefficients up to -403 +/- 3 uV/K at 300 K are demonstrated using the superlattice approach.

Full microfabricated thermoelectric generators made using bulk SiGe and Ge/SiGe superlattices will be presented. Whilst improved ZT values can be achieved for the superlattices, the lower thermal conductivity makes thermal impedance matching more difficult. Power outputs up to a few 100 mWs have been demonstrated with 10s of K difference in temperature across the legs of the generator. A full review will be presented of the latest results and methods to optimise the material and generator further.