Thermoelectric Properties of Mg2Si Thin Films by Computational Approaches

The semiclassical Boltzmann approach coupled with density functional theory calculations has been used to investigate the structural, electronic, and thermoelectric properties of Mg2Si thin films with 001, 111, and 110 orientations. The most stable slab is found to be that with the 110 orientation. The electronic band gap vanishes for the 001 and 111 thin films whereas for the 110 orientation the film is semiconducting with a band gap ranging from 0.27 to 0.36 eV depending on the number of atomic planes used to model the thin film. The energy gap decreases when the number of planes increases. As a consequence of the electronic band structure, the 110 semiconducting thin film exhibits the highest thermoelectric performance, especially the Seebeck coefficient (-350 mu V K-1 at 600 K). The lower the number of atomic planes the larger the Seebeck coefficient. By comparing experimental data for the electrical conductivity sigma to our calculated value of sigma/tau at 600 K we have determined the electron relaxation time tau to be about 5 x 10(-16) s. Using this value, and assuming a thermal conductivity of 2-3 W m(-1) K-1 for the 110 Mg2Si thin film, we estimate that the figure of merit ZT at 600 K lies in the range of 0.4-0.6.