Polycrystalline Mg2Si thin films: A theoretical investigation of their electronic transport properties

Abstract

The electronic structures and thermoelectric properties of a polycrystalline Mg2Si thin film have been investigated by first-principle density-functional theory (DFT) and Boltzmann transport theory calculations within the constant-relaxation time approximation. The polycrystalline thin film has been simulated by assembling three types of slabs each having the orientation (001), (110) or (111) with a thickness of about 18Å. The effect of applying the relaxation procedure to the thin film induces disorder in the structure that has been ascertained by calculating radial distribution functions. For the calculations of the thermoelectric properties, the energy gap has been fixed at the experimental value of 0.74eV. The thermoelectric properties, namely the Seebeck coefficient, the electrical conductivity and the power factor, have been determined at three temperatures of 350K, 600K and 900K with respect to both the energy levels and the p-type and n-type doping levels. The best Seebeck coefficient is obtained at 350K: the Syy component of the tensor amounts to about ±1000μVK−1, depending on the type of charge carriers. However, the electrical conductivity is much too small which results in low values of the figure of merit ZT. Structure–property relationship correlations based on directional radial distribution functions allow us to tentatively draw some explanations regarding the anisotropy of the electrical conductivity. Finally, the low ZT values obtained for the polycrystalline Mg2Si thin film are paralleled with those recently reported in the literature for bulk chalcogenide glasses.