Importance of relativistic effects in electronic structure and thermopower calculations for Mg_2Si, Mg_2Ge, and Mg_2Sn

摘要

We present a theoretical study of the influence of relativistic effects on the electronic band structure and thermopower of Mg_2X (X = Si, Ge, Sn) semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used, and a detailed comparison between the fully relativistic and semirelativistic electronic structure features is done. We show that the spin-orbit (S-O) interaction splits the valence band at F point in good agreement with the experimental data, and this effect strongly depends on X atom. The S-O modifications of the topology of the Γ-centered holelike Fermi surface pockets lead to a change in electron transport properties, which are investigated using the Boltzmann approach. In addition, a simple and efficient method is presented for the calculation of density of states effective mass m~*, and then used to examine the impact of relativistic effects on m~*. It is found that the S-O coupling of the valence bands reduces the effective mass and therefore significantly lowers the thermopower, primarily in Mg_2Sn, but also in Mg_2Ge. A detrimental influence of the S-O interaction on the thermoelectric performance of p-type Mg_2X is analyzed as a function of temperature (10-900 K) and carrier concentration (10~(18)-10~(22) cm~(-3)). Interestingly, similar calculations in n-type Mg_2X, show a negligible effect of the S-O interaction on the lowest conduction bands and, consequently, also on the Seebeck coefficient.