First-principles Study of Large Seebeck Coefficients in Fe-doped Si-Ge Alloys

Si-Ge alloys are one of the cheapest nontoxic thermoelectric materials utilized at high temperatures, but their dimensionless figure of merit, ZT, is relatively small. To improve their low ZT values, there are some attempts to modify an electronic band structure by doping Fe, and it has been reported that a high ZT value, ZT>1.88 (at T=873K), as well as a large Seebeck coefficient, |S|>517μV/K (at T=673K), were produced in the nanostructured Si_0.55Ge_0.35P_0.10Fe_0.01 sample [1]. It is believed that they originate from a strong peak at the edge of the conduction band generated by the Fe-doping (a so-called impurity state). However, an occurerence of the impurity state in the Fe-doped Si-Ge system has not been confirmed yet. In this work, therefore, the impurity state in the Fe-doped Si-Ge alloys is calculated from an electronic band structure calculation, and the reported large Seebeck coefficient is reproduced with the use of the linear response theory. Using a special quasi-random structure with a hybrid functional (HSE06), the impurity state was successfully produced, and computed Seebeck coefficients showed good agreement with the experimental data.
[1] K. Delime-Codrin, et al., Appl. Phys. Express 12, 045507 (2019).