Band effective masses of cubic (GeTe)₁₀Sb₂Te₃ and its anisotropy

Abstract

Recent progress observed in the band engineering of thermoelectric GeTe-based materials is significantly dependent on the enhancement of its electronic band degeneracy and anisotropic effective mass. Here, we evaluated the anisotropic effective mass of cubic (GeTe)₁₀Sb₂Te₃ according to first-principles calculation, based on the Korringa-Kohn-Rostoker coherent-potential-approximation method, by comparing it with cubic GeTe. We found alloying GeTe with Sb₂Te₃ decreased the band gap energy and shifted the valence band maxima closer to the Fermi level, indicating straightforward convergence of the multiple Σ, L, and Δ valence bands. The obtained band structure suggested that the Δ band is expected to contribute electronic transport properties at the experimental carrier concentration which was reported previously. The Δ band had unique characteristics with a heavier densityof-states effective mass and higher band anisotropy than the conventional Σ and L bands, possibly leading to enhancement in Seebeck coefficient of (GeTe)₁₀Sb₂Te₃ . The alloying with Sb₂Te₃ did not significantly change band anisotropies while it increased overall band effective masses of Σ, L, Δ valence bands. Therefore, it is suggested that alloying GeTe with Sb₂Te₃ enhances its band degeneracy and band effective masses while keeping its anisotropy.