Demonstrating the quasilocalized nature of the Sn impurity states in Bi2Te2Se
Résumé
Binary M2X3 compounds (M = Sb, Bi and X = S, Se, Te) and their solid solutions are among the most efficient materials for thermoelectric applications near room temperature. In the canonical compound Bi2Te3, Sn has been shown to induce a resonant level (RL) 15 meV below the valence band edge, leading to enhanced thermopower values with respect to those achieved with rigid-like impurities. However, the question of whether Sn is still a resonant impurity in ternary alloys and notably, in the topological insulator Bi2Te2Se, remains unsettled. Here, we demonstrate experimentally that Sn loses its resonant character in Bi2Te2Se. Alloying with Pb in Bi1.99-ySn0.01PbyTe2Se rigidly shifts the chemical potential towards the valence bands and reveals a metal-insulator transition at low temperatures, which can be well described by an Sn impurity level that dominates the electrical conduction up to y = 0.020. Further increasing y to 0.025 and 0.030 progressively restores a conventional valence band conduction. The presence of the Sn impurity level that forms a density-of-states peak disconnected from the valence band manifold is supported by electronic band structure calculations. Due to the quasi-localized nature of this impurity level, the addition of Sn does not enhance the thermopower values compared to those measured in Sn-free samples, resulting in comparable thermoelectric performance, with a peak ZT value of 0.70 achieved at 425 K for y = 0.01.
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