Band structure engineering in Sn1.03Te through an In-induced resonant level

Abstract

Narrow-band-gap IV–VI semiconductors represent a historically important class of thermoelectric materials. As one of the representative compounds of this class, SnTe has been reinvestigated over the last few years demonstrating its potential as a high-temperature p-type thermoelectric material. Here, we present a detailed study of the influence of very low doping levels of In, from 0.05% up to 2%, on the high-temperature transport properties of the self-compensated Sn_1.03Te compound. Our results evidence a strong impact of In on the transport properties, consistent with the presence of an In-induced resonant level (RL) in the valence bands of Sn_1.03Te. This peculiar behavior is confirmed by electronic band structure calculations performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) revealing a narrow and sharp peak in the density of states (DOS) induced by the hybridization of the In s-states with the electronic states of Sn_1.03Te. This distortion in the DOS results in a spectacular increase in both the thermopower and electrical resistivity at 300 K. Although the influence of the RL is somewhat lessened at higher temperatures, a significant enhancement in the ZT values is nevertheless achieved with a peak ZT of 0.75 at 800 K which represents an increase of 35% over the values measured in Sn_1.03Te. Of relevance for practical applications, the weak dependence of the RL on temperature leads to an enhanced average ZT value.