Strategies for the High Average zT in the Electron-Doped SnTe

We propose a new strategy to achieve a high average zT in electron-doped SnTe by applying the two-band (TB) and single parabolic band (SPB) models to the electronic transport properties of Sn0.97M0.03Te (M=Ga, In, Bi, and Sb) reported in the literature. To achieve a high average zT at temperatures from 300 to 823 K, both zT at 300 and 823 K should be high with a steadily increasing zT over the temperatures. The p-type SnTe is known to have a light valence band and a heavy valence band that are approximately 0.40 eV apart. The Bi-doped SnTe exhibits one of the highest zT among all the other doped samples at 300 K (0.09) and the highest zT at 823 K (0.9), with a steadily increasing zT in between. The TB model confirms the presence of the resonant state at 300 K which is responsible for the high zT at 300 K. The B-factor, which is related to the theoretical maximum zT, calculated by the SPB model indicates a steady increase in zT with increasing temperature. The temperature-dependent B-factor of the Bi-doped SnTe suggests that the initial position of its Fermi level at 300 K calculated by the TB model may be responsible for the temperature coefficient of the B-factor, which determines the zT at 823 K. According to the SPB model, experimental zT of 0.9 of the Bi-doped SnTe can be further improved to 1.03 (14% improvement) at 823 K upon carrier concentration optimization. To achieve a high average zT in SnTe, electron doping with a dopant that forms a resonant state and placing the Fermi level at the light valence band in the vicinity of the heavy valence band maximum are both essential.