Effects of Sb and Bi Doping in SnTe Layers Grown by Molecular Beam Epitaxy

SnTe is an interesting narrow-band-gap semiconductor due to its good thermoelectric performance, near-room-temperature phase transition and ferroelectricity, and topologically nontrivial band structure. Yet, the negative formation energy of Sn vacancies results in SnTe thin films always being heavily p-type, which limits their potential in thermoelectric and spintronic applications. While attempts have been made to develop n-type SnTe bulk crystals, these efforts have not extended to thin films. In this work, we report on the effect of incorporating Sb and Bi dopants in SnTe thin films by molecular beam epitaxy. We found that Sb is an unsuitable electron dopant and has a detrimental effect on the SnTe surface morphology. However, by incorporating Bi into SnTe films, a 2.5× reduction in free hole concentrations is observed and a smooth surface is retained. Unfortunately, high Bi fluxes lead to the formation of twin defects. Using first-principles calculations with density functional theory, we show that the preferred substitutional site of the Sb and Bi dopants depends on the growth conditions: positively charged Sb_Sn⁺ impurity is only found in heavily Sn-poor conditions, while Bi_Sn⁺ is favored in all but the Sn-rich growth conditions. Moreover, for both Sb and Bi dopants, the substitutional sites may form complexes, with Sn vacancies acting as shallow acceptors, which would be a significant factor limiting the reduction of free hole concentrations. Although the films remain p-type, this work is the first step toward developing n-type SnTe thin films.