First-Principles Study on Doping of SnSe2 Monolayers.

Doping is a vitally important technique that can be used to modulate the properties of two-dimensional materials. In this work, by using first-principles density functional calculations, we investigated the electrical properties of SnSe2 monolayers by p-type/n-type and isoelectronic doping. Substitution at Sn/Se sites was found to be easy if the monolayer was grown under Sn-/Se-poor conditions. Substitutions at Sn sites with metallic atoms (e.g. Ga, Ge, In, Bi, Sb, Pb) resulted in positive substitution energies, which indicated that they were not effective doping candidates. For substitutions at Se sites with nonmetallic atoms, no promising candidates were found for p-type doping (e.g., N, P, As). Among these, N and As showed positive substitution energies. Although P had a negative substitution energy under Sn-rich conditions, it introduced trap states within the band gap. For n-type doping (e.g., F, Cl, Br), all the calculated substitution energies were negative under both Sn- and Se-rich conditions. Br was proven to be a promising candidate, because the impurity introduced a shallow donor level. Finally, for isoelectronic doping (e.g., O, S, Te), the intrinsic semiconducting features of the SnSe2 monolayer did not change, and the contribution from the impurity to the states near the band edge increased with the atomic number.