Crystal structure and electrical and optical properties of two-dimensional group-IV monochalcogenides
Mateus B. P. Querne, Jean M. Bracht, Juarez L. F. Da Silva, Anderson Janotti, Matheus P. Lima
Abstract
Two-dimensional (2D) semiconductor materials offer a platform for unconventional applications such as valleytronics, flexible nanoelectronics, and hosts of quantum emitters. Many of these materials and their electronic properties remain to be explored. Using ab initio simulations based on the density functional theory, we investigate group-IV monochalcogenides $MQ$ ($M=\mathrm{Si}, \mathrm{Ge}, \mathrm{Sn}$ and $Q=\mathrm{S}, \mathrm{Se}, \mathrm{Te}$), an emerging class of 2D materials, with two competing crystal structures: (i) phosphorenelike ($Pmn{2}_{1}$), which has already been synthesized, and (ii) $\mathrm{SiTe}$-type ($P\overline{3}m1$), which has been much less explored. Except for $\mathrm{SnS}$, we find that the $\mathrm{SiTe}$ type is the lowest-energy structure and has higher structural stability, motivating efforts to synthesize this less explored $P\overline{3}m1$ phase. Regarding the optoelectronic properties of these two phases, in the $P\overline{3}m1$ phase, $MQ$ compounds have band gaps around the sunlight spectrum peak and show narrower variations in band gap with the composition and higher absorption coefficients for lighter chalcogens. In contrast, in the $Pmn{2}_{1}$ phase, $MQ$ compounds have wider band gaps and show a band gap variation of up to 72% with composition, higher absorption coefficients with $\mathrm{Te}$ atoms, and potential for valleytronics. In particular, $\mathrm{SiS}$ shows interesting high optical anisotropy among all the investigated materials. Furthermore, the optical spectra present peaks that are particular to each phase or composition, making the refractive index a distinguishing parameter for identifying the different $MQ$ compounds. Finally, a phase transition from monolayer to bulk due to an interaction between the layers is observed. Thus, the present results straighten out the role of the crystalline phase in the optoelectronic properties of these monochalcogenides.