Novel exciton behavior in low-dimensional polarized MoSSe/GaN heterostructures: A first-principles study

Excitonic effects are important in many low-dimensional materials due to a reduced dielectric screening which generally enhances exciton binding strengths. For the important class of two-dimensional layered materials, we examine monolayers of GaN, $\mathrm{Mo}{\mathrm{S}}_{2}$, and MoSSe as well as heterojunctions composed of two layers: GaN and MoSSe. Employing many-body perturbation theory for the quasiparticle band structure (GW theory) and solving the Bethe-Salpeter equation for the excitons, we systematically study the optical properties of these systems. We also include spin-orbit coupling in our analysis and elucidate its influence on the frontier orbitals. It is found that the lowest four excitons in MoSSe monolayers appear at about 1.65 eV and have a binding energy of about 0.67 eV. These excitons are either red-shifted or blue-shifted when forming heterojunctions, depending on the polarization of the junction. These properties, as well as the binding strength and spatial localization (interlayer versus intralayer) of the excitons are systematically correlated with the system's polarization and dielectric strength, opening the way to tune optical properties in such systems by varying polarization and dielectric strength.