Anisotropic electronic and excitonic properties of monolayer SiP₂ from the first-principles GW-BSE calculations

We investigate electronic structures and excitonic properties of monolayer SiP₂ within the framework of first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations. Within the G₀W₀ approximation, monolayer SiP₂ is identified as a direct-gap semiconductor with an electronic gap of 3.14 eV, and the excitons exhibit a hybrid-dimensional character similar to that of the bulk counterpart. The optical absorption spectra reveal pronounced excitonic effects with strong anisotropy: the first bright exciton has a binding energy of 840 meV under x-polarized light, compared with 450 meV under y-polarized light. We further analyze the symmetry origins of the polarization-dependent optical selection rules through group theory. This binding energy difference arises from the intrinsic nature of the excitons: flat-band excitons under x-polarized light and conventional excitons localized at a single k point under y-polarized light. Our work enhances the understanding of excitonic behavior in monolayer SiP₂ and highlights its potential for polarization-sensitive and directionally tunable optoelectronic applications.