Anisotropic electronic and excitonic properties of monolayer SiP ₂ from the first-principles GW-BSE calculations

Abstract We investigate electronic structures and excitonic properties of monolayer SiP 2 within the framework of first-principles GW plus Bethe–Salpeter equation (GW-BSE) calculations. Within the G 0 W 0 approximation, monolayer SiP 2 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 SiP 2 and highlights its potential for polarization-sensitive and directionally tunable optoelectronic applications.