Phonon-Assisted Photoluminescence and Ultrafast Exciton Dynamics in Two-Dimensional Silicon Carbide

Phonon-assisted photoluminescence provides a direct window into exciton-phonon interactions in semiconductors. Using fully ab initio many-body perturbation theory, including finite-momentum Bethe-Salpeter calculations, we investigate phonon-assisted emission and exciton dynamics in two-dimensional (2D) hexagonal silicon carbide (h-SiC) and benchmark its response against 2D h-boron nitride (h-BN). By explicitly resolving exciton-phonon matrix elements, we identify an electron-phonon scattering channel mediated by \({{\rm{E}}}^{{\prime} }\) high-energy longitudinal and transverse optical phonons as the dominant contributors to sideband formation and quantify their spectral weights. We find that h-SiC exhibits pronounced phonon-assisted sidebands comparable to h-BN, despite a smaller exciton-phonon energy separation and fewer resolved replicas. The bright K - K exciton governs both the near-UV zero-phonon emission and the phonon-assisted sidebands through symmetry-allowed longitudinal and transverse optical-phonon coupling. Temperature-dependent scattering rates reveal an ultrashort bright-exciton lifetime of approximately 300 fs at 10 K, highlighting rapid exciton relaxation driven by intrinsic phonon channels.