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 (hSiC) and benchmark its response against 2D h boron nitride (hBN). By explicitly resolving exciton phonon matrix elements, we identify an electron-phonon scattering channel mediated by A$^\prime$ 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 hBN, despite a smaller exciton phonon energy separation and fewer resolved replicas. The bright \textbf{K}\textbf{K} exciton governs near UV zero phonon emission, while intervalley excitons acquire radiative character through symmetry allowed 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.