Quasiparticle energies and optical excitations of 3C-SiC divacancy from GW and GW plus Bethe-Salpeter equation calculations

Excitons localized around point defects in semiconductors are promising candidates for long-lived and photon-addressable qubits. However, their microscopic origin is difficult to characterize due to the computational complexity of studying large systems with defects. Here we study the quasiparticle and optical absorption spectrum of the divacancy defect in 3C-SiC, a prototypical defect for quantum information applications, by means of large-scale GW and GW plus Bethe-Salpeter equation calculations. Despite the presence of localized unoccupied quasiparticle states in the gap, we find that the low-energy excitonic states are made primarily of transitions from occupied defect states to continuum conduction states from SiC, especially from the X point of the Brillouin zone. The mixed character of defect states and bulk states of these low-energy exciton states is in contrast with the NV- center in diamond and the divacancy in 4H-SiC, where the deep defect levels are well separated from bulk states. Our calculations provide a quantitative prediction of the defect quasiparticle energy levels and a physical understanding of the zero-phonon absorption. They highlight the important role of frontier conduction bands in the optical properties and formation of low-energy excitons in 3C-SiC divacancy.