Ab Initio Molecular Dynamics Study of the Interaction between Defects during Nonradiative Recombination

Herein, we apply ab initio molecular dynamics simulations based on a multireference description of the electronic structure to model the excited-state dynamics of models of the silicon (111) surface containing two neighboring triplet-coupled dangling bond defects at varying distances. We find that the system explores regions of near-zero energy gap between the ground and first excited triplet potential energy surfaces of 40-60 fs after excitation. Minimal energy conical intersection optimizations confirm that conical intersections exist in these regions. These intersections are characterized by geometric distortion of a single dangling bond site. The second dangling bond site maintains the geometric and electronic structures of a dangling bond defect in its ground electronic state. These simulations indicate that even when separated by very short distances (ca. 4 Å), dangling bond defects do not strongly interact during recombination. In fact, the recombination mechanism is essentially identical to that found in the previous work on isolated dangling bond defects. The absence of an interaction between apparently degenerate sites is attributed to the breaking of degeneracy by local geometric distortions on a single site. Though site-site interaction is not observed during the relaxation to the ground state, ultrafast (ca. 10 fs) energy transfer between nearest-neighbor dangling bonds is predicted to occur with a significant yield prior to recombination.