Tight-binding molecular dynamics and density-functional simulations reveal detailed diffusion mechanisms of the compact silicon tri-interstitials I 3 b . The diffusion pathway of I 3 b s can be visualized as a five defect-atom object both translating and rotating in a screw-like motion along <111> directions. Density functional theory yields a diffusion constant of ~10 -5 exp (- 0.49 eV / k B T) cm 2 /s. The diffusion path of I 3 b s suggests a similar collective diffusion for the ground state di-interstitial I 2 a s. While I 2 a s perform a translation/rotation step with a 0.3 eV barrier to diffuse along <111> bond directions, an additional reorientation step with a 90 meV barrier allows I 2 a s to achieve isotropic diffusion through the crystal. The resulting diffusion constant of I 2 a is ~10 -4 exp (- 0.3 eV / k B T) cm 2 /s. The 0.3 eV diffusion barrier of I 2 a s is consistent with the experimental value of 0.6 ± 0.2 eV. The low-diffusion barriers of I 2 a and I 3 b may be important in the growth of ion-implantation-induce extended interstitial defects.
I also calculate a low-lying transition path connecting the three lowest-energy silicon tri-interstitials I 3 b , I 3 c and ground state I 3 a . An examination of transition rates reveals that at an annealing temperature of 815 o C, the relative populations of the three silicon tri-interstitials reach thermal equilibrium within 1 μs. In particular, I estimate the transition rate from I 3 b to I 3 a to be 7.8 THz exp (- 1.4 eV / k B T). I find that the I 3 -chain structure rapidly decays to I 3 a by a strongly exothermic reaction with an activation of only 0.1 eV. The I 4 -chain, while not the lowest energy I 4 structure, is a deep local minimum of the total energy with escape barriers of 0.6 eV. I find that it can easily form by an exothermic reaction of I 3 a plus a single interstitial. Because {311} planar defects are comprised of parallel interstitial-chain structures, this reaction may be an important step of the growth process by which {311} defects form.