The diffusion of Si was studied using secondary ion mass spectroscopy and transmission electron microscopy after implantation into GaAs at energies ranging from 20 to 200 keV and at doses ranging from 1 x 10(13) to 1 X 10(14) cm-2 followed by furnace annealing. Little or no diffusion occurred in the higher energy implants (>100 keV) while, in general, samples implanted at lower energies (<100 keV) exhibited appreciable dopant redistribution, regardless of peak implant concentration. Both concentration dependent and concentration independent diffusion was observed. Dislocation loop density varied inversely with the amount of diffusion as a function of implant energy. The Monte Carlo computer program TRIM is able to predict the trends of the implant energy dependence of the diffusion by considering the excess point defect dose produced upon implantation. The influence of this excess defect dose together with surface effects on the diffusion of Si is shown to be consistent with a vacancy assisted Si diffusion mechanism.