Experimental observations of dopant diffusion and defect formation are reported as a function of ion energy and implant temperature in Si implanted GaAs. In higher energy implants (>100 keV), little or no diffusion occurs, while at energies less than 100 keV, the amount of dopant redistribution is inversely proportional to energy. The extended defect density shows the opposite trend, increasing with increasing ion energy. Similarly, the diffusion of Si during post implant annealing decreases by a factor of 2.5 as the implant temperature increases from -2 to 40°C. In this same temperature range, the maximum depth and density of extrinsic dislocation loops increases by factors of 3 and 4, respectively. Rutherford Backscattering (RBS) channeling measurements indicate that Si implanted GaAs undergoes an amorphous to crystalline transition at Si implant temperatures between -51 and 40°C. A unified explanation of the effects of ion energy and implant temperature on both diffusion and dislocation formation is proposed based on the known differences in sputter yields between low and high energy ions and crystalline and amorphous semiconductors. The model assumes that the sputter yield is enhanced at low implant energies and by amorphization, thus increasing the excess vacancy concentration. Estimates of excess vacancy concentration are obtained by simulations of the diffusion profiles and are quantitatively consistent with a realistic sputter yield enhancement. Removal of the vacancy rich surface by etching prior to annealing completely suppresses the Si diffusion and increases the dislocation density, lending further experimental support to the model.