We have investigated the effect of GaAs surface conditions prior to plasma enhanced chemical vapor deposition of a silicon nitride cap on the activation efficiency of implanted Si in GaAs. The oxygen plasma treatment improved the activation efficiency of implanted Si by ∼35% over (1:10) NH4OH:H2O treatment. X-ray photoelectron spectroscopy (XPS) analysis of the oxygen plasma treated GaAs surface indicated the formation of ∼25Å thick oxide layer consisting of Ga2O3, As2O3, As2O5 and elemental As. During the activation anneal, the arsenic-containing oxides react with the GaAs substrate to form Ga2O3 and elemental As. The presence of excess As between the GaAs and the nitride cap film increases the probability that the implanted Si incorporates in the Ga sites over the As sites, and thereby improves the activation efficiency. This surface-related mechanism suggests that the variation in activation efficiency is mostly attributed to variation in surface conditions, and may explain the wide variety of reported values of activation efficiency.