The behavior of Ar plasma‐induced defects deactivating Si donors in GaAs has been studied. We have applied photoreflectance spectroscopic analysis combined with stepwise wet etching to the depth profiling of defects and succeeded in determining the concentrations and profiles of defects in the sub‐surface layer for the first time. We found that the point defects responsible for deactivation undergo electronically enhanced diffusion under the circumstance of photoexcited carriers, demonstrating that the ultraviolet light from plasma is a cause of the deep penetration of defects far beyond the stopping range of ions. We also found that the generation of these point defects is enhanced by photoexcited carriers and that diffusing point defects are trapped by the background defects or impurities, forming immobile complexes. We propose a model in which self‐interstitials are the most probable point defects responsible for deactivation. ©1995 American Institute of Physics.