The incorporation of Si in ion‐implanted and thermally annealed GaAs has been studied by local vibrational mode spectroscopy. Raman scattering and Fourier transform IR absorption have been used to analyze the Si site distribution both in the near surface region and averaged over the whole implanted layer, respectively. The samples implanted with doses of 5×1014–1016cm−2were annealed with various techniques using different capping layers. The Si site distribution is found to depend strongly on the details of the annealing. In particular, capping with SiO2leads to the formation of the so‐called Si‐Xdefect complex in addition to the incorporation of Si on both lattice sites and the formation of nearest‐neighbor Si pairs.