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The physical nature of the electrically inactive phosphorus in silicon is investigated by double crystal X‐ray diffraction measurements. This analysis is performed on laser annealed supersaturated samples, doped by ion implantation up to 5 × 1021 cm−3. After isothermal heat treatments, these solid solutions show marked reductions in the electrically active phosphorus concentration. In particular, 850°C heatings give rise to a carrier concentration which corresponds to the phosphorus solubility in equilibrium with the inactive dopant. This dopant is characterized by means of lattice strain measurements: they are found consistent with the presence of perfectly coherent cubic SiP precipitates. This result is in agreement with the one obtained in preceeding works by electrical measurements and transmission electron microscopy observations and contradicts the hypothesis that the excess dopant atoms are, at least in part, charged point defects (E‐centres).

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