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The As diffusion coefficient as a function of its concentration was determined by Boltzmann–Matano analysis of the profiles of the dopant diffusing out of its conjugate phase precipitates during furnace annealing at 900 and 1050 °C of samples heavily doped by ion implantation. This method allowed to assure a constant diffusion source of As and to investigate a doping range attaining 3×1021 cm−3. Along the same lines, the diffusivity versus concentration of specimens heavily implanted with P was determined at 900 and 1000 °C. Dopant profiles were determined by secondary neutral mass spectroscopy. The diffusivity of both As and P increases with dopant content, attaining a maximum at a concentration which closely corresponds to the saturation value of the carrier density, ne, which we previously determined by equilibration annealing of specimens with excess dopant. This finding demonstrates that ne represents the limiting value of the concentration of unclustered dopant at the diffusion temperature. On the contrary, a diffusivity monotonically increasing with dopant concentration up to its solubility limit, was observed in the case of B and Sb, which do not cluster. Finally, we report the results of a simulation model which can accurately describe the evolution of the As profile upon annealing, by using our diffusivity data and taking into account both the precipitation and clustering phenomena.

The physical nature of the electrically inactive phosphorus in silicon was investigated by annealing experiments performed on laser annealed specimens doped by ion implantation up to 5×1021 at/cm3. The hypothesis of point defects, which compensate or make the excess dopant electrically inactive, is contradicted by the experimental results. It was verified that phosphorus solubility corresponds to the electrically active concentration in equilibrium with the inactive dopant, and that the latter is precipitated phase. This was confirmed by transmission electron microscopy (TEM) examinations with the weak beam technique, which detected a high density of very small coherent precipitates. This method allowed us to observe particles of the same kind even on specimens thermally predeposited in conditions typical of device technology. In both cases the amount of precipitates was consistent with the inactive dopant concentration. In addition these experiments show that precipitation is associated with a high enhancement of the diffusivity of the dopant. This phenomenon can account for the plateau region of the carrier profiles after thermal predeposition.

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The physical nature of the electrically inactive arsenic in silicon was investigated by annealing experiments performed on specimens doped in a wide range of concentration, up to 4 x 1021 cm ~, obtained by ion implantation and laser annealing. Thermal treatments of these alloys at temperatures of 800 ~ 900 ~ and 1000~ provided solid solubility values which correspond to the carrier density in equilibrium with excess dopant. Additiona] confirmation that the electrically inactive arsenic is a precipitated phase was obtained by the kinetics of the annealing process. The occurrence of precipitation in an amount which is consistent with that of the electrically inactive dopant was confirmed by small angle x-ray scattering which provided also details on the size and shape of the particles. The latter are in form of thin platelets and match the structure of the silicon lattice. These results clearly contradict the models developed to account for the difference between carrier and arsenic concentration which hypothesize the formation of a high equilibrium concentration of complex point defects making the excess dopant electrically inaetive.