  To explain the effect of electron density saturation at high phosphorus
concentrations the model of negatively charged phosphorus clusters was compared
with the experimental data. A number of negatively charged clusters
incorporating a point defect and phosphorus atoms were investigated at
temperatures of 920 and 1000 Celsius degrees. It was established that for
clusters incorporating more than one phosphorus atom calculated electron
density reached a maximum value and then monotonically and slowly decreased
with increasing the total dopant concentration. If the cluster incorporates 3-4
phosphorus atoms the calculated dependencies of carrier concentration agree
well with the experimental data for both temperatures regardless of the
negative charge state of the cluster (- or 2-). Moreover, for all investigated
doubly negatively charged clusters the fitting parameter is independent on the
temperature. It means that formation of doubly negatively charged clusters is
most likely but it is difficult to choose between clusters incorporating two,
three or four phosphorus atoms. From a good agreement of the calculated curves
with the experiments it follows that the model based on the formation of
negatively charged clusters can be used in simulation of high concentration
phosphorus diffusion.
