  An analysis of the equations used for modeling thermal arsenic diffusion in
silicon has been carried out. It was shown that for arsenic diffusion governed
by the vacancy-impurity pairs and the pairs formed due to interaction of
impurity atoms with silicon self-interstitials in a neutral charge state, the
doping process can be described by the Fick's second law equation with a single
effective diffusion coefficient which takes into account two impurity flows
arising due to interaction of arsenic atoms with vacancies and silicon
self-interstitials, respectively. Arsenic concentration profiles calculated
with the use of the effective diffusivity agree well with experimental data if
the maximal impurity concentration is near the intrinsic carrier concentration.
On the other hand, for higher impurity concentrations a certain deviation in
the local regions of arsenic distribution is observed. The difference from the
experiment can occur due to the incorrect use of effective diffusivity for the
description of two different impurity flows or due to the formation of
nonuniform distributions of neutral vacancies and neutral self-interstitials in
heavily doped silicon layers.
