The damage profile in P+‐ion‐implanted Si is investigated by ellipsometry. Ellipsometric data are analyzed based on a three‐layer model, i.e., SiO2surface layer–implantation‐induced amorphous Si (a‐Si) layer–crystal Si (c‐Si) substrate. Thea‐Si layer thickness is estimated for three implantation energies, 100, 50, and 30 keV at a fixed dose of 5×1015cm−2. It is shown that a consistent interpretation of the data is possible taking into account the finite‐width transition region betweena‐Si andc‐Si. Assuming the transition layer to be random mixtures ofa‐Si andc‐Si particles, the optical constant distribution in the transition layer is calculated using the effective medium theory for small particle composites. This model is used to estimate transition layer width.