A phosphorus implanted p-Si layer with dose slightly higher than its critical value was studied by EER combined with stripping-etching technique. The concept of a certain optimal wavelength, named "Specially Effective Wavelength" (SEW), is introduced for the optical measurements of structurally disordered materials. At SEW, the absorption coefficients of a material with different degree of disorder, from crystalline to amorphous, are all equal. For Si, the SEW is approximately 3490 ?, i. e. hω～3.55 eV (in the E1, E1 + △1 region), and the corresponding absorption coefficient is 106cm-1 independent of its degree of disorder. The depth profile of (△R/R)3.55ev is related to that of disorder degree, and the characteristic relations △R/R(D) were obtained for all of the three depth regions. The resulting curve for the implant-damaged region is similar to that reported by Anderson et al. for GaAs layers implanted with inert elements ions. By use of stripping, the observation of the disorder degree dependence of the 3.4 eV structure lineshapes has been realized in a wide range with only one sample. A comparison is made with the lineshape features of vacuum evaporated Ge films. Some conclusions have been drawn from these measurements about the mixed lattice interaction origin of the 3.4 eV spectral region of silion. It has been confirmed that EER is a powerful technique with extremely high sensitivity for the investigation of ion implanted semiconductor layers.