Optical properties of fully amorphous silicon

We have determined the complex dielectric function, epsilon>(*) over bar * (E)= epsilon (1)(E)+ i epsilon (2)(E), for self-implanted amorphous silicon (a-Si) by spectroscopic ellipsometry (SE) in the 1.5-5.2-eV photon-energy range at room temperature. The measured SE spectra show a single broad peak in epsilon (2)(similar to 26.6) at E similar to3.45 eV, which is typically observed in amorphous tetrahedral semiconductors. The SE epsilon>(*) over bar * (E) data, together with the literature data which exhibit the largest peak in epsilon (2)(similar to 30) at E similar to3.7 eV and have been widely used as a reference of ''dense" a-Si, are analyzed by means of the Bruggeman effective-medium approximation. The results clearly indicate that the self-implanted a-Si sample is in the fully amorphous state, while the literature data contain microcrystalline component (muc-Si) in its spectra. The volume fraction of muc-Si is estimated to be about 53%. The optical spectra of muc-Si are found to be quite different from those of single-crystalline silicon, especially in the vicinity of the sharp critical-point features. The SE epsilon>(*) over bar * (E) data of the self-implanted a-Si are successfully parametrized using Jellison-Modine's dispersion expression.