Vibrational excitations of defect sites in amorphous group-V semiconductors

A study of the vibrational excitations of structural defects in amorphous pnictide semiconductors is presented. Amorphous As is chosen as a prototype for this study. The local phonon densities of states, infrared absorption, and Raman-scattering spectra of twofold coordinated As atoms, fourfold coordinated As atoms, intimate valence alternation pairs, and vacancy defects are presented. The effects of relaxations, disorders, and distortions upon the vibrational states of the defects are discussed. The results suggest that only the relaxed twofold coordinated As atoms (or vacancylike defects) give rise to localized vibrational modes with optical activities consistent with sharp weak features observed in the experimental infrared and Raman spectra of bulk amorphous As. The implications of our results for amorphous P and Sb, as well as the relationships between spectroscopic measurements and other optical measurements are also discussed.