LOCALIZED VIBRATIONAL-STATES IN AMORPHOUS-SILICON

Using a valence-force-field model as interatomic potential, vibrational eigenstates have bean computed in the harmonic approximation. The density of vibrational states and their inverse participation ratio are compared for crystalline silicon, fully-coordinated amorphous silicon (a-Si) and a-Si with voids. Voids of various sizes and concentrations have been introduced into an a-Si structure that was generated with a vacancy model. The presence of voids increases the local strain in the a-Si network and causes substantial changes in the vibrational density of states. Localization occurs not only for high frequency modes but also for band edge states. At low frequencies, the deviation from a Debye density of states is due to such localized extra modes depending on void size and concentration.