LOCALIZED VIBRATIONAL-STATES IN AMORPHOUS-SILICON

Using a valence-force-field model as interatomic potential, vibrationa l eigenstates have bean computed in the harmonic approximation. The de nsity 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 densit y of states. Localization occurs not only for high frequency modes but also for band edge states. At low frequencies, the deviation from a D ebye density of states is due to such localized extra modes depending on void size and concentration.