Vibrational resonance modes in amorphous silicon alloys

The vibrational structures of hydrogenated, deuterated, fluorinated, and chlorinated amorphous silicon alloys in the (0-600)-${\mathrm{cm}}^{-1\mathrm{}}$ frequency regime are examined with the use of the generalized cluster-Bethe-lattice approach and a valence-force-field representation of the near-neighbor interactions. Correlation functions and one-phonon densities of states are calculated for Si—H, Si—D, Si—F, and Si—Cl bonding groups in two different local atomic environments. It is found that inband resonance modes associated with each of the bonding groups are spatially localized and are relatively insensitive to the topology of the host amorphous Si network. Our results also indicate that the Si—H, Si—D, and Si—F groups are characterized by an infrared-active in-phase stretching mode of $A$-like symmetry with a frequency near 200 ${\mathrm{cm}}^{-1\mathrm{}}$. A similar resonance mode occurs near 128 ${\mathrm{cm}}^{-1\mathrm{}}$ for the Si—Cl group. In addition, each group is found to exhibit an infrared-active shear-type vibrational mode in the (400-500)-${\mathrm{cm}}^{-1\mathrm{}}$ frequency regime.