Boson peak in amorphous silicon: A numerical study

The low-frequency part of the phonon spectrum of a-Si is investigated for a microscopic model of this amorphous solid even for frequencies below 50 cm(-1). This requires system sizes up to 64000 atoms. The variation of a model parameter allows us to generate structures with different degrees of disorder. The vibrational properties are calculated in a harmonic approximation to the Stillinger-Weber potential. The less disordered of our models already show an enhancement of the low-frequency vibrational density of states g(omega) (VDOS) compared to the crystal. For the more disordered structures additionally a peak in g(omega)/omega (2) versus omega is observed. The deviation from the Debye omega (2) behavior is caused by at least two effects: a shift of the crystalline transverse acoustic peak toward lower frequency and a broadening of the same. Consequently the occurrence of the bosom peak in the Raman spectrum of a-Si can be explained already in the harmonic approximation (at least in part) by a complex of phenomena of the low-frequency VDOS.