Vibrational modes of three-membered self-interstitial clusters in silicon

A selection of the vibrational modes of two variants, compact and non-compact, of three-membered self-interstitial clusters in crystalline Si is studied by first-principles calculations in order to contribute to the possible identification of a three-bodied cluster with the W centre in Si. Internal vibrational modes of the compact clusters are found between 19 meV ('translation' modes) and 47 meV (breathing mode), in fair agreement with the phonon replica spectrum of W. However, the compact cluster does not possess the local mode above the bulk phonon range associated with W. A local mode is instead exhibited by the non-compact cluster variant. The latter, though, is energetically disfavoured over the compact one. The combined results on the vibration and energetics suggest that neither of the two clusters can be identified with W.