Ab Initio Studies of Local Vibrations of Small Self-Interstitials Aggregates in Silicon

We report on the energetics, electrical and optical activity of small self-interstitial (I3 and I4) clusters in Si, found from ab-initio density-functional modeling studies. I4 possesses nine local vibrational modes above the Raman edge, which account for up to three dipole-allowed vibronic transitions observed in recent experiments associated with the X-photoluminescent line. Another prominent photoluminescent line (known as the W-line) that shows a trigonal stress-induced splitting pattern, has been previously assigned to I3. Our analysis of the LVMs of a metastable form of I3 support this assignment.