Interstitial O centers were compared by using an analysis that was based upon first-principles total-energy determinations of the potential energy surface of the centers, and upon a calculation of their respective low-energy excitations and infra-red absorption spectra. The total-energy calculations revealed unambiguously that interstitial O was quantum-delocalized; with the delocalization being essentially different in Si and Ge. The O in Si lay at the bond-center site in a highly anharmonic potential well. In the case of Ge, it was found to rotate almost freely around the original Ge-Ge bond which it broke. This difference in delocalization was the cause of large differences in the low-energy excitation spectra. There was a clear de-coupling of rotation and vibration excitations in Ge; thus giving different energy scales (1/cm for rotation, 200/cm for the 2 mode). On the other hand, the 2 motions were non-trivially mixed in Si; in a common energy scale of about 30/cm. Calculations of the vibrational spectra of the defect revealed the existence of vibrational modes (related to the 1 mode) which had never been experimentally observed - due to their weak infra-red activity. It was found that the combination of these modes with the well-known 3 asymmetrical stretching ones was the cause of experimentally well-characterized modes at frequencies above the 3 mode frequency.

E.Artacho, F.Yndurain: Materials Science Forum, 1995, 196-201, 103-8