Theory of interstitial oxygen in silicon and germanium

The interstitial oxygen centers in silicon and germanium are reconsidered and compared in an analysis based on the first-principles total-energy determination of the potential-energy surface of the centers, and a calculation of their respective low energy excitations and infrared absorption spectra. The total-energy calculations reveal unambiguously that interstitial oxygen is quantum delocalized, the delocalization being essentially different in silicon and in germanium. Oxygen in silicon lies at the bond center site in a highly anharmonic potential well, whereas in germanium it is found to rotate almost freely around the original Ge-Ge bond it breaks. This different delocalization is the origin of the important differences in the low energy excitation spectra: there is a clear decoupling in rotation and vibration excitations in germanium, giving different energy scales (1 cm(-1) for the rotation, 200 cm(-1) for the nu(2) mode), whereas both motions are non-trivially mixed in silicon, in a common energy scale of around 30 cm(-1). The calculation of the vibrational spectra of the defect reveals the existence of vibrational modes (related to the nu(1) mode) never been experimentally observed due to their weak infrared activity. It is found that the combination of these modes with the well established nu(3) asymmetric stretching ones is the origin of the experimentally well characterized modes at frequencies above the nu(3) mode frequency.