Interstitial oxygen in germanium and silicon

The microscopic structure of interstitial oxygen in germanium and its associated dynamics are studied both experimentally and theoretically. The infrared absorption spectrum is calculated with a dynamical matrix model based on first-principles total-energy calculations describing the potential energy for the nuclear motions. Spectral features and isotope shifts are calculated and compared with available experimental results. From new spectroscopic data on natural and on quasimonoisotopic germanium samples, new isotope shifts have been obtained and compared with the theoretical predictions. The low-energy spectrum is analyzed in terms of a hindered rotor model. A fair understanding of the center is achieved, which is then compared with interstitial oxygen in silicon. The oxygen atom is nontrivially quantum delocalized both in silicon and in germanium, but the physics is shown to be very different: while the Si-O-Si quasimolecule is essentially linear, the Ge-O-Ge structure is puckered. The delocalization in a highly anharmonic potential well of oxygen in silicon is addressed using path-integral Monte Carlo simulations, for comparison with the oxygen rotation in germanium. The understanding achieved with this new information allows us to explain the striking differences between both systems, in both the infrared and the far-infrared spectral regions, and the prediction of the existence of hidden vibrational modes, never directly observed experimentally, but soundly supported by the isotope-shift analysis.