The microscopic structure of interstitial O in Ge, and its associated dynamics, were studied experimentally and theoretically. The infra-red absorption spectrum was calculated by using a dynamic matrix model that was based upon first-principles total energy calculations which described the potential energy for nuclear motions. Spectral features and isotope shifts were calculated and were compared with available experimental data. From new spectroscopic data on natural and quasi mono-isotopic Ge samples, new isotope shifts were obtained and were compared with theoretical predictions. The low-energy spectrum was analyzed in terms of a hindered-rotor model. A reasonable understanding of the center was achieved, and this was then compared with interstitial O in Si. The O atom was non-trivially quantum delocalized in both Si and Ge, but the details were very different. That is, while the Si-O-Si quasi-molecule was essentially linear, the Ge-O-Ge structure was puckered. The delocalization in a highly anharmonic potential well of O in Si was studied by using path-integral Monte Carlo simulations and was compared with the O rotation in Ge. The understanding which was thereby achieved permitted an explanation to be offered for the marked differences between the systems in both the infra-red and far-infrared spectra. The prediction of hidden vibrational modes, which had never been directly observed experimentally, was firmly supported by the isotope-shift analysis.

E.Artacho, F.Ynduráin, B.Pajot, R.Ramírez, C.P.Herrero, L.I.Khirunenko, K.M.Itoh, E.E.Haller: Physical Review B, 1997, 56[7], 3820-33