The vibrational dynamics of bond-centered protons (HBC+) and deuterons (DBC+) in mono-isotopic (28Si, 29Si and 30Si) crystals were considered; based upon joint infra-red absorption measurements and ab initio modeling studies. Protons and deuterons were implanted below 20K, and in situ–type infra-red absorption measurements were then performed at 8K. A major absorption line was observed at 1998/cm, after proton implantation, which had previously been attributed to a local mode of HBC+. It was found that the HBC+ mode at 1998/cm displayed an anomalous (positive) frequency shift when the Si isotope mass was increased, unlike the analogous DBC+ mode at 1448/cm, which showed a negative shift. This effect could not be described by a purely harmonic model. It was shown that the mode frequencies were accurately accounted for by a simple model which was based upon a linear Si-H-Si structure when anharmonicity, volumetric effects due to the host-isotope mass, and the coupling of the Si-H-Si unit to the lattice were taken into account. Interstitial-O (Oi) atoms in Si, also located at the bond-center site, were also investigated. The relative contributions, of the different terms in the vibrational model, to the mode frequency of HBC+ and Oi were compared. The anomalous (positive) isotope shift of HBC+ resulted from mixing via anharmonicity of A2u and A1g modes of the Si-H-Si unit. This showed that a reliable vibrational model had to take account of the local structure of the defect. The mode frequency of the Oi defect exhibited the normal (negative) isotope shift because the relatively modest contribution made by anharmonicity diminished the importance of mode-mixing. The effect of defect-lattice coupling upon the stretching-mode frequencies of HBC+ and Oi was also considered.

Anharmonicity and Lattice Coupling of Bond-Centered Hydrogen and Interstitial Oxygen Defects in Mono-Isotopic Silicon Crystals. R.N.Pereira, B.Bech Nielsen, J.Coutinho, V.J.B.Torres, R.Jones, T.Ohya, K.M.Itoh, P.R.Briddon: Physical Review B, 2005, 72[11], 115212 (13pp)