Anharmonicity and lattice coupling of bond-centered hydrogen and interstitial oxygen defects in monoisotopic silicon crystals

We discuss the vibrational dynamics of bond-centered protons (HBC+) and deuterons (DBC+) in monoisotopic (Si28, Si29, and Si30) silicon crystals, based on joint infrared absorption measurements and ab initio modeling studies. Protons and deuterons have been implanted at temperatures below 20K, and in situ-type infrared absorption measurements have subsequently been performed at 8K. A major absorption line is observed at 1998cm-1 after proton implantation, which has previously been ascribed to a local mode of HBC+. We find that the HBC+ mode at 1998cm-1 displays an anomalous (positive) frequency shift when the Si isotope mass is increased, unlike the analogous DBC+ mode at 1448cm-1, which shows a negative shift. This effect cannot be described with a purely harmonic model. We show that the mode frequencies are accurately accounted for with a simple model based on 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 are taken into account. Interstitial oxygen (Oi) atoms in silicon, also located at the bond-center site, are as well investigated in a parallel way. The relative contributions of the different terms of the vibrational model to the mode frequency of HBC+ and Oi are compared. The anomalous (positive) isotope shift of HBC+ results from mixing via anharmonicity of A2u and A1g modes of the Si-H-Si unit, which shows that a reliable vibrational model has to take into consideration the local structure of the defect. The mode frequency of the Oi defect exhibits the normal (negative) isotope shift, because the relatively modest contribution of anharmonicity diminishes the importance of the mode mixing. The effect of the defect-lattice coupling on the stretch-mode frequencies of HBC+ and Oi is also discussed.