Theory of anharmonicity on bond-centered hydrogen oscillators in silicon

An anomalous positive shift when the mass of the Si host isotope is increased has been observed recently [Pereira et al., Physica B 340-342 (2003) 697] for the asymmetric stretch frequency of the bond-centered proton in Si (HBC+). On the other hand, the usual downward isotope shift was observed for the analogous bond-centered deuteron (DBC+). Based on phenomenological and ab initio modeling studies, we explain the observed puzzling effect. We introduce a Si-H-Si linear model that accounts well for the observations when anharmonicity, volumetric effects due to the host-isotope mass change, and the coupling of the Si-H-Si unit to the lattice are taken into account. The positive isotope shift for HBC+ results from anharmonic A2u+A1g mode mixing and volumetric effects.