Host isotope effect on the localized vibrational modes of oxygen in isotopically enriched ²⁸Si, ²⁹Si, and ³⁰Si single crystals

A high-resolution infrared absorption study of the localized vibrational modes (LVM's) of oxygen in isotopically enriched ²⁸Si, ²⁹Si, and ³⁰Si single crystals is reported. Isotope shifts of LVM frequencies from those in natural Si are clearly observed not only due to the change of the average mass in the nearest-neighbor silicon atoms, but also to the combined effect of the (i) change in Si masses of the second and beyond nearest neighbors and (ii) change in the lattice constants of the host Si crystals. These conclusions have been drawn based on a direct comparison between the experimental results and theoretical calculations assuming harmonic potentials for localized vibrations of oxygen. However, the LVM linewidths of the A_2u mode in the enriched samples are much narrower than those in natural Si, despite the fact that the harmonic approximation predicts very little dependence of the width on the host Si isotopic composition. This observation suggests that both anharmonicity and inhomogeneous broadening due to isotopic disorder are playing important roles in the determination of oxygen LVM linewidths. Moreover, a new series of oxygen LVM peaks is observed clearly in the isotopically enriched samples thanks to the small degree of mass disorder.