Local vibrational modes of bond-centered H in ²⁸Si, ²⁹Si, and ³⁰Si crystals

The local vibrational mode of positively charged bond-centered hydrogen (H_BC⁺) has been investigated in Si crystals enriched with ²⁸Si,²⁹Si, or ³⁰Si isotopes. Protons and deuterons were implanted into each sample at temperatures below 20 K, and in-situ-type infrared absorption measurements were subsequently performed at 8 K. The isotope shifts observed for the hydrogen and deuterium modes display opposite behavior with increasing mass of the silicon isotope. This behavior cannot be accounted for with a purely harmonic potential. We apply a simple model based on the linear molecule Si-H-Si, where the potential associated with the stretching of each Si-H bond is approximated by the Morse potential. The local mode frequencies and the isotope shifts obtained with our model are in excellent agreement with those observed.