Vibrational dynamics of hydrogen in silicon and germanium

The vibrational lifetime of bond-center hydrogen (HBC +) at 1998 cm-1 in silicon is measured to be 7.8 +/- 0.2 ps using transient bleaching spectroscopy. The temperature dependence of the lifetime shows that decay into vibrational modes does not occur by a lowest-order energy conserving process, but involves some low-frequency vibrational modes. Vibrational lifetimes of bond-center hydrogen (HBC +) at 1794 cm-1 and the two-fold degenerate mode of hydrogen near the tetrahedral site (H-) at 745 cm-1 in germanium are measured with infrared absorption spectroscopy. The HBC+ line shape measured at 10 K for the case of low hydrogen concentrations gives a lifetime in the range of 15--23 ps. The lifetimes of HBC+ in Si and Ge are similar in magnitude, which indicates that the lifetime of the bond-centered mode is largely insensitive to the phonon frequency distributions of the host material. This suggests that the stretch mode of HBC+ does not decay by the lowest-order, energy-conserving, multiphonon process but may involve low-frequency localized modes. Pure dephasing dynamics of hydrogen in silicon and germanium were studied by measuring the line shape of the 1998, 1794, and 745 cm-1 features as a function of temperature. These measurements were analyzed using a recently formulated extended exchange model for vibrational dephasing. The temperature dependence of the 1998 cm-1 and 1794 cm -1 line shapes are attributed to thermal fluctuations in the occupation number of an exchange mode at 114 +/- 3 cm-1 and 75 +/- 2 cm-1, respectively. The exchange modes are assigned to pseudo-localized modes associated with the distortion of the germanium atoms surrounding the hydrogen atom. The temperature dependence of the 745 cm-1 line shape is attributed to thermal fluctuations in the occupation number of an exchange mode at 77 +/- 4 cm-1 .