Huge isotope effect on the vibrational lifetimes of an H2(C) defect in Si

The hydrogenation of C-rich Si leads to the formation of two (almost) energetically degenerate H2(C) complexes, each containing one substitutional C (Cs) and two interstitial H atoms which are located at a bond-centered (bc) and an antibonding (ab) site, respectively. The two defects are trigonal: Cs-Hbc · · ·Si-Hab and Hab-Cs · · ·Hbc-Si. Fourier-transform infrared (FTIR) absorption spectra of these two defects should show two Cs-H and two Si-H stretch modes, but the Hab-Cs mode was absent in earlier studies. The missing line has now been observed by FTIR in especially C rich Si material. The line is unexpectedly broad, suggesting a very short vibrational lifetime. Partial D substitutions result in the formation of a Hab-Cs · · ·Dbc-Si center. In this defect, the Hab-Cs line shifts by only 0.3 cm−1 but becomes very sharp, suggesting a long lifetime. The IR line widths show that the vibrational lifetime of the Hab-Cs mode in Hab-Cs · · ·Dbc-Si is about 16 times longer than that of the same Hab-Cs mode in Hab-Cs · · ·Hbc-Si. This paper contains experimental data and first-principles calculations which explain this isotope effect.