Huge isotope effect on the vibrational lifetimes of an H*2 (C) defect in Si

The hydrogenation of C-rich Si leads to the formation of two (almost) energetically degenerate H*(2) (C) complexes, each containing one substitutional C (C-s) and two interstitial H atoms which are located at a bond-centered (bc) and an antibonding (ab) site, respectively. The two defects are trigonal: C-s-Hbc...Si-H-ab and H-ab-Cs...H-bc-Si. Fourier-transform infrared (FTIR) absorption spectra of these two defects should show two C-s-H and two Si-H stretch modes, but the H-ab-C-s 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 H-ab-C-s ... D-bc-Si center. In this defect, the H-ab-C-s 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 H-ab-C-s mode in H-ab-C-s ... D-bc-Si is about 16 times longer than that of the same H-ab-C-s mode in H-ab-C-s ... H-bc-Si. This paper contains experimental data and first-principles calculations which explain this isotope effect.