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 (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-H_bc Si-H_ab and H _ab-C_s 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 0.3cm^-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.