First-principles calculations of vibrational lifetimes in silicon
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Time-resolved infrared absorption spectroscopy of the local vibrational modes (LVMs) of light impurities in crystalline silicon reveal that the vibrational lifetimes of almost identical LVMs sometimes differ by up to two orders of magnitude. Indeed, at low temperatures, the lifetimes of the 2062cm-1 mode of H2*, the 1998cm-1 mode of H+BC, and the 2072cm-1 mode of V2H2 are 4,8, and 295ps, respectively. Since the optical phonon of Si is about 530cm-1, these decays should all involve at least four phonons and have long and comparable lifetimes. More surprising still, the measured lifetime of the asymmetric stretch of interstitial O in Si changes by almost an order of magnitude with the isotope of O or one of its Si neighbors. In this work, ab-initio molecular dynamic simulations in periodic supercells are used to calculate the temperature dependence of vibrational lifetimes. The theoretical approach developed for these calculations can be applied to the calculation of any vibrational lifetime in any crystal. The calculations predict accurate lifetimes of the various defects in the range 50<T<200K and provide critical insight into the decay processes.