Host isotope effects on midinfrared optical transitions in silicon

The effects of changing the host-lattice isotopes from natural silicon (approximately ²⁸Si) to ³⁰Si are reported for some of the important local vibrational modes (LVMs) observed in as-grown and electron-irradiated Czochralski silicon. We show that the quanta of the LVMs shift to lower energy in ³⁰Si compared to natural silicon by amounts varying from 3.5 to 27 cm ^-1, in very good agreement with predictions from density functional theory. The 2767 cm ^-1 ("3.6 μm") transition of the negative divacancy is also found to shift to lower energy with increasing silicon mass, but we demonstrate, using an empirical method of predicting the shifts of zero-phonon lines, that the line is a zero-phonon line. The empirical method used is verified by analyzing the shifts of the very-low energy 3942 cm ^-1 vibronic band.