Isotope Dependence of the Lifetime of the 1136-cm-1 Vibration of Oxygen in Silicon

By simply changing the isotopes of the Si atoms that neighbor an oxygen O_i atom in crystalline silicon, the measured decay rate τ of the asymmetric-stretch vibration (ν₃=1136cm^-1) of oxygen (O_i) in silicon changes by a factor of ∼2.5. These data establish that ν₃ decays by creating one ν₁ symmetric-stretch, local-vibrational mode of the Si-O_i-Si structure. If the residual energy (ν₃-ν₁) is less than the maximum frequency ν_m of the host lattice, as for Si28-O16-Si28 in natural silicon, then it is emitted as one lattice mode, and τ depends on the density of one-phonon states at ν₃-ν₁. If (ν₃-ν₁)>ν_m, as for O16 in single-isotope Si30 silicon, two lattice modes are created in addition to ν₁, increasing τ. Prediction of τ for a particular defect clearly requires a detailed knowledge of that defect.