Optical properties of triplet states of excitons bound to interstitial-carbon interstitial-oxygen defects in silicon

Observation of photoluminescence from spin triplet states of excitons bound to interstitial carbon-oxygen complexes (Ci-Oi) in silicon is reported. New luminescence peak labeled as CT line emerges at the energy 2.64 meV below the well-known luminescence from the no-phonon transition of a Ci-Oi singlet state situating at 790 meV (C line). Observations of local vibrational modes associated with CT line and the temperature dependence of the relative intensity between CT and C lines lead to unambiguous identification of CT line as the no-phonon line from Ci-Oi defects. In addition, the host silicon isotope shift of CT line is equal to that of C line, indicating that CT line is no-phonon luminescence as well. Furthermore, our photoluminescence measurements carried out in magnetic field show that CT line is associated with an isotropic spin triplet state due to quenching of orbital angular momentum of the hole composing the bound exciton.