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