The $T$-line luminescence system is created in Si by annealing at 400–600 °C. Shifts and splitting of the spectral features with ${}^{13}\mathrm{C}$ and D isotope substitution identify the presence of two C atoms and one H atom in the center. Uniaxial stress and magnetic field measurements show that the $T$ center has monoclinic $I$ symmetry and possesses an acceptor ( $-/0\mathrm{}$) level at 0.2 eV below the conduction band. Ab initio cluster calculations lead to a structure in which an interstitial C-H defect binds with a substitutional C atom. The calculated vibrational modes are in good agreement with those observed.

• Received 2 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4812