Spectroscopic study and structural characterization of a Li-related photoluminescence center in neutron-irradiated Si

We report on a new Li-related photoluminescence center with zero-phonon lin e at 879.3 meV. The center is created at 550-600 degrees C in the final sta ges of annealing out radiation-induced point defects in float-zone silicon. Isotope and chemical correlation data establish that the center contains L i and C atoms. The isotope shift from Li-6 to Li-7, Delta E = E(Li-7) - E(L i-6) = 0.18 meV, is similar, per Li atom, to that observed for other Li-rel ated centers in silicon. Uniaxial stress measurements establish the symmetr y as monoclinic I, with only small departures from trigonal symmetry. A sim ple method for the transition is introduced to fit simultaneously the energ ies, polarizations, and relative intensities of the stress-split components . The transition's dipole is shown to be close to a bonding direction in th e plane perpendicular to the characteristic [110] axis of the monoclinic I center. The vibronic sideband is produced by coupling to modes of 16, 31, a nd 36 meV, with a Huang-Rhys factor S = 1.1. This value can be predicted si mply from the uniaxial-stress data. The temperature dependence of the zero- phonon line can be fitted precisely using the spectrum of coupled phonons d erived from the vibronic band shape, plus the approximation that the differ ences in frequencies of the phonons in the ground and excited electronic st ates are proportional to the phonon frequency. The luminescence from the ce nter is reversibly quenched with increasing temperature, with an activation energy E-a = 32+/-5 meV, however, we show that this result does not arise from the excited state containing a shallow particle. Although the center i s created at 600 degrees C, it is rapidly destroyed at room temperature thr ough passivation by the capture of one mobile Li atom.