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

Abstract

We report on a new Li-related photoluminescence center with zero-phonon line at 879.3 meV. The center is created at 550--600 \ifmmode^\circ\else\textdegree\fi{}C in the final stages of annealing out radiation-induced point defects in float-zone silicon. Isotope and chemical correlation data establish that the center contains Li and C atoms. The isotope shift from ${}^{6}\mathrm{Li}$ to ${}^{7}\mathrm{Li},$ $\ensuremath{\Delta}{E=E(}^{7}\mathrm{Li})\ensuremath{-}{E(}^{6}\mathrm{Li})=0.18\mathrm{meV},$ is similar, per Li atom, to that observed for other Li-related centers in silicon. Uniaxial stress measurements establish the symmetry as monoclinic I, with only small departures from trigonal symmetry. A simple method for the transition is introduced to fit simultaneously the energies, polarizations, and relative intensities of the stress-split components. The transition's dipole is shown to be close to a bonding direction in the plane perpendicular to the characteristic 〈110〉 axis of the monoclinic I center. The vibronic sideband is produced by coupling to modes of 16, 31, and 36 meV, with a Huang-Rhys factor $S=1.1.$ This value can be predicted simply from the uniaxial-stress data. The temperature dependence of the zero-phonon line can be fitted precisely using the spectrum of coupled phonons derived from the vibronic band shape, plus the approximation that the differences in frequencies of the phonons in the ground and excited electronic states are proportional to the phonon frequency. The luminescence from the center is reversibly quenched with increasing temperature, with an activation energy ${E}_{a}=32\ifmmode\pm\else\textpm\fi{}5\mathrm{meV},$ however, we show that this result does not arise from the excited state containing a shallow particle. Although the center is created at 600 \ifmmode^\circ\else\textdegree\fi{}C, it is rapidly destroyed at room temperature through passivation by the capture of one mobile Li atom.