Synthetic crystals were grown, at high pressures and temperatures, from metallic melts which contained Si. Their absorption and photoluminescence spectra were investigated at temperatures which ranged from 1.8 to 77K. A 12-line fine structure was observed close to 1.682eV. This could be divided into 3 similar groups, each of which contained 4 components. The relative strengths of the optical absorptions for the 3 groups of lines were found to be the same as the ratios of the abundances of the natural isotopes: 28Si, 29Si and 30Si. This showed that the 1.682eV center was related to a Si impurity. Changes in the relative intensities of the 4 component lines which were associated with 28Si indicated that the center had 2 ground-state energy levels, with a separation of 0.0002eV. The occupancies of 2 excited-state levels with a separation of 0.00107eV tended to reach thermal equilibrium after optical excitation and before luminescence occurred. The degeneracies of the ground-state levels were the same, and the degeneracies of the excited-state levels were also equal.

C.D.Clark, H.Kanda, I.Kiflawi, G.Sittas: Physical Review B, 1995, 51[23], 16681-8