Al-doped is investigated with respect to optical absorption, magnetic circular dichroism, and the optical detection of electron paramagnetic resonance. Two paramagnetic centers are formed under near-bandgap illumination at low temperatures. Magneto-optics permits their description in terms of holes more or less delocalized (slightly different environment) on the oxygens next to Al at the tetrahedral Si site. We demonstrate that the photoinduced optical absorption in the 0.5–1.2-eV spectral range is due to the charge-transfer transitions The thermal energies associated with the two acceptor levels are found to be 0.16 and 0.40 eV above the valence-band edge by monitoring and modeling of the thermal bleaching of the IR bands. The associated optical energies are determined by isochronal annealing at 0.95 and 0.78 eV, and the photoionization cross sections of holes are estimated. In the colored state the respective hole populations are in the ratio 1:3.5 for the two states. The large ratio between the optical and thermal energies indicates the occurrence of a strong electron–lattice coupling. Finally, besides original spectroscopic assignments, we demonstrate that optical measurements on the charge-transfer bands of and related sillenite crystals provide information similar to that given by thermally stimulated conductivity or luminescence.
[Optical Society of America ]