OPTICAL AND MAGNETOOPTICAL CHARACTERIZATION OF THE AL ACCEPTOR LEVELSIN BI12SIO20

Al-doped Bi12SiO20 is investigated with respect to optical absorption, magnetic circular dichroism, and the optical detection of electron pa ramagnetic resonance. Two paramagnetic centers are formed under near-b andgap illumination at low temperatures. Magneto-optics permits their description in terms of holes more or less delocalized (slightly diffe rent 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 Al-0 h nu --> Al- + h(upsilon),. The thermal energies associated with the t wo Al-/0 acceptor levels are found to be 0.16 and 0.40 eV above the va lence-band edge by monitoring and modeling of the thermal bleaching of the IR bands. The associated optical energies are determined by isoch ronal annealing at 0.95 and 0.78 eV, and the photoionization cross sec tions 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 rati o between the optical and thermal energies indicates the occurrence of a strong electron-lattice coupling. Finally, besides original spectro scopic assignments, we demonstrate that optical measurements on the ch arge-transfer bands of Bi12SiO20:Al and related sillenite crystals pro vide information similar to that given by thermally stimulated conduct ivity or luminescence. (C) 1998 Optical Society of America.