Luminescence of impurity-bound excitons in corundum

The luminescence characteristics of M ³⁺: Al₂O₃ crystals, where M ³⁺ stands for an isoelectronic cation impurity with a filled electron shell, namely, Sc³⁺, Y³⁺, or La³⁺, were studied. The luminescence of excitons bound (BE) to these impurities was detected. The position of the BE energy states at the long-wavelength absorption edge as a function of the M ³⁺ ionic radius was established. The energies of the long-wavelength BE creation threshold and of the maximum of the BE luminescence band were approximated empirically by third-order polynomials of the Toyozawa polynomial type, which describes electron-phonon interaction. The energy and spatial structure of the BE was found to be similar to that of a self-trapped exciton (STE). The BE and STE states are separated by an energy barrier, and energy transfer from the STE to BE is frozen at low temperatures.