It was noted that recent experiments had indicated that current models for the structures of light-induced defects in hydrogenated amorphous material were inadequate. Photoconductivity measurements showed that light-induced defects were different to native dangling-bond defects. Electron spin-echo envelope-modulation studies showed that light-induced defects were formed in H-depleted regions and were separated from each other by more than 10nm. Structures were proposed, for light-induced defects, that agreed with these observations. These were valence alternation pairs that consisted of a charged O and a charged Si dangling bond. They were isolated from each other and from native dangling bonds, and could occur in H-depleted regions. The respective charged O partner of the defect complex remained charged and diamagnetic because its energy state was close to the valence band (for O-) or the conduction band (for O3+), while the Si dangling bond could change its charge state. These light-induced recombination centers were affected by the Coulomb potential of their charged O partners. An observed correlation between the annealing energies of metastable defects, and their effectiveness as recombination centers, was suggested to be associated with the separation of the defect pair. It could still not be explained why the saturation concentration (107 to 2 x 107/cm3) of light-induced defects was largely independent of the O concentration. This saturation value was not satisfactorily explained by other models.

H.Fritzsche: Journal of Non-Crystalline Solids, 1995, 190[1-2], 180-4