The Gueorgi Nadjakov Lecture Negative-U defects in chalcogenide glasses: Elusive or non-existent?

The negative-U model for chalcogenide glasses has traditionally been interpreted in terms of charged co-ordination defects that lead to discrete electronic energy levels in the band gap. However, the failure of these levels to be revealed in the optical absorption spectra of pure a-As₂S ₃ samples has recently led to a questioning of the actual occurrence of such defects. We therefore review the relevant experimental evidence and point out that, since the density of the charged defects is lower than the density of localized tail states, failure to resolve the former in direct optical spectroscopy does not constitute sufficient grounds for doubting their presence in the material.