The nature of deep defect states, in general, and those associated with group III elements (Ga, In, Tl) in narrow band-gap IV-VI semiconductors (PbTe and PbSe), in particular, have been of great interest over the past three decades. We present ab initio electronic structure calculations that give a new picture of these states compared to the currently accepted model in terms of a negative-U Hubbard model. The Fermi surface pinning and why In-doped PbTe and related compounds show excellent high temperature thermoelectric behavior can be understood within the new picture.
