It was noted that understanding the detailed electronic structure of deep defect states in narrow band-gap semiconductors was a challenging problem. Recently, self-consistent ab initio calculations within density functional theory using super-cell models had been successful in tackling this problem. Such calculations were carried out here for PbTe, a well-known narrow band-gap semiconductor, for a large class of defects: cationic and anionic substitutional impurities of different valence, and cationic and anionic vacancies. For the cationic defects, the chemical trends in the position of defect levels were studied by looking at series of compounds RPb2n–1Te2n, where R was vacancy or monovalent, divalent, or trivalent atom. Similarly, for anionic defects, the compounds, MPb2nTe2n–1, were studied where M was a vacancy, S, Se or I. It was found that the density of states near the top of the valence band and the bottom of the conduction band get significantly modified for most of these defects. This suggested that the transport properties of PbTe in the presence of impurities may not always be interpreted by simple carrier doping (from bound impurity states in the gap) concepts, confirming such ideas developed from qualitative and semi-quantitative arguments.

Ab initio Studies of the Electronic Structure of Defects in PbTe. S.Ahmad, S.D.Mahanti, K.Hoang, M.G.Kanatzidis: Physical Review B, 2006, 74[15], 155205 (13pp)