  Understanding the detailed electronic structure of deep defect states in
narrow band-gap semiconductors has been a challenging problem. Recently,
self-consistent ab initio calculations within density functional theory (DFT)
using supercell models have been successful in tackling this problem. In this
paper, we carry out such calculations in 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, we study a series of compounds
RPb2n-1Te2n, where R is vacancy or monovalent, divalent, or trivalent atom; for
the anionic defects, we study compounds MPb2nTe2n-1, where M is vacancy, S, Se
or I. We find that the density of states (DOS) near the top of the valence band
and the bottom of the conduction band get significantly modified for most of
these defects. This suggests that the transport properties of PbTe in the
presence of impurities can not be interpreted by simple carrier doping
concepts, confirming such ideas developed from qualitative and
semi-quantitative arguments.
