The nature of defect states associated with group III impurities (Ga, In, Tl) in PbTe, a narrow band-gap semiconductor, was studied within density functional theory and super-cell model. For all 3 impurities (both substitutional, at the Pb site and interstitial, at the tetrahedral site), there was a hyper-deep defect state which lay 0.5 to 1.0eV below the valence band. It was a highly localized bonding state between the impurity s-orbital and the surrounding p-orbitals of the Te atoms. The corresponding anti-bonding state, denoted as the deep defect state, lay in the band-gap region. Its precise position vis-à-vis the conduction- and valence-band extrema controlled the unusual properties exhibited by these defects.

Deep Defect States in Narrow Band-Gap Semiconductors. S.D.Mahanti, K.Hoang, S.Ahmad: Physica B, 2007, 401-402, 291-5