Deep defect states in narrow band-gap semiconductors

The nature of defect states associated with group III impurities (Ga, In, and Tl) in PbTe, a narrow band-gap semiconductor, has been studied within density functional theory and supercell model. For all three impurities (both substitutional—at the Pb site and interstitial—at the tetrahedral site), there is a hyper-deep defect state which lies about 0.5–1.0 eV below the valence band. It is 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, lies in the band-gap region. Its precise position vis-à-vis the conduction- and valence-band extrema controls the unusual properties exhibited by these defects.