| Understanding the detailed electronic structure of deep defect states in narrow band-gap semiconductors has been a challenging problem. Self-consistent ab initio calculations within density functional theory (DFT) using supercell models have been quite successful in tackling this problem. In this thesis, we carry out such calculations in PbTe and SnTe, two well-known narrow band-gap semiconductors, 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 the chemical trends in the position of defect levels by looking at a series of compounds RQ 2n-1Te2n, where Q is Pb or Sn, R is vacancy or monovalent, divalent, or trivalent atom. Similarly, for 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 and SnTe 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.;Transport coefficient calculations using the Boltzmann equation within energy-dependent relaxation time approximations have been carried out for n-type PbTe over a wide temperature (T) and concentration (n) range, 300 K ≤ T ≤ 900 K and 1 ≤ n/n0 ≤ 10, n0 = 5 x 1019cm-3. The nonparabolic Kane model for the energy dispersion was used in these calculations following the earlier work of Ravich et. al. Although the T dependence of the electrical conductivity sigma comes from several sources (band structure parameters, chemical potential mu, scattering relaxation time tau), we find that the T dependence of tau dominates. We represent the temperature dependence and the energy of the electrons (epsilon) dependence of the total relaxation time tautot by a scaling function t∼aT-pb+ c3r, where a, b, c, p, r are T and epsilon independent parameters but depend on the carrier concentration. Using this simple scaling function in the calculation of sigma, we find that for these concentrations changing the parameter r which governs the energy dependence of scattering does not appreciably affect the T dependence of sigma. In addition to the study of the T dependence of sigma, a careful electronic thermal conductivity calculation (both at constant current J and constant electric field E) was done to reexamine the validity of the Wiedemann-Franz (WF) law in PbTe, extending the earlier work of Bhandari and Rowe. We point out that using the standard WF law to estimate the electronic contribution of the thermal conductivity (kappael) usually overestimates this contribution by more than 0.5 W K-1m -1, and hence underestimates the lattice thermal conductivity. This has important implications in the question of how low the lattice thermal conductivity can go in the presence of nanostructures in PbTe. |
