| This thesis concerns mainly with atomic and electronic structures of novel ternary and quaternary chalcogenide narrow band-gap semiconductors which are of great interest for infrared devices, photovoltaics, and high temperature thermoelectrics. More specifically, it presents studies of charge ordering and self-assembled nanostructures in a class of quaternary systems and their phase diagram, defect clustering and nanostructure formation in bulk thermoelectrics, atomic and electronic structures of ternary chalcogenides, and nature of defect states in narrow band-gap semiconductors.; Studies are carried out using Monte Carlo (MC) and ab initio methods to understand the nanostructuring phenomenon observed in AgPbmSbTem+2 and similar systems. MC simulations in these quaternaries using an ionic model show a distinct phase diagram and a variety of structural orderings depending on the concentration of the monovalent and trivalent atoms as a result of the long-range nature of the Coulomb interaction. Ab initio density functional theory (DFT) based calculations also show that monovalent and trivalent impurities in PbTe, SnTe, and GeTe-based bulk thermoelectric materials like to come close to each other and form clusters or some sort of embedded nanostructures.; Interplay of atomic and electronic structures and band gap formation in I-V-VI2 and TI-based III-V-VII2 tertiary chalcogenides (I=Ag, Cu, Au, Na, K; V=As, Sb, Bi; VII=S, Se, Te) are studied using ab initio electronic structure calculations. These calculations have been able to identify low energy ordered structures which are consistent with experiments. Several intriguing physical properties of these materials can be understood in terms of the calculated electronic structure. This thesis suggests how to modify a certain ternary by replacing its constituting elements) such that the electronic structure shows desired features for different applications.; Comprehensive studies of the nature of defect-induced electronic states associated with a large class of substitutional impurities and native point defects in PbTe, SnTe, and GeTe are carried out using DFT and supercell models. Calculations are also carried out in PbTe thin films and nanoclusters to study how the defect states change in going from one geometry to another. This thesis also concerns with energetics of the defects, particularly defect formation energy, which may be able to give some information on the doping mechanism and the distribution of the defects in these systems. Based on the calculated electronic structures, one can explain the peculiar properties of PbTe doped with group III (Ga, In, T1) impurities and the observed transport properties of PbTe, SnTe, and GeTe-based thermoelectrics. |
