Bletskan M. Influence of polymorphism and defects on the electronic structure and photoelectric properties of tin chalcogenides

The thesis is devoted to the first-principle investigation of electronic structure and nature of chemical bonds in the known polymorphic phases of tin monochalcogenides and Sn2S3, PbSnS3, SnGeS3 compounds with mixed cation valency. Group-theoretical analysis of the electronic spectra for the low- and high-temperature orthorhombic phases of tin monochalcogenides was performed, allowed one to determine the symmetry of wave functions, to find the structures of band representations of the valence and conduction bands, to determine the actual Wyckoff positions, to analyze the appearance of Davydov splitting and to establish the selection rules for optical transitions. Quantum-chemical modeling of the influence of intrinsic point defects (vacancies in the cation and anion sublattices) and substitutional impurities (M - Sn, M = P, Sb, Bi) on the electronic structure of a-SnS was performed within the supercell approximation. The self-compensation phenomenon of antimony impurity atoms by doubly ionized tin vacancies VSn was detected in Sb-doped SnS crystals, accompanied by a sharp increase of resistivity as well as by an appearance of photosensitivity and photoemf. Some correlation between the growth conditions of the SnGeS3 crystals and their electrical and photoelectrical properties was established.