StatusThe thesis was presented on the 7 April, 2017
Approved by NCAA on the 31 May, 2017
Abstract– 2.42 Mb / in romanian
6.92 Mb /
The thesis consists of introduction, four chapters, general conclusions including recommendations, and bibliography. The thesis includes 161 text pages, 89 figures, 29 tables, 56 formulas, and 361 references. It also contains 6 annexes with 35 text pages, 24 figures, 9 tables, 4 formulas. The obtained results are published in 74 scientific materials.
Research field: 1.13 Physical Sciences.
The main aim of the thesis consist of elaboration of technological procedures for obtaining materials, based on undoped and Eu-doped AIIIBVI layered semiconductors and nanolamellar composites with AIIBVI semiconductors, with relevant optical, photoelectrical and luminescent properties.
The objectives of the thesis are: the growth of GaS, GaSe, GaTe, InSe, GaS:Eu, and GaSe:Eu single crystals; the determination of homogeneity of europium distribution in GaS(Se) crystals and the influence of dopant concentration on the PL and TSL (intensity and relaxation mechanisms); the compositional studies for materials obtained by thermal annealing of AIIIBVI crystals in Cd, and Zn vapors, as well as investigations of structural and morphological changes on the surface of elementary packing; the study of absorption spectra in the fundamental absorption edge region for Eu-doped GaS(Se) crystals and determination of interaction mechanisms between excitons and different Eu ion (different ionization states); the study of FTIR and Raman spectra for primary single crystals and their composites with AIIBVI semiconductors; the investigation of phonons’ spectra; the study of photoelectrical properties and generation-recombination mechanisms for nonequilibrium charge carriers in composite nanostructures containing AIIIBVI and AIIBVI semiconductors; and determination of the energy diagram influenced by doping and intercalation.
Novelty and scientific originality: thermal annealing of AIIIBVI single crystals (GaS, GaSe, GaTe and InSe), in Zn, and Cd vapors, at temperatures of 670-1070 K, results in modification of primary crystals and forming of Ga, In, Cd, and Zn, micro- and nanocomposites with optical and luminescent characteristics specific to the contained crystallites; the anti-Stokes photoluminescence of the composites containing Ga, In, Cd and Zn chalcogenides, is determined by the presence of the nanometric crystallites; the nanostructuration and composites formation lead to the creation of recombination and trapping levels which determine the photosensitivity and PL kinetics; it has been proven that Eu atoms localized in the Van der Waals space of GaSe crystals, form Eu-Se bonds and determine the electrical conductivity; in GaSe, Eu concentration increase from 0.025 at.% up to 3 at.%, results in structural defects density increase at both elementary packings surface and interface; it was demonstrated that structural and compositional defects determine the photoluminescence and photosensitivity bands structure.
The scientific problem - solved: the preparation of lamellar nanostructures consisting of AIIIBVI layered semiconductors and Cd, and Zn chalcogenides; the identification of the composites consisting of Ga, In, Cd, and Zn chalcogenides forming mechanism; the characterization of crystal structure, polymorphic forms, surface morphology, dopant/intercalant distribution homogeneity in Ga and In chalcogenides; the determination of the localized states energy in the AIIIBVI layered structures and in the composites with Cd, and Zn chalcogenides; the investigation of generation-recombination mechanism for nonequilibrium charge carriers, optical transitions, and photoelectrical processes behavior, which determine the anisotropy of optical and electric properties.
The theoretical and practical significance of the thesis: the Cd and Zn intercalant, and Eu as a dopant for
GaS, and GaSe crystals, form chemical bonds between elementary packings which determine the electrical
anisotropy of these materials. Identification of the generation-recombination mechanisms of nonequilibrium
charge carriers in the layered compounds and the controlling of this process in the GaSe:Eu crystals, and in
GaTe-ZnTe, and GaTe-CdTe, nano- and microcomposites. The determination of crystal structure and lattice
vibrations type for nano– and microcomposites obtained by thermal annealing of AIIIBVI layered compounds in
Cd, and Zn vapors. The recombination and trapping levels diagram was established by the analysis of
photoconductivity and photoluminescence bands, their relaxation curves, and also TSL. The preparation of
materials having high PL output in the NUV – NIR region of the spectrum based on GaSe:Eu crystals and
composites of Ga, In, Cd and, Zn chalcogenides. The preparation of composite materials, based on AIIIBVI and
AIIBVI semiconductors, having high dielectric permittivity.