Bondar M. Exciton procesess and percolation transitions in two-phases mismatch semiconductor II-VI quantum structures.

The thesis is devoted to investigation of excitonic processes and percolation transitions in two-phase mismatch and disordered low-dimensional structures with such quantum objects: quantum wells, superlattices, borosilicate matrices with ZnSe or CdS quantum dots or quantum discs on a basis of chalcogenides (ZnS, ZnSe, ZnO, CdS and ZnTe). It is shown how energies of electrons, holes and excitons are connected with parameters of the epitaxially growth processes. This proves that the excitonic energy in those systems is determined by the spatial and dielectric confinement of electrons and holes, and also by interphase mismatch. The latter is due to disagreement between the material parameters of matrix and semiconductor. Interphase mismatch appears as the lattice, thermal or dielectric ones. The latter mismatches give rise to interface disorder in quantum objects and excitonic localized states of samples. The transition manifests itself as radical changes in optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. Excitonic emission from a percolation cluster of bound quantum dots as a fractal object is observed for the first time. Analysis of the structure of the photoluminescence spectra shows that the spectra are determined by the contribution of exciton states that belong to different structural elements of the percolation cluster, specifically, to the skeleton (backbone), dangling (dead) ends, and internal hollow spaces. A qualitative model is proposed to interpret the dependence of the exciton energy in these structural elements on the concentration of quantum dots in the material. Understanding of nature of these transitions makes it possible to simulate processes in real mesoscopic and microscopic systems: thick-films, sensor-based systems, mixture of conductive and nonconductive grains, porous materials. The proposed mechanism of the excitonic percolation transition in samples with various volume fraction and topologie is based on assumpion of excitonic quantum percolation. This allows to characterize the formation of the percolation level with the quantum dots volume fraction, less than 0.1 of the matrix volume.