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Relaxation processes of nonequilibrium charge carriers in bulk and quantumdimensional semiconductor structures

Author: Dobîndă Igor
Degree:doctor of Physics
Speciality: 01.04.10 - Semiconductors physics and engineering
Scientific advisers: Sveatoslav Moscalenco
doctor habilitat, Institute of Applied Physics of the ASM
Dneprovskii Vladimir
dr. hab. în şt. fiz.-mat., prof. univ., USM “ M. Lomonosov”,Rusia
Institution: Institute of Applied Physics of the ASM


The thesis was presented on the 24 December, 2015
Approved by NCAA on the 25 February, 2016


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172 pages


GaSe, exciton, scattering, electron-hole plasma (EHP), plasmon, band gap renormalization (BGR), exchange and correlation interactions, Mott transition, GaAs, quantum well, kinetics of photoluminescence (PL) spectra, quantum dot, CdSe, quantum size levels, PL excitation spectra, „phonon bottleneck”, differential transmission, pump- probe technique, ultrashort light pulse


Publications: The results of the study have been presented as 19 scientific publications (7 articles, 5 conference proceedings, and 7 abstracts), three of which without coauthors.

Structure of the work: The thesis consists of the Introduction, five Chapters, Conclusions, Recommendations, the Bibliography of .280.references, and Summary in three languages. The work contains 172 pages of the text body, 49 Figures, and 8 Tables.

The purpose of the thesis. The study of the nonequilibrium charge carriers (NCC) relaxation in bulk GaSe at low and middle levels of excitation, as well as at high excitation in GaSe EHP, in GaAs/Al0,3Ga0,7As quantum wells. and in CdSe/ZnS quantum dots .

Novelty and scientific originality. For the first time, in the PL of GaSe the contribution of the “phononless” radiative Auger-recombination of two indirect excitons is established. The novel technique of the time-resolved spectra is applied to study the EHP PL. The new theory of the PL spectra kinetics is submitted for consideration: after short time, the spontaneous emission from direct and indirect bands, which is attributed to NCC thermalization, is replaced by the PL from the indirect band, which is determined by the electron-electron scattering and then by the PL from the indirect band, which is related to the exciton-exciton scattering. The PL decay characteristic time is measured. In addition, for the first time, constants of the processes of the electron-electron and exciton-exciton scatterings were evaluated. In GaSe, the NCC density of a Mott transition was calculated in a new way – from the equality of values of the exciton Rydberg and the BGR. By the direct measuring of the EHP stimulated emission decay characteristic time, the transient quantum well BGR decay characteristic time is determined. At high concentrations of the excited electron–hole pairs, the retardation of charge carrier relaxation over the quantum size energy levels in CdSe/ZnS quantum dots was recorded experimentally.

The scientific issues considered in the thesis: in bulk GaSe, at different levels of excitation, new relaxation channels of NCC were revealed; the GaAs quantum well transient BGR decay characteristic time was determined; the fast zero-phonon energy relaxation of hot electrons in CdSe/ZnS quantum dots was observed and the retardation of the charge carrier relaxation over the quantum size energy levels in quantum dots at high concentrations of excited electron–hole pairs was detected.

The research objects are the NCC relaxation processes in GaSe crystal, in GaAs/Al0,3Ga0,7As quantum wells, and in CdSe/ZnS quantum dots.

Scientific significance of the thesis consist in the discovery of a number of fundamentally new results, which are important for understanding of both the NCC relaxation processes in bulk and quantumdimensional structures, and of the influence of the large density NCC relaxation processes on the respective structures optical properties.

The implementation and practical significance. The obtained results may be used to create optical switching elements on the base of plasma reflection, which could shape ultrashort pulses of specified parameters in the terahertz frequency range. In addition, a next promissing possible application is the creation of saturable absorbers and optical modulators for both active and passive modelocking in the compact ultrafast solid-state lasers.