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Time-resolved spectroscopy of semiconductor nanostructures
Kořínek, Miroslav ; Trojánek, František (advisor) ; Herynková, Kateřina (referee) ; Němec, Hynek (referee)
Title: Time-resolved spectroscopy of semiconductor nanostructures Author: RNDr. Miroslav Kořínek Department: Department of Chemical Physics and Optics Supervisor: doc. RNDr. František Trojánek, Ph.D., Department of Chemi- cal Physics and Optics, Faculty of Mathematics and Physics, Charles University in Prague Abstract: This Ph.D. thesis is focused on the study of relaxation and re- combination processes in silicon nanocrystals embedded in dielectric (SiO2, SiC and Si3N4) matrices that were fabricated within the EU 7th Framework Pro- gramme Silicon Nanodots for Solar Cell Tandem (NASCEnT). Different host ma- terials, in which silicon nanocrystals are incorporated, not only provide different transport properties, but also differently affect the nanocrystal surfaces. A large part of the thesis is devoted to research of SiO2-embedded silicon nanocrystals having various sizes (in the order of nanometers) and various inter-nanocrystal separation, that differ also in the manner of an additional annealing. Experimen- tal techniques of optical spectroscopy are used to monitor the photoexcited charge carrier dynamics on a wide time scale from picoseconds to milliseconds. The initial (picosecond) dynamics is characterized very well using a relevant rate equation. We propose a theoretical description of how the...
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Studium optických nelinearit v polovodičích a polovodičových nanostrukturách
Kozák, Martin ; Trojánek, František (advisor) ; Herynková, Kateřina (referee) ; Kužel, Petr (referee)
This Ph.D. thesis is focused on the study of optical nonlinearities and dynamics of excited charge carriers in monocrystalline diamond, nanocrystalline diamond and silicon. The dynamics of high density carriers in bulk diamond is investigated in detail (the transition from excitons and free carriers to electron-hole liquid or plasma). We study the picosecond dynamics of electron-hole liquid condensation using several techniques of time-resolved optical spectroscopy and demonstrate its evaporation by femtosecond laser pulses. We also propose two new optical techniques for measurement of lifetime, diffusion coefficient and surface recombination velocity of excitons in diamond. The results obtained by these techniques are described theoretically using diffusion equation and compared with the results obtained by the transient grating diffraction measurement. Further we study two- and three- photon absorption and nonlinear refractive index in diamond. In nanocrystalline diamond we study the second and third harmonic generation and its physical origin. In superlattices of silicon nanocrystals in SiO2 matrix we investigate the nonlinear transient absorption dynamics and carrier diffusion.
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Silicon nanocrystals, photonic structures and optical gain
Ondič, Lukáš ; Herynková, Kateřina (advisor) ; Oswald, Jiří (referee) ; Lauret, Jean-Sebastien (referee)
Silicon nanocrystals (SiNCs) of sizes below approximately 5 nm are a material with an efficient room-temperature photoluminescence (PL) and optical gain. Optical gain is a pre- requisite for obtaining stimulated emission from a pumped material, and the achievement of stimulated emission (and lasing) from Si-based nanostructures is of particular interest of the field of silicon photonics. The aim of this work was (i) to investigate fundamental optical properties of SiNCs, (ii) to design and prepare a photonic crystal with enhanced light ex- traction efficiency and (iii) to explore a possibility of enhancing optical gain of light-emitting SiNCs by combining them with a two-dimensional photonic crystal. First, free-standing oxide (SiOx/SiO2)-passivated SiNCs were prepared by electrochemical etching of a Si wafer. Their optical properties were studied by employing time-resolved spectroscopy, also at cryogenic temperatures. The fast blue-green emission band of these SiNCs was linked with the quasi- direct recombination of hot electrons and holes in the vicinity of the Γ-point. Furthermore, the spectral shift of the slow orange-red band (of these SiNCs) as a function of temperature was explained on the basis of an interplay between tensile strain and bulk Si temperature-induced indirect bandgap shift. The...
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Morfology and surface chemical composition of porous silicon prepared at various conditions
Konečný, Martin ; Dian, Juraj (advisor) ; Herynková, Kateřina (referee)
Title: Morfology and surface chemical composition of porous silicon prepared at various conditions Author: Bc. Martin KONEČNÝ Author's e-mail: konecmar@seznam.cz Department: Department of Chemical Physics and Optics Supervisor: Doc. RNDr. Juraj Dian, CSc. Supervisor's e-mail: Juraj.Dian@mff.cuni.cz Abstract: Porous silicon is a silicon-based material prepared mainly by anodic etching of crystalline silicon in hydrofluoric acid. Physical and chemical properties of porous silicon are governed by structures with sizes of the order of ones to tens of nanometers. Properties of nanostructure material are affected - as compared to macroscopic counterparts - by quantum confinement effect and enormous internal surface. According to type of silicon substrate (type of dopant, conductivity, crystallographic orientation) and technological conditions a material with different mean size of pores (macro-, meso- and nanoporous silicon) and surface chemical composition (different ratio of Si-O and Si-H bond) can be prepared. Morphology and surface chemical composition predestinated application potential of porous silicon for sensors of chemical species by taking advantage of strong sensitivity of physical properties of silicon nanocrystals - especially of photoluminescence - on the chemical state of a surface. Detection of...
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Study of semiconductors by methods of laser spectroscopy
Dzurňák, Branislav ; Trojánek, František (advisor) ; Oswald, Jiří (referee) ; Herynková, Kateřina (referee)
Title: Study of semiconductors by methods of time resolved laser spectroscopy: Luminescence spectroscopy of nanocrystalline diamond Author: Branislav Dzurňák Department: Department of Chemical Physics and Optics Supervisor: doc. RNDr. František Trojánek, Ph.D. Abstract: The PhD thesis is focused on optical properties of nanocrystalline diamond prepared by chemical vapour deposition method. Photoluminescence of nanocrystalline diamond samples and effects of ambient temperature, pressure, pH and UV irradiation on it are studied by laser spectroscopy. Results suggest the keyrole of water and air adsorbates which affect the energy states in the sub-bandgap region of diamond. Photoluminescence decay of samples of different surface termination and structure and its dependency on ambient pressure and temperature is studied by methods of ultrafast (picosecond and nanosecond scale) laser spectroscopy. Results are analysed by power-law decay function which fits well the luminescence decay curves and also describes the dynamics of charge carriers in states localised within the bandgap. The model of interaction of nanocrystalline diamond with air adsorbates is proposed. Non-linear optical properties of nanocrystalline diamond are also studied, namely the generation of second and third harmonic frequency. The thesis...
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Dynamics of photoexcited charge-carriers in diamond
Popelář, Tomáš ; Malý, Petr (advisor) ; Herynková, Kateřina (referee)
Title: Dynamics of photoexcited charge carriers in diamond Author: Bc. Tomas Popelar Department: KCHFO MFF UK Supervisor: prof. RNDr. Petr Maly DrSc., KCHFO MFF UK Abstract: In this work we examine the luminescence from diamond in the strong excitation regime by one-photon or two-photon absorption of femtosecond laser beam. Measured sample was very pure type IIa diamond prepared by CVD method which was held in cryostat in order to measure temperature dependant luminescence in range from 12 K to 300 K. The signal was collected and analyzed either by spectrograph or streak camera so we gained time-integrated and time-resolved results. We analyzed only the part of the spectrum containing contributions from electron-hole liquid (for low temperatures), free excitons and probably exciton complexes. For higher temperatures where the condensation is not possible the contribution from e-h plasma was too weak compared to free excitons and was only detectable by time-resolved measurements. Other time-dependant results were obtained by pump and probe experiment with which we examined a change of life- times based on mode of excitation (one-photon or two-photon one) and also an influence of other beams on the condensation into electron-hole drops. Keywords: CVD diamond, dynamics, e-h liquid, excitons
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Ultrafast laser spectroscopy of semiconductor nanostructures
Neudert, Karel ; Trojánek, František (advisor) ; Nikl, Martin (referee) ; Herynková, Kateřina (referee)
This doctoral thesis deals with analysis of carrier dynamics in semiconductor nanocrystals and nanostructures by methods of ultrafast laser spectroscopy. In three chapters silicon, InAs/GaAs, InAs/AlAs and CdSSe materials are studied. Silicon nanocrystals prepared by sol-gel method were measured by time-resolved luminescence method with various time resolutions, two-photon absorption and Z-scan method. A model describing findings was suggested. InAs was well measured by upconversion method and gained knowledge appeared to be fully consistent with quantum model. CdSe material was examined in two basic forms - thin films of nanocrystals prepared by chemical deposition and nanocrystals in comercial filters. In both cases all questions regarding influencing optical properties of nanocrystals by modification of their growth were answered.
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