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Charge transport in semiconductors
Pipek, Jindřich ; Belas, Eduard (advisor) ; Doležal, Zdeněk (referee) ; Oswald, Jiří (referee)
This thesis is focused on the study of charge transport in semiconductors. Monte Carlo simulation is combined with the numerical solution of the coupled drift-diffusion equation with Poisson's equation. These numerical simulations of charge transport are applied to experimental data to obtain charge transport parameters of the detector, such as electric field profile, space charge density, defect level energy, capture cross section and concentration, charge carrier drift mobility and diffusion coefficient, and other parameters. Semiconductor samples prepared from semi-insulating GaAs, CdZnTe, and CdZnTeSe single crystals are studied using electrical, spectroscopic, and optical characterization techniques.
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Photo-Hall effect spectroscopy and laser-induced transient currents in CdTe-based semiconductor radiation detectors
Musiienko, Artem ; Grill, Roman (advisor) ; Dubecký, František (referee) ; Oswald, Jiří (referee)
Title: Photo-Hall effect spectroscopy and laser-induced transient currents in CdTe-based semiconductor radiation detectors Author: Artem Musiienko Department / Institute: Institute of Physics, Faculty of Mathematics and Physics, Charles University Supervisor of the doctoral thesis: Prof. RNDr. Roman Grill, CSc, Institute of Physics, Faculty of Mathematics and Physics, Charles University Abstract: Cadmium Telluride, Cadmium Zinc Telluride, and Cadmium Manganese Telluride are important semiconductors with applications in radiation detection, solar cells, and electro-optic modulators. Their electrical and optical properties are principally controlled by defects forming energy levels within the bandgap. Such defects create recombination and trapping centers capturing photo- created carriers and depreciating the performance of the detector. Simultaneously, the changed occupancy of levels leads to the charging of detector's bulk, which results in the screening of applied bias and the loss of detector's sensitivity. Detailed knowledge of crystal defect structure is thus necessary for the predictable detector work and also for the possibility to reduce the structural defects concentration. This thesis reports on the investigation of deep energy levels in CdTe-based high resistivity and detector-grade materials by...
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Photoluminescence of CdTe crystals
Procházka, Jan ; Hlídek, Pavel (advisor) ; Toušek, Jiří (referee) ; Oswald, Jiří (referee)
Title: Photoluminescence of CdTe crystals Author: Jan Procházka Department: Institute of Physics of Charles University in Prague Supervisor: Doc. RNDr. Pavel Hlídek, CSc. Abstract: Energy levels connected with defects in nominally undoped crystals CdTe, indium- doped crystals and chlorine-doped crystals were studied using low-temperature photoluminescence. The crystals are intended for X- and gamma- ray detectors operated at room temperature. An effect of annealing in cadmium or tellurium vapor on luminescence spectra was investigated. Some changes were interpreted by filling of vacancies not only by atoms coming from gaseous phase but also by impurities from defects like interstitials, precipitates, inclusions, grain boundaries etc. The luminescence bands assigned to defects important for compensation mechanism were examined, namely A-centers (complexes of vacancy in cadmium sublattice and impurity shallow donor) and complexes of two donors bound to a vacancy. It was shown, that temperature dependence of the luminescence bands results from more complicated processes than a simple thermal escape of bound excitons or thermal excitation of electrons (holes) from defects to bands. We observed expressive "selective pair luminescence" bands (SPL) on partially compensated In-doped samples during sub-gap...
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Ultrafast laser spectroscopy of semiconductors
Popelář, Tomáš ; Malý, Petr (advisor) ; Oswald, Jiří (referee) ; Žídek, Karel (referee)
This Ph. D. thesis is focused on the study of modern semiconductor materials, diamond, and silicon nanocrystals, by methods of laser spectroscopy. A new experimental setup, the generator of mid-infrared laser pulses, was built to study diamond in detail. With its help, we observed the dynamics of high-density excited carriers, their condensation into electron-hole liquid, and their interaction with light around their plasma frequency. Thanks to the high sensitivity of these measurements on the excited carrier temperature we could observe the thermalisation dynamics which was described by the quantum theory of free carrier absorption. In the doped silicon nanocrystals embedded into SiO2 matrix, we studied the potential presence of free carriers which could be used to create a PN junction. We also found an important recombination channel in these samples which manifests at high excitation intensities. We identified its origin and described its interaction with nanocrystals. At last, we analyzed the influence of incorporated boron atoms on the distribution of decay times according to the emitted wavelength.
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Interaction of ultrashort laser pulses with semiconductor nanostructures
Preclíková, Jana ; Malý, Petr (advisor) ; Potůček, Zdeněk (referee) ; Oswald, Jiří (referee)
The thesis is devoted to the optical spectroscopy (photoluminescence, transmission, ultrafast photoluminescence) of nanocrystalline diamond films and silver nanoparticles in titanium dioxide matrix. We studied the influence of the laser irradiation on optical properties of both materials under various conditions. Nanocrystalline diamond exhibits strong subpicosecond and picosecond photoluminescence parameters of which (intensity and decay rate) change with the laser irradiation and ambient air pressure. The effects are accompanied by a change in optical thicknesses of the nanocrystalline diamond films. We assigned the phenomena to light induced adsorption processes which influence the subgap states originating from surface and grainboundaries atoms. We implemented and optimized the preparation technique of the nanocomposite Ag-TiO2films exhibiting the multicolour photochromic effect. The analysis of the initial stages of the photochromic transformation revealed that during the laser irradiation the plasmon frequency of the resonating silver nanoparticles was blue shifted. The photoluminescence spectroscopy confirmed an increase in number of the Ag+ ions during the photochromic transformation. The photoluminescnece of the nanocrystalline TiO2 is due to the two types of radiative transitions: the...
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