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Centra rekombinace v semiizolačním CdTe
Zázvorka, Jakub ; Franc, Jan (advisor) ; Fiederle, Michael (referee)
Title: Recombination centers in semiinsulating CdTe Author: Jakub Zázvorka Department / Institute: Institute of Physics of Charles University Supervisor of the master thesis: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles University Abstract: The properties of CdTe for application as a radiation detector are influenced through the presence of deep levels in the bang gap. These energy levels complicate the charge collection and the detector efficiency. Contactless resistivity mapping (COREMA) represents a good option for material characterization without the necessity of metal contacts application. The time-dependent charge measurement was investigated on an adjusted apparatus in FMF Freiburg. Theoretical model of charge transport based on band bending on the sample surface was proposed and a non-exponential behavior was calculated. Using this, the resulted parameter tendencies and their connection with deep level trap or recombination center were explained. A correlation was observed between resistivity, photoconductivity and a near midgap level photoluminescence. Parameter profiles were explained using the theory of Fermi level shift relative to the near midgap level. Three deep levels were observed on samples grown at the Charles University in Prague. Their photoluminescence supports the...
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Mapping of photoelectric effects in semiinsulating CdTe
Korcsmáros, Gabriel ; Franc, Jan (advisor) ; Sedláková, Vlasta (referee)
The main advantage of CdTe and CdZnTe materials is the fact that the envolving elements have big atomic numbers and density, this is reflected in a high absortion coeficient. which is a very preferent feature. It enables to detect low energy photons and means high quantum effiency. Mentioned features can be managed at room temperature, therefore these matrials are very perspective for gamma-ray detection. For the detector sensitivity is important to have low dark current, and to have big fold of carrier lifetime and mobility. These properties can be reached in some parts of the materials. Finding the causes of these quality degradation and the signal loss are the main priorities of the exploration. The main problem is the quality of these materials, the inhomogenities are decreasing the detection capability. In this work we will study lux-amper characteristics and analyze photoconductivity maps to better our understanding how inhomogenities influence these parameters. Two main methods will be used, contactless and contact methods with Au applied as contact metal. These maps will be compared. Photoconductivity maps can increase our understanding of charge transport inside the material.
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Photoelectric transport in high resistivity CdTe for gamma ray detectors
Dědič, Václav ; Franc, Jan (advisor) ; Šikula, Josef (referee)
CdTe semiconductor is a good material for the construction of X-ray and gamma ray detectors. Its physical properties are strongly influenced by an existence of deep levels in the forbidden band. This thesis deals with an influence of deep levels to the photoelectric transport in high resistivity CdTe. Experimental part of this thesis consits of measurement of slopes of Lux-Ampere characteristics of variously doped CdTe samples depended on voltage and energy of excitation. Gradients of measured guidelines of Lux-Ampere characteristics show strong dependency on an electric charge accumulated on deep levels. This thesis also contains numerical models of photoconductivity for various parameters of material.
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Point defects in materials for detection of X-ray and gamma radiation
Rejhon, Martin ; Franc, Jan (advisor) ; Oswald, Jiří (referee) ; Toušek, Jiří (referee)
Title: Point defects in materials for detection of X-ray and gamma radiation Author: Martin Rejhon Department: Institute of Physics of Charles University Supervisor: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles Uni- versity Abstract: Cadmium telluride and its compounds are suitable materials for pro- duction of X-ray and gamma-ray detectors working at room temperature. How- ever, the detector quality is affected by material imperfections, such as crystal defects and impurities. It results into forming of deep levels which act as re- combination and trapping centers. Then, the accumulated space charge at these deep levels influences electric and spectroscopic properties of the detector. In the end it may result in the polarization effect, when the electric field is localized in vicinity of one contact and detection properties are decreased. This thesis reports a complex study of a detector band structure by various meth- ods with focus on differences between CdTe, CdZnTe, CdTeSe and CdZnTeSe. The electro-optic Pockels effect is used to investigate the influence of the illumi- nation in range 900 − 1800 nm on the inner electric field. The temperature and time evolutions of the electric field after application of bias or switching of the additional light at 940 nm were measured to determine deep levels...
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Influence of external fields on electric field and photocurrent in CdTe detectors
Rejhon, Martin ; Franc, Jan (advisor)
This thesis is focused on a study of CdTe and CdZnTe semiconductor detectors working under high flux of radiation. We studied experimentally an influence of high flux of X-rays and optical radiation on polarization of the detector. The polarization phenomenon decreases the efficiency of the detector due to a screening of an applied electric field by a space charge accumulated at deep levels due to a trapping of photogenerated carriers. In order to measure the electric field profiles in the detectors we employed a method based on cross polarizers technique and Pockels effect. The main objective of this work was to study the possibilities of an optical de-polarization of CdTe and CdZnTe detectors for different photon energies of additional light, its dynamics and physical origin. We have found that detectors can be de-polarized by above bandgap light. Moreover, CdZnTe detector can be depolarized by near infrared light and in a pulse mode. The de- polarization is associated with a compensation of the space charge at deep traps.
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Conservation procedure for the protection and treatment of chromolithographic prints on paper refined by coating
Chadimová, Martina ; Vlčková, Lucie ; Franc, Jan ; Vávrová, Petra ; Ruml Fortelná, Irena ; Kubíčková, Markéta
In conservation and restoration practice, the issue of chromolithographic prints on refined paper is a non-trivial area to which systematic attention has not yet been paid. The absence of reference sources and case studies on this subject results from the fact that these objects – artifacts and documents – have not been preserved in great numbers in collecting and memorial institutions, and not much importance has been attributed to them within such collections. The rarity of the preservation of valuable chromolithographs in collecting institutions is complicated by their often problematic condition and the frequent damage resulting from the specifics of the chromolithographic process. Although after continuous improvement it was possible to print on almost anything, the most valuable picture posters were printed almost exclusively on “refined” paper. The “refined” coating on this paper, characteristic of chromolithography, as well as the final, transparent “lacquer” layer, cause many problems in terms of restoration, and many physical and chemical risks arise from the layering and interaction of the materials used. The established conservation and restoration procedures in the common practice of collecting institutions have not yet taken into account these very specific issues. The conservation procedure itself involves both theoretical and practical aspects. The section dealing with theoretical aspects summarizes the knowledge necessary to determine how to design conservation and restoration interventions and what constitutes the proper care of chromolithographic prints on refined paper. The introductory section provides many tools for the identification of such prints, including an historical excursion into printing technology and the production of refined coated paper. Rich pictorial documentation serves as a visual aid for recognizing the basic (most common) types of chromolithographs and for identifying paper types based on the damage incurred. The theoretical aspects deal in more detail with the process of refining paper by coating it, with regard to the chemical composition of the substances used, and provides an overview of such binders and pigments. An overview of the factors influencing the degradation of paper is summarized in a separate text dealing with external degradation factors (humidity, temperature, light) and internal ones (hydrolysis of the glycosidic bond, oxidation and photochemical reactions of cellulose, oxidation and photooxidation of hemicelluloses, photolysis and the photooxidation of lignin). The theoretical starting points for the conservation procedures in practice therefore explain the operation of the methods applied in the procedure. The focus is on the preservation of prints containing acidic materials, i.e., on de-acidification methods, such as in situ de-acidification in vacuum packages, the Papersave Swiss technology, and the MMMK method. The optimal variants of the conservation procedures for chromolithographic prints on refined paper are presented in a practical methodological manual based on theoretical analysis of the issue. The first practical step is the macroscopic and microscopic analysis of the physical condition of the examined print and production of a digital record. The output of that procedure is the identification of the material and a thorough assessment of the recorded types of damage, as well as the stage of degradation. As a useful practical guide, a categorization of types of damage according to the conservation procedure they require has been prepared. The record of these facts is the starting point for each conservation protocol and yields supporting information for the choice of a subsequent conservation procedure. The proposed conservation procedures include pH measurement and the subsequent adjustment, gluing and follow-up care. The procedure concludes with a proposal for optimal storage.
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Spectral dependency of the charge generation in semiconductor detectors using nano-second laser pulses
Raja, Marek ; Belas, Eduard (advisor) ; Franc, Jan (referee)
This work deals with the study of charge transport in a semiconductor detector made of CdZnTe material. Theoretical models of charge density distribution are based on a drift-diffusion equation with consideration of infinite and finite lifetime of a charge carrier caused by a shallow and deep trap. The shapes of the measured waveforms with the L-TCT method are fitted by the Monte Carlo method. The obtained values of drift mobility, electric field profile, charge passage time and surface recombination rate are obtained by fitting with the OriginPro program.
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Point defects in materials for detection of X-ray and gamma radiation
Rejhon, Martin ; Franc, Jan (advisor) ; Oswald, Jiří (referee) ; Toušek, Jiří (referee)
Title: Point defects in materials for detection of X-ray and gamma radiation Author: Martin Rejhon Department: Institute of Physics of Charles University Supervisor: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles Uni- versity Abstract: Cadmium telluride and its compounds are suitable materials for pro- duction of X-ray and gamma-ray detectors working at room temperature. How- ever, the detector quality is affected by material imperfections, such as crystal defects and impurities. It results into forming of deep levels which act as re- combination and trapping centers. Then, the accumulated space charge at these deep levels influences electric and spectroscopic properties of the detector. In the end it may result in the polarization effect, when the electric field is localized in vicinity of one contact and detection properties are decreased. This thesis reports a complex study of a detector band structure by various meth- ods with focus on differences between CdTe, CdZnTe, CdTeSe and CdZnTeSe. The electro-optic Pockels effect is used to investigate the influence of the illumi- nation in range 900 − 1800 nm on the inner electric field. The temperature and time evolutions of the electric field after application of bias or switching of the additional light at 940 nm were measured to determine deep levels...
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Analysis of the training process Martin Jakš in the years 2010 - 2016
Franc, Jan ; Gnad, Tomáš (advisor) ; Jindra, Matouš (referee)
Title: The training process analysis of Martin Jakš in 2010 - 2016 Objectives: The purpose of this study is to pedagogically evaluate the training process of Martin Jakš, the member of the Czech cross-country skiing team. Methods: This study is designed as a case study. The study addresses a qualitative research, which was based on an analysis of training process of an elite cross-country skier. Training indicators and sport physical examination were analyzed and assessed with regard to racer's performance in the studied term. Results: The results of this study showed a tendency to decrease of athlete's performance in the studied term. Key terms: cross-country skiing, yearlong training cycle, training indicators, sport physical examination, performance
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Photoconductivity, photoluminescence and charge collection in semiinsulating CdTe and CdZnTe
Zázvorka, Jakub ; Franc, Jan (advisor) ; Humlíček, Josef (referee) ; Oswald, Jiří (referee)
Title: Photoconductivity, photoluminescence and charge collection in semiinsulating CdTe and CdZnTe Author: Jakub Zázvorka Department: Institute of Physics of Charles University Supervisor of the doctoral thesis: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles University. Abstract: Cadmium telluride and its compounds with zinc are the material of choice in spectroscopic room temperature high energy radiation detectors. The development of the final device is influenced by many parameters, including material impurities and defects, homogeneity and surface preparation. This thesis offers a comprehensive investigation of the detector fabrication process and of the parameters and physical effects influencing the spectroscopic resolution and performance of the detector. Structure of deep levels is investigated through photoluminescence and correlated with other electro-optical measurements dealing with the impact of structural imperfections of the material and their effect. The influence of resistivity and photoconductivity homogeneity on the detector performance is studied through electrical measurement of the charge carrier transport and charge collection of the sample. Obtained results are explained using the Fermi level shift theory and confronted with a theoretical model and calculations. The...
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