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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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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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Photoelectric transport in high resistivity CdTe for gamma ray detectors
Dědič, Václav ; Šikula, Josef (referee) ; Franc, Jan (advisor)
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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The Ancient Egypt Resources of the Coptic Religiosity: The Selected Subjects
Franc, Jaroslav ; Gebelt, Jiří (referee) ; Janák, Jiří (advisor)
The master thesis titled The Ancient Egypt Resources of the Coptic Religiosity: the Selected Subjects is in the hypothetic possibility focused on the continuity of the religious symbolism from Ancient Egypt times to the time of The Coptic Church. The paper is based on the personal experience from within The Coptic Church in Egypt as well as on the research in libraries and museums. The attention is drawn mainly to the theme of breastfeeding woman - Eset, Isis, the Virgin Mary.
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Photoelectric spectroscopy of deep electronic levels in high-resistance CdTe
Kubát, Jan ; Franc, Jan (advisor) ; Sopko, Bruno (referee)
CdTe is one of the most interesting X-ray and g-ray detectors' material. This work deals with influence of deep levels to photoelectric properties of CdTe. PICTS, Lux-Ampere and spectral dependences measurements at room temperature and low temperature 10K were performed on one undoped and several variously doped (Cl, Sn and Ge) samples and applied electrical fields up to 800V.cm-1. Experimental setups are introduced. Room temperature numerical solution of sample photoelectrical properties for typical midgap level using driftdiffusion and Poisson equation was performed and results are discussed. The experimentally observed slopes of Lux-ampere characteristics and energy shifts of the main photoconductivity peak with the applied voltage are explained based on a model of screening of electric field by charge accumulated on deep levels. Finally comparison with acquired experimental data is performed yielding estimates of maximum total concentration of deep levels in the samples.
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