National Repository of Grey Literature 38 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
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...
Influence of Si doping in different layers on luminescence properties of InGaN/GaN multiple quantum well structure
Hájek, František ; Hospodková, Alice ; Oswald, Jiří ; Slavická Zíková, Markéta
Luminescence of InGaN/GaN multiple quantum well (MQW) structure is strongly affected by spontaneous and piezoelectric polarizations. To suppress them, doping with shallow impurities (e. g. Si) can be used. This works presents the effects of Si doping in different layers around the MQW area. On the basis of photoluminescence and cathodoluminescence measurements and band structure simulation, the piezoelectric field is most efficiently reduced when both layers under and over MQW area are Si doped.\n
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...
Room-temperature semiconducting detectors
Pekárek, Jakub ; Belas, Eduard (advisor) ; Oswald, Jiří (referee) ; Štekl, Ivan (referee)
Semiconducting material CdTe/CdZnTe has a huge application potential in spectroscopic room temperature radiation detection due to its properties. Such detectors can be used in medical applications, homeland security and for monitoring of nuclear facilities. However, the final device quality is influenced by many parameters. One crucial stage in detector fabrication is the proper surface treatment. The detailed study of surface treatments and their effect on final detector device is reported. Another crucial fact is the polarization of the detector caused by high radiation fluxes which negatively affects the use of such devices. The polarization occurs by capturing the photogenerated holes at the deep levels inside the semiconductor. The possible detector depolarization by infrared illumination during the detector operation has been experimentally verified and the obtained results are shown in this thesis. For optimal technology of preparation, it is also necessary to develop the fast characterization method for prepared detectors. The last aim of the thesis is to study the resulting quality of prepared planar and co-planar detectors by transient-current-technique (TCT). TCT is an electro-optical method allowing to determine variety of transport properties of radiation detectors, such as internal electric...
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...
Influence of Deep Levels on Charge Transport in CdTe and CdZnTe
Dědič, Václav ; Franc, Jan (advisor) ; Oswald, Jiří (referee) ; Štekl, Ivan (referee)
CdTe and CdZnTe are promising materials for room temperature semiconductor X-ray and gamma ray detectors. The accumulation of a space charge at deep energy levels due to a band bending at contacts with Schottky barriers and due to trapped photogenerated charge may result in time dependent change of charge collection efficiency in CdTe and CdZnTe detectors known as polarization effect. This thesis is mainly focused on a study of electric field profiles in detectors under dark and high photon flux conditions simulating detector operation using crossed polarizers technique exploiting the electro-optic (Pockels) effect. It also deals with a study of deep levels responsible for the polarization and influence of contact metals on charge accumulation. Several experimental results are supported by theoretical simulations. The measurements were performed on three sets of samples equipped with different contact metals (Au, In) cut from three different n-type CdTe and CdZnTe materials. Energy levels were detected using methods based on the Pockels effect and discharge current measurements. Detailed study of internal electric field profiles has revealed a fundamental influence of near midgap energy levels related to crystal defects and contact metals on the polarization in semiconductor detectors under high radiation...
Influence of external fields on electric field and photocurrent in CdTe detectors
Rejhon, Martin ; Franc, Jan (advisor) ; Oswald, Jiří (referee)
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.
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...
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...
Photoelectric transport in high resistivity CdTe
Kubát, Jan ; Franc, Jan (advisor) ; Šikula, Josef (referee) ; Oswald, Jiří (referee)
Title: Photoelectric transport in high resistivity CdTe Author: Ing. RNDr. Jan Kubát Institute: Institute of Physics of Charles University Supervisor: Doc. Ing. Jan Franc, DrSc. Supervisor's e-mail address: Abstract: CdTe is one of the most promising material for fabrication of X-ray and gamma ray detectors. Despite a considerable effort invested in material development there are still problems remain to be solved and influence efficiency of charge collection. We focus on study of polarization of the sample due to space charge accumulated on deep levels in this work. Samples of CdTe doped with Cl, Sn, In and Ge were investigated. Measurements of spectral dependence of photocurrent and lux- ampere characteristics were done. We performed mathematical modeling of measured data using an approach based on a three level compensation model, and solution of drift-diffusion and Poisson equations. Concentration of deep levels 1011 -1013 cm-3 was revealed in semiinsulating CdTe by modeling. Contact method measurement for determination of µτ product using I-A characteristics and Hecht relation was applied. Mapping of CdTe and CdZnTe samples via contact and contactless method was performed and measurements were compared. Correlation analysis of maps of electric resistivity and photoconductivity has...

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