National Repository of Grey Literature 39 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
Studium elektrického pole v detektorech záření pomocí Pockelsova jevu
Hakl, Michael ; Franc, Jan (advisor) ; Richter, Ivan (referee)
Study of electric field in radiation detectors by Pockels effect (Master Thesis) by Michael Hakl Abstract Cadmium Telluride (CdTe) is a convenient candidate for room tem- perature detection of X-ray and gama radiation due to 1.5 eV band- gap energy and high atomic mass. Since CdTe has the highest linear electro-optical coefficient among II-VI compounds, the detector rep- resents a Pockels cell. Transmittance of the crystal is modulated by the internal electric field. Processing of infrared camera photographs results in an electric field profile between biasing electrodes. The elec- tric field in semi-insulting CdTe is influenced with deep level traps causing charge polarization under the electrodes. Occupation of traps is dependent on metal-semiconductor interface. Relation of charge accumulation and band bending for gold and indium contacts was studied. Repolarization/depolarization induced by additional illumi- nation with sub/above bandgap excitation laser was observed and ex- ploited for determination of the deep level energy. Results obtained by the Pockels-effect method were supported with luminescence measure- ments. Correlation between the occurrence of deep levels and surface point defects was discovered. Keywords: Pockels electro-optical effect, Cadmium Telluride ra- diation detector, Electric field, Schottky...
Effect of surface of semiconductor nanocrystals on their properties: investigation by laser spectroscopy
Preclíková, Jana ; Malý, Petr (advisor) ; Franc, Jan (referee)
The topic of the diploma thesis is chemical treatment of the surfaces of semiconductor nanocrystals CdS and CdSe in the form of thin nanocrystalline films prepared by chemical bath deposition. The evolution of photoluminescence and absorption spectra was investigated under treatment by water, toluene, butylamine, pyridine and hydrogen peroxide. The strongest changes were observed by samples soaked in water, pyridine and aqueous solution of pyridine. The observed effects were explained by partly dissolving of nanocrystals by water and affecting behaviour of photoexcited holes in the presence of the strong hole acceptor pyridine. The second part of this diploma thesis deals with photochromic properties of silver nanoparticles in a titanium oxide matrix. The photochromic behaviour of the samples was examined in detail. A phenomenological model of photochromic changes was suggested in analogy with the hole burning spectroscopy. In order to better understand the microscopic processes in this composite material other experiments were realized - e.g. samples were UV irradiated, or laser illuminated under lowered air pressure, pump & probe experiment. Under laser irradiation optical extinction of some nanoparticles was decreased. Simultaneously, changes of plasmon resonances of the rest of nanoparticels were...
Electrooptic Pockels effect in X-ray radiation detectors
Rejhon, Martin ; Franc, Jan (advisor) ; Moravec, Pavel (referee)
In this work, we have added a temperature controller to an apparatus for measuring Pockels effect, which comprises of an integrated circuit and a Peltier element. Then the course of the electric field was measured in the sample of high resistance CdTe, a suitable detector on the high-energy radiation, depending on the voltage and temperature of the sample. Also the charge distribution in the sample was determined depending on time after the voltage and sample's temperature were reached. Finally, the activation energies of deep levels were determined, which are responsible for the polarization. Powered by TCPDF (
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.
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 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.
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...
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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See also: similar author names
3 Franc, J.
4 Franc, Jakub
3 Franc, Jaroslav
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