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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: jan.franc@mff.cuni.cz 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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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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Non-Linear Electro-Ultrasonic Spectroscopy of Resistive Materials
Tofel, Pavel ; Koktavý, Bohumil (referee) ; Smulko, Janusz (referee) ; Šikula, Josef (advisor)
Elektro-ultrazvuková spektroskopie je založena na interakci dvou signálů, elektrického střídavého signálu s frekvencí fE a ultrazvukového signálu s frekvencí fU. Ultrazvukový signál mění vzdálenost mezi vodivými zrny ve vzorku a tím mění jeho celkový elektrický odpor R. Změna odporu R je proměnná s frekvencí ultrazvukového signálu fU. Vzorek, který obsahuje mnoho defektů ve své struktuře, vykazuje vysokou změnu odporu R v porovnání se vzorkem bez defektů při stejné hodnotě ultrazvukového a elektrického signálu. V disertační práci je popsána elektro-ultrazvuková metoda na tlustovrstvých rezistorech, hořčíkových slitinách, monokrystalech Si a CdTe, varistorech a také jeden z prvních pokusů aplikace elektro-ultrazvukové spektroskopie na horninové vzorky a tak diagnostikovat jejich stav poškození. V našem případě byl proměřen vzorek žuly. Jelikož se jedná o nedestruktivní metodu testování, tak má tato metoda velmi perspektivní budoucnost. Tato metoda je citlivá na všechny defekty ve vzorku. Její výhodou je, že se měří velikost signálu ne frekvenci danou rozdílem nebo součtem budících frekvencí fE a fU a tím se dá dosáhnout vysoké citlivosti. V mém případě byl vždy měřen signál na rozdílové frekvenci fi = fE-fU.
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Electronic Noise of Piezoceramic Sensors of Acoustic Emission
Majzner, Jiří ; Šikula, Josef (advisor)
In our work the analysis of electrical and noise characteristics of piezoceramic acoustic emission sensors is accomplished. The objective of our work is to analyze and optimize the signal-to-noise ratio. The starting point is the explanation of the noise origin in the acoustic emission sensors. The voltage fluctuation is caused by the dipole vibrations due to their interaction with phonons. The frequencies of dipoles vibrations have statistical distribution and the total energy of these vibrations is proportional to the temperature. The statistical distribution of vibration frequencies leads to the origination of the 1/f type noise spectral density. The interaction between the phonons and electric dipoles is characterized by the imaginary part of susceptibility which is related to the transformation of electric energy to the mechanical energy of vibrations. This process is irreversible and this forms important theoretical question whether the Callen-Welton fluctuation dissipation theorem could be used for the description of fluctuation processes in the acoustic emission sensors. In our work the influence of the real and imaginary part of the susceptibility on the noise and electrical characteristics is solved, the dissipation of electrical energy characterized by the imaginary part of susceptibility is described and the connection between the imaginary part of susceptibility and the noise power spectral density is discussed. Due to the fact that these processes originate in the interaction between electrical dipoles and phonons, we give account of the temperature dependencies of equivalent series resistance and power spectral density of noise voltage, respectively. Piezoceramics stiffness contribute significantly to the resonance creation hence the pressure influence on the sensor noise characteristics was studied. The signal-to-noise ration improvement requires the piezoceramic sample diameter increase for its constant thickness. The ratio of the noise spectral density and sensitivity is independent on the sample thickness. The noise voltage is proportional to the square root of spectral density and frequency bandwidth that is why for the high signal-to-noise ratio it is necessary to minimize the signal amplifier frequency bandwidth. The noise voltage power spectral density increases with the temperature while the activation energy is 20 meV for the temperature 300 K, and 80 meV for the temperature 400 K, respectively. The power spectral density of planar oscillations decreases with increasing pressure and simultaneously the resonant frequency increases. The bandwidth of the normalized spectral density increases with the pressure for the planar oscillations while is invariable for the thickness oscillations. For the examination of the influence of the piezoceramic electrical polarization on the electrical and noise characteristics the experimental study of these dependencies was accomplished for samples without polarization, and samples polarized by electric field EP = 500V/mm and 1000V/mm, respectively. The samples without polarization show the noise of 1/f type only which corresponds to the Callen-Welton fluctuation dissipation theorem. The polarization leads to the generation of planar and thickness oscillations and the power spectral density of voltage fluctuation on the electrodes is proportional to the temperature, and inversely proportional to the imaginary part of permittivity, to the sample area S, and the frequency f.
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