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Optical methods for characterization of semiconductor radiation detectors
Pekárková, Katarína ; Belas, Eduard (advisor) ; Humlíček, Josef (referee) ; Ledinský, Martin (referee)
CdZnTe and perovskites are promising materials for radiation detectors. Here, we address the main obstacles to their application, CdZnTe charging, and perovskite stability by photocurrent measurements. In the first part, models describing space-charge-limited photocurrents are developed, and an excellent agreement with measured data is obtained. The Drift-diffusion model provided us with a detailed defect analysis of CdZnTe. A linear rise of photocurrent at low voltage originates in the trapping of injected holes close to the cathode. We propose a simple procedure to evaluate space-charge density from photocurrents. The influence of space-charge, photoconductive gain, and shallow levels result in inaccurate evaluation of mobility-lifetime product via photoconductivity. The second part focuses on light-induced changes in perovskites. Laser-induced transient current measurements show improvements in hole lifetime and surface recombination in CH3NH3PbBr3 single crystals. We further study phase segregation in mixed-halide perovskites by Fourier-Transform photocurrent spectroscopy. The recorded gradual formation of an I-rich phase correlates with an increase in deep defect concentration, which we attribute to charged iodide interstitials. Phase segregation is associated with iodide migration through interstitial...
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Charge transport in semiconductors
Pipek, Jindřich ; Belas, Eduard (advisor) ; Doležal, Zdeněk (referee) ; Oswald, Jiří (referee)
This thesis is focused on the study of charge transport in semiconductors. Monte Carlo simulation is combined with the numerical solution of the coupled drift-diffusion equation with Poisson's equation. These numerical simulations of charge transport are applied to experimental data to obtain charge transport parameters of the detector, such as electric field profile, space charge density, defect level energy, capture cross section and concentration, charge carrier drift mobility and diffusion coefficient, and other parameters. Semiconductor samples prepared from semi-insulating GaAs, CdZnTe, and CdZnTeSe single crystals are studied using electrical, spectroscopic, and optical characterization techniques.
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Charge transport in halide perovskites affected by the surface treatment and metalization.
Šlechta, Miroslav ; Belas, Eduard (advisor) ; Grym, Jan (referee)
In this thesis, I study charge transport in halide perovskites. This is done using the characterization method, which here is the I-V characteristics. It studies the effect of current and voltage on the prepared sample, also at different temperatures. A significant sensitivity of the material to the measurement history was found. Derived evaluations of this measurement technique further provide possibilities for the detection of hysteresis behavior as well as the evaluation of measurements over time to determine the temporal stability of carrier collection. This also proved the phenomenon of electromigration of defects. Furthermore, transport parameters were calculated from the time measurements, which point to their non-monotonic development in increasing temperatures. A theoretical model explaining such behavior was proposed as a possibility for further research on perovskites and development of this topic.
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Transport properties of perovskites
Raja, Marek ; Belas, Eduard (advisor) ; Holovský, Jakub (referee)
This work studies charge transport in halide perovskites made of methylammonium lead tribromide CH3NH3PbBr3. By finding and using bipolar pulsation parameters, we describe the transport properties of both holes and electrons. The shapes of the measured current waveforms with the L-TCT method are simulated by the Monte Carlo simulations. Theoretical models of charge density distribution are based on a drift-diffusion equation with consideration of the infinite and finite lifetime of a charge carrier caused by a shallow and deep trap. Theobtained values ofdrift mobility, electric field profile, transit time, and surfacerecombination rate are obtained by Monte Carlo simulation. We have successfully shown the effect of pulsing with unipolarand bipolar biases. By finding thepulsation parameters at which the sample does not polarize, we calculated the hole mobility around 13 cm2 V-1 s-1 . We arrived at the ambiguity of determining the effect of the expanding deep trap region and the effect of space charge formation. Thus, we found multiple possible models to describe the measured current waveforms. This work confirms the high sensitivity of perovskites to the method and history of measurement.
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Effect of the laser pulse illumination on charge collection efficiency in radiation detectors.
Betušiak, Marián ; Belas, Eduard (advisor)
The main focus of this thesis is the characterization of the charge transport in CdZnTe radiation detectors and the study of the effect of the detector illumination on charge transport. The transport properties are evaluated using Laser-induced Transient Current Technique and the Monte Carlo simulation is used for fitting the measured current waveforms. The properties of the detector prepared from semi-insulating CdZnTe single crystal with a platinum Schottky contacts were measured in the dark in the unpolarized and polarized state and under the anode and cathode continuous LED above-bandgap illumination.
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Electrical and optical properties of SiC single crystals
Brynza, Mykola ; Belas, Eduard (advisor)
Silicon carbide is a semiconductor with a wide bandgap of up to 3.2 eV and is capable of operating in extreme conditions, high temperature and high energy modes. This work focuses on the investigation of electrical and optical properties of monocrystalline SiC by various methods including Raman spectroscopy, volt-ampere characteristics, L-TCT and spectroscopic techniques. The adhesion of contacts and the influence of different contact materials on the ability to detect ionizing radiation are also studied to optimize the technology of preparation of quality SiC-based radiation detectors.
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Structural defects in II-VI semiconductors
Šedivý, Lukáš ; Belas, Eduard (advisor)
Title: Structural defects in II-VI semiconductors Author: Lukáš Šedivý Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc., Institute of Physics, Faculty of Mathematics and Physics, Charles University Abstract: The single crystalline CdTe doped by chlorine is an excellent material for man- ufacturing x-ray and gamma-ray room-temperature semiconductor detectors thanks to the large linear attenuation coefficient, the possibility to make it high-resistive at the room temperature, and good electron mobility and the lifetime. This thesis aimed to examine the effect of annealing in well-defined ambient component pressure, Cd or Te, on the crys- tal's defect structure. The first experimental is devoted to eliminating the second phase defects - inclusions - present in CdTe : Cl, which significantly decay the crystal quality and detection performance. The following experimental parts are focused on the detailed inves- tigation of the point defect structure of CdTe : Cl. The annealing interval bisection method for reaching high resistivity material is introduced. The equilibrium defect structure is in- vestigated using the in-situ high-temperature Hall effect measurements. The results are interpreted through an advanced model of the defect structure considering also dissocia-...
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Electromigration of defects in (CdZn)Te single crystals.
Pekárek, Jakub ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In the present work we study effect of electromigration of defects in CdTe material in external electric field at various temperatures and biases. The aim was to verify this effect using a set of electric contacts arranged linearly along the sample. These contacts as a potential probes was used for the detection of the drift of the local resistance modulation. We try to determine under what conditions and why it happens and then decide how this effect can be used for purification of semi-conducting samples and for preparation of semi-insulating material.
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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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