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Electrical and optical properties of SiC single crystals
Brynza, Mykola ; Belas, Eduard (advisor) ; Štekl, Ivan (referee)
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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Diffusion of native defects and impurities in CdTe/CdZnTe
Šedivý, Lukáš ; Belas, Eduard (advisor) ; Šíma, Vladimír (referee)
Title: Diffusion of native defects and impurities in CdTe/CdZnTe. Author: Lukáš Šedivý Author's e-mail address: luky.sedivy@seznam.cz Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc. Supervisor's e-mail address: belas@karlov.mff.cuni.cz Abstract: In this thesis, the influence of structural defects on the electrical and de- tection characteristics of CdTe material was investigated. The performed research fo- cused on the reduction of structural defects in the material by annealing in Cd or Te vapor, while preserving acceptable features for X-ray and γ-ray detection. The mate- rial was characterized by measurement of the electrical resistivity and concentration and mobility of free carriers. Tellurium and cadmium inclusions were studied using infrared microscope. The static and dynamic properties of defect structures at high temperatures and de- fined Cd pressures was investigated, as well, and chemical diffusion coeficients describing the dynamic properties of these defects were experimentally determined. Keywords: monocrystal CdTe, structural defects in semiconductors, annealing in Cd or Te, chemical diffusion coefficient, γ-ray detectors. 1
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Effect of the laser pulse illumination on charge collection efficiency in radiation detectors.
Betušiak, Marián ; Belas, Eduard (advisor) ; Oswald, Jiří (referee)
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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Structural defects in II-VI semiconductors
Šedivý, Lukáš ; Belas, Eduard (advisor) ; Hulicius, Eduard (referee) ; Schneeweiss, Oldřich (referee)
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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Transport and optical properties of CdTe/CdZnTe single crystals
Uxa, Štěpán ; Belas, Eduard (advisor) ; Humlíček, Josef (referee) ; Drbohlav, Ondřej (referee)
Title: Transport and optical properties of CdTe/CdZnTe single crystals Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Eduard Belas, PhD, Institute of Physics of Charles University Abstract: The thesis is focused on a study of internal electric field and temperature dependence of the absorption edge of CdTe and CdZnTe samples. In the first part of the thesis the transient-current technique (TCT) is used for investigations of electric fields within planar radiation detectors. The original comprehensive theory that links together TCT measurements with measurements of detector's charge-collection efficiency (CCE) is presented. This approach results in the development of two new iterative methods for processing of experimental data which can be used in any situation when the internal electric field can be approximated by a linear profile. In the second part of the thesis high temperature measurements of transmittance of thin polished CdTe samples are presented, leading to the estimation of the temperature dependence of the bandgap energy from calculated spectra of absorption coefficient. This is for the first time when measurements in a Cd overpressure have been performed which significantly reduces sample sublimation. Keywords: CdTe, transient-current technique,...
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Structure defects in SiC radiation detectors
Zetek, Matyáš ; Belas, Eduard (advisor)
Silicon carbide (SiC), is a wide band gap (2.4 eV < Eg < 3.3 eV) semiconducting material well known for its potential applications in high-temperature, high-power, high-frequency or hard radiation resistant devices. In this thesis, we are broadening elementary knowledge about this material. We identify energy levels in the material, using Photo-Hall effect spectroscopy supported by the temperature dependency of classic Hall effect measurement and temperature dependent photoluminescence. This knowledge is essential to allow SiC application as a radiation detector.
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Charge transport in semiconducting radiation detectors
Pipek, Jindřich ; Belas, Eduard (advisor)
This thesis is focused on study of charge transport in semiconducting radiation detectors. Theoretical calculations of current waveforms based on continuity equation and drift-diffusion equation are done. Useful approximations of current waveforms for detector with shallow electron trap are discussed. Monte Carlo simulation of the current waveforms is proposed and applied to fit experimental current waveforms measured using laser-induced transient current technique and for evaluation of charge transport parameters of the detector such as electric field profile, trapping and detrapping time of traps, drift mobility and other parameters. Detectors prepared from semi-insulating GaAs and CdZnTe single crystals are tested using electrical, spectroscopic and optical characterization techniques.
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Charge transport optical characterization in semiconductor radiation detectors
Ridzoňová, Katarína ; Belas, Eduard (advisor)
Measurements of DC photocurrent-voltage characteristic, photocurrents spectral response and laser induced transient current technique enable investigation of surface recombination centers, bulk trap levels and distribution of the inner electric field. In the presented work, the n- type planar CdZnTe detectors with quasi Ohmic contacts were studied by above mentioned methods. It has been shown that in the case of strongly absorbed light under the DC regime of illumination not only surface recombination influences the detector's transport properties. The effect of the space charge, created as a consequence of carriers trapped by the impurity levels, must be taken into account. Therefore some new theoretical models were created in order to describe measured photocurrent-voltage dependencies. Obtained data were fitted with the new theory and the mobility and surface recombination velocity for electrons were determined.
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Charge transport in semiconducting radiation detectors
Pipek, Jindřich ; Belas, Eduard (advisor)
This thesis is focused on study of charge transport in semiconducting radiation detectors. Theoretical calculations of current waveforms based on continuity equation and drift-diffusion equation are done. Useful approximations of current waveforms for detector with shallow electron trap are discussed. Monte Carlo simulation of the current waveforms is proposed and applied to fit experimental current waveforms measured using laser-induced transient current technique and for evaluation of charge transport parameters of the detector such as electric field profile, trapping and detrapping time of traps, drift mobility and other parameters. Detectors prepared from semi-insulating GaAs and CdZnTe single crystals are tested using electrical, spectroscopic and optical characterization techniques.
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Effect of the laser pulse illumination on charge collection efficiency in radiation detectors.
Betušiak, Marián ; Belas, Eduard (advisor) ; Oswald, Jiří (referee)
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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