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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...
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Study of the charge collection efficiency affected by metalization in Gamma and X- ray detectors
Pipek, Jindřich ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In this thesis, the influence of metal contacts, prepared on CdZnTe-based semiconductor material by electroless deposition using aqueous or alcohol based solution, on the detection quality of Gamma and X-ray detectors. The quality of metal contacts is also affected by surface treatments. From single crystal of CdZnTe the testing detector was made. Electrical, spectroscopic and optical characterization techniques were used and their results were compared for two mentioned types of contacts. The aim was to show the possibility of preparing high quality metal contacts prepared from alcohol based solution of Gold(III) chloride. Alcohol based contacts are new method for preparing chemical contacts and promise different properties from standard chemical contacts prepared from aqueous based solution of Gold(III) chloride.
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Charge transport optical characterization in semiconductor radiation detectors
Ridzoňová, Katarína ; Belas, Eduard (advisor) ; Toušek, Jiří (referee)
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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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...
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Gamma and X- ray CdTe/CdZnTe detectors
Pekárek, Jakub
In the present work we studied an influence of different types of surface etching and surface passivation of high resistivity CdZnTe-based semiconductor detector material. The aim was to find the optimal conditions to improve the properties of metal-semiconductor contact. The main effort was to reduce the leakage current and thus get better X-ray and gamma-ray spectrum, ie to create a detector operating at room temperature based on this semiconductor material with sufficient energy resolution and the maximum charge collection efficiency. Individual surface treatments were characterized by volt-ampere characteristics, spectral analysis and by determination of the profile of the internal electric field.
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Influence of Deep Levels on Charge Transport in CdTe and CdZnTe
Dědič, Václav
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...
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Gamma and X- ray CdTe/CdZnTe detectors
Pekárek, Jakub ; Belas, Eduard (advisor) ; Doležal, Zdeněk (referee)
In the present work we studied an influence of different types of surface etching and surface passivation of high resistivity CdZnTe-based semiconductor detector material. The aim was to find the optimal conditions to improve the properties of metal-semiconductor contact. The main effort was to reduce the leakage current and thus get better X-ray and gamma-ray spectrum, ie to create a detector operating at room temperature based on this semiconductor material with sufficient energy resolution and the maximum charge collection efficiency. Individual surface treatments were characterized by volt-ampere characteristics, spectral analysis and by determination of the profile of the internal electric field.
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Defects limiting charge collection in semiinsulated CdZnTe
Zajac, Vít ; Franc, Jan (advisor) ; Šikula, Josef (referee)
Title: Defects limiting charge collection in semiinsulated CdZnTe Author: Vít Zajac Department: Institute of Physics of Charles University Supervisor: doc. Ing. Jan Franc, DrSc., Institute of Physics of Charles University Abstract: We achieved to detect photoluminescence transitions deep in the band gap in 4 samples cut from 2 different crystals of semiinsulating Cd1-xZnxTe (x = 0,02 - 0,18) in a row of points along the growth axis. The spectral peaks give evidence of the presence of deep levels in the sample and the intensity of the photoluminescence peaks is to a certain extent proportional to the concentration of these levels. A comparison between resistivity and photoconductivity that were measured by a contact-less method showed that the change of photoluminescence intensity of deep levels does not bring about an unambigous change of neither resistivity nor photoconductivity. Correlation analysis of resistivity and photoconductivity of 6 samples from 4 different crystals confirmed the following model: A shift of the Fermi level within the band gap induced by a change of donor-acceptor compensation is accompanied by an unambigous change of resistivity and results in a change in occupation of the deep levels. This causes a change in the photoconductivity of the crystal because the occupation factor of...
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Gamma- ray and X- ray detection by detectors based on (CdZn)Te single crystals.
Drobný, Miloslav ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In this thesis, the influence of metal contacts, prepared on semiconductor detectors made of high resistive Cd0,85Zn0,15Te, on the quality of detected signal was investigated. The goal of thesis was to prepare ohmic contacts. Two materials (gold, platinum) and two methods of the preparation (chemical deposition, evaporation) were tested. It was found that improved detection properties were obtained on detectors prepared with gold contacts. Current-voltage characteristics of these contacts were found to be pseudoohmic. Moreover, when the Au contacts are prepared by evaporation, the value of the surface leakage current is lower.
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Noise, Transport and Structural Properties of High Energy Radiation Detectors Based on CdTe
Šik, Ondřej ; Lazar, Josef (referee) ; Navrátil, Vladislav (referee) ; Grmela, Lubomír (advisor)
Poptávka ze strany vesmírného výzkumu, zdravotnictví a bezpečnostního průmyslu způsobila v posledních letech zvýšený zájem o vývoj materiálů pro detekci a zobrazování vysokoenergetického záření. CdTe a jeho slitina CdZnTe. jsou polovodiče umožnují detekci záření o energiích v rozsahu 10 keV až 500 keV. Šířka zakázaného pásma u CdTe / CdZnTe je 1.46 -1.6 eV, což umožňuje produkci krystalů o vysoké rezistivitě (10^10-10^11 cm), která je dostačující pro použití CdTe / CdZnTe při pokojové teplotě. V mé práci byly zkoumány detektory CdTe/CdZnTe v různých stádiích jejich poruchovosti. Byly použity velmi kvalitní spektroskopické detektory, materiál s nižší rezistivitou a výraznou polarizací, detektory s asymetrií elektrických parametrů kontaktů a teplotně degenerované vzorky. Z výsledků analýzy nízkofrekvenčního šumu je patrný obecný závěr, že zvýšená koncentrace defektů způsobí změnu povahy původně monotónního spektra typu 1/f na spektrum s výrazným vlivem generačně-rekombinačních procesů. Další výrazná vlastnost degenerovaných detektorů a detektorů nižší kvality je nárůst spektrální hustoty šumu typu 1/f se vzrůstajícím napájecím napětí se směrnicí výrazně vyšší než 2. Strukturální a chemické analýzy poukázaly, že teplotní generace detektorů způsobuje difuzi kovu použitého při kontaktování a stopových prvků hlouběji do objemu krystalu. Část mé práce je věnována modifikaci povrchu svazkem argonových iontů a jejímu vlivu na chemické složení a morfologii povrchu.
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