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Optical gating of epitaxial graphene
Fridrišek, Tomáš ; Dědič, Václav (advisor)
Silicon carbide (SiC) is a wide band gap semiconductor with high thermal, chemical and mechanical resistance. By thermal decomposition of silicon from SiC, it is possible to grow high quality epitaxial graphene on the surface. This production can be industrially scaled. This work investigates whether it is possible to optically gate epitaxial graphene and what effect the surrounding environment has on the electrical stability of epitaxial graphene. The experimental part of the work consists of optimizing preparation of the sample contacts using electron beam lithography. Further investigates the influence of surrounding atmosphere on the electrical properties of graphene, measuring the dependence of photocurrent on optical power and spectral dependence of photoconductivity of graphene. The significant effect of adsorption of molecules from the environment on the electrical properties of graphene has been demonstrated.
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Study of electric fields in silicone carbide
Sanitrák, Jakub ; Dědič, Václav (advisor) ; Trojánek, František (referee)
Silicon carbide (SiC) is due to its unique electrical and mechanical properties very important material for photonics and electronics. Hexagonal polytypes of SiC are being used as substrates for the growth of epitaxial graphene. SiC shows Pockels linear electro- optic effect. The goal of this thesis was to use Pockels effect and evolve the method to study profiles of inner eletric fields and accumulations of electric charges inside SiC, whose variations may influence functions of optoelectronic components. Our method con- sists in complex evaluation of optical transmittances of crystals inserted between crossed polarizers. It has been shown that the excitation light with photon energy near the ban- dgap of SiC significantly influences profiles of inner electric field due to accumulation of photogenerated charge on traps. The conclusion of the thesis concentrates mainly on the discussion of causes affecting the accuracy of this new method. 1
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Optical gating of epitaxial graphene
Fridrišek, Tomáš ; Dědič, Václav (advisor) ; Nádvorník, Lukáš (referee)
Silicon carbide (SiC) is a wide band gap semiconductor with high thermal, chemical and mechanical resistance. By thermal decomposition of silicon from SiC, it is possible to grow high quality epitaxial graphene on the surface. This production can be industrially scaled. This work investigates whether it is possible to optically gate epitaxial graphene and what effect the surrounding environment has on the electrical stability of epitaxial graphene. The experimental part of the work consists of optimizing preparation of the sample contacts using electron beam lithography. Further investigates the influence of surrounding atmosphere on the electrical properties of graphene, measuring the dependence of photocurrent on optical power and spectral dependence of photoconductivity of graphene. The significant effect of adsorption of molecules from the environment on the electrical properties of graphene has been demonstrated.
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Study of spin dynamics in magnetic metals using terahertz and optical spectroscopy
Jechumtál, Jiří ; Nádvorník, Lukáš (advisor) ; Dědič, Václav (referee)
Understanding of the ultrafast spin dynamics is essential for application and further development of new spintronic devices. The ultrafast spintronics is a promising path towards faster and less energetically demanding technologies comparing to current electronics. Recent studies have reported on a novel possi- bility of investigation of ultrafast out-of-plane spin transport using the technique of spintronic generation of terahertz (THz) pulses. This work focuses on deter- mining the nature of out-of-plane spin transfer in spintronic THz emitters by direct experimental quantification of the spin current relaxation length and the speed of spin propagation. Our results suggest that the ultrafast spin currents are of the ballistic nature and the spin current relaxation time is limited by the scaterring time and the relaxation length by the mean free path. Another recentely published method, also used in this work, is the the contactless THz measurement of anisotropic magnetoresistance (AMR). In a thin layer of cobalt- iron, we demonstrate that the AMR, a central phenomenon commonly used in the antiferromagnetic spintronics to read the state of magnetic order, can be investigated by the THz spectroscopy even if dominated by the crystalline term. We complement the study by a spectral analysis of the crystalline...
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Photoelectric transport in high resistivity CdTe for gamma ray detectors
Dědič, Václav ; Franc, Jan (advisor) ; Šikula, Josef (referee)
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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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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Measurement of the electric field in semiconductor detector with non-trivial contact geometry
Fridrišek, Tomáš ; Dědič, Václav (advisor) ; Trojánek, František (referee)
The main aim of this bachelor thesis was to investigate whether it is possible to use a new method based on the electro-optical Pockels effect, to study the internal electric field in the coplanar grid detector. The Pocklels coefficient for CdZnTe was determined by measuring the detector with trivial contact geometry. Subsequently, the found Pockels coefficient was used for evaluation of 2D vector field for samples with non-trivial contact geometry. This method was successful to evaluate the electric field in almost entire volume in coplanar grid detectors.
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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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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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