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Tunable materials and structures for terahertz spectral range
Skoromets, Volodymyr ; Kužel, Petr (advisor) ; Hlídek, Pavel (referee) ; Pashkin, Alexei (referee)
This thesis is devoted to an experimental study of dielectric properties of incipient fer- roelectrics. The terahertz time-domain spectroscopy was used to investigate the complex permittivity spectra in both single crystals and various strained thin film structures versus temperature and applied electric bias. Namely, it allowed characterizing the ferroelectric soft-mode dynamics and its coupling to a central mode. An electric-field tunability of bulk single crystals of SrTiO3 was determined up to room temperature for the first time. The phenomenon is governed by soft-mode stiffening under applied field. As an application we proposed and characterized a tunable one-dimensional photonic structure with a thin SrTiO3 plate inserted as a defect layer. The importance of the soft-mode dynamics was stressed also in the study of bulk high density KTaO3 ceramics. A systematic study was performed of a KTaO3 thin film and especially of a set of strained multilayers consisting of SrTiO3/DyScO3 bilayers grown on DyScO3 substrate. Strain-induced ferroelectric transition was observed in these films governed by the soft mode coupled to a lower-frequency relaxation. A general model was developed describing the whole family of the studied samples. Effect of the composition stoichiometry of SrTiO3 films grown on DyScO3 was...
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Photoluminescence of CdTe crystals
Procházka, Jan ; Hlídek, Pavel (advisor) ; Toušek, Jiří (referee) ; Oswald, Jiří (referee)
Title: Photoluminescence of CdTe crystals Author: Jan Procházka Department: Institute of Physics of Charles University in Prague Supervisor: Doc. RNDr. Pavel Hlídek, CSc. Abstract: Energy levels connected with defects in nominally undoped crystals CdTe, indium- doped crystals and chlorine-doped crystals were studied using low-temperature photoluminescence. The crystals are intended for X- and gamma- ray detectors operated at room temperature. An effect of annealing in cadmium or tellurium vapor on luminescence spectra was investigated. Some changes were interpreted by filling of vacancies not only by atoms coming from gaseous phase but also by impurities from defects like interstitials, precipitates, inclusions, grain boundaries etc. The luminescence bands assigned to defects important for compensation mechanism were examined, namely A-centers (complexes of vacancy in cadmium sublattice and impurity shallow donor) and complexes of two donors bound to a vacancy. It was shown, that temperature dependence of the luminescence bands results from more complicated processes than a simple thermal escape of bound excitons or thermal excitation of electrons (holes) from defects to bands. We observed expressive "selective pair luminescence" bands (SPL) on partially compensated In-doped samples during sub-gap...
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Spin and Lattice Excitations in Multiferroics
Skiadopoulou, Styliani ; Kamba, Stanislav (advisor) ; Bordács, Sándor (referee) ; Hlídek, Pavel (referee)
The scientific significance of dynamical magnetoelectric coupling, combined with the eminent implication of its technological applications, prompted our investigation of spin and lattice excitations of a series of widely known, as well as entirely novel, multiferroic materials. The magnetoelectric excitations of the celebrated room temperature multiferroic BiFeO3 were investigated. Among the previously reported, two new infrared active spin excitations were discovered. Polar phonons in BiFeO3 ceramics, as well as in a thin film epitaxially grown on TbScO3 substrate, were investigated in a broad temperature range. The novel double perovskite Pb2MnTeO6 was prepared and described for the first time. Its crystal and magnetic structure classify it in the rare family of antipolar antiferromagnets. The spin-induced colossal magnetoelectric effect in Ni3TeO6 stimulated us to discover electromagnons in this material and to prepare and study Mn and Co doped samples. All Ni3-xBxTeO6 (B=Mn, Co) compounds present the non-centrosymmetric R3 space group symmetry, collinear antiferromagnetic spin structure, and spin-induced electric polarization below TN. . Mn-doping results in increase of the Néel temperature, whereas Co-doping in decrease of the magnetic field value of the spin-flop transition, at which the...
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Silicon solar cells: methods for experimental study and evaluation of material parameters in advanced structures
Holovský, Jakub ; Vaněček, Milan (advisor) ; Sládek, Petr (referee) ; Hlídek, Pavel (referee)
This work concerns with today's challenges of photoelectrical characterization methods in the research and development of thin film silicon solar cells. Relevant results are obtained only when photocurrent spectroscopy and measurement of current-voltage characteristics, are applied on the real structures that can however be multi-layered, multi-junction devices with nanostructured interfaces. Analytical and numerical optical models comprising light scattering are used for analysis of light absorption and for evaluation of optical absorption coefficient of silicon layers in sub-gap region. The slope of absorption edge and residual absorption in mid-gap indicate material disorder and defect density. Based on the investigation of electrical interaction between sub-cells in the dual-junction solar cell we developed new methods of evaluation of photocurrent spectra and current-voltage characteristics individually for each sub-cell with no need to contact them directly. Usability of Fourier Transform Photocurrent Spectroscopy as a robust method for photocurrent spectroscopy of amorphous silicon is thoroughly analyzed here. The issues of frequency dependence are addressed in detail and comparison with photothermal deflection spectroscopy is made.
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Photovoltaic silicon solar cells: study of materials and solar structures by the method of Fourier photoconductive spectroscopy
Holovský, Jakub ; Toušek, Jiří (advisor) ; Hlídek, Pavel (referee)
The use of Fourier transform infrared spectrometer for the measurement of Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently reported and it is basis of this thesis. The main task of the thesis is extension of the FTPS to Surface Photovoltage (SPV) method. This method gives an unique possibility to evaluation of effective diffusion length L and effective surface recombination velocity (SRV) for thin or non-symmetrical semiconductor structures that can't be measured by standard techniques. These parameters play key role in good performance of solar cells absorbers. The experiments were carried out on c-Si material with surface passivated by silicon nitride. Sample is illuminated from the passivated side and the SPV signal is detected between the back capacitive contact and non-illuminated ohmic contact. One-dimensional continuity equation is solved to fit the measured spectrum. We have to ensure linearity of measured signal versus incident light intensity and account for frequency dependence. One measurement takes 30 seconds - that enables arbitrary variations. The influence of surface barrier height on SRV was assumed and additional constant illumination (light bias) was used to investigate its behavior and new relations were found. Beside SPV new elegant technique for interference...
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Spin and Lattice Excitations in Multiferroics
Skiadopoulou, Styliani ; Kamba, Stanislav (advisor) ; Bordács, Sándor (referee) ; Hlídek, Pavel (referee)
The scientific significance of dynamical magnetoelectric coupling, combined with the eminent implication of its technological applications, prompted our investigation of spin and lattice excitations of a series of widely known, as well as entirely novel, multiferroic materials. The magnetoelectric excitations of the celebrated room temperature multiferroic BiFeO3 were investigated. Among the previously reported, two new infrared active spin excitations were discovered. Polar phonons in BiFeO3 ceramics, as well as in a thin film epitaxially grown on TbScO3 substrate, were investigated in a broad temperature range. The novel double perovskite Pb2MnTeO6 was prepared and described for the first time. Its crystal and magnetic structure classify it in the rare family of antipolar antiferromagnets. The spin-induced colossal magnetoelectric effect in Ni3TeO6 stimulated us to discover electromagnons in this material and to prepare and study Mn and Co doped samples. All Ni3-xBxTeO6 (B=Mn, Co) compounds present the non-centrosymmetric R3 space group symmetry, collinear antiferromagnetic spin structure, and spin-induced electric polarization below TN. . Mn-doping results in increase of the Néel temperature, whereas Co-doping in decrease of the magnetic field value of the spin-flop transition, at which the...
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Photoluminescence of CdTe crystals
Procházka, Jan ; Hlídek, Pavel (advisor) ; Toušek, Jiří (referee) ; Oswald, Jiří (referee)
Title: Photoluminescence of CdTe crystals Author: Jan Procházka Department: Institute of Physics of Charles University in Prague Supervisor: Doc. RNDr. Pavel Hlídek, CSc. Abstract: Energy levels connected with defects in nominally undoped crystals CdTe, indium- doped crystals and chlorine-doped crystals were studied using low-temperature photoluminescence. The crystals are intended for X- and gamma- ray detectors operated at room temperature. An effect of annealing in cadmium or tellurium vapor on luminescence spectra was investigated. Some changes were interpreted by filling of vacancies not only by atoms coming from gaseous phase but also by impurities from defects like interstitials, precipitates, inclusions, grain boundaries etc. The luminescence bands assigned to defects important for compensation mechanism were examined, namely A-centers (complexes of vacancy in cadmium sublattice and impurity shallow donor) and complexes of two donors bound to a vacancy. It was shown, that temperature dependence of the luminescence bands results from more complicated processes than a simple thermal escape of bound excitons or thermal excitation of electrons (holes) from defects to bands. We observed expressive "selective pair luminescence" bands (SPL) on partially compensated In-doped samples during sub-gap...
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Tunable materials and structures for terahertz spectral range
Skoromets, Volodymyr ; Kužel, Petr (advisor) ; Hlídek, Pavel (referee) ; Pashkin, Alexei (referee)
This thesis is devoted to an experimental study of dielectric properties of incipient fer- roelectrics. The terahertz time-domain spectroscopy was used to investigate the complex permittivity spectra in both single crystals and various strained thin film structures versus temperature and applied electric bias. Namely, it allowed characterizing the ferroelectric soft-mode dynamics and its coupling to a central mode. An electric-field tunability of bulk single crystals of SrTiO3 was determined up to room temperature for the first time. The phenomenon is governed by soft-mode stiffening under applied field. As an application we proposed and characterized a tunable one-dimensional photonic structure with a thin SrTiO3 plate inserted as a defect layer. The importance of the soft-mode dynamics was stressed also in the study of bulk high density KTaO3 ceramics. A systematic study was performed of a KTaO3 thin film and especially of a set of strained multilayers consisting of SrTiO3/DyScO3 bilayers grown on DyScO3 substrate. Strain-induced ferroelectric transition was observed in these films governed by the soft mode coupled to a lower-frequency relaxation. A general model was developed describing the whole family of the studied samples. Effect of the composition stoichiometry of SrTiO3 films grown on DyScO3 was...
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