PhD theses defended at the Department since 1993
- Pavel Friš. Construction of a far-infrared ellipsometer and an experimental study of optical properties of ferromagnetic cobaltites. April 2018.
- Dominique Geffroy. Theoretical study of electronic properties of high-Tc superconductors and other materials with strongly correlated electrons. June 2017.
- Petr Klenovský. Optical response of low-dimensional structures. October 2013.
- Přemysl Maršík. Advanced ellipsometric techniques and studies of low-k dielectric films. October 2009.
- Jan Krčmář. Standing-wave grazing-incidence x-ray diffraction from polycrystalline multilayers. June 2009.
- Jiří Chaloupka. Microscopic gauge-invariant theory of the c-axis infrared response of bilayer HTCS. February 2009.
- Richard Štoudek. Optical characterization of defects in silicon. September 2008.
- Vlastimil Křápek. Excitonic structure of absorption edge in quantum dots. March 2008.
- Ondřej Caha. Lateral compositional modulation in InAs/AlAs superlattices. April 2007.
- Pavel Klang. X-ray study of structural defects in silicon monocrystals. December 2006.
- Jiří Novák. Structural investigations of nano-islands using X-ray diffraction techniques. August 2006.
- Adam Dubroka. Optical properties of high-temperature superconductors and manganites. July 2006.
- Petr Cásek. Electronic structure of transition metal oxides. January 2004.
- Radek Pavelka. Defects effect to optical properties of thin films. January 2003.
- Martin Friák. Electronic structure, phase transformations and stability of phases. September 2002.
- Mojmír Meduňa. X-ray reflectivity on laterally structured multilayers. September 2002.
- Jan Grim. Diffuse X-ray scattering by relaxed epitaxial layers. January 2001.
- Jan Šik. Spectroscopic ellipsometry of layered structures. July 2000.
- Petr Mikulík. X-ray reflectivity from planar and structured multilayers. March 1997.
- Jaroslav Hora. Optická spektroskopie objemových materiálů a vrstevnatých struktur. 1996.
- František Vižďa. Reflectance and transmittance of thin films with rough boundaries. February 1996.
- Dominik Munzar. Electronic structure of semiconductor superlattices. September 1994.
- Zdeněk Bochníček. Multicrystal high temperature x-ray diffractometry. May 1993.
- Petr Pánek. Optical spectroscopy of semiconductor heterostructures. 199?.
Defended in April 2018. Supervisor: Adam Dubroka. Language: English.
Full text (pdf) is available here.
Abstract. This dissertation thesis addresses two topics: the construction and performance of a vacuum variable-angle far-infrared ellipsometer and the optical properties of ferromagnetic cobaltites. First, the physical background required for both topics is described. Next, we describe the construction of the ellipsometer and its major components. The ellipsometer is based on a Fourier transform spectrometer, an in-house built chamber with a computer controlled theta–2theta goniometer and a closed-cycle bolometer. The ellipsometer operates in the rotating analyzer mode between 50–680 cm−1. We identified and corrected artifacts like the spectrometer beam inhomogeneity, and the finite reflectivity of mirrors. The performance of the ellipsometer was tested on a SrTiO3 crystal and the results agree very well with those obtained on a commercial mid-infrared ellipsometer in the region of overlap. The ellipsometer can be supplemented with a He closed-cycle cryostat that enables measurements between 5–400 K. The thesis also includes a description of the software which controls the ellipsometer and processes the measured data. In the second part, we present results of ellipsometric measurements of the optical response of ferromagnetic cobaltite La0.7Sr0.3CoO3 between 7–300 K. The cobaltite thin films were measured using three ellipsometers: Woollam V-VASE, Woollam IR-VASE, and the in-house built far-infrared ellipsometer that yielded data in a broad energy range between 0.01–6.5 eV. The ellipsometry data were complemented by x-ray diffraction, magnetic and transport measurements. This comprehensive set of data shows that below the Curie temperature a spectral weight is transferred from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. This transfer clearly shows that the driving force for the ferromagnetism in hole doped cobaltites is a saving of the effective kinetic energy which is a key aspect of the so-called double exchange mechanism. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by t2g orbitals.
Defended in June 2017. Supervisor: Dominik Munzar. Language: English.
Full text (pdf) is available here.
Abstract. In the first part of the thesis (chapters 1 to 7), we briefly introduce the topics of our research and we summarize some important tools used in studies of strongly correlated electron systems. Next (in chapters 8 and 9) we report on our main results concerning the electronic structure of the high-T c cuprate superconductors, and the excitonic condensation in the Hubbard model, respectively. The electronic dispersion of the high-T c cuprate superconductors has been investigated using the fully self-consistent version of the phenomenological model, where charge quasi- particles are coupled to spin fluctuations. The inputs we use —the underlying (bare) band structure and the spin susceptibility χ— are extracted from fits of angle resolved pho- toemission and inelastic neutron scattering data of underdoped YBa 2 Cu 3 O 6.6 reported by T. Dahm and coworkers [T. Dahm et al., Nat. Phys. 5, 217 (2009)]. Our main results are: (i) We have confirmed the finding by T. Dahm and coworkers that the main nodal kink is, for the present values of the input parameters, determined by the upper branch of the hour-glass of χ. We demonstrate that the properties of the kink depend qualitatively on the strength of the charge-spin coupling. (ii) The effect of the resonance mode of χ on the electronic dispersion strongly depends on its kurtosis in the quasimomentum space. A low (high) kurtosis implies a negligible (considerable) effect of the mode on the dispersion in the near-nodal region. (iii) The energy of the kink decreases as a function of the angle θ between the Fermi surface cut and the nodal direction, in qualitative agreement with recent experimental observations. We clarify the trend and make a specific prediction concerning the angular dependence of the kink energy in underdoped YBa 2 Cu 3 O 6.6 . We have discovered a mechanism that leads to the appearance of a k-space spin texture in the non-degenerate two-band Hubbard model, due to a spontaneous symmetry breaking driven by electronic correlations. Using dynamical mean-field theory, we show that doping a spin-triplet excitonic insulator provides a means of creating new thermodynamic phases with unique properties. The numerical results are interpreted using analytic calculations within a generalized double-exchange framework. Spin textures in k-space are of particular interest because they find numerous applications in spintronics. Our result represents a significant step forward, insofar as it shows that such patterns can be generated even in the absence of spin-orbit coupling.
Abstract. This thesis deals with theoretical and experimental investigations of type-II quantum dots (QDs). We have studied dots fabricated from two material systems, (i) InAs/GaAs QDs capped by GaAsSb layer, and (ii) SiGe/Si QDs. We have calculated the electronic and excitonic structure of these systems. For the former we have used the envelope function approach based on 8-band k.p theory, the latter has been done using the configuration interaction method. These QDs were experimentally investigated by the photoluminescence spectroscopy. For experimental investigation of these QDs we have used the photoluminescence spectroscopy. The InAs QDs capped by GaAsSb exhibit, with increasing Sb content, a type-I to type-II transition, i.e., transition from a state when holes are confined in the dot volume to a state where they are located in the ternary layer. This transition is known to be associated with a large redshift of the emission from the dots. We have studied the type-I to type-II transition for these dots, simulated the redshift of emission and explained its origin. Furthermore, we have found that the position of the hole wavefunction depends on the degree of uniformity of Sb distribution in the layer, on the size and aspect ratio of the QD. In either case, if the hole wavefunction is located close to the base of the QD or above it, it is composed of two segments due to the effect of piezoelectric potential. These segments are oriented along  and [1-10] crystallographic directions if the hole is close to the base of the dot and above it, respectively. In order to confirm our findings, we have proposed two experiments that might elucidate the vertical position of the holes in real structures. Furthermore, the hole wavefunction was found to form quantum-molecule-like states in type-II GaAsSb capped InAs QDs that might be used as a quantum gate device. We have also calculated some of the properties of these quantum molecules. Finally, we have provided an alternative explanation of the experimentally observed blueshift of the photoluminescence spectra of GaAsSb capped InAs QDs based on the strongly blueshifted emission of biexcitons. The fairly large blueshift of the emission of biexcitons seems to be, in our understanding, an intrinsic property of type-II structures. We have further tried to test this hypothesis by measurements of the excitation power dependence of the photoluminescence spectra of SiGe QDs grown on prepatterned Si(001) substrates, a different type-II system exhibiting a similar blueshift with pumping. By a thorough analysis of the spectra by fitting them with Gauss-Lorentz profiles, we arrived at a conclusion that rather than a blueshift of the whole spectra, additional bands appear at their high energy ends as the excitation intensity increases. These new bands exhibit a superlinear, almost quadratic, dependence of their oscillator strength on pumping intensity. Our calculations of the excitonic structure we have performed for this system suggest that these bands correspond to the recombination of strongly blueshifted biexcitons. We belive that this is the first observation of the recombination of multi-excitonic states in SiGe/Si QDs.
Defended in October 2009. Supervisor: Josef Humlíček. Language: English.
Full text (pdf) is available here.
Abstract. Thesis covers two related topics: ellipsometric methodology and studies of dielectric thin films. The work included reconstruction on an ellipsometric instrument and development of the necessary data acquisition and analysis software. In the first part, the necessary theoretical background is reviewed and the basic ellipsometric configurations are described. We focus on two ellipsometric configurations: the rotating analyzer (RAE) and the rotating compensator (RCE). In the case of RAE, a calibration process for unknown analyzer and polarizer offset is developed and implemented. In the case of RCE, we formulated elegant equations for data reduction and proposed algorithm for polarizer, analyzer and compensator offset calibration. We detected spurious artifacts in the actual data, related to a compensator imperfection. A possible source of the artifacts is discussed and the interpretation is validated by simulations. Finally, the impact of the artifacts on the calibration and data reduction procedure is examined and a possible treatment of the data is proposed. The second part of the work is focused on the ellipsometric determination of the optical properties of low dielectric constant (low-k) films (for application as inter-metal dielectric in microelectronics) and their behavior under various conditions typical for the technology. We studied porous SiCOH materials deposited by plasma-enhanced chemical vapor deposition. The porosity is generated by co-deposition of the SiCOH matrix material with sacrificial organic phase (porogen). After the deposition, the porogen is removed by UV- assisted annealing (UV-cure), creating the pores. The low-k materials are exposed to various plasmas in the microprocessor fabrication technology. All these technological steps have effect on the optical properties of the films. We interpreted the observed behavior in terms of changes of composition and supported our conclusions by additional measurements: infrared absorption and ellipsometric porosimetry.
Defended in June 2009. Supervisor: Václav Holý. Language: Czech.
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Abstract. In this dissertation we present a new method, which uses the concept of x-ray standing wave in the grazing-incidence geometry, in which both the incidence and exit angles αi,f are close to the critical angle αc of total external reflection. If the angle of incidence αi of the primary x-ray wave onto a periodic multilayer is close to αc or to a "Bragg-like" maximum on the reflectivity curve, the interference of the transmitted and reflected waves creates a standing wave pattern in the multilayer volume, the period of which equals the multilayer period. If the multilayer contains crystalline grains (crystallites), the intensity of diffraction from these crystallites depends on the mutual position of the crystallites and the antinodes of the standing wave. Similarly, the wave diffracted by the crystallites is reflected from the multilayer interfaces, which results in a standing wave pattern as well. The standing wave pattern is shifted by changing αi or αf so that from the measured dependence of the diffracted intensity on αi,f it is possible to determine the position of the diffracting crystallites. Moreover, measuring the dependences of the diffracted intensity on the in-plane scattering angle 2θ at various αi it is possible to determine the lateral sizes of the crystallites in different depths in the multilayer. The theoretical description of the scattering process is based on the Distorted Wave Born approximation. In this work we prove the feasibility of this concept by measuring the standing-wave effects in C/Ni3N multilayers using synchrotron radiation, and we were able to determine the sizes of the crystallites in different depths in the multilayer and the thicknesses of amorphous Ni3N transition layers at the C/Ni3N interfaces. We demonstrate that this method can be carried out also at a modified x-ray diffractometer, and it is capable of studying the structure of crystalline layers in a periodic multilayer at a laboratory. We have used this method for the investigation of the structure of Nb/Si multilayers, and we obtained the thicknesses of amorphous and crystalline parts of the Nb layers. These thicknesses agree very well with the transmission electron microscopy observations. Presented method is very suitable for a detailed study of sizes and positions of crystallites in periodic multilayers, as well as for the determination of the depth profile of the crystallite size.
Defended in February 2009. Supervisor: Dominik Munzar. Language: English.
Full text (pdf) is available here.
Abstract. The thesis deals with a theoretical description of the far-infrared optical response of the high-temperature cuprate superconductors. The emphasis is put on the gauge-invariance of the theory, which is intimately related to the charge continuity. Both the in-plane optical conductivity as well as the c-axis response are studied. In the first case we study the role of the vertex corrections, which are omitted in most of the present calculations because of their complexity. The main objective in the latter case is the interpretation of the c-axis experimental spectra in terms of a properly designed microscopic model. We focus on the so-called bilayer cuprates with two closely spaced copper-oxygen planes in a unit cell. The key ingredients of the model are: (i) two bilayer-split bands renormalized by the coupling to spin fluctuations, (ii) local field effects due to charges accumulating at the copper-oxygen planes, (iii) vertex corrections ensuring the gauge-invariance of the theory, and (iv) a weak coupling between the bilayers. The optical response resulting from this model is studied in detail and an interpretation of two superconductivity-induced peaks in the experimental data of the real part of the c-axis conductivity is proposed. The peak located around 400/cm is attributed to a collective mode of the bilayer regions that is an analogue of the Bogolyubov-Anderson mode. The peak at ca 1000/cm is interpreted as a pair-breaking peak that is related to the coupling through the spacing layers separating the bilayers.
Defended in September 2008. Supervisor: Josef Humlíček. Language: Czech.
Full text (pdf) is available here.
Abstract. In this dissertation we deal with optical diagnostics of defects in silicon lattice.
Single-crystal Si still stays the most used material of semiconductor industry. Our goal was to use the infrared absorption to study point defects (interstitial oxygen, substitutional carbon, nitrogen) in the Czochralski and floating-zone silicon, and also other defects resulting from annealing in Czochralski silicon (thermodonors, oxygen precipitates).
The accuracy of determination of the concentration of both interstitial oxygen and substitutional carbon by measurement of transmittance of silicon wafers in the mid-infrared range is higher at the temperature of liquid nitrogen than at the room temperature. This can be used particularly for the floating-zone silicon, which usually contains very small concentrations of impurity atoms.
In the case of annealed samples of Czochralski silicon we could separate the contributions of interstitial oxygen and oxygen precipitates using low-temperature measurements. The absorption spectra of the SiOx precipitates were analysed using effective-medium models, allowing us to specify the shape, volume fraction and stoichiometry of inclusions. With the use of infrared absorption spectroscopy, we are able to identify spherical and disc-shaped precipitates of the stoichiometry close to SiO2; however, it is impossible to determine their dimensions and concentrations.
This method has been applied to three series of annealed samples of the Czochralski silicon, which was always slightly doped by boron. We have examined the influence of different parameters on the precipitation of the interstitial oxygen. These were parameters related to growth conditions (position of the sample in ingot, nitrogen doping), and also related to subsequent annealing operations (e.g., nucleation pre-annealing). The consistence of our analysis is confirmed by the fact that, for almost all annealed samples, the decrease of concentration of interstitial oxygen is in very good agreement with the amount of the oxygen fixed in the SiOx precipitates.
Defended in March 2008. Supervisor: Josef Humlíček. Language: English.
Full text (pdf) is available here.
Abstract. The goal of the thesis is twofold: (i) to develop a theoretical model describing electronic structure and optical properties of quantum dots, and (ii) to apply it to some interesting topics of the quantum dot physics.
Effective mass theory was chosen as the appropriate theoretical method for a description of quantum dots, which represents a reasonable trade-off between accuracy and completeness on the one hand, and simplicity, easy interpretation of results and short calculation time on the other hand. In the framework of this approach I have developed a method of approximate variable separation, which enables a numerically simple retrieval of wave functions and energies of bound states in flat quantum dots of arbitrary shape. For a set of important characteristic quantum dot shapes (cone, lens, cylinder), I further implemented a variational method of solution of the Schrödinger equation, which gives more accurate results at the price of slower calculations and lower generality. Both methods provide qualitatively correct results and, regarding limited knowledge of quantum dot structure, are suitable for the interpretation of experimental data. An implementation of eight band k ⋅ p theory developed at Technische Universität Berlin was used for comparison.
An important goal of quantum dot technology is to achieve long wavelength luminescence from InAs/GaAs quantum dots, namely communication wavelengths of 1.3 μm and 1.55 μm. Multilayer quantum dot systems may be used for this purpose. The red shift is caused by the wave function extension over the dots in the stack, and by the influence of the lower layer to the upper dot layer growth – the dots in upper layers are usually larger. Compaing experimental data with our model, we determine the proportion of these factors in contributing to the red shift (differentiating further between the increased lateral or vertical dimensions for the case of increased volume) for various values of the spacer layer thickness and number of layers in the system.
Long luminescence wavelengths can also be achieved by using ternary InGaAs strain-reducing capping layers. The red shift is due to a lower strain influencing the band energies, lower potential barrier on the capping layer side, and increased dot volume due to modified strain and chemical composition during the growth of the capping layer. Comparing experimental data with the model results, we again determine the importance of the factors mentioned above.
The last part of the thesis is dedicated to magneto-optical properties of quantum dots. We discuss the effect of structural properties on the magnetooptical properties and a possibility to determine the shape of the dots from measured magnetooptical spectra. We demonstrate a fair sensitivity of the spectra to the lateral elongation of the wave functions, which may be connected with lateral elongation of the quantum dots or with piezoelectric fields.
Defended in April 2007. Supervisor: Václav Holý. Language: English.
Full text (pdf) is available here.
Abstract. The thesis is devoted to a morphology of InAs/AlAs superlattices grown on InP (001) substrate. We have studied an evolution of a surface profile using numerical growth simulations and x-ray scattering experiments.
An elastic strain field in the sample plays substantial role in both numerical growth simulations and an analysis of x-ray experiments. We describe two methods based on a continuum elasticity theory and one atomistic methods for the strain calculation. Results obtained by various computational methods in the system of short-period superlattice are compared and a good agreement is found.
In the following, numerical simulations based on a continuum approach of a surface diffusion are presented. On systems of a single heteroepitaxial layer and superlattice we have studied effects of various nonlinear terms on a resulting superlattice morphology. We focused especially on a nonlinear conservative KPZ term and a wetting effect.It was found that, in contrast to KPZ term, a wetting effect plays a substantial role in the resulting lateral composition modulation.
Diffuse x-ray scattering experiments in high-angle x-ray diffraction and grazing-incidence geometries were used to study morphology of the superlattices.In a theoretical section we present an approximative method allowing a direct determination of lateral modulation parameters from a grazing-incidence diffraction experiments. Experiments were performed on two types of samples. The first sample has 100 superlattice periods; on this sample we studied the morphology of a laterally modulated superlattice. The second type was a series of samples grown under the same deposition condition, the samples vary with the number of superlattice periods. The results obtained on the series of samples were compared to the numerical simulations. We have found good quantitative agreement of the numerical simulations to the experimental data in a lateral modulation period as well as modulation amplitude.
Defended in December 2006. Supervisor: Václav Holý. Language: Czech.
Full text (pdf) is available here.
Abstract. In this dissertation I focus on the study of structural defects in silicon monocrystals. Silicon is the most common material used in the semiconductor industry. Due to the increasing requirements on the quality of this semiconducting material the study of the defects is gradually growing in importance. I focus on the characterization of defects in annealed wafers, which come from silicon monocrystals grown by Czochralski method. This method has a 95% share in the world production of monocrystalline silicon at present.
The main method used in the analysis of the samples is a high-resolution x-ray diffraction. We focused on the diffuse scattering in the defects close to the reciprocal lattice point in the symmetric diffraction.
It is possible to discern four basic types of diffuse scattering in measured reciprocal maps indicating the distribution of intensity diffracted on the crystal containing defects: without diffuse scattering, with circular symmetric and asymmetric one and the scattering concentrated into streaks in distinctive directions. In this dissertation we first made an analysis of these four types of maps and then we demonstrated which defects these maps correspond to and which parameters of the defects can be gained from the maps in question.
To interpret the measured data correctly it was necessary to complete the results of the theory of x-ray diffuse scattering with calculations on the displacement field in the locality of the defects based on the theory of elasticity. We used the Krivoglaz theory of x-ray scattering on crystals with defects and the calculation of the displacement field around defects based on results of Dederichs and Burgers to simulate x-ray diffuse scattering on point defects, their clusters, dislocation loops and stacking faults.
The dominant type of defects was determined by comparing the experiment with simulation. Maps without diffuse scattering correspond to a crystal without defects and circularly symmetric scattering correspond to the scattering on spherical defects which do not deform the surrounding matrix. In case of asymmetry of measured maps the surrounding of the defect is deformed and we can determine that the defect is either interstitial- or vacancy-type (stressing or expanding the lattice). We are also able to determine the defect strength (indicative of how much the defect deforms the lattice) and their concentration. In the case of the map with the streaks the defects were Frank's stacking faults. Their radius was determined from the width of the streaks.
We applied these results to the corresponding maps measured on the samples from three series we had analyzed. The aim of the analysis of the samples of the first series was to determine the influence of nitrogen doping on parameters of defects in silicon wafers. The second series was concerned with the inclusion of the nucleation preannealing as the starting procedure in the technological wafer processing and its influence on the behavior of the wafer. The third series focused on the effect of two-step nucleation annealing with the subsequent high temperature processing.
The results of this research enabled us to determine the type of defects in the crystal using the symmetry of the measured map of diffuse scattering. The determination of the other properties of the defects depends on the type of the experimental map. Thanks to this analysis we confirmed the favourable effects of nitrogen doping and nucleation preannealing on the defect formation in silicon wafers. These defects serve as traps for gettering metal contaminants in particular.
Abstract. The thesis deals with structural characterization of quantum dots and nano-islands by means of X-ray diffraction. Coplanar high-angle X-ray diffraction and experiments in grazing incidence diffraction geometry are applied for investigations of nano-islands grown in Stranski-Krastanow growth mode. In particular we dealt with the following systems: (i) InAs/GaAs (001) quantum-dots grown by molecular beam epitaxy and capped with GaAs, (ii) 11-stack multilayer of laterally ordered and unordered InAs/GaAs quantum-dots grown by molecular beam epitaxy, (iii) Ge/Si(001) nano-islands forming a three-dimensional island crystal, and (iv) In(Ga)As/ GaAs(001) islands grown by low-pressure metal-organic vapour-phase epitaxy. For the latter we studied effects of island capping with Ga(In)As, and effects of annealing. The results of X-ray analysis are correlated with results obtained by atomic force microscopy, transmission electron microscopy, photoluminescence, and band gap calculations. Furthermore, we discuss and apply new methods for analysis of X-ray diffraction data to obtain structural information on the studied nano-islands: (i) A generalization of the iso-strain scattering method for buried nano-islands is introduced. The method allows for the characterization of strain fields in buried islands and for obtaining the geometry (size and shape) of islands from X-ray data measured in grazing-incidence diffraction geometry without any model assumption on the island shape. (ii) A method for extracting structural information on laterally ordered and unordered nano-islands in cases when X-ray scattering signals from both classes of islands are mixed in the detected data. The procedure also allows for determination of the mean displacement of the ordered nano-islands from the two-dimensional lattice sites. (iii) The combination of an analytical solution of the equilibrium equations of linear elasticity with kinematical X-ray scattering theory is used for structural analysis of a three-dimensional island crystals.
Defended in July 2006. Supervisor: Josef Humlíček. Language: English.
Abstract. The thesis concerns three topics from the field of high-temperature superconductors and manganese oxides (manganites). First, we examine the infrared response of artificial epitaxial layered heterostructures composed of YBa2Cu3O7 and La0.7Ca0.3MnO3 at nanometer scale. We have found that the far infrared response of conducting and superconducting electrons in these heterostructures is weak and the spectra display an unexpectedly strong mid infrared band. This band exhibits signatures of metal-insulator transition and superconductivity-induced changes. The observations are in a qualitative agreement with results of model calculations of the response of a heterogeneous medium. The heterogeneity is probably caused by irregularities in the samples that do not strongly suppress superconductivity or metal-insulator transition but create the lateral dielectric contrast. In the second part, we use Raman and infrared spectroscopy to study a series of manganites La1-ySryMn1-xMxO3 in which the manganese ions have been partially substituted by other transition metal elements. Broad bands in the Raman spectra and the dc resistivity extracted from the infrared reflectivity suggest that electronic properties of these compounds are mainly influenced by the radius of the substituent. The Cu-substituted compounds exhibit an anomaly suggesting that in this case, additionally, electronic and/or magnetic properties of the substituent play an important role. The Raman spectra display an additional Ag-like mode which we attribute to the local breathing mode of oxygen atoms in a close vicinity of the substituent. In the last part, we study superconductivity-induced changes of the infrared response along the c-axis of multilayer superconductors. Our model calculations of the coupling between phonons and the superconducting condensate explain the spectral changes of the in-phase and out-of-phase vibrations of planar oxygens observed in the trilayer superconductors Tl-2223 and Bi-2223. We discuss also the infrared response of the normal and superconducting electrons and propose a picture which may account for the temperature development of the spectra.
Defended in January 2004. Supervisor: Dominik Munzar. Language: English.
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The whole abstract is available as pdf file. ...to be added...
Defended in January 2003. Supervisor: Ivan Ohlídal. Language: Czech.
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Defended in September 2002. Supervisor: V. Humlíček. Language: Czech.
Abstract. The present thesis is focused on the calculation of electronic structure, elastic properties and structural behavior of various magnetic phases of iron and two transition metal disilicides, MoSi2 and WSi2, with C11b structure. For all ab initio electronic structure calculations, the all-electron full potential linearized augmented plane wave method (FLAPW) within the framework of density functional theory (DFT) implemented in Wien97 package is used....
The whole abstract is available as pdf file.
Defended in September 2002. Supervisor: V. Holý. Language: Czech.
Abstract. The thesis is devoted to the structural investigation of interface morphology in Ge/Si and SiGe/Si superlattices. We focus on the self-assembled islands, dots and ripples formed at interfaces during the epitaxial growth.
Laterally structured multilayers grown by a self-assembling mechanism are of increasing significance in nanotechnologies. Although the fabrication of quantum dots and wires by means of lithographic techniques is quite often the method of choice, the self-assembled growth is a very promising method in strained heteroepitaxial systems. Especially for optoelectronical applications, it is important to avoid electrically active defects, which can be a problem in lithographically defined and etched structures. On the other hand, using self-assembled growth, it is a main challenge to produce structures of appropriate dimensions with very small fluctuations of their structural parameters. Information on their size, shape and correlation properties is very important for their subsequent development and growth.
Experimental methods such as x-ray scattering under grazing incidence, x-ray diffraction, and methods of scanning microscopy, especially atomic force microscopy, are used for the investigation of the structural properties of surfaces and interfaces with laterally ordered objects. These methods are independent, since they investigate the morphology of the surface (interface) by means of different physical interactions.
In the thesis, we present results from x-ray reflectivity measurements and their simulations performed in Ge/Si and SiGe/Si multilayers. The first half of the thesis is directed on the theoretical aspects of the scattering theory for x-ray reflectivity where methods of calculation and examples of some scattering problems are shown. In particular, we have shown a calculation procedure of the scattering at the interfaces with the roughness obtained as a superposition of an simple roughnesses. And next, we have presented a new method of computational comparison of the surface morphology determined from atomic force microscopy with the data obtained from x-ray reflectivity. These theoretical procedures are used in the results of this work.
In particular, we have studied the structural and orientational investigation of one-dimensional self-assembled patterns (ripples) occurring in Ge/Si superlattices. The samples were grown by solid source molecular beam epitaxy on vicinal substrates Si (001) at 620oC. The azimuthal miscut direction was close to . Using the method of non-specular x-ray reflectivity under grazing incidence and atomic force microscopy, we have found that the ripples are not oriented perpendicularly to the miscut but almost parallel to it, namely along . On the basis of a model of sequence of self-similar random ripples with hyperbolic cross-sections and with a fractal roughness superimposed, we have determined the structural parameters of the ripples from specular and non-specular x-ray reflection. The parameters and the orientation of the ripples are explained by a zigzag structure of the atomic steps formed on the surface due to its 2x1 reconstruction and a good coincidence was found between x-ray measurements, atomic force microscopy and the surface reconstruction.
The second part of the experimental work is focused on two periodic multilayers Si0.55Ge0.45/Si with different directions and values of the surface misorientation (miscut), using x-ray reflectivity under grazing incidence and atomic force microscopy. The samples were grown by molecular beam epitaxy on silicon substrates with (001) surface orientation and with miscut directions parallel to  and . X-ray reflectivity measurements in different azimuths are compared to the data from atomic force microscopy, which are used for simulations of the x-ray experiments. With this combination of different experimental techniques, we have determined the structural properties, in particular the ordering, of different objects (dots, holes, islands and terraces) at the sample surface and inside the multilayer at the SiGe/Si interfaces. The hexagonal ordering of the dots in lateral direction has been confirmed by sample miscut parallel to  and by rectangular one by sample miscut parallel to . The good coincidence of x-ray results with atomic force microscopy confirms very good homogeneity of the samples in lateral and vertical directions.
Defended in January 2001. Supervisor: V. Holý. Language: Czech.
Defended in July 2000. Supervisor: V. Humlíček. Language: Czech.
Defended in March 1997. Supervisors: V. Holý and T. Baumbach. Language: English.
Thesis "under double leadership" worked out at both Masaryk University and Université J. Fourier (Grenoble, France).
Please, visit the WWW page with the table of contents. In addition, abstract, introduction and conclusion of the thesis and introduction to each chapter are available in both English and French.
Abstract. The X-ray reflection from planar and structured multilayers is presented using different theoretical approaches. The scattering phenomena studied are the specular reflection from planar multilayers with various stacking sequences (single layer, periodic, quasiperiodic), the diffuse scattering from rough multilayers, and the scattering from surface gratings and from multilayer gratings. The theories employed for the calculation are: the kinematical theory, the distorted-wave Born approximation, the dynamical theory and various approximations of the dynamical theory (the single-reflection approximation, the two-beam approximation and the multiple-beam approximation), developed in one unified formalism. This unified formalism enables all these theories to be discussed and compared in a consistent and methodological way. Numerical calculations are applied to fit the experimental curves in order to reveal the structural parameters of miscellaneous types of layered samples.
Defended in September 1994. Supervisor: J. Humlíček. Language: Czech.
Defended in 1996. Supervisor: J. Humlíček. Language: Czech.
Defended in February 1996. Supervisor: I. Ohlídal. Language: Czech.
Defended in May 1993. Supervisor: V. Holý. Language: Czech.
Abstract. New high temperature multicrystal high resolution x-ray diffractometer has been designed and it was used for studying of thermomechanical properties of thin films deposited on single crystal substrates.
The goniometer has been tested in details and it provides a sufficient temperature and mechanical stability for high temperature experiments up to 850 deg in 99% inert gas atmospheric pressure ambient. The goniometer has been computerized using PC computer.
We have measured stress (from the curvature of the substrate), and strain (from the substrate — layer separation) in ZnTe layer. The new triple crystal method for measurement of thermal expansion of substrates has been proposed. This method eliminates mechanical instabilities of high temperature goniometer. All measurements have been performed in temperature range 20–350 deg. Using experimental data we have determined the elastic constants c11 and c12 and thermal expansion coefficient of the layer. It was shown that the layer fully relaxed during deposition, the room temperature stress was a result of thermal expansion mismatch.
The stress measurement in temperature range 20–850 deg was used for studying of densification and viscous flow in amorphous SiO2 layer. The thermomechanical properties of the layer have become thermal oxide — like after the first annealing in duration of six hours. The viscous flow appeared in temperatures above 750 deg and viscosity depends strongly on thermal history.
Hardware and software of the computerization are described in appendixes.
Defended in 199?. Supervisor: F. Lukeš. Language: Czech.