Dizertační (doktorské, Ph.D.) obhájené na ústavu od roku 1993

Abstract. We address three topics: (i) phonon anomalies and superconducting correlations in the copper-oxygen ladder compound Sr2Ca12Cu24O41 (SCCO), (ii) interlayer optical conductivity in the spin polaron model, and (iii) superconductor--insulator transition in Josephson junction chains. The chapter on the first topic is meant to supplement our published article (appended) where we analyzed the temperature dependence of the optical conductivity spectra of SCCO. Then, by comparing the spectra to those of YBa2Cu3O6+x (YBCO), we found striking similarities between a-axis (electric field polarized along the rungs of the ladders) response in SCCO and c-axis (perpendicular to the CuO2 plane) of YBCO, and between c-axis (along the legs of the ladders) of SCCO and a-axis (along the Cu-O bonds in the CuO2 planes) of YBCO. These findings support the scenario of charge carriers in the CuO2 planes segregating into ladder-like stripes. The chapter starts with an introduction to the electronic and optical properties of SCCO. Then, our phenomenological model, that describes the anomalous doping- and temperature-dependence of the phonon features in the a-axis response, is presented. In the experimental data, the lineshapes of several phonon modes change drastically with decreasing temperature and several new phonon modes appear. In our model, this is due to a gap developing in the electronic response and the formation of a (hole pair) crystal or density wave whose excitations couple to optically inactive phonons. The chapter concludes with numerical calculations of singlet correlations and off-diagonal spectral functions in the t-J ladders. In (ii), we report on the results of our study of the c-axis (interplane) optical conductivity of a pair of weakly coupled copper oxygen planes with a single doped hole. The planes are described using the t-J model and approximations of the well known spin polaron approach are employed. The polaron problem is solved using a simple variational technique on the subspace, where the hole is dressed by a single spin excitation. The calculated dispersion relation is roughly consistent with that of the self-consistent Born approximation. The variational ansatz allows us to analyze the optical transitions, evaluate the corresponding matrix elements, and estimate the temperature dependence of the conductivity. Under certain conditions, the conductivity displays a pseudogap opening with decreasing temperature, associated with a spectral weight shift from low energies to a band around 2J. Our results provide a qualitative explanation for the trends in the data of underdoped cuprates. In (iii), we provide additional background for our study (appended) of the superconductor--insulator quantum phase transition in a dissipationless and disorder-free Josephson junction chain. There, we employed a path integral Monte Carlo algorithm and renormalization group theory to map the zero-temperature phase diagram. In the chapter, we first motivate the study of Josephson junction arrays. Then, we describe the Hamiltonian and the Monte Carlo algorithm. Further, the relation of the superconductor-insulator transition to those in other systems and the Berezinskii-Kosterlitz-Thouless universality class, to which the transition belongs, are discussed, together with the renormalization group theory.

Jan Dvořák

Abstract. This dissertation thesis addresses the topic of SPR (Surface Plasmon Resonance), the detection technique mainly used for the detection of thin (biological) layers and for a measurement of kinetic and binding properties of biological substances. The surface plasmon resonance detector was built and extended for the possibility of the relative phase measurements using total internal reflection ellipsometry (TIRE). In the first chapter of the presented work we describe the physical principles which are needed to understand the function of these experimental methods. In the second part we deal extensively with the optical properties of metals, especially gold. Chapter explaining the principles of SPR, ellipsometry and TIRE follows. Relatively comprehensive chapter is devoted to the experimental setup and detail explanation of the experiments. The last chapter presents several concrete experiments to illustrate usefulness of proposed SPR-TIRE data processing methods.

Jakub Rozbořil

Abstract. Two different classes of functional molecules are investigated in the thesis: organic semiconductors and single molecule magnets. Thin films of organic semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene (TES-ADT) are deposited by spin-coating. Structural changes of the films during ageing at room temperature and during thermal annealing are investigated by X-ray scattering techniques and optical spectroscopy. Morphology of the films is investigated by means of stylus profilometry and polarization optical microscopy. Conditions for formation of five different polymorphs of TES-ADT are described. The lattice parameters and their thermal evolution for three of the polymorphs are reported. Most surprisingly, a thickness limited phase transformation was observed for the TES-ADT films. The unusual behavior of the films is rationalized by anisotropic surface energies of the crystallites and by a two-step nucleation process. The second part of the thesis deals with deposition and characterization of ultra-thin films of terbium and dysprosium bis-phthalocyanines which belong to the class of single molecule magnets. The molecular films were deposited by Langmuir--Blodgett technique. Goal of this part of the thesis was to produce homogeneous films exhibiting well defined molecular orientation with respect to the sample surface normal. Comparing various deposition strategies, the best quality of multilayer films was obtained when the investigated molecules were codeposited with stearic acid. Structural properties of the films were characterized by \mbox{X-ray} reflectivity and atomic force microscopy. Magnetization and orientation of the molecules in the thin films are investigated by magnetic circular dichroism measurements. An exceptional sensitivity of the technique was demonstrated by detecting a signal from a molecular monolayer.

Pavel Friš

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.

Dominique Geffroy

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.

Jiří Vašátko

Abstract. In this thesis we address three problems from the field of electronic structure of the high temperature cuprate superconductors. First, the influence of Zn and Ni impurities on the normal-state pseudogap of underdoped high-Tc cuprate superconductors is studied using exact diagonalization of effective t-J-like Hamiltonians describing low energy electronic excitations of the CuO(2) plane with some of the copper ions replaced with Zn/Ni. Our results clarify the trends observed in the c-axis infrared conductivity data of Zn and Ni substituted (Sm,Nd)Ba(2)Cu(3)O(7-x) crystals and support the point of view that the pseudogap is caused by strong spin correlations. Second, we address the formulas frequently used to describe the c-axis infrared response of the coupled electron-phonon system of bilayer cuprate superconductors. They have been originally obtained at the level of the phenomenological multilayer model and have been providing important insights into the physics of the cuprates. Here we derive them using diagrammatic perturbation theory. The third part of this work is focused on magnetic effects at the interface between superconducting YBa(2)Cu(3)O(7-x) and ferromagnetic La(2/3)Ca(1/3)MnO(3). It is known from earlier studies that the interfacial Cu ions acquire a ferromagnetic moment antiparallel to the one of Mn. This has been explained in terms of an antiferromagnetic exchange interaction between the Cu ions and Mn ions. Here we show that the Cu moments can alternatively be explained as a result of the so-called orbital reconstruction of the Cu ions.

Petr Klenovský

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 [110] 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.

Přemysl Maršík

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.

Jan Krčmář

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 α_i,f are close to the critical angle α_c of total external reflection. If the angle of incidence α_i of the primary x-ray wave onto a periodic multilayer is close to α_c 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 α_i or α_f so that from the measured dependence of the diffracted intensity on α_i,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 α_i 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.

Jiří Chaloupka

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.

Richard Štoudek

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 SiO_x 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 SiO_x precipitates.

Vlastimil Křápek

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.

Ondřej Caha

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.

Pavel Klang

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.

Jiří Novák

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.

Adam Dubroka

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.

Petr Cásek

Abstract. to be added
The whole abstract is available as pdf file. ...to be added...

Radek Pavelka

Abstract. to be added

Martin Friák

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 C11_b 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.

Mojmír Meduňa

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 620^oC. The azimuthal miscut direction was close to [010]. 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 [100]. 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 Si_0.55Ge_0.45/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 [100] and [110]. 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 [110] and by rectangular one by sample miscut parallel to [100]. The good coincidence of x-ray results with atomic force microscopy confirms very good homogeneity of the samples in lateral and vertical directions.

Jan Grim

Jan Šik

Petr Mikulík

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.

Dominik Munzar

Jaroslav Hora

František Vižďa

Zdeněk Bochníček

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 °C 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 °C. 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 °C 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 °C and viscosity depends strongly on thermal history.

Hardware and software of the computerization are described in appendixes.

Petr Pánek