The study of surface and interface structure of thin film devices is becoming increasingly important in industrial applications today. In this thesis, the technique of grazing incidence x-ray scattering (GEXS) is developed and its validity examined for many different materials. In addition, the technique of Born wave analysis introduced by Lagally has been extended, showing that in certain cases surface roughness can be obtained without full simulations of the data. GEXS measurements performed on float glass revealed that the density of the tin and air sides were 2.7 ± 1 and 2.3 ± 1 g/cc respectively across the entire ribbon. For all samples studied a surface layer on the order of a few nanometers was in evidence. When similar measurements were made on container glass it was seen to be layer free. In addition, excellent agreement was found between the roughness deduced from the simulations and the Born wave analysis. In all samples, the surface roughness deduced from the diffuse scatter was less than that from the specular, indicating that a vertical density gradient was present. When x-ray scattering measurements were performed on CVD layers deposited on glass it was noted that two sets of Yoneda wings existed in the diffuse scatter. From these it was concluded that lateral density variations existed in the SnO(_2) layer which were on the order of 0.5mm in diameter. Furthermore, excellent specular simulations were obtained. These indicated that the control of both layer thickness and interface roughness between samples was on the Angstrom level. Specular and diffuse x-ray scattering measurements were performed on Cu/Co multilayers grown on sapphire using a Nb buffer. Here the effect of anomalous dispersion was employed in order to highlight the scattering from the Cu/Co interfaces. From these data Born wave analysis was used to characterise the roughness of the samples. It was found that little change took place for anneals of 45 minutes at temperatures up to 300 C. In addition, it would appear that the measured OMR bore no relationship to the component of uncorrelated roughness within the samples. GIXS measurements perfumed on Cu/Co multilayers grown on Si, using a Cu(_3)Si buffer, revealed an asymmetry in the diffuse scatter, which was attributed to terracing within the layers. Simulations were obtained to the data using a slight modification of the fractal model for several scattering vectors with a single set of parameters. From this, a model of step bunching has been proposed which allows the diffuse scatter to be described using such a fractal surface. Diffuse x-ray scattering studies performed on Al(_2)O(_3), which had been ground in different ways, indicted that the surface density as measured from the Yoneda wings followed the bearing area as measured from optical techniques. Comparisons performed between these results and those from optical microscopy and surface acoustic wave techniques allowed a model to be proposed which was consistent with all the findings. Finally, GIXS measurements were performed on GaAs which had been subjected to various polishing techniques. Simulations revealed that a nanometer scale surface layer was present on all samples. Born wave analysis was successfully applied to the diffuse scatter and the effect of surface layers on the deduced roughness was investigated. In addition, symmetric and asymmetric triple axis diffraction studies revealed that the distribution of strains was independent of the type of polishing used whereas the tilts were not. A model has been proposed in which the surface consists of unstrained mosaic blocks separated by cracks.

In the last two decades, advances in the dark field detectors and microscopes of scanning transmission electron microscopy (STEM) have inspired a resurgence of interest in quantitative STEM analysis. One promising avenue is the use of STEM as a nanothermometric probe. In this application, thermal diffuse scattering, captured by a CCD camera or an annular dark field detector, acts as an indirect measurement of the specimen temperature. One challenge with taking such a measurement is achieving adequate sensitivity to quantify a change in scattered electron signal on the order of 1% or less of the full electron beam. Another difficulty is decoupling the thermal effect on electron scattering from scattering changes due to differing specimen thicknesses and materials. To address these issues, we have developed a method using STEM, combined with electron energy loss spectroscopy (EELS), to produce a material-specific calibration curve. On silicon, across the range 89 K to 294 K, we measured a monotonically increasing HAADF signal ranging from 4.0% to 4.4% of the direct beam intensity at a thickness-to-mean-free-path ratio of 0.5. This yielded a calibration curve of temperature versus full-beam-normalized, thickness-normalized HAADF signal. The method enables thermal measurements on a specimen of varying local thickness at a spatial resolution of a few nanometers. We demonstrated the potential of the technique for testing electron scattering models by applying single-electron scattering theory to the data collected to extract a measurement of the mean atomic vibration amplitude in silicon at 294 K. The measured value, 0.00738 ± 0.00002 nm, agrees well with reported measurement using X-rays.

Channel impulse responses collected on the Virginia Tech campus show combinations of specular multipath and diffuse scattering at LMDS frequencies. An algorithm is presented that estimates link performance based on the channel impulse response. Presented and analyzed are representative impulse responses (one is primarily specular in nature and one shows significant diffuse scattering) to show that the proposed algorithm is appropriate for analyzing channels exhibiting either of these characteristics. Monte Carlo simulations logged the sequence number of each bit error to gain an understanding of the distribution of errors over time. The results show that for these static channels the errors occur randomly rather than in bursts, leading to the conclusion that average bit-error rate statistics are appropriate for channel characterization. Zero-Forcing (Z-F) and Minimum Mean Square Error (MMSE) equalizers employed on these channels had a significant impact on the link quality. In many cases, the performance of the MMSE equalizer performed only slightly better than the Z-F equalizer. However, when deep nulls were present in the channel response, the MMSE equalizer performed significantly better. Algorithms for determining the number of taps necessary to approach an optimum equalization are presented for both types of equalizers and a '98%' rule of thumb is presented. The algorithm's role in adaptive and cognitive radio systems is discussed and two applications are presented to illustrate its utility. / Ph. D.

Recently, Heusler alloys are studied for their interesting magnetic and electronic properties. These properties are strongly dependent on the crystallographic struc- ture. This work deals with Heusler alloys of the A2BC type. We have powder samples and single-crystaline samples for our study as well. An object of interest was a description of crystallographic structure of the samples, site occupation numbers of each type of atoms and their possible occupation disorder. Pow- der difraction and EXAFS have been measured on the powder samples. Clasical single-crystal diffractions has been meassured on the single-crystaline samples. In the case of a modulated structure in the samples, satelite difractions have been measured too. 1

Neste trabalho é estudado o efeito do carbono na formação de defeitos em silício Czochralski crescido na direção em amostras submetidas a tratamentos térmicos variados. Medidas de espalhamento difuso de raios-X, espectroscopia de infravermelho, medidas de resistividade, topografia de raios-X e microscopia eletrônica de transmissão mostraram que os defeitos nas amostras \"como crescidas\" podem ser relacionados com os microdefeitos tipo B. Tratamento térmico a 450ºC mostrou a presença de vacâncias nas amostras com baixa concentração de carbono enquanto que nas amostras com alta concentração de carbono ocorre a inibição da formação dos doadores térmicos (\"Thermal Donors - TD\"). Os resultados confirmam os modelos de Newman e Mathiot para a geração dos TD. Para tratamento térmico a 650ºC o carbono promove a formação de Novos Doadores (\"New Donors - ND\"). Os resultados mostram que estes defeitos são de natureza predominante de vacância e concordam com os modelos de geração que envolvem átomos de oxigênio substitucional. Os doadores observados a 550ºC puderam ser relacionados aos Novos Doadores Térmicos (\"New Thermal Donors - NTD\") observados por Kamiura et al.. / Effect of carbon concentration upon defect formation in oxygen rich Czochralski grown silicon has been investigated by combining various furnace thermal anneals. Diffuse X-ray scattering, infrared spectroscopy, resistivity, x-ray topography, and transmission electron microscopy have shown that defects in as-grown samples could be related to the B swirls. 450ºC anneals have shown the presence of vacancies in low carbon samples while high carbon concentration inhibited Thermal Donor (TD) formation. Our results confirm models by Newman and Mathiot for thermal donors generation. For 650ºC anneals carbon promotes New Donors (ND) formation. Our results show that these defects are mainly vacancy in nature and agrees with the substitutional oxygen models proposed for these donors. Donor formation was observed at 550ºC which could be related to New Thermal Donors (NTD) proposed by Kamiura et al..

Neste trabalho é estudado o efeito do carbono na formação de defeitos em silício Czochralski crescido na direção em amostras submetidas a tratamentos térmicos variados. Medidas de espalhamento difuso de raios-X, espectroscopia de infravermelho, medidas de resistividade, topografia de raios-X e microscopia eletrônica de transmissão mostraram que os defeitos nas amostras \"como crescidas\" podem ser relacionados com os microdefeitos tipo B. Tratamento térmico a 450ºC mostrou a presença de vacâncias nas amostras com baixa concentração de carbono enquanto que nas amostras com alta concentração de carbono ocorre a inibição da formação dos doadores térmicos (\"Thermal Donors - TD\"). Os resultados confirmam os modelos de Newman e Mathiot para a geração dos TD. Para tratamento térmico a 650ºC o carbono promove a formação de Novos Doadores (\"New Donors - ND\"). Os resultados mostram que estes defeitos são de natureza predominante de vacância e concordam com os modelos de geração que envolvem átomos de oxigênio substitucional. Os doadores observados a 550ºC puderam ser relacionados aos Novos Doadores Térmicos (\"New Thermal Donors - NTD\") observados por Kamiura et al.. / Effect of carbon concentration upon defect formation in oxygen rich Czochralski grown silicon has been investigated by combining various furnace thermal anneals. Diffuse X-ray scattering, infrared spectroscopy, resistivity, x-ray topography, and transmission electron microscopy have shown that defects in as-grown samples could be related to the B swirls. 450ºC anneals have shown the presence of vacancies in low carbon samples while high carbon concentration inhibited Thermal Donor (TD) formation. Our results confirm models by Newman and Mathiot for thermal donors generation. For 650ºC anneals carbon promotes New Donors (ND) formation. Our results show that these defects are mainly vacancy in nature and agrees with the substitutional oxygen models proposed for these donors. Donor formation was observed at 550ºC which could be related to New Thermal Donors (NTD) proposed by Kamiura et al..

<div>X-ray crystallography is a foundation of the modern structural biology. Thus, refinement of crystallographic structures remains an important and actively pursued area of research. We have built a software solution for refinement of crystallographic protein structures using X-ray diffraction data in conjunction with state-of-the-art MD modeling setup. This solution was implemented on the platform of Amber 16 biomolecular simulation package, making use of graphical processing unit (GPU) computing. The proposed refinement protocol consists of a short MD simulation, which represents an entire crystal unit cell containing multiple protein molecules and interstitial solvent. The simulation is guided by crystallographic restraints based on experimental structure factors, as well as conventional force-field terms. We assessed the performance of this new protocol against various refinement procedures based on the Phenix engine, which represents the current industry standard. The evaluation was conducted on a set of 84 protein structures with different realizations of initial models; the main criterion of success was free R-factor, R_free. Initially, we performed the re-refinement of the models deposited in the PDB bank. We found that in 58% of all cases our protocol achieved better R_free than Phenix. As a next step, we conducted the refinement on three different sets of lower-quality models that were manufactured specifically to test the competing algorithms (average C^α RMSD from the target structures 0.75, 0.89, and 1.02 Å). In these tests, our protocol outperformed the refinement procedures available in Phenix in up to 89% of all cases. Aside from R-factors, we also compared geometric qualities of the models as measured by MolProbity scores. It was found that our protocol led to consistently better geometries in all of the refinement comparisons.</div><div>Recently, a number of attempts have been made to fully utilize the information encoded in protein diffraction data, including diffuse scattering, which is dependent on molecular dynamics in the crystal. To understand the nature of this dependence, we have chosen three different crystalline forms of ubiquitin. By post-processing the MD data, we separated the effects from different types of motion on the diffuse scattering profiles. This analysis failed to identify any features of the diffuse scattering profiles that could be uniquely linked to certain specific motional modes (e.g. small-amplitude rocking motion of protein molecules in the crystal lattice). However, we were able to confirm the previous experimental observations, made in the combined X-ray diffraction and NMR study, suggesting that the amount of motion in the specific crystal is reflected in the amplitude of diffuse scattering.</div>

Nous avons étudié les transitions de phases structurales dans une série d'approximant 1/1 de quasicristaux de type CdYb. Les approximants 1/1 partagent le même environnement local que les quasicristaux, mais sont périodiquement ordonnés. L'approximant 1/1 Cd6Tb est particulièrement intéressant puisqu'il présente un ordre magnétique à longue distance en dessous de 20K. Une transition structurale est observée à Tc=192K. Nous avons déterminé la structure atomique de la phase cubique haute température ainsi que celle de la phase monoclinique observée en dessous de Tc. La dépendance en température de la mise en ordre du tétraèdre central a été étudiée en détail entre 192 et 180K. La distorsion de l'icosaèdre Tb12 joue certainement un rôle important dans la formation de l'ordre magnétique à longue distance. Nous avons confirmé que la transition de phase dans Cd6Yb n'est pas de type [110] mais de type [111] avec un groupe d'espace de la phase basse température P2/m. De plus l'influence de l'ordre chimique sur la transition a été étudié sur Cd6Pr and (Cd-Mg)6Pr avec 10% at. Mg. Dans ces deux cas la mise en ordre se fait le long de la direction [110]. Des simulations de dynamique moléculaire ont été conduite en utilisant des potentiels de pairs oscillant pour les phases approximantes Zn6Sc 1/1, Zn85.12Sc14.88 5/3, et Zn85.16Sc14.84 8/5. Le système Zn-Sc est isostructural au système Cd-Yb et les approximant 5/3 et 8/5 ont une maille suffisamment grande pour simuler un quasicrystal de manière réaliste. Dans les trois systèmes nous avons simulés les diagrammes de diffusion diffuse pour une température comprise entre 500 et 100K. Dans l'approximant 1/1 Zn6Sc l'évolution du signal de diffusion diffuse est en accord avec une augmentation de l'ordre à courte distance lorsque la température diminue. La comparaison de la diffusion diffuse entre l'approximant 1/1 et l'approximant 5/3 montre clairement un excès de diffusion diffuse pour l'approximant 5/3 qui est interprété comme résultant des fluctuations de phasons. Dans les trois approximants nous avons mis en évidence un phénomène de diffusion atomique entre le tétraèdre et le dodécaèdre. Pour l'approximant 1/1 cette diffusion se produit le long des diagonales de la maille cubique avec une énergie d'activation de l'ordre de 0.1 eV. Les simulations sur les approximants 5/3 et 8/5 ont montré que la dynamique du tétraèdre et la diffusion atomique dépendent de la complexité de la structure. Cette étude monte que la structure des amas atomiques et leur environnement local jouent certainement un rôle important pour les mécanismes de stabilisation de ces phases. / We have studied the structural phase transitions occurring in a series of periodic 1/1 approximant to the CdYb type quasicrystals. 1/1 approximants share the same local environment as the parent quasicrystal, yet with a periodic long range order. The Cd6Tb 1/1 approximant is of particular interest owing to its long range magnetic order observed below 20K. A structural phase transition is observed at Tc= 192K. The atomic structures of the high temperature cubic phase and the low temperature monoclinic phase has been determined. The temperature dependence of the central tetrahedron ordering and its connection to the phase transition has been studied in details between 192 and 180K. The distortion of the Tb12 icosahedron resulting from the ordering of the tetrahedron at low temperature is supposed to play a crucial role in the formation of the long range magnetic order. Diffuse scattering, characteristic of a short range order has also been observed above Tc. For the Cd6Yb approximant we have confirmed that the phase transition does not correspond to an ordering along [11 ̅0]. The space group of the low temperature phase is proposed as P2/m. Furthermore, the effects of chemical order on the ordering mechanism of the tetrahedra are investigated on Cd6Pr and (Cd-Mg)6Pr with 10% at. Mg. The ordering scheme of both Cd6Pr and (Cd-Mg)6Pr is of the [110] type instead of the previously claimed [111] type. Molecular dynamic simulations are carried out on the Zn6Sc 1/1, Zn85.12Sc14.88 5/3, and Zn85.16Sc14.84 8/5 approximants using an effective oscillating pair potential. The Zn-Sc system is isostructural to the Cd-Yb one and the 5/3 and 8/5 approximant have a unit cell large enough to mimic a real quasicrystal. In the three systems we have simulated the diffraction pattern as the temperature goes from 500K to 100K. In the Zn6Sc 1/1 approximant, the temperature dependence of the simulated diffuse scattering evidenced a short range order forming in the low temperature phase. It most likely correspond to the pretransitional short range order as observed experimentally. The comparison of the simulated diffuse scattering between the 1/1 and 5/3 approximants demonstrates an excess of diffuse scattering in the latter phase which is likely the contribution from phasons modes. We have evidenced an atomic diffusion taking place between tetrahedra and dodecahedra in all three approximants. The long range atomic diffusion in the 1/1 approximant is only observed along space diagonal directions of the cubic cell, with an activation energy of 0.1 eV. Further investigation on the 5/3 and 8/5 approximants have shown that the orientational dynamics of the central tetrahedra as well as the atomic diffusion are dependent on the complexity of the structures. The configuration of the clusters and their local environment are considered to play a crucial role in the stabilization mechanism of the quasicrystals and their periodic approximants.

Nous avons étudié les transitions de phases structurales dans une série d'approximant 1/1 de quasicristaux de type CdYb. Les approximants 1/1 partagent le même environnement local que les quasicristaux, mais sont périodiquement ordonnés. L'approximant 1/1 Cd6Tb est particulièrement intéressant puisqu'il présente un ordre magnétique à longue distance en dessous de 20K. Une transition structurale est observée à Tc=192K. Nous avons déterminé la structure atomique de la phase cubique haute température ainsi que celle de la phase monoclinique observée en dessous de Tc. La dépendance en température de la mise en ordre du tétraèdre central a été étudiée en détail entre 192 et 180K. La distorsion de l'icosaèdre Tb12 joue certainement un rôle important dans la formation de l'ordre magnétique à longue distance. Nous avons confirmé que la transition de phase dans Cd6Yb n'est pas de type [110] mais de type [111] avec un groupe d'espace de la phase basse température P2/m. De plus l'influence de l'ordre chimique sur la transition a été étudié sur Cd6Pr and (Cd-Mg)6Pr avec 10% at. Mg. Dans ces deux cas la mise en ordre se fait le long de la direction [110]. Des simulations de dynamique moléculaire ont été conduite en utilisant des potentiels de pairs oscillant pour les phases approximantes Zn6Sc 1/1, Zn85.12Sc14.88 5/3, et Zn85.16Sc14.84 8/5. Le système Zn-Sc est isostructural au système Cd-Yb et les approximant 5/3 et 8/5 ont une maille suffisamment grande pour simuler un quasicrystal de manière réaliste. Dans les trois systèmes nous avons simulés les diagrammes de diffusion diffuse pour une température comprise entre 500 et 100K. Dans l'approximant 1/1 Zn6Sc l'évolution du signal de diffusion diffuse est en accord avec une augmentation de l'ordre à courte distance lorsque la température diminue. La comparaison de la diffusion diffuse entre l'approximant 1/1 et l'approximant 5/3 montre clairement un excès de diffusion diffuse pour l'approximant 5/3 qui est interprété comme résultant des fluctuations de phasons. Dans les trois approximants nous avons mis en évidence un phénomène de diffusion atomique entre le tétraèdre et le dodécaèdre. Pour l'approximant 1/1 cette diffusion se produit le long des diagonales de la maille cubique avec une énergie d'activation de l'ordre de 0.1 eV. Les simulations sur les approximants 5/3 et 8/5 ont montré que la dynamique du tétraèdre et la diffusion atomique dépendent de la complexité de la structure. Cette étude monte que la structure des amas atomiques et leur environnement local jouent certainement un rôle important pour les mécanismes de stabilisation de ces phases. / We have studied the structural phase transitions occurring in a series of periodic 1/1 approximant to the CdYb type quasicrystals. 1/1 approximants share the same local environment as the parent quasicrystal, yet with a periodic long range order. The Cd6Tb 1/1 approximant is of particular interest owing to its long range magnetic order observed below 20K. A structural phase transition is observed at Tc= 192K. The atomic structures of the high temperature cubic phase and the low temperature monoclinic phase has been determined. The temperature dependence of the central tetrahedron ordering and its connection to the phase transition has been studied in details between 192 and 180K. The distortion of the Tb12 icosahedron resulting from the ordering of the tetrahedron at low temperature is supposed to play a crucial role in the formation of the long range magnetic order. Diffuse scattering, characteristic of a short range order has also been observed above Tc. For the Cd6Yb approximant we have confirmed that the phase transition does not correspond to an ordering along [11 ̅0]. The space group of the low temperature phase is proposed as P2/m. Furthermore, the effects of chemical order on the ordering mechanism of the tetrahedra are investigated on Cd6Pr and (Cd-Mg)6Pr with 10% at. Mg. The ordering scheme of both Cd6Pr and (Cd-Mg)6Pr is of the [110] type instead of the previously claimed [111] type. Molecular dynamic simulations are carried out on the Zn6Sc 1/1, Zn85.12Sc14.88 5/3, and Zn85.16Sc14.84 8/5 approximants using an effective oscillating pair potential. The Zn-Sc system is isostructural to the Cd-Yb one and the 5/3 and 8/5 approximant have a unit cell large enough to mimic a real quasicrystal. In the three systems we have simulated the diffraction pattern as the temperature goes from 500K to 100K. In the Zn6Sc 1/1 approximant, the temperature dependence of the simulated diffuse scattering evidenced a short range order forming in the low temperature phase. It most likely correspond to the pretransitional short range order as observed experimentally. The comparison of the simulated diffuse scattering between the 1/1 and 5/3 approximants demonstrates an excess of diffuse scattering in the latter phase which is likely the contribution from phasons modes. We have evidenced an atomic diffusion taking place between tetrahedra and dodecahedra in all three approximants. The long range atomic diffusion in the 1/1 approximant is only observed along space diagonal directions of the cubic cell, with an activation energy of 0.1 eV. Further investigation on the 5/3 and 8/5 approximants have shown that the orientational dynamics of the central tetrahedra as well as the atomic diffusion are dependent on the complexity of the structures. The configuration of the clusters and their local environment are considered to play a crucial role in the stabilization mechanism of the quasicrystals and their periodic approximants.

Growing environmental concerns, coupled with increasing regulatory restrictions, are requiring industries to develop non-lead-based compositions of ferroelectric and piezoelectric materials. These materials—now widely used in sensors, actuators, and transducers—are for the most part lead-based compounds such as Pb(Zr,Ti)O₃ (PZT). Indeed, PZT represents the dominant market share for use in these technologies. Moreover, next generation compounds, which include Pb(Mg_1/3Nb_2/3)O₃-xat%PbTiO₃ (PMN-x%PT) crystals with ultrahigh piezo-/electromechanical properties, are also Pb-based systems and thus are problematic for meeting more restrictive environmental standards. As alternative, Pb-free ferroelectrics such as NBT-derived single crystals represent viable next-generation materials for use in ferro-/piezoelectric applications. Development of these types of NBT-based crystals has made important advancements in the last decade. In fact, the performances of NBT-based materials are beginning to approach the properties of the widely used commercial PZT ceramic material. Nonetheless, additional studies are needed before it being able to compete with PMN-x%PT and PZN-x%PT crystals in next-generation applications. As a new type of piezoelectric material, much remains to be learned about Pb-free piezoelectric crystals. For instance, in addition to enhancing our understanding the nature of the piezoelectric third-rank tensor coefficients such as d₃₃ and d₁₅, a thorough knowledge of the Curie temperature, leakage current, and electromechanical properties is also essential for increasing the applications potential of these crystals. As detailed herein, multiple dopants may have to be incorporated into NBT to modify its microstructure and properties to meet these specific requirements, which may further complicate its chemical structure-property relationships. This study, therefore, was designed to investigate the heterogeneous structure of NBT-based single crystals, using x-ray diffraction, transmission electron microscopy, and neutron inelastic scattering, with the goal of investigating the mechanism coupling of morphotropic phase boundary (MPB) and the maximum property responses in A-site disordered perovskite Pb-free piezoelectric systems. Using the framework of polar nanoregions and adaptive phase theory, I sought to determine how the nanostructure of these single crystals change with temperature and composition—and how these factors impact its properties. Diffuse scattering, domain morphology, and phonon dispersions were used to investigate both the static and dynamic properties of these heterogeneous structures. / Ph. D.