Behaviour of Group III (Al) and IV (Sn) metals on Si(100) surface was studied by Scanning Tunnelling Microscopy in a temperature range from 115 K to 350 K. Evolution of the length of Sn chains at room and elevated temperatures was studied by the method of repeated line scans. Activation energies and the respective frequency prefactors for detachment of different individual atoms from the chains' terminations were obtained by means of statistical processing. Kinetic Monte Carlo simulations were used to obtain activation energies for hops of Sn adatoms on the Si(100) surface at room temperature by fitting experimentally measured growth characteristics of Sn/Si(100). Three basic Al objects observed by Scanning Tunnelling Microscopy at room temperature on Si(100) were identified and thoroughly described. Direct observation of Al adatom hopping on Si(100)-c(4x2) at 115 K was used to evaluate the activation energies for individual hops in directions parallel with Si dimer rows and perpendicular to Si dimer rows. Kinetic Monte Carlo simulations were used to obtain activation energies for hops of Al adatoms on the Si(100) surface at room temperature by fitting experimentally measured growth characteristics of Al/Si(100).

No description available.

L'objectif de cette étude est de caractériser la redistribution d'éléments d'alliages et de dopants au cours des premiers stades de formation des siliciures de Ni. Pour cela, nous avons étudié la nature, la séquence et la cinétique des phases formées, dans un premier temps pour les systèmes binaires Pd/Si, Pt/Si et Ni/Si, puis pour les systèmes ternaires (Ni,Pt)/Si et Ni/(Si, As) présentant un intérêt technologique pour la nanoélectronique. Ainsi, nous avons couplé des techniques de caractérisation originales (calorimétrie différentielle à balayage sur films minces, sonde atomique tomographique, diffraction des rayons X in situ) pour mesurer la redistribution du Pt dans les phases formées et leurs cinétiques de croissance. Nous avons pu développer un modèle pour décrire les premiers stades de croissance de ces siliciures alliés et dégager les mécanismes mis en jeu ainsi que les facteurs limitant la redistribution des éléments d'alliage et des dopants.

The compression behavior of a newly developed near eutectic Al-Si based cast alloy with three different microstructures has been investigated in the present work. Microstructures with modified and unmodified Si particles and matrix with different tempers are investigated. The main objective of this work is to understand the effect of heat treatment and modification on the fracture behavior of the alloy under compression. This alloy is subjected to compressive loading at different strain rates and temperatures during the operation of the engines. Hence, the effect of strain rates and temperatures is also considered. The compression tests are carried out at different strain rates from quasi-static to dynamic viz., 3*10-4 to 102/s and three different temperatures RT, 100°C and 200°C. Microstructure of the alloys studied predominantly consists of eutectic colonies of α-Al and Si with a few interspersed α-Al dendrites. Modified alloy has more globular Si particles than unmodified alloy. Heat treated alloys are found to have hardening precipitates S’ & Al7Cu4Ni and 3-7 atomic layer thick zones, which may be precursors to S’ phase. A variety of large intermetallics, viz., plate like particles Al4.5FeSi, Chinese script like particles Al19Fe4MnSi2 and bulky phase Al3NiCu are also observed in the alloys. Mechanical behavior of the alloys is found to be different for different microstructures. Modification improves strength and ductility. Heat treatment improves strength of the alloy at the expense of ductility. A transition in mechanical behavior is observed after a particular strain rate for all the alloys studied. This transition strain rate is dependent on heat treatment, Si particle size and temperature. This transition can be explained on the basis of dislocation-precipitate and dislocation-Si particle interactions. Work hardening behavior of the alloys depends on the matrix microstructure in the unmodified alloys, and both matrix and particles play a role in the modified alloy. A statistically robust quantitative micro structural analysis has been carried out after compressing the samples at various strain rates and temperatures. The unique contribution of this work is the understanding of combined effect of strain rate and temperature on Si particle fracture characteristics in the alloy with different microstructures. From the fracture characteristics of Si particles, it is concluded that both dislocation pile-up mechanism and fibre loading are responsible for particle fracture in the modified alloy, whereas the fibre loading mechanism alone is sufficient to explain the particle fracture characteristics in the unmodified alloy. Si particles in the modified condition are found to cleave along the lowest surface energy planes {112} & {110} and the particles with orientations {112} & {111} are more prone to fracture. In addition to Si particle fracture, elongated Fe rich intermetallic particles are also seen to show peculiar fracture behavior. The Al4.5FeSi intermetallics with (100) as the plane of the plate cleave along (100) planes. This is a novel finding in this work and could have immense implications on the role of Fe impurities in the fracture behavior of these alloys. Moreover, since these cleavage fractures are seen to be more than 200 microns in size (which implies that the real penny shaped crack would be even larger) their role cannot be assumed to be negligible, as was previously thought. The load sharing between the Al matrix and eutectic Si particles are simulated by microstructure based finite element modeling. The program OOF (Object-Oriented Finite element analysis) is used to generate the finite element meshes for real microstructures with different Si morphology. The experimentally obtained stress – strain properties of the alloy is given as an input to describe the plastic behavior of the Al matrix, in the finite element simulation. This analysis helps to understand the effect of particle size, shape, orientation & clustering and matrix temper on the stress transferred to the Si particles. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to validate the stress states in Si particles with {111} orientations. The stress at fracture of Si particles is also estimated from Raman technique. Even though the alloys with different microstructures show different mechanical behavior, the sequence of fracture mechanisms is found to be same for all the alloys. The failure occurs in three stages: cracking of Si particles at low strains, micro-crack formation along the fractured particles, micro-crack coalescence and propagation leading to final failure. Thus, the proposed analysis links various deformation mechanisms ranging from nano precipitate-dislocation interactions to micro short-fiber theory of load sharing by eutectic silicon along with coupled effect of strain rate and temperature. In addition, negative strain rate sensitivity is also observed in the lower strain rate regimes (3*10-4, 10-3& 102/s) at RT and 100°C for all the three alloys, and serrated flow is also observed in the same strain rate and temperature regimes. Some of the features of serrated flow can be explained by the dynamic strain aging model and some other features by precipitate shearing.

Utilizando métodos de primeiros princípios, baseados na Teoria do Funcional da Desidade, investigamos, de forma sistemática, problemas de interesse na física de semicondutores: (i) A liga SixGe1-x: observa-se um pequeno desvio da linearidade, para o parâmetro de rede da liga, em função da concentração, sendo que as distâncias entre átomos de Ge são as que mais variam. O comportamento de vacâncias nessa liga se mostrou intermediário entre o Ge e o Si, e a energia de formação (EF) das vacâncias variou entre 2,06 eV e 2,90 eV, dependendo da vizinhança dessa. Propusemos um modelo para a difusão de Ge nessa liga. O fato das vacâncias com mais átomos de Ge como vizinhos ter menor energia de formação, faz com que a difusão das vacâncias se dê por caminhos lembrando o Ge puro, ou seja, a tendência será de que os vizinhos dessa vacância sejam de Ge, para ligas ricas em Ge; (ii) Superfícies: estudamos a adsorção de monômeros e dímeros de Ge sobre Si (100). Verificamos que, para monômeros, podem existir diversos mínimos locais da superfície de energia potencial para a mesma posição (x, y) na superfície, cuja diferença está na inclinação dos dímeros da superfície perto deste átomo. Mostramos que isso pode ser identificado em imagens teóricas de STM (Scanning Tunelling Microscopy) e propomos que isso seja verificado experimentalmente. Este tipo de efeito também ocorre com dímeros, sendo que isso já foi observado experimentalmente, mas com uma interpretação errônea. Através do nosso estudo podemos propor uma estrutura mais condizente com as imagens experimentais. O comportamento de átomos e dímeros de Si e Ge perto de degraus também foi analisado. Devido à diferença no parâmetro de rede do Si e do Ge, mostramos que a adsorção se torna diferente quando somente dois átomos estão na superfície. Através desses resultados, propomos um modelo para explicar a reversão da rugosidade dos degraus durante o crescimento de Ge sobre Si(100); (iii) Impurezas de Mn em Si: analisamos o comportamento de impurezas de Mn no cristal de Si e sobre a superfície. No cristal, observamos que a EF de sítios intersticiais é menor do que os sítios substitucionais. Sobre a superfície, existe um sítio substitucional que possue a mesma EF de um sítio intersticial. Esse é um fato importante para o desenvolvimento de semicondutores ferromagnéticos à base de Si. Propomos um método para que esse fato seja verificado experimentalmente, através de imagens de STM. / Using ab initio methods, based on Density Functional Theory, we have systematically investigated problems of interest for semiconductor physics: (i)The SixGe1-x alloy: we observe a small bowing in the lattice parameter, as a function of concentration, and that a larger variation in interatomic distances occurs for Ge-Ge bonds. The behavior of vacancies in this alloy is intermediate between Si and Ge, with formation energies (EF) varying from 2,06 eV to 2,90 eV, depending on the neighborhood of the vacancy. We propose a model for the diffusion of Ge in this alloy. As the vacancy with four Ge atoms as first neighbors hás smaller formation energy, the diffusion of the vacancies Will take place along paths that resemble purê Ge, for Ge-rich alloys; (ii)Surfaces: we study the adsorption of Ge monomers and dimers on Si(100). For monomers, there are several local mínima of the potential energy surface, for the same (x, y) position in the surface, depending on the buckling of the surface dimers near the adatom. We show that this can be identified in theoretical STM (Scanning Tunelling Microscopy) images. This kind of effect also occurs for dimers, and hás already been observed experimentally, but with a wrong interpretation. Through our study, we propose a new structure for the experimental images. The behavior of Si and Ge atoms and dimers near steps is also analyzed. Due to the differencein the lattice parameter of Si and Ge, we show that the adsorption becomes different when only two atoms are in the surface. Through these results, we propose a model to explain the reversion of step roughness due to Ge deposition on Si(100); (iii)Mn impurities in Si: we analise the behavior of Mn impurities in bulk Si ando n the surface. For the bulk, we observe that EF is lower for intersititial than for substitutional sites. On the surface, there is a substitutional site that hás the same EF AS A INTESTITIAL ONE. This fact may be important to the development of Si based ferromagnetic semiconductors. A method is proposed to verify this fact experimentally, through STM images.

Dans le cadre de la réduction des dimensions des technologies CMOS, le module de jonction apparaît comme un point bloquant pour l'amélioration des performances. En particulier, la hauteur de barrière entre le siliciure et le silicium limite le courant passant du transistor. Cette thèse adresse spécifiquement la problématique du contrôle de la hauteur de barrière suivant deux directions. D'une part, nous étudions l'intérêt d'une modification du métal formant le siliciure. D'autre part, nous évaluons le potentiel des techniques de ségrégation de dopants pour la modulation de la hauteur de barrière. Dans un premier temps, nous démontrons les difficultés liées à l'intégration des siliciures de type n (ErSi). Par ailleurs, nous mettons en évidence le fort potentiel du siliciure de platine (PtSi). En effet, ce matériau présente une stabilité thermique supérieure au siliciure de référence (NiSi) et montre une faible barrière à l'injection de trous. De plus, nous montrons que les techniques de ségrégation de dopants permettent d'obtenir de faibles hauteurs de barrières pour l'injection des électrons. Le PtSi apparaît donc comme un candidat à fort potentiel pour les futures technologies CMOS. Après avoir montré les inconvénients majeurs posés par l'intégration auto-alignée du PtSi grâce au procédé standard par eau régale, nous proposons une nouvelle méthode de retrait sélectif basée sur la transformation du métal non réagi en un germaniure facilement retiré par des chimies conventionnelles. En conclusion, nous intégrons le PtSi dans un procédé de fabrication industriel afin de démontrer des performances électriques à l'état de l'art des technologies CMOS les plus avancées.

Cette thèse a été consacrée à l’étude du Si poreux comme matériaux à faible conductivité thermique (k) pour application aux dispositifs thermoélectriques à base de Si. D’autres paramètres thermoélectriques, comme par exemple le coefficient Seebeck de ce matériau, ont été également étudiés.Si poreux est un matériau complexe composé de nanostructures de Si séparées de vide. Quand la porosité est élevée, sa conductivité thermique est bien inférieure à celle de Si cristallin. Nous avons étudié la conductivité thermique de Si poreux de différentes morphologies et porosités dans la gamme de températures 4.2-350K. Les mesures à T<20K sont les premières dans la bibliographie et ont montré une saturation de k en fonction de T pour ces températures. A des températures supérieures à 20K, k augmente régulièrement avec la température. La dépendance de température de k de Si poreux a été interprétée en considérant des modèles théoriques, basées sur la nature “fractal” de Si poreux. Nous avons calculé la dimension fractale de Si poreux par des images de microscopie électronique à balayage (SEM) et l’algorithme de “box counting”.Deux méthodes différentes ont été utilisées pour mesurer k: la méthode à courant direct (dc) combinée avec une analyse FEM et la méthode 3ω. Nous avons proposé deux approches améliorées pour extraire k du signal de potentiel 3ω en fonction de la fréquence. La première considère l’accord des résultats expérimentaux avec la solution asymptotique intégrale de la formule de Cahill, et la seconde fait une analyse des résultats expérimentaux en solvant l’équation temporelle de transfert de chaleur par des éléments finis. Plus précise est la méthode 3ω combinée avec des éléments finis. Les résultats correspondants sont en bon accord avec ceux obtenus par la méthode dc.Nous avons aussi étudié le Si poreux comme matériau thermoélectrique. Dans ce cas, le Si poreux peut être intéressant si il a une faible porosité, car le matériau à haute porosité est très résistive. Dans ce but, nous avons déterminé le coefficient Seebeck (S) des membranes de Si poreux de différentes porosités dans la gamme 40-84%, en utilisant un dispositif de mesure spécialement développé à cet effet. Pour des échantillons de porosité 51%, la valeur de coefficient S est de 1mV/K, bien supérieure à celle le Si cristallin. La dépendance de S de la porosité n’est pas monotone, et ceci est attribué à une combinaison des effets de filtrage d’énergie, des collisions des phonons et interactions phonon-porteurs électriques. Les résultats obtenus sont basées sur des mesures de photoluminescence (PL) et observations microscopiques à transmission (TEM). Nous avons enfin conclue que, malgré le coefficient S très élevé, le Si poreux n’est pas adéquat comme matériau thermoélectrique à cause de sa faible conductivité électrique, qui diminue en augmentant la porosité à cause de la résultante déplétion de porteurs.Nous avons aussi étudié des films minces polycristallins dopés avec du Bore. Ces films sont très intéressants comme matériaux thermoélectrique, car ils sont compatibles avec les procédés de fabrication des circuits intégrés de Si. Leur performance thermoélectrique est améliorée par diminution de la taille des grains. Des films minces polycristallins d’épaisseur entre 100 et 500nm ont été étudiés. Tous leurs paramètres thermoélectriques ont été mesurés et nous avons trouvé que le facteur de performance thermoélectrique zT augmente d’un facteur 3 en diminuant l’épaisseur de 500 à 100nm ceci étant attribué à la diminution de la taille des grains dans les films, conduisant à zT = 0.033, qui est la meilleure valeur reporté dans la littérature.Ce résultat compétitif augmente le potentiel d’utilisation des films polycristallins dans des dispositifs thermoélectriques efficaces, compatibles à la technologie de Si. / This thesis is devoted to the thermal conductivity and other thermoelectric properties of porous silicon (PSi) and thin polycrystalline Si films (thickness: 100-500 nm).PSi is a complex material composed of a Si skeleton of interconnected nanowires and dots, separated by voids. When it is highly porous, its thermal conductivity is very low, even below that of the amorphous Si. This makes it a good material for use as a thermal isolation platform on the Si wafer. In addition, its Seebeck coefficient is much higher than that of bulk c-Si.We studied k of PSi layers with different morphologies and porosities, in the temperature range 4.2-350K. The measurements below 20K are the first reported in the literature. A plateau-like dependence on temperature was observed for T below 20K, while above this temperature a monotonic increase with T is observed. The observed behaviour was interpreted using known theoretical models, based mainly on the fractal nature of PSi. PSi was characterized as a fractal material by calculating its fractal dimension using SEM images and the box counting algorithm.Two different methods were used to determine porous Si thermal conductivity: the DC method combined with FEM analysis and the 3ω method. Concerning the 3ω method, two improved approaches were proposed for extracting k from the 3ω voltage as a function of frequency: the first uses a fitting of the experimental data to the asymptotic solution of the Cahill’s integral formula, and the second is based on the analysis of the experimental data by combining them with a solution of the transient heat transfer equation using FEM analysis. The results in this second case were more accurate and in very good agreement with the DC method.We also measured the Seebeck coefficient (S) of PSi membranes with porosities 40-84% using a home-built setup, which was fabricated, calibrated and tested within this thesis. A value as high as 1mV/K was obtained for the 51% porosity sample. An anomalous porosity dependence of S was obtained, which was attributed to the interplay between energy filtering, phonon scattering and phonon drag effects. The results were explained by combining them with PL and TEM measurements, used for the determination of nanocrystal sizes. We concluded that, despite of the extremely low k and the high S of PSi, the material with the studied high porosities is not adequate for use as a “good thermoelectric” material, because of its significantly low electrical conductivity, which decreases with increasing porosity, resulting from carrier depletion during formation.We also studied the thermoelectric properties of thin, boron-doped, polycrystalline silicon films, which are much more attractive for use as Si-based thermoelectrics than porous Si. Their thermoelectric performance is improved by decreasing film thickness, due to a decrease in polysilicon grain size. Thin films with thickness between 100-500nm were investigated. We measured their thermal conductivity, resistivity and Seebeck coefficient and extracted their thermoelectric figure of merit, which showed threefold increase by reducing film thickness down to 100nm. A value as high as 0.033 was achieved, which is the highest reported in the literature so far for boron-doped polysilicon films at room temperature. This increase is attributed to a decrease in the grain size of the material. The obtained value shows the interest of nanocrystalline Si films for integration in efficient Si-based thermoelectric generators, compatible with CMOS processing.

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Agradecemos a compreensão. on 2018-11-09T17:16:20Z (GMT) / Submitted by Guilherme Felisberto (guilherme.felisberto@hotmail.com) on 2018-11-12T09:32:46Z No. of bitstreams: 1 Tese Guilherme online.pdf: 2540417 bytes, checksum: cc06d51bdd9f4e0c02a5806a4459259d (MD5) / Approved for entry into archive by Neli Silvia Pereira null (nelisps@fcav.unesp.br) on 2018-11-12T17:03:26Z (GMT) No. of bitstreams: 1 felisberto_g_dr_jabo.pdf: 2540417 bytes, checksum: cc06d51bdd9f4e0c02a5806a4459259d (MD5) / Made available in DSpace on 2018-11-12T17:03:26Z (GMT). No. of bitstreams: 1 felisberto_g_dr_jabo.pdf: 2540417 bytes, checksum: cc06d51bdd9f4e0c02a5806a4459259d (MD5) Previous issue date: 2018-10-05 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A deficiência de zinco (Zn) leva ao estresse oxidativo, causando prejuízos na produção e qualidade do produto colhido em diversas espécies de plantas cultivadas. O silício (Si) por sua vez é um elemento conhecido por mitigar diversos tipos de estresses, podendo ser uma importante ferramenta na mitigação da deficiência de Zn. Contudo, não há relatos na literatura sobre sua eficácia na mitigação da deficiência do micronutriente em nível de produção, nem se há diferenças entre fornecê-lo via solução nutritiva (radicular) ou via aplicação foliar. Possivelmente em função de que a adoção da aplicação foliar de Si ainda é pequena, justamente por apresentar problemas com a polimerização do Si da calda. Isso se dá possivelmente em função da utilização de concentrações, pH e misturas inadequadas. Para tentar minimizar esses problemas, novas fontes de Si têm sido desenvolvidas, mas pouco se sabe sobre seu comportamento e sua eficácia em relação ao silicato de potássio, considerado fonte padrão nos estudos com Si. Nesse sentido foram realizados dois estudos, sendo o primeiro para avaliar o comportamento e a eficácia de fontes de Si (silicato de potássio, nanosílica, silicato de sódio e potássio estabilizado e ácido silícico estabilizado) e concentrações (0; 0,5; 1,0; 1,5 g L-1 de Si) no crescimento e produção de grãos de soja e de arroz; e o segundo para avaliar o efeito do Si aplicado via foliar ou radicular (via solução nutritiva) na mitigação da deficiência de Zn em soja e em arroz. No primeiro estudo foi obervado que as fontes foram eficientes no fornecimento de Si. Na maioria das variáveis estudadas o silicato de sódio e potássio estabilizado se equiparou ou foi superior o silicato de potássio. A concentração de aproximadamente 1 g L-1 de Si apresentou melhor desempenho para a maioria das variáveis analisadas. No segundo estudo foi observado que o Si fornecido via radicular mitigou a deficiência de Zn da soja por ter diminuído os teores de malondialdeído e mantendo a produção de grãos a níveis semelhantes das plantas sem deficiência. O arroz não teve a deficiência de Zn mitigada pelo Si, embora a atividade da enzima superóxido dismutase tenha aumentado com o fornecimento radicular do elemento benéfico. Contudo independentemente se em condição de suficiência ou deficiência de Zn a aplicação radicular ou foliar de Si, promoveu ganhos na produção em relação à não aplicação de Si. Por fim acredita-se as novas fontes apresentem potencial em fornecer Si eficientemente às plantas sendo uma alternativa ao uso do silicato de potássio e que tanto a aplicação foliar como radicular promova ganhos consideráveis na soja e no arroz, seja sob deficiência de Zn ou não. / Zinc (Zn) deficiency leads to oxidative stress, causing losses in yield and quality of the product harvested in several species of cultivated plants. Silicon (Si) is an element known to mitigate several types of stress, and can be an important tool in the mitigation of Zn deficiency. However, there are no reports in the literature about its effectiveness in mitigating Zn deficiency at yield level, nor whether there are differences between providing it via root or leaf. The adoption of the foliar application of Si is still poor due to problems with the Si polymerization in the solution, much as a result of the inadequate concentrations use, pH and mixtures. Recently new sources of Si have been developed but little is known about their behavior and their efficacy compared to potassium silicate, considered a standard source in studies with Si. In this sense, two studies were carried out. The first one to evaluate behavior and efficacy of Si sources (potassium silicate, nanosilica, stabilized sodium and potassium silicate and stabilized silicic acid) and concentrations (0, 0.5, 1.0, 1.5 g L -1 of Si) in the growth and production of soybeans and rice; and the second to evaluate the effect of Si applied by foliar or root in the mitigation of Zn deficiency in soybean and rice. In the first study, it was observed that the sources were efficient in the supply of Si. In most of the variables studied, the stabilized sodium and potassium silicate was equal or superior to the potassium silicate. The concentration of approximately 1 g L-1 of Si presented better performance for most of the analyzed variables. In the second study it was observed that Si provided by the root mitigated the Zn deficiency of soybean by reducing the MDA levels and maintaining grain production at similar levels of plants without deficiency. Rice did not have the Zn deficiency mitigated by Si, although SOD activity increased with root supply of the beneficial element. However independently if in Zn sufficiency or deficiency condition, the root or leaf application of Si promoted production gains in relation to the non-application of Si. Finally, it is believed that the new sources have potential to provide Si efficiently to plants, being an alternative to the use of potassium silicate and that both foliar and root application promote considerable gains in soybean and rice, whether under Zn deficiency or not.

Orientador: Renato de Mello Prado / Resumo: A deficiência de zinco (Zn) leva ao estresse oxidativo, causando prejuízos na produção e qualidade do produto colhido em diversas espécies de plantas cultivadas. O silício (Si) por sua vez é um elemento conhecido por mitigar diversos tipos de estresses, podendo ser uma importante ferramenta na mitigação da deficiência de Zn. Contudo, não há relatos na literatura sobre sua eficácia na mitigação da deficiência do micronutriente em nível de produção, nem se há diferenças entre fornecê-lo via solução nutritiva (radicular) ou via aplicação foliar. Possivelmente em função de que a adoção da aplicação foliar de Si ainda é pequena, justamente por apresentar problemas com a polimerização do Si da calda. Isso se dá possivelmente em função da utilização de concentrações, pH e misturas inadequadas. Para tentar minimizar esses problemas, novas fontes de Si têm sido desenvolvidas, mas pouco se sabe sobre seu comportamento e sua eficácia em relação ao silicato de potássio, considerado fonte padrão nos estudos com Si. Nesse sentido foram realizados dois estudos, sendo o primeiro para avaliar o comportamento e a eficácia de fontes de Si (silicato de potássio, nanosílica, silicato de sódio e potássio estabilizado e ácido silícico estabilizado) e concentrações (0; 0,5; 1,0; 1,5 g L-1 de Si) no crescimento e produção de grãos de soja e de arroz; e o segundo para avaliar o efeito do Si aplicado via foliar ou radicular (via solução nutritiva) na mitigação da deficiência de Zn em soja e em arroz. N... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Zinc (Zn) deficiency leads to oxidative stress, causing losses in yield and quality of the product harvested in several species of cultivated plants. Silicon (Si) is an element known to mitigate several types of stress, and can be an important tool in the mitigation of Zn deficiency. However, there are no reports in the literature about its effectiveness in mitigating Zn deficiency at yield level, nor whether there are differences between providing it via root or leaf. The adoption of the foliar application of Si is still poor due to problems with the Si polymerization in the solution, much as a result of the inadequate concentrations use, pH and mixtures. Recently new sources of Si have been developed but little is known about their behavior and their efficacy compared to potassium silicate, considered a standard source in studies with Si. In this sense, two studies were carried out. The first one to evaluate behavior and efficacy of Si sources (potassium silicate, nanosilica, stabilized sodium and potassium silicate and stabilized silicic acid) and concentrations (0, 0.5, 1.0, 1.5 g L -1 of Si) in the growth and production of soybeans and rice; and the second to evaluate the effect of Si applied by foliar or root in the mitigation of Zn deficiency in soybean and rice. In the first study, it was observed that the sources were efficient in the supply of Si. In most of the variables studied, the stabilized sodium and potassium silicate was equal or superior to the potassium si... (Complete abstract click electronic access below) / Doutor

Utilizando métodos de primeiros princípios, baseados na Teoria do Funcional da Desidade, investigamos, de forma sistemática, problemas de interesse na física de semicondutores: (i) A liga SixGe1-x: observa-se um pequeno desvio da linearidade, para o parâmetro de rede da liga, em função da concentração, sendo que as distâncias entre átomos de Ge são as que mais variam. O comportamento de vacâncias nessa liga se mostrou intermediário entre o Ge e o Si, e a energia de formação (EF) das vacâncias variou entre 2,06 eV e 2,90 eV, dependendo da vizinhança dessa. Propusemos um modelo para a difusão de Ge nessa liga. O fato das vacâncias com mais átomos de Ge como vizinhos ter menor energia de formação, faz com que a difusão das vacâncias se dê por caminhos lembrando o Ge puro, ou seja, a tendência será de que os vizinhos dessa vacância sejam de Ge, para ligas ricas em Ge; (ii) Superfícies: estudamos a adsorção de monômeros e dímeros de Ge sobre Si (100). Verificamos que, para monômeros, podem existir diversos mínimos locais da superfície de energia potencial para a mesma posição (x, y) na superfície, cuja diferença está na inclinação dos dímeros da superfície perto deste átomo. Mostramos que isso pode ser identificado em imagens teóricas de STM (Scanning Tunelling Microscopy) e propomos que isso seja verificado experimentalmente. Este tipo de efeito também ocorre com dímeros, sendo que isso já foi observado experimentalmente, mas com uma interpretação errônea. Através do nosso estudo podemos propor uma estrutura mais condizente com as imagens experimentais. O comportamento de átomos e dímeros de Si e Ge perto de degraus também foi analisado. Devido à diferença no parâmetro de rede do Si e do Ge, mostramos que a adsorção se torna diferente quando somente dois átomos estão na superfície. Através desses resultados, propomos um modelo para explicar a reversão da rugosidade dos degraus durante o crescimento de Ge sobre Si(100); (iii) Impurezas de Mn em Si: analisamos o comportamento de impurezas de Mn no cristal de Si e sobre a superfície. No cristal, observamos que a EF de sítios intersticiais é menor do que os sítios substitucionais. Sobre a superfície, existe um sítio substitucional que possue a mesma EF de um sítio intersticial. Esse é um fato importante para o desenvolvimento de semicondutores ferromagnéticos à base de Si. Propomos um método para que esse fato seja verificado experimentalmente, através de imagens de STM. / Using ab initio methods, based on Density Functional Theory, we have systematically investigated problems of interest for semiconductor physics: (i)The SixGe1-x alloy: we observe a small bowing in the lattice parameter, as a function of concentration, and that a larger variation in interatomic distances occurs for Ge-Ge bonds. The behavior of vacancies in this alloy is intermediate between Si and Ge, with formation energies (EF) varying from 2,06 eV to 2,90 eV, depending on the neighborhood of the vacancy. We propose a model for the diffusion of Ge in this alloy. As the vacancy with four Ge atoms as first neighbors hás smaller formation energy, the diffusion of the vacancies Will take place along paths that resemble purê Ge, for Ge-rich alloys; (ii)Surfaces: we study the adsorption of Ge monomers and dimers on Si(100). For monomers, there are several local mínima of the potential energy surface, for the same (x, y) position in the surface, depending on the buckling of the surface dimers near the adatom. We show that this can be identified in theoretical STM (Scanning Tunelling Microscopy) images. This kind of effect also occurs for dimers, and hás already been observed experimentally, but with a wrong interpretation. Through our study, we propose a new structure for the experimental images. The behavior of Si and Ge atoms and dimers near steps is also analyzed. Due to the differencein the lattice parameter of Si and Ge, we show that the adsorption becomes different when only two atoms are in the surface. Through these results, we propose a model to explain the reversion of step roughness due to Ge deposition on Si(100); (iii)Mn impurities in Si: we analise the behavior of Mn impurities in bulk Si ando n the surface. For the bulk, we observe that EF is lower for intersititial than for substitutional sites. On the surface, there is a substitutional site that hás the same EF AS A INTESTITIAL ONE. This fact may be important to the development of Si based ferromagnetic semiconductors. A method is proposed to verify this fact experimentally, through STM images.