Nous étudions les propriétés optiques de nanostructures (Al,Ga)N/GaN crûes selon diverses orientations cristallographiques. Les orientations concernées sont : le plan non-polaire (1-100) ou plan m ; le plan semi-polaire (1-101) ou plan s ; et le plan semi-polaire (11-22). Dans un premier temps, nous nous consacrons à l'étude de l'anisotropie de la réponse optique de puits quantiques crûs selon les plans m et s. Dans un deuxième temps, nous évaluons les effets de la température sur les propriétés optiques de ces puits quantiques en utilisant la technique de photoluminescence résolue en temps qui permet d'obtenir des informations concernant les phénomènes radiatifs et non-radiatifs. S'agissant des durées de vie radiatives, nous avons mis en évidence la contribution de deux régimes de recombinaison: celui des excitons localisés, lequel est caractérisé par une durée de vie constante; et celui des excitons libres dont la durée de vie croit linéairement avec la température. Pour tous les échantillons que nous avons étudié, le régime d'excitons localisés domine à basse température alors que le régime d'excitons libres domine à haute température. Nous avons ainsi caractérisé la qualité des interfaces des puits quantiques (Al,Ga)N/GaN à partir de la détermination d'un modèle de la densité d'états de localisation. Nous trouvons qu'elle est, dans nos échantillons, encadrée par des valeurs comprises entre 10^11 - 10^12 cm-2. Notre étude montre que les puits orientés (11 22) présente la plus faible densité, et que les puits orientés selon le plan s sont les moins affectés par les phénomènes non-radiatifs. Dans un troisième temps, nous nous sommes intéressés à la caractérisation de nanostructures crûes selon le plan (11-22) pour diverses conditions de croissance. En faisant varier celles-ci, il est possible d'obtenir des boites quantiques, des fils quantiques, ou des puits quantiques. L'étude de la dynamique de recombinaison des excitons dans ces nanostructures (11-22) montre une dépendance en température de la durées de vie radiative en fonction du degré de confinement : constante pour les boîtes quantiques; proportionnelle à racine de T pour les fils ; linéaire pour les puits. Cette étude démontre la richesse de possibilités de nanostructures crûes sur des orientations non-traditionnelles elle mets en perspective de nouvelles études de croissance cristalline de nano-objets pour des applications inédites en optoélectroniques. / We study the optical properties of (Al,Ga)N/GaN nanostructures grown along several crystallographic orientations. The involved orientations are: the non-polar (1-100) plane or m-plane; the semi-polar (1-101) or s-plane; and the semi-polar (11-22) plane. First, we focus on the study of the anisotropy of the optical response of quantum wells grown in m- and s-planes. Second, we evaluate the effects of the temperature on optical properties of these quantum wells by extensive utilization of the time-resolved photoluminescence technique. It allows to obtain information regarding the evolution of radiative and non-radiative phenomena with temperature. Concerning radiative decay times, we have discriminated the contributions of two recombination regimes: the recombinations of localized excitons characterized by a constant decay time; and the recombinations of free excitons whose decay time increases linearly with the temperature. For all samples studied here, the regime of recombination of localized excitons dominates at low temperature and the regime of recombination of free excitons dominates at high temperature. In addition, we characterized the quality of (Al,Ga)N/GaN interfaces by the determination of the density of localization states. The values are ranging between 10^11 cm-2 and 10^12 cm-2 in our samples. This study demonstrates that (11-22)-oriented quantum wells exhibit the lowest density, and we find that the optical properties of s-plane oriented wells are the less impacted by the non-radiative phenomena. Third, we concentrated on the characterization of nanostructures grown along (11-22) plane direction under very different growth conditions. By modifying them, it is possible to obtain either quantum dots, or quantum wires or quantum wells. The study of the exciton recombination dynamics in these (11-22)-oriented nanostructures reveals a temperature dependence of radiative decay times correlated with the dimensions of the confining potentials: it is constant for the quantum dots; proportional to square root of T for quantum wires; and linear for quantum wells. This study demonstrates the potentialities of the nanostructures grown on non-traditional orientations for optoelectronic applications.

Les nanofils semiconducteurs sont des nano-objets dont la longueur peut aller jusqu'à quelques microns et dont la section peut être inférieure à la dizaine de nanomètre. En particulier, les nanofils de nitrures d'éléments III (GaN, AlN, InN, leurs alliages ternaires et leurs hétérostructures) sont extrêmement prometteurs en vue du développement d’une nouvelle génération de dispositifs d’électronique et d’optoélectronique tels que photodétecteurs, nanotransistors, biocapteurs, source de lumière, cellules solaires, etc.Dans ce travail, nous présentons la fabrication et la caractérisation de deux types de dispositifs à base de nanofils de nitrures III-V : des photodétecteurs d’une part et des dispositifs émetteurs de lumière d’autre part. Tout d'abord, nous avons réalisé et caractérisé un photodétecteur UV aveugle à la lumière du jour à base de nanofils de GaN verticalement alignés sur un substrat de Si(111) contenant une jonction p-n. Nous avons montré que ces dispositifs présentent une réponse supérieure à celle de leurs homologues en couches minces. Ensuite, nous avons fait la démonstration de photodétecteurs UV à base de nanofils uniques contenant des disques quantiques GaN / AlN multi-axiales insérés dans une région non intentionnellement dopé. Les résultats obtenus par spectroscopie de photoluminescence (PL) et cathodoluminescence (CL) montrent des contributions spectrales en-dessous et au-dessus de la bande interdite du GaN attribuées a la variation de l'épaisseur des disques. Les spectres de photocourant montrent un pic sous la bande interdite lié à l'absorption inter-bande entre les états confinés dans les disques les plus larges. Enfin, nous présentons une étude de photodétecteurs et émetteurs de lumière à base de nanofils de GaN contentant une hétérostructure cœur-coquille InGaN / GaN. Les fils utilisés comme photodétecteurs ont montré une contribution en dessous de la bande interdite de GaN. D’autre part, les mesures OBIC démontrent que ce signal provient exclusivement de la région active. Les fils de type LED basés sur la même structure montrent une forte émission d'électroluminescence et un décalage vers le rouge lorsque le taux d’indium présent dans les disques quantiques augmente, en accord avec les résultats de photoluminescence et de cathodoluminescence. / Semiconductor nanowires are nanostructures with lengths up to few microns and small cross sections (10ths of nanometers). In the recent years the development in the field of III-N nanowire technology has been spectacular. In particular they are consider as promising building in nanoscale electronics and optoelectronics devices; such as photodetectors, transistors, biosensors, light source, solar cells, etc. In this work, we present fabrication and the characterization of photodetector and light emitter based devices on III-N nanowires. First we present a study of a visible blind photodetector based on p-i-n GaN nanowires ensembles grown on Si (111). We show that these devices exhibit a high responsivity exceeding that of thin film counterparts. We also demonstrate UV photodetectors based on single nanowires containing GaN/AlN multi-axial quantum discs in the intrinsic region of the nanowires. Photoluminescence and cathodoluminescence spectroscopy show spectral contributions above and below the GaN bandgap according to the variation of the discs thickness. The photocurrent spectra show a sub-band-gap peak related to the interband absorption between the confined states in the large Qdiscs. Finally we present a study of photodetectors and light emitters based on radial InGaN/GaN MQW embedded in GaN wires. The wires used as photodetectors showed a contribution below the GaN bandgap. OBIC measurements demonstrate that, this signal is exclusively generated in the InGaN MQW region. We showed that LEDs based on this structure show a electroluminescence emission and a red shift when the In content present in the QWs increases which is in good agreement with photoluminescence and cathodoluminescence results.

Os resultados apresentados neste trabalho estão sistematizados em três partes. Em uma primeira etapa, efetuamos um estudo sistemático do comportamento da estrutura eletrônica em super-redes de deltas em função do período da super-rede e em função da concentração planar de dopantes. Uma nova abordagem, que se baseia no método celular e na solução autoconsistente das equações de Schroedinger e de Poisson, foi desenvolvida e aplicada para super-redes com dopagem planar tipo n em GaAs e em silício. Em ambos os casos, foi observada a transição de um comportamento eletrônico de caráter bi- para tridimensional conforme o período da super- rede diminui. No caso de super-redes de deltas de Si em GaAs foi empreendido o cálculo da energia de corte nos espectros de fotoluminescência de excitação. Uma boa concordância com as medidas experimentais foi obtida. O estudo da estrutura eletrônica para o caso de super-rede de deltas de Sb em Si foi pioneiro. Isto tornou os resultados de nossa investigação teórica de importância fundamental para experimentais e teóricos atuando na 6rea. A concordância entre nossas previsões teóricas e dados experimentais da literatura demonstram a consistência e o poder da abordagem desenvolvida. Em uma segunda etapa, foi efetuado o estudo da distribuição espacial do campo elétrico interno em heteroestruturas contendo camadas tipo \"bulk\", compostas por GaAs e (A1Ga)As. Uma nova abordagem foi desenvolvida para a execuqi3o de cálculos dos perfis de potencial eletrostático e de campo elétrico, sem assumir a ionização total dos dopantes e a não-degenerescência do material. Nosso método transforma a equação de Poisson em uma equação integral que deve ser resolvida autoconsistentemente. Os exemplos numéricos demonstram a aplicabilidade de nossa abordagem a sistemas reais. Perfis do campo elétrico calculados são usados para interpretar os espectros de fotorefletância. Em uma terceira etapa, a teoria geral da fotorefletância de heteroestruturas semicondutoras foi desenvolvida neste trabalho para tornar a interpretação de espectros de fotorefletância precisa e de aplicação eficiente. Um novo metodo de cdculo do coeficiente de reflexgo na presenga de inomogeneidade espacial da funggo dieletrica no interior de cada camada fmeceu um novo patamar de cornpreens20 dos espectros de fotorefletiincia. Este metodo e baseado na construgiio de uma matriz de transferhcia que iraclui as inomogeneidades no interior da camada de um mod0 integral. Portanto, para descrever uma camada de heteroestrutura e preciso ter apenas uma ma& de transferencia. 0s resultados de simulag6es numericas de espectros da fotoreflethcia estilo em uma concordhcia bastante boa com aqueles obtidos atravb de medidas opticas. A eficiencia de nosso metodo o torna aplicavel a simulag6es tip0 \"on-line\". 0s resultados dos metodos anteriores sgo reproduzidos como casos limites de nossa abordagem geral. / The results presented in this work can be displayed along the following three lines. In the first we performed a systematical study of the electronic structure behavior in delta superlattices as a function of superlattice period and sheet doping concentration. A new approach, based on the cellular method and on the selfconsistent solution of Schroedinger and Poisson equations, was developed and applied to superlattices with n-type delta doping in GaAs and silicon. In both cases, a transition from bi- to three- dimensional electronic behavior with the decrease of superlattice period was observed. For Si delta-doping superlattices in GaAs we performed calculations of the energy threshold in the photoluminescence excitation spectra. A good agreement with experimentally measured values was observed. Our investigation of the electronic structure of Sb delta-doping superlattices in Si was a pioneer theoretical study. Due to thls fact, the results of our work are of great importance for experimentalists and theoreticians acting in this area. The agreement between our theoretical predictions and the available experimental data demonstrates the consistency and the power of the developed approach. Along the second line we studied electric field spatial distribution inside of heterosinctures containing bulk layers of GaAs and (A1Ga)As. A new approach was developed to calculate the electrostatic potential and electric field profiles, providing the possibility to take .into account the incomplete ionization of impurities and the degeneracy of the materials. Our method transforms the Poisson equation into an integral equation, which must be solved selfconsistently. Numerical examples show the way to apply our approach to real systems. Internal electric field proiiles, calculated by means of our method are used to interpret photoreflectance spectra. In the third line, a general theory of photoreflectance for semiconductor heterostructures was developed in this work to make the interpretation of fotoreflectance spectra more precise and straightfornard. A new method to calculate the reflection coefficient in the presence of weak spatial inhomogenities of the dielectrical function inside each layer, provided us with a new degree of comprehension of the photoreflectance spectra. This method is based on the construction of a transfer matrix which includes the inhomogenities inside the layer in an integral way. This explains why we need only one matrix to describe one layer of the heterostructure. Results of our numerical simulations are in very good agreement with data of optical measurements. The efficiency of our method makes it suitable for on-line simulations. The results of previous methods emerge from our general approach as limit cases.

Efficiency droop is a critical issue for the Group III-nitride based light-emitting diodes (LEDs) to be competitive in the general lighting application. Carrier spill-over have been suggested as an origin of the efficiency droop, and an InAlN electron-blocking layer (EBL) is suggested as a replacement of the conventional AlGaN EBL for improved performance of LED. Optimum growth condition of InAlN layer was developed, and high quality InAlN layer was grown by using metalorganic chemical vapor deposition (MOCVD). A LED structure employing an InAlN EBL was grown and its efficiency droop performance was compared with a LED with an AlGaN EBL. Characterization results suggested that the InAlN EBL delivers more effective electron blocking over AlGaN EBL. Hole-injection performance of the InAlN EBL was examined by growing and testing a series of LEDs with different InAlN EBL thickness. Analysis results by using extended quantum efficiency model shows that further improvement in the performance of LED requires better hole-injection performance of the InAlN EBL. Advanced EBL structures such as strain-engineered InAlN EBL and compositionally-graded InAlN EBLs for the delivery of higher hole-injection efficiency were also grown and tested.

The two-dimensional electronic systems (2DESs) have kept surprising physicists for the last few decades. Examples include the integer and fractional quantum Hall effects, cuprate superconductivity, and graphene. This thesis is intended to develop suitable theoretical tools which can be generalized to study new types of 2DESs with strong correlation feature. The first part of this thesis describes the investigation of heterostructures made by Mott insulators. This work is mostly motivated by the significant improvement of techniques for layer-by-layer growth of transition metal oxides in the last few years. We construct a toy model based on generalized Hubbard model complemented with long-ranged Coulomb interaction, and we study it by Hartree-Fock theory, dynamical mean-field theory, and Thomas-Fermi theory. We argue that interesting 2D strongly correlated electronic systems can be created in such heterostructures under several conditions. Since these 2D systems are formed entirely due to the gap generated by electron-electron interaction, they are not addiabatically connected to a noninteracting electron states. This feature makes these 2D systems distinguish from the ones created in semiconductor heterostructures, and they may be potential systems having non-Fermi liquid behaviors. The second part of this thesis is devoted to the study of collective excitations in high-temperature superconductors. One important achievement in this work is to develop a time-dependent mean-field theory for t-U-J-V model, an effective low energy model for cuprates. The time-dependent mean-field theory is proven to be identical to the generalized random-phase approximation (GRPA) which includes both the bubble and ladder diagrams. We propose that the famous 41 meV magnetic resonance mode observed in the inelastic neutron scattering measurements is a collective mode arising from a conjugation relation, which has been overlooked in previous work, between the antiferromagnetic fluctuation and the phase fluctuation of the d-wave superconducting order parameter near momentum ([pi, pi]). Furthermore, we find that this collective mode signals the strength of the antiferromagnetic fluctuations which are responsible for the suppression of the superfluid density in the underdoped cuprates even at zero temperature. Finally, we perform a complete analysis on an effective model with parameters fitted by experimental data of Bi2212 within the GRPA scheme and conclude that the short-range antiferromagnetic interactions which are a remnant of the parent Mott-insulator are more likely the pairing mechanism of the High-T[subscript c] cuprates. / text

When materials approach the size of few nanometers, they show properties which are significantly different from their bulk counterpart. Such unique/improved properties make them potential candidate for several emerging applications. At the reduced dimension, controlling the shape of nanocrystals provides an effective way to tune several material properties. In this regard, wet chemical synthesis has been established as the ultimate route to synthesize nanocrystals at ultra-small dimensions with excellent control over the morphology. However, the use of surfactant poses a barrier into efficient realization of its application as it requires a clean interface for better performance. Exercise of available cleaning protocols to clean the surface often leads to coarsening of the nanoparticles due to their inherent high surface curvature. For anisotropic nanomaterials, rounding of the shape is an additional problem. Anchoring nanomaterials onto substrates provides an easy way to impart stability. In this thesis, ultrathin Au nanowires, that are inherently unstable, have been shown to grow over a wide variety of substrates by in-situ functionalization. Use of nanomaterials as device component holds promise into miniaturization of electronics. But device fabrication in such cases require manipulation of nanomaterials with enhanced control. Dielectrophoresis offers an easy way to assemble nanomaterials in between contact pads and hence evolved as a promising tool to fabricate device with a good level of precision. Herein, directed assembly of ultrathin Au nanowires by dielectrophoresis, has been shown as an efficient strategy to fabricate devices based on the wires. Combining more than one nanocrystal, to form a heterostructure, often has the advantage of synergism and/or multifunctionality. Therefore, synthesis of heterostructure is highly useful in enhancing and/or adding functionalities to nanomaterials. There are several routes available in literature for synthesis of heterostructures. Newer strategies are being evolved to further improve performance in an application specific way. In that regard, a good understanding of mechanism of formation is crucial to form the desired product with the required functionality. For example, Au due to high electron affinity has been known to undergo reduction rather than cation exchange with chalcogenides. In this thesis, it has been shown that the final product depends on the delicate balance of reaction conditions and the system under study using CdS-Au as the model system. In yet another case, PdO nanotubes have been shown to form, on reaction of PdCl2 with ZnO at higher starting ratio of the precursors. In-situ generation of HCl provides an effective handle for tuning of the product from the commonly expected hybrid to hollow. Graphene has evolved as a wonder material due to its wide range of practical applications. Its superior conductivity with high flexibility has made it an important material in the field of nanoelectronics. In this thesis, an interesting case of packed crumpled graphene has been shown to sense a wide variety of strain/pressure which has applications in day to day life. The study reported in the thesis is organized as follows: Chapter 1 presents a general introduction to nanomaterials followed by the review of the available strategies to synthesize various 1D nanomaterials. Subsequently, a section on the classification of hybrid followed by the different synthetic protocols adopted in literature to synthesize them, have been provided. A review on the available methodologies for directed assembly of nanomaterials has been presented. Chapter 2 provides a summary of the materials synthesized and the techniques used for characterization of the materials. A brief description of all the synthetic strategy adopted has been provided. The basic principle of all the characterization techniques used, has been explained. A section explaining the principle of dielectrophoresis has also been presented. Chapter 3 presents a general method to grow ultrathin Au nanowires over a variety of substrates with different nature, topography and rigidity/flexibility. Ultrathin nanowires of Au (~2 nm in diameter) are potentially useful for various catalytic, plasmonic and device applications. Extreme fragility on polar solvent cleaning was a limitation in realizing the applications. Direct growth onto substrate was an alternative but poor interfacial energy of Au with most commercial substrates lead to poor coverage. In this chapter, in-situ functionalization of the substrates have been shown to improve Au nucleation dramatically which lead to growth of dense, networked nanowires over large area. Catalysis and lithography-free device fabrication has been demonstrated. Using the same concept of functionalization, SiO2 coating of the nanowires have been shown. A comparative study of thermal stability of these ultrafine Au nanowires in the uncoated and coated form, has been presented. Chapter 4 demonstrates an ultrafast device fabrication strategy with Au nanowires using dielectrophoresis. While dense growth of Au nanowires is beneficial for some applications, it is not so for some others. For example, miniaturization of electronics require large number of devices in a small area. Therefore, there is a need for methods to manipulate nanowires so as to place them in the desired location for successful fabrication of device with them. In this chapter, dielectrophoresis has been used for assembling nanowires in between and at the sides of the contact pads. Alignment under different conditions lead to an understanding of the forces. Fabrication of a large number of devices in a single experiment has been demonstrated. Chapter 5 presents a simple route to synthesize CdS-Au2Sx hybrid as a result of cation-exchange predominantly. Au due to high electron affinity has been shown in literature to undergo reduction rather than cation exchange with CdS. In this chapter, it has been shown that cation exchange may be a dominant product. The competition between cation exchange and reduction in the case of CdS-Au system has been studied using EDS, XRD, XPS and TEM. Thermodynamic calculation along with kinetic analysis show that the process may depend on a delicate balance of reaction conditions and the system under study. The methodology adopted, is general and may be applied to other systems. Chapter 6 presents an one pot, ultrafast microwave route to synthesize PdO hollow/hybrid nanomaterials. The common strategy to synthesize hollow nanomaterials had been by nucleation of the shell material on the core and subsequent dissolution of the core. In this chapter, a one step method to synthesize hollow PdO nanotubes, using ZnO nanorods as sacrificial template, has been shown. By tuning the ratio of the PdCl2 (PdO precursor) to ZnO, ZnO-PdO hybrid could be obtained using the same method. The PdO nanotubes synthesized could be converted to Pd nanotubes by NaBH4 treatment. Study of thermal stability of the PdO nanotubes has been carried out. Chapter 7 demonstrates a simple strategy to sense a variety of strain/pressure with taped crumpled graphene. Detection of ultralow strain (10-3) with high gauge factor is challenging and poorly addressed in literature. Taped crumpled graphene has been shown to detect such low strain with high gauge factor (> 4000). An ultra-fast switching time of 20.4 ms has been documented in detection of dynamic strain of frequency 49 Hz. An excellent cyclic stability for >7000 cycles has been demonstrated. The same device could be used to detect gentle pressure pulses with consistency. Slight modification of the device configuration enabled detection of high pressure. Simplicity of the device fabrication allowed fabrication of the device onto stick labels which could be pasted on any surface, for instance, floor. Hard pressing, stamping with feet and hammering shocks do not alter the base resistance of the device, indicating that it is extremely robust. Sealed arrangement of the graphene allowed operation of the device under water in detection of water pressure. Presence of trapped air underneath the tape enabled detection of air pressure both below and above atmospheric pressure.

Le polytype cubique du carbure de silicium (3C-SiC) est un matériau très prometteur pour les applications MEMS. En plus de sa tenue mécanique et chimique, il peut être épitaxié sur des substrats Si de faible coût. De plus, l'hétéroépitaxie multiple, c’est-à-dire quand on empile plusieurs couches Si et 3C-SiC peut ouvrir des pistes pour de nouveaux dispositifs à base de 3C-SiC. Vue la complexité de développer de telles hétérostructures, nous avons procédé à l'amélioration de la qualité de chaque couche séparément. De plus, nous avons mené une étude approfondie sur la nature des défauts dans chaque couche. Après le développement de l'hétérostructure complète, nous avons procédé à la fabrication de microstructures à base de cet empilement. Nous présentons une méthode inédite pour former des membranes de 3C-SiC auto-supportées. Cette technique simplifie considérablement le procédé de fabrication de membranes tout en réduisant le temps de fabrication et le coût. En outre, elle aide à surmonter plusieurs problèmes techniques. / Due to its outstanding physico-chemical properties, the cubic polytype of silicon carbide (3C-SiC) gained significant interest in several fields. In particular, this material emerged as a potential candidate to replace Si in MEMS devices operating in harsh environment. The development of 3C-SiC/Si/3C-SiC heterostructures on top of Si substrate can pave the road towards original and novel MEMS devices profiting from the properties of the 3C-SiC. However, such epitaxial system suffers from wide range of defects characterizing each layer. Thus, we first tried to improve the quality of each layer in this heterostructure. This was achieved relying on two levers; (i) the optimization of the growth parameters of each layer and (ii) the understanding of the nature of defects present in each layer. These two key points combined together allowed an in-depth understanding of the limit of improvement of the overall quality of this heterostructure. After the development of the complete heterostructure, the fabrication of 3C-SiC microstructures was performed. Furthermore, we presented an unprecedented method to form free-standing 3C-SiC membranes in-situ during its growth stage. This novel technique is expected to markedly simplify the fabrication process of suspended membranes by reducing the fabrication time and cost.

L'augmentation des performances des circuits intégrés s'est effectué durant les trentes dernières années par la miniaturisation du composant clé à savoir le transistor MOSFET. Cette augmentation de la densité d'intégration se heurte aujourd'hui à plusieurs verrous, notamment celui de la puissance consommée qui devient colossale. Il devient alors nécessaire de travailler sur de nouveaux composants, les transistors à effet tunnel, où les porteurs sont injectés par effet tunnel bande à bande permettant de limiter considérablement la puissance consommée en statique. Les nanofils semiconducteurs sont de bons candidats pour être intégrés comme canaux de ces nouveaux composants de part la possibilité de moduler leur gap et leur conductivité au cours de la croissance. Dans ce contexte, cette thèse traite de la croissance d'hétérostructures axiales Si/Si1-xGex élaborés par croissance VLS par RP-CVD. Tout d'abord, nous identifions les conditions de croissance pour réaliser des interface Si/Si1-xGex et Si1-xGex/Si abruptes. Les deux heterointerfaces sont toujours asymétrique quelle que soit la concentration en Ge ou le diamètre des nanofils ou des conditions de croissance. Deuxièmement, nous étudions les problématiques impliquées par l'ajout d'atomes dopants. Nous discutons de l'influence des paramètres de croissance (le rapport flux de gaz (Si / Ge), et la pression partielle de dopants) sur la morphologie des nanofils et la concentration de porteurs. Grâce à cette étude, nous avons été capable de faire croitre des hétérojonctions P-I-N. Troisièmement, nous présentons une technique basée sur la microscopie à sonde locale pour caractériser les hétérojonctions. / After more than 30 years of successful scaling of MOSFET for increasing the performance and packing density, several limitations to further performance enhancements are now arising, power dissipation is one of the most important one. As scaling continues, there is a need to develop alternative devices with subthreshold slope below 60 mV/decade. In particular, tunnel field effect transistors, where the carriers are injected by quantum band to band tunneling mechanism can be promising candidate for low-power design. But, such devices require the implementation of peculiar architectures like axial heterostructured nanowires with abrupt interface. Using Au catalyzed vapor-liquid-solid synthesis of nanowires, reservoir effect restrains the formation of sharp junctions. In this context, this thesis addresses the growth of axial Si and Si1-xGex heterostructured nanowire with controlled interfacial abruptness and controlled doping using Au catalyzed VLS growth by RP-CVD. Firstly, we identify the growth conditions to realize sharp Si/Si1-xGex and Si1-xGex/Si interfacial abruptness. The two heterointerfaces are always asymmetric irrespective of the Ge concentration or nanowire diameter or growth conditions. Secondly, we study the problematics involved by the addition of dopant atoms and focus on the different approaches to realize taper free NWs. We discuss the influence of growth parameters (gas fluxes (Si or Ge), dopant ratio and pressure) on NW morphology and carrier concentration. With our growth process, we could successfully grow p-I, n-I, p-n, p-i-n type junctions in NWs. Thirdly, we present scanning probe microscopy to be a potential tool to delineate doped and hetero junctions in these as-grown nanowires. Finally, we will integrate the p-i-n junction in the NW in omega gate configuration.

Utilizou-se um Hamiltoniano KP (6x6) do tipo Kane para, se estudar a estrutura de bandas e níveis de Landau para heteroestruturas de semicondutores zincblende dos grupos III-V e II-VI. Os efeitos do acoplamento entre as bandas de condução e valência, da mistura dos estados da banda de valência, da não-parabolicidade dos níveis, da total degenerescência dos níveis, do warping e das descontinuidades das massas efetivas nas heterointerfaces são levados em conta. Mostrou-se que a interação entre as bandas de condução e valência não pode ser desprezada, mesmo para semicondutores de gap largo, como citado em trabalhos existentes na literatura. Para um estudo sistemático do modelo, utilizou-se um poço quântico de GaAs Ga(Al)As e então aplicou-se o modelo a um sistema de semicondutores semi-magnéticos (poço quântico de CdTe Cd(Mn)Te). / A Kane-like (6x6) KP Hamiltonian is used to study the subband structure and Landau levels for group III-V and group II-VI zincblende semiconductor heterostructures. The effects of conduction-valence band coupling, valence band states mixing, nonparabolicity of the levels, the full degeneracy of the levels, warping and effective masses discontinuities at the heterointerfaces are taken into account. It is shown that the interaction between conduction-valence bands cannot be neglected, even so the semicondutctor have wide gap, as claimed in previous work in the literature. GaAs-Ga(Al)As quantum well was used as a model for a systematic study of the effects of each effective KP parameters. Then, it was applied to the study the subband structure of semi-magnetic semiconductor system (a quantum well of CdTe-Cd(Mn)Te.

Orientador: Bernardo Laks / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T12:54:31Z (GMT). No. of bitstreams: 1 Araujo_AlexandreAbdalla_D.pdf: 3128592 bytes, checksum: cb2ac303a68b8fb439a89a0ee1627986 (MD5) Previous issue date: 2008 / Resumo: A Física de superfícies, interfaces e filmes finos vem se desenvolvendo muito rapidamente nas últimas décadas com o aparecimento de inúmeras técnicas experimentais para estudo das propriedades de superfície. Por outro lado, tem ocorrido um grande avanço dos equipamentos de informática e dos métodos computacionais, com o desenvolvimento de novos algoritmos, os quais já permitem o estudo de sistemas mais complexos como interfaces, defeitos, filmes-finos e nanofios, contendo um número cada vez maior de átomos. Um considerável interesse em superfícies e na deposição de filmes finos sobre superfícies, envolvendo metais, tem sido motivado pela possibilidade de se conseguir novas propriedades magnéticas e eletrônicas, incluindo temperaturas acima da temperatura ambiente, visando avanços tecnológicos em dispositivos eletrônicos. Nosso trabalho representa uma estratégia bastante promissora nessa área, pois nele identificamos claramente a possibilidade de produção de filmes finos com caráter ferromagnético half-metallic (isto é, com um canal de condução eletrônico semicondutor e outro metálico). Conforme pudemos mostrar, este caráter foi atingido a partir de pequenas variações de parâmetro de rede, de espessura de filme e de composição atômica. As propriedades observadas em nossos resultados teóricos sinalizam a importância de aplicação de diferentes materiais tais como CrAs, CrTe, CrAs(1-x)Sex, CrAs(1-x)T e x, CrSe(1-x) Tex, objetivando suas utilizações em Spintrônica. Desta forma, realizamos um estudo sistemático desses materiais, verificando suas propriedades eletrônicas e magnéticas e suas viabilidades de aplicações em novos dispositivos. Dois métodos de cálculo de estrutura eletrônica: o RS-LMTO-ASA (Real-Space ¿ Linear Muffin-Tin ¿ Atomic Sphere Approximation) e o FLAPW (Full Potential - Linearized Augmented Plane wave), assim como o método da Matriz Transferência foram utilizados em nossas investigações. Em primeiro lugar, apresentamos estudos teóricos sobre as fases estruturais e magnéticas observadas nas primeiras camadas de filmes finos de CrAs, crescidos sobre substratos de GaAs(001). Esses estudos englobaram processos de otimização de geometria, realizados através do método FLAPW, baseados em cálculos autoconsistentes de primeiros princípios, levando em consideração a polarização de spin. Em segundo lugar, estudamos as propriedades eletrônicas e magnéticas das superfícies CrAs(001) através do RS-LMO-ASA e determinamos as dispersões dos estados eletrônicos de superfície segundo direções de alta simetria na zona de Brillouin bidimensional. A seguir, como os resultados apontaram a possibilidade de obtermos mais materiais com comportamento ferromagnético half-metallic, passamos a investigar toda uma classe de materiais com estruturas volumétricas ou de filmes finos envolvendo os elementos Cr, As, Te, e Se, arranjados em ligas binárias (CrAs, CrSe, CrTe) e ternárias (CrAs(1-x)Sex, CrAs(1-x)T ex, CrSe(1-x)Tex), em diferentes concentrações e diferentes regiões superficiais. Como conseqüência, um amplo conjunto de resultados interessantes foi conseguido, confirmando nossas expectativas de que pequenas variações de parâmetro de rede, de espessura e de composição atômica são ingredientes fundamentais a serem considerados para se atingir uma transição do regime ferromagnético metálico para half-metallic e que isto representa uma área bastante promissora, que deverá estimular novos experimentos, com a produção de novos tipos de filmes finos, com espessura e composição controladas. Por último, apresentamos um estudo teórico do composto Fe2CoAl, no qual a precisão de nossos cálculos é comparada a medidas experimentais / Abstract: In the last decades, the Physics of Surfaces, Thin Films and Interfaces has motivated a great advance of the experimental techniques applied to study surface properties. In addition, a fast progress in the computational area has also occurred, with the development of powered computers, new methods of calculations, and new algorithms, which already allow the description of more complex systems, such as interfaces, defects, thin films and nanowires. A considerable interest in the deposition of thin films on surfaces, involving metals, has been motivated by the possibility of producing new devices using the fascinating electronic and magnetic properties, in order to produce technological advances in electronic devices. This work represents a promising strategy in this area, because we identify, clearly, the possibility of producing thin films with half-metallic character (that is, with a semiconductor electronic spin channel and a metallic spin channel, simultaneously). As we showed, this character was attained from small variations of lattice parameter, film thickness or atomic composition. The results of our theoretical calculations have pointed the importance of some materials such as CrAs, CrTe, CrAs(1-x)S ex, CrAs(1-x)Tex, CrS e(1-x)Tex to be used in the Spintronic branch. So, we carry out a systematic analysis of these new materials, emphasizing its structural, electronic and magnetic properties and the viability of using these materials in new electronic devices. Two different methods of electronic structure calculations: the RS-LMTO-ASA (Real-space - Linear Muffin-Tin - Atomic Sphere Approximation) and the LAPW (Linearized Augmented Plane-Wave), as well as the Matrix Transfer method have been used in our studies. Initially, we present the theoretical results of the structural and magnetic phases, observed in the first layers of thin films of orthorhombic CrAs, grown on a GaAs(001) substrate. Two geometry optimization processes have done with the Full-Potential Linearized Augmented Plane-Wave (FLAPW) method, based on first principles, self-consistent calculations, taking in account the spin polarization, at the scalar relativistic level. Secondly, we study the electronic and magnetic properties of the CrAs(001) surfaces, via the RS-LMTO-ASA, and determined the energy dispersion of the electronic surface states along two highly symmetric directions in the two-dimensional Brillouin zone. Then, as the results suggested the possibility of obtaining new thin films, with ferromagnetic half-metallic behavior, we started to investigate a large class of materials, with volumetric and thin films structures, of binary (CrAs, CrSe, CrTe) and ternary (CrAs(1-x)Sex, CrAs(1-x) Te, CrSe(1-x)Tex) systems, in different atomic concentrations and with different superficial regions. Consequently, a large quantity of interesting results was obtained for these ferromagnetic materials, confirming that small variations of lattice parameters, film thickness and atomic composition are the fundamental ingredients to be considered, in order to reach the transition from metallic regime to ferromagnetic half-metallic regime and that our results can stimulate new experiments with the aim of producing new thin films, with controlled thicknesses and atomic compositions. Finally, we present a theoretical study of the inter-metallic compound Fe2CoAl, by comparing the precision of our calculations with experimental measurements / Doutorado / Física da Matéria Condensada / Doutor em Ciências