The formation of Ge nanowires in V-grooves has been studied experimentally as well as theoretically. As substrate oxide covered Si V-grooves were used formed by anisotropic etching of (001)Si wafers and subsequent oxidation of their surface. Implantation of 1E17 Ge+ cm^-2 at 70 keV was carried out into the oxide layer covering the V-grooves. Ion irradiation induces shape changes of the V-grooves, which are captured in a novel continuum model of surface evolution. It describes theoretically the effects of sputtering, redeposition of sputtered atoms, and swelling. Thereby, the time evolution of the target surface is determined by a nonlinear integro-differential equation, which was solved numerically for the V-groove geometry. A very good agreement is achieved for the predicted surface shape and the shape observed in XTEM images. Surprisingly, the model predicts material (Si, O, Ge) transport into the V-groove bottom which also suggests an Ge accumulation there proven by STEM-EDX investigations. In this Ge rich bottom region, subsequent annealing in N2 atmosphere results in the formation of a nanowire by coalescence of Ge precipitates shown by XTEM images. The process of phase separation during the nanowire growth was studied by means of kinetic 3D lattice Monte-Carlo simulations. These simulations also indicate the disintegration of continuous wires into droplets mediated by thermal fluctuations. Energy considerations have identified a fragmentation threshold and a lower boundary for the droplet radii which were confirmed by the Monte Carlo simulation. The here given results indicate the possibility of achieving nanowires being several nanometers wide by further growth optimizations as well as chains of equally spaced clusters with nearly uniform diameter.

The formation of Ge nanowires in V-grooves has been studied experimentally as well as theoretically. As substrate oxide covered Si V-grooves were used formed by anisotropic etching of (001)Si wafers and subsequent oxidation of their surface. Implantation of 1E17 Ge+ cm^-2 at 70 keV was carried out into the oxide layer covering the V-grooves. Ion irradiation induces shape changes of the V-grooves, which are captured in a novel continuum model of surface evolution. It describes theoretically the effects of sputtering, redeposition of sputtered atoms, and swelling. Thereby, the time evolution of the target surface is determined by a nonlinear integro-differential equation, which was solved numerically for the V-groove geometry. A very good agreement is achieved for the predicted surface shape and the shape observed in XTEM images. Surprisingly, the model predicts material (Si, O, Ge) transport into the V-groove bottom which also suggests an Ge accumulation there proven by STEM-EDX investigations. In this Ge rich bottom region, subsequent annealing in N2 atmosphere results in the formation of a nanowire by coalescence of Ge precipitates shown by XTEM images. The process of phase separation during the nanowire growth was studied by means of kinetic 3D lattice Monte-Carlo simulations. These simulations also indicate the disintegration of continuous wires into droplets mediated by thermal fluctuations. Energy considerations have identified a fragmentation threshold and a lower boundary for the droplet radii which were confirmed by the Monte Carlo simulation. The here given results indicate the possibility of achieving nanowires being several nanometers wide by further growth optimizations as well as chains of equally spaced clusters with nearly uniform diameter.

Le regain d'attention des industriels pour les mémoires non volatiles intégrant des nanocristaux, illustré par l'introduction sur le marché de la Flexmemory de Freescale en technologie 90 nm, incite à poursuivre des études sur ce type de systèmes. Pour cela, nous avons mis au point des cellules mémoires élémentaires, à savoir des transistors MOS dont l'oxyde de grille contient une grille granulaire formée par un plan de nanocristaux de silicium (Si-ncx) stockant la charge électrique.Ce travail présente les principaux résultats issus de ces travaux, ceux-ci allant du procédé de fabrication à la caractérisation fine des dispositifs mémoires. Le parfait contrôle de l'élaboration de la grille granulaire de Si-ncx par implantation ionique à très basse énergie (ULE-IBS) est accompagné de caractéristiques « mémoires » répondant aux normes industrielles d'endurance et d'une discrimination des pièges responsables du chargement. Le stockage majoritaire par les Si-ncx est démontré, ce qui est essentiel pour la rétention de la charge. Nous avons développé une technique électrique permettant d'extraire à la fois la quantité de charge stockée par les Si-ncx mais également leurs principales caractéristiques structurales (taille, densité, position dans l'oxyde). Cette extension de la technique électrique de « pompage de charges », non destructive et in-situ permet de suivre l'état du composant en fonctionnement et de caractériser des pièges (e.g. les Si-ncx) pour la première fois au-delà de 3 nm de profondeur dans l'oxyde. Ces résultats ont été validés par des observations TEM. La résolution du pompage de charge étant le piège unique, nous avons alors couplé l'ULE-IBS avec la lithographie « Stencil » pour réduire latéralement le nombre de Si-ncx synthétisés. Cette technique nous permet pour le moment de contrôler la synthèse locale à la position désirée dans l'oxyde de « poches » de Si-ncx de 400 nm. La synthèse de « quelques » Si-ncx est envisagée à très court terme. Nous serons alors en mesure de fabriquer des mémoires à nombre choisi de nanocristaux (par SM-ULE-IBS), dont les propriétés structurales (taille, densité, position) et électriques (quantité de charge stockée) seront vérifiées par pompage de charge, offrant ainsi des outils puissants pour la fabrication et la caractérisation de mémoires à nombre réduit de nanocristaux, notamment pour des longueurs de grilles inférieures à 90 nm / The aim of this thesis has been to fabricate and electrically characterize elementary memory cells containing silicon nanocrystals (Si-ncs), in other words MOSFET which insulating layer (SiO2) contains a Si-ncs array storing the electrical charge. We have shown that we perfectly control the synthesis of a 2D array of 3-4 nm Si-ncs embedded into the MOSFET oxide by low-energy ion implantation (1-3 keV) Reaching this goal implied two key steps: on the one hand develop a reliable MOSFET fabrication process incorporating the Si-ncs synthesis steps and on the other hand develop tools and methods for both memory window and Si-ncs array itself characterizations. We have developed an in-situ characterization technique based on the well-known charge pumping technique, allowing for the first time the extraction of traps depth (e.g. the Si-ncs array) further than 3 nm into the oxide layer leading to the characterization of both position of these Si-ncs into the SiO2 matrix and their structural properties (diameter, density). These results have been confirmed by EF-TEM measurements. Finally, we have worked on the improvement of controlled local synthesis of Si-ncs pockets by combining low-energy ion implantation and stencil lithography. We reduced the size of these pockets down to about 400 nm using this parallel, low cost and reliable technique and identified the limiting effect for the pockets size reduction. These results pave the way for memory cells containing a few Si-ncs with a well-defined position into the oxide and a well-controlled number of ncs

Metal-Oxide-Silicon Field-Effect-Transistors with a layer of electrically isolated Si nanocrystals (NCs) embedded in the gate oxide are known to improve conventional floating gate flash memories. Data retention, program and erase speeds as well as the memory operation voltages can be substantially improved due to the discrete charge storage in the isolated Si NCs. Using ion beam synthesis, Si NCs can be fabricated along with standard CMOS processing. The optimization of the location and size of ion beam synthesized Si NCs requires a deeper understanding of the mechanisms involved, which determine (i) the built-up of Si supersaturation by high-fluence ion implantation and (ii) NC formation by phase separation. For that aim, process simulations have been conducted that address both aspects on a fundamental level and, on the other hand, are able to avoid tedious experiments. The built-up of a Si supersaturation by high-fluence ion implantation were studied using dynamic binary collision calculations with TRIDYN and have lead to a prediction of Si excess depth profiles in thin gate oxides of a remarkable quality. These simulations include in a natural manner high fluence implantation effects as target erosion by sputtering, target swelling and ion beam mixing. The second stage of ion beam synthesis is modeled with the help of a tailored kinetic Monte Carlo code that combines a detailed kinetic description of phase separation on atomic level with the required degree of abstraction that is necessary to span the timescales involved. Large ensembles of Si NCs were simulated reaching the late stages of NC formation and dissolution at simulation sizes that allowed a direct comparison with experimental studies, e.g. with electron energy loss resolved TEM investigations. These comparisons reveal a nice degree of agreement, e.g. in terms of predicted and observed precipitate morphologies for different ion fluences. However, they also point clearly onto impact of additional external influences as, e.g., the oxidation of implanted Si by absorbed humidity, which was identified with the help of these process simulations. Moreover, these simulations are utilized as a general tool to identify optimum processing regimes for a tailored Si NC formation for NC memories. It is shown that key properties for NC memories as the tunneling distance from the transistor channel to the Si NCs, the NC morphology, size and density can be adjusted accurately despite of the involved degree of self-organization. Furthermore, possible lateral electron tunneling between neighboring Si NCs is evaluated on the basis of the performed kinetic Monte Carlo simulations.

Metal-Oxide-Silicon Field-Effect-Transistors with a layer of electrically isolated Si nanocrystals (NCs) embedded in the gate oxide are known to improve conventional floating gate flash memories. Data retention, program and erase speeds as well as the memory operation voltages can be substantially improved due to the discrete charge storage in the isolated Si NCs. Using ion beam synthesis, Si NCs can be fabricated along with standard CMOS processing. The optimization of the location and size of ion beam synthesized Si NCs requires a deeper understanding of the mechanisms involved, which determine (i) the built-up of Si supersaturation by high-fluence ion implantation and (ii) NC formation by phase separation. For that aim, process simulations have been conducted that address both aspects on a fundamental level and, on the other hand, are able to avoid tedious experiments. The built-up of a Si supersaturation by high-fluence ion implantation were studied using dynamic binary collision calculations with TRIDYN and have lead to a prediction of Si excess depth profiles in thin gate oxides of a remarkable quality. These simulations include in a natural manner high fluence implantation effects as target erosion by sputtering, target swelling and ion beam mixing. The second stage of ion beam synthesis is modeled with the help of a tailored kinetic Monte Carlo code that combines a detailed kinetic description of phase separation on atomic level with the required degree of abstraction that is necessary to span the timescales involved. Large ensembles of Si NCs were simulated reaching the late stages of NC formation and dissolution at simulation sizes that allowed a direct comparison with experimental studies, e.g. with electron energy loss resolved TEM investigations. These comparisons reveal a nice degree of agreement, e.g. in terms of predicted and observed precipitate morphologies for different ion fluences. However, they also point clearly onto impact of additional external influences as, e.g., the oxidation of implanted Si by absorbed humidity, which was identified with the help of these process simulations. Moreover, these simulations are utilized as a general tool to identify optimum processing regimes for a tailored Si NC formation for NC memories. It is shown that key properties for NC memories as the tunneling distance from the transistor channel to the Si NCs, the NC morphology, size and density can be adjusted accurately despite of the involved degree of self-organization. Furthermore, possible lateral electron tunneling between neighboring Si NCs is evaluated on the basis of the performed kinetic Monte Carlo simulations.

Les aciers appelés ODS (pour Oxide Dispersion Strengthened), renforcés par une dispersion homogène de nano-oxydes, sont des matériaux de structure avancés pour les futurs réacteurs nucléaires de fusion et de fission. En effet ces nano-oxydes, à base d’Y et Ti, servent comme centres de recombinaison de défauts ponctuels et d'obstacles aux mouvements des dislocations, améliorant de ce fait leur résistance aux radiations et aux températures élevées. La fabrication conventionnelle des aciers ODS est réalisée par broyage mécanique suivi de traitements thermo-mécaniques, et ne permet pas facilement de comprendre les mécanismes physiques conduisant à la précipitation des nano-oxydes, ce qui serait potentiellement utile pour optimiser leur production. La cinétique de formation de ces nano-oxydes peut être étudiée en utilisant une technique alternative, à savoir la synthèse par faisceaux d’ions, qui présente de nombreux avantages, notamment le contrôle précis des paramètres expérimentaux et la possibilité de décorréler divers facteurs contribuant à la cinétique de précipitation. Au cours de cette thèse, cette technique a été utilisée pour étudier la coprécipitation d'ions métalliques (Y et/ou Ti) et d'oxygène implantés dans un alliage modèle Fe-Cr de composition proche de celle typique des aciers ODS commerciaux. Des ions de Y, Ti et O à basse énergie ont été implantés dans des échantillons d'alliage Fe10wt%Cr de haute pureté à température ambiante. Les échantillons implantés ont ensuite été recuits à diverses températures entre 600 à 1100°C pour favoriser la précipitation de nano-oxydes, conformément au principe de cette technique. La microscopie électronique à transmission a été utilisée pour caractériser la structure cristallographique et la composition chimique des nano-oxydes formés lors de trois séries d'expériences. Tout d'abord, l'implantation séquentielle d'ions Ti et O a été mise en œuvre. Un recuit ultérieur a révélé qu’il n’y avait pas de précipitation d'oxyde de titane jusqu’à des températures inférieures à 1000°C, mais la présence de nano-oxydes riches en chrome avec une structure hexagonale de type corundum, qui contiennent une certaine quantité de Ti à des températures suffisamment élevées. Ce n’est qu’après le recuit à 1100°C que des nano-oxydes d’un autre type à cœur enrichi en Ti et coquille enrichie en Cr ont également été observés. Deuxièmement, l'implantation séquentielle d’ions Y et O a entraîné la formation à 800°C de nano-oxydes probablement riches en yttrium. Le recuit à 1100°C a favorisé la croissance des particules identifiées comme étant des nano-oxydes d’yttrium avec une coquille enrichie en Cr. Enfin, une implantation ionique séquentielle de deux ions métalliques (Y et Ti) a été réalisée, suivie d'une implantation d’O. L'ordre d'implantation des ions métalliques s'est révélé crucial pour la précipitation de nano-oxydes lors du recuit ultérieur. Lors de la séquence avec une implantation de Ti en premier, une précipitation d'oxyde riche en chrome de structure corundum hexagonale a été observée, très similaire au cas de l'implantation d’ions Ti et O. En revanche, la séquence avec une implantation d’ions Y en premier a produit des nano-oxydes d'yttrium-titane qui possèdent une structure non identifiable. En résumé, l’étude a démontré la faisabilité de la formation de nano-oxydes de Y, Ti et (Y, Ti) par implantation ionique. La thèse présente la caractérisation détaillée de ces nano-oxydes, ainsi que certaines de leurs caractéristiques spécifiques, telles que la présence de relations d'orientation entre les nano-oxydes et la matrice FeCr, qui ont été observées même dans le cas de nano-oxydes de type corundum riches en Cr. Enfin, les résultats obtenus, combinées avec les données de la littérature, sont discutées pour une meilleure compréhension des mécanismes impliqués dans la formation des nano-oxydes dans les aciers ODS. / Oxide Dispersion Strengthened (ODS) steels, that is steels reinforced with a homogeneous distribution of (Y,Ti) oxide nano-particles, are advanced structural materials for nuclear applications. The oxide particles serve as point defect recombination centres and obstacles to dislocation motion thereby improving radiation resistance and high-temperature strength of these steels making them perfect candidate materials for future fusion and fission nuclear reactors. The conventional fabrication of ODS steels is achieved by mechanical alloying followed by thermomechanical heat treatments. This way of ODS steel production seems complicated to understand the physical mechanisms leading to the precipitation of nano-oxide particles. The kinetics of nanoparticle formation can be much better studied using an alternative technique of nanoparticle growth, namely Ion Beam Synthesis (IBS). This approach has many advantages including the precise control of experimental parameters and the ability to de-correlate various factors contributing to precipitation kinetics. A better knowledge gained in this way would be potentially helpful for optimization of ODS steel production routines. In the course of this PhD study, the IBS approach was applied to investigate the co-precipitation of metal (Y and/or Ti) and oxygen ions implanted into a model Fe-Cr alloy with the composition close to those typical for commercial ODS steels. Following the standard IBS schedule, consisting of ion implantation followed by high-temperature heat treatment, ions of Y, Ti and O at low energies were implanted into high-purity Fe10wt%Cr alloy samples at room temperature. The implanted samples were then annealed at various temperatures ranging from 600 to 1100°C to promote the precipitation of nano-oxide particles. A range of Transmission Electron Microscopy techniques were used to characterize the crystallographic structure and chemical composition of the nanoparticles. The study has been performed following three sets of experiments. First of all, the sequential implantation of Ti and O ions was implemented. Subsequent annealing at temperatures below 1000°C revealed that precipitation of titanium oxide was suppressed. Instead, chromium-rich nano-oxide particles with corundum hexagonal structure were found to precipitate. At sufficiently high temperatures these corundum particles were found to contain certain amount of Ti. Only after annealing at the highest temperature of 1100°C, particles of another type with Ti enriched core and Cr enriched shell were additionally fixed. Secondly, sequential Y and O ion implantation resulted in the formation of probable yttrium-rich oxides at 800°C. Annealing at 1100°C promoted their growth to larger sized yttria (Y₂O₃) particles with a Cr enriched shell. Finally, sequential ion implantation of both metal ions (Y and Ti) was performed, followed by O implantation. The order of metal ion implantation has been found to be crucial for subsequent oxide precipitation at the annealing stage. With the Ti implantation first in the sequence, the precipitation of corundum hexagonal chromium-rich oxide was observed, very similar to the case of Ti and O implantation. In contrast, implantation starting with Y produced yttrium-titanium oxide particles with unidentifiable structure. Summing up, the study has demonstrated the feasibility of the formation of Y, Ti and (Y,Ti) oxides by ion implantation. The thesis presents the detailed characterization of the nanoparticles, as well as the discovered specific features of precipitated particles, such as the presence of orientation relationships between the particles and the FeCr matrix, which was observed even for the case of Cr-rich corundum particles. Finally, the implications of the obtained results, in conjunction with the already known data from the existing literature, for the better understanding of the mechanisms involved in the formation of nano-oxide particles in ODS steels are discussed.