<p>Due to its close tissue-equivalence, high radiation sensitivity, dose and dose-rate linearity, diamond is a very promising detector for radiation therapy applications. The present thesis focuses on the development of a chemical vapour deposited (CVD) diamond detector with special attention on the arrangement of the electrodes and encapsulation having minimal influence on the measured signal. Several prototype detectors were designed by using CVD-diamond substrates with attached silver electrodes.</p><p>Interface effects in the electrode-diamond-electrode structure are investigated using the Monte Carlo (MC) code PENELOPE. The studies cover a wide range of electrode and diamond thicknesses, electrode materials and photon beam energies. An appreciable enhancement of the absorbed dose to diamond was found for high-Z electrodes. The influence of the electrodes diminishes with decreasing atomic number difference and layer thickness, so that from this point of view thin graphite electrodes would be ideal. The effect of encapsulation, cable and electrical connections on the detector response is also addressed employing MC techniques. For Co-60, 6 and 18 MV photon beam qualities it is shown that the prototypes exhibit energy and directional dependence of about 3% and 2%, respectively. By modifying the geometry and using graphite electrodes the dependencies are reduced to 1%.</p><p>Although experimental studies disclose some limitations of the prototypes (high leakage current, priming effect and slow signal stabilisation), diamonds of higher quality, suitable for dosimetry, can be produced with better-controlled CVD process. With good crystals and a well-designed encapsulation, the CVD-diamond detector could become competitive for routine dosimetry. It is then important for correct dose determination to use a collision stopping power for diamond incorporating proper mean excitation energy and density-effect corrections. A new mean excitation energy of 88 eV has been calculated.</p>

Il s'agit de dépôt de silice utilisant le silane et l'oxygène dans un système plasma haute densité. L'influence des paramètres du procédé et du système d'injection du silane sur les propriétés des matériaux sont étudiés par ellipsométrie spectroscopique, spectroscopie infrarouge à transformée de Fourier, spectroscopie de transmission et microscopie à force atomique. On étudie la phase gazeuse par spectroscopie d'émission optique et spectrométrie de masse quadripolaire avec pompage différentiel. La création de l'eau dans le système et son incorporation dans les couches pendant le dépôt sont étudiées utilisant l'injection du silane par jet capillaire. L'absorption du Si-OH dans les couches est mesurée en différentes positions sur le substrat. Les simulations de flux de gaz par la technique simulation directe Monte-Carlo sont utilisées pour étudier le flux des espèces sur le porte substrat. On a trouvé que le flux de l'eau est uniforme tandis que le flux de silane varie sur le porte substrat, ce qui explique la faible quantité de silanol dans la couche dans les régions déposées à grande vitesse. Une comparaison entre les résultats expérimentaux et les simulations montre que les systèmes plasma haute densité peuvent seulement être considérés comme bien mélangés quand il n'y a pas une élimination rapide des espèces en phase gazeuse.

Dans le cadre d'une approche bottom-up, la fabrication de nanofils par une croissance catalysée ouvre la voie à nombres d'applications: nano--transistors verticaux à grille enrobantes, heterostructures cœur--coquilles... Avec ces nouveaux objets, de nouvelles interrogations apparaissent quant à l'influence du catalyseur et de la surface sur les propriétés électroniques des nanofils. Mon travail basé sur une étude spectroscopique via des expériences de photoluminescence a mis en évidence le rôle prépondérant de la surface sur les propriétés électroniques des nanofils. La passivation des états de surface a permis d'observer la recombinaison radiative des porteurs libres d'une phase dense : le liquide électron-trou, dans des nanofils catalysés par de l'or et du cuivre. Cette phase liquide a la particularité d'être stable thermodynamiquement et sa densité est constante. Cette propriété unique dans les semiconducteurs a conduit à l'étude quantitative de l'influence de la surface via la modification du ratio surface/volume. Une méthode originale de mesure de la vitesse de recombinaison de surface (VRS) a ainsi été développée et des VRS relativement faibles ont été mesurées indiquant une excellente passivation des états de surface. Les propriétés de volume de nanofils catalysés 'or' sont très similaires à celles d'un silicium massif utilisé en micro-électronique. Enfin, l'oxydation sacrificielle du silicium a permis d'obtenir des nanofils de diamètre inférieur à 10 nm. L'oxydation progressive des nanofils a permis d'observer un décalage de la raie vers le rouge attribué à la présence de contraintes, puis l'augmentation du gap est corrélée au confinement quantique des porteurs.

<p>SiC is an attractive material since it has remarkable properties. For several years efforts have been put primarily in electronic applications. High power and high frequency devices can be fabricated on SiC due to its wide band gap, high breakdown field and high thermal conductivity. SiC devices can be used in harsh environment since its operation temperature is significantly high (about 1200 ). SiC bulk growth has been improved by seeded physical vapour transport (PVT) during last decades. However, the quality and doping concentration of SiC bulk are not good enough to be used as an active layer for devices. SiC epilayer growth by chemical vapour deposition (CVD) was established in the last three decades. Only about 5 µm/h growth rate is achieved by CVD with a standard process. Long deposition time is required to grow ≥100µm thick epilayer for high voltage devices. The main problem in standard CVD is the formation of silicon (Si) droplets due to supersaturation of Si-species on the growth surface or in the gas-phase, which is detrimental for devices performance. To solve the problem of Si-droplets, chloride-based CVD was introduced. Chlorinated species can dissolve the silicon aggregates through the formation of strong bonds to silicon species compared to Si-Si bonds. Typical chlorinated precursors are hydrogen chloride (HCl) and methyltrichlorosilane (MTS). In this thesis study, HCl was mainly used as chlorinated precursors. Distinct chlorinated precursors result in different chemical reactions which affect the epilayer growth appreciably. The Cl/Si ratio, which is the ratio of the amount of chlorinated precursors to silicon precursors, is a very critical growth parameter for morphology, growth rate and background doping concentration. The C/Si ratio and Si/H₂ ratio also affect the epilayer growth appreciably. Besides, growth temperature, growth pressure and temperature ramp up condition are other important growth parameters. In the CVD reaction chamber, the temperature profile and gas species distribution are not uniform along the whole susceptor length, which leads to different thickness of epilayer, morphology and doping concentration at different area of the reaction chamber. The polarity and off-angle of substrates can bring about complete different grown epilayers. Epitaxial defects are mainly replicated from the substrate. Therefore, the quality of substrates is very important as well. Deep energy levels can be introduced by adding transition metal such as vanadium (V), chromium (Cr) or tungsten (W). There are some limits which are needed to be overcome for a complete development of SiC. 4” SiC wafers are commercially available on the market, larger diameter would be very useful for the industrial development of SiC. High growth rate and good quality with controlled uniformity are desired for electronic applications. In this thesis, the influences of growth parameters such as C/Si and Cl/Si ratios, comparison between different precursors, growth condition in different areas of reaction chamber and effects of substrate polarity are discussed. Intentional incorporation of tungsten atoms is investigated by deep-level transient spectroscopy measurement and thermodynamic analysis.</p>

<p>Thin films of refractory metals and carbides have been studied extensively over many years because of their wide range of application. The two major techniques used are Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD). These can result in the deposition of two-dimensional blanket or patterned thin films. Laser-assisted Chemical Vapour Deposition (LCVD) can provide a maskless alternative for localised deposition in two and three dimensions. This thesis describes LCVD of micrometer-sized tungsten, molybdenum and tungsten carbide rods. The kinetics, phase composition and microstructure have been studied as a function of in situ measured laser induced deposition temperature.</p><p>Tungsten and molybdenum rods were deposited by hydrogen reduction of their corresponding hexafluorides, WF6 and MoF6, respectively. Single crystal and polycrystalline tungsten rods were obtained, depending on the H2/WF6 molar ratio and deposition temperature. The molybdenum rods were either single crystals or dendritic in form depending on experimental conditions. The field emission characteristics of the tungsten single crystals were investigated. The results showed LCVD to be a potential fabrication technique for field emitting cathodes.</p><p>Nanocrystalline tungsten carbide rods were deposited from WF6, C2H4 and H2. TEM analysis showed that the carbide rods exhibited a layered structure in terms of phase composition and grain size as a result of the temperature gradient induced by the laser beam. With decreasing WF6/C2H4 molar ratio, the carbon content in the rods increased and the phase composition changed from W/W2C to WC/WC1-x and finally to WC1-x/C.</p>

<p>One important driving force in nanotechnology today is the change which can be made in the properties of a material when the dimensions of its individual building blocks are decreased below approximately 100 nm. Such small building blocks, typically nanoparticles, may induce new and unique properties compared to those of the corresponding bulk material. The challenge in nanotechnology is to make nanoparticles with a discrete particle size within the range 1-10 nm. It is also important to develop appropriate assembly methodologies in order to construct devices composed of such small building blocks.</p><p>This thesis reports iron nanoparticle synthesis using laser-assisted photolysis of ferrocene. The particles were protected against oxidation by a carbon shell formed in situ during their growth. By varying the experimental conditions such as fluence, repetition rate and laser beam area, particles could be synthesized in the size range 1 to 100 nm. Their size was measured using a differential mobility analyser (DMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD). DMA was also used successfully to size-select particles to facilitate the deposition of monodisperse nanoparticle films.</p><p>A theoretical "residence time approach (RTA)" model was developed to relate particle volume to the laser parameters used. The growth of these particles was studied in situ using optical emission spectroscopy; the results were compared with those from quantum mechanical calculations. The particles were characterised ex situ by TEM, convergent beam electron diffraction, XRD, X-ray photoelectron spectroscopy and Raman spectroscopy. Results from the TEM investigations revealed that the carbon shell was graphitic close to the iron core, while the outer part of the carbon shell was amorphous, indicating different growth mechanisms. Both bcc and fcc iron particles were observed. </p>

MEMS-based gas turbine engines are currently under development at MIT for use as a button-sized portable power generator or micro-aircraft propulsion sources. Power densities expected for the micro-engines require very high rotor peripheral speeds of 300-600m/s and high combustion gas temperatures of 1300-1700K. These harsh requirements for the engine operation induce very high stress levels in the engine structure, and thus call for qualified refractory materials with high strength. Silicon carbide (SiC) has been chosen as the most promising material for use due to its high strength and chemical inertness at elevated temperatures. However, the state-of-the art microfabrication techniques for single-crystal SiC are not yet mature enough to achieve the required level of high precision of micro-engine components. To circumvent this limitation and to take advantage of the well-established precise silicon microfabrication technologies, silicon-silicon carbide hybrid turbine structures are being developed using chemical vapor deposition (CVD) of thick SiC (up to ~70µm) on silicon wafers and wafer bonding processes. Residual stress control of thick SiC layers is of critical importance to all the silicon-silicon carbide hybrid structure fabrication steps since a high level of residual stresses causes wafer cracking during the planarization, as well as excessive wafer bow, which is detrimental to the subsequent planarization and bonding processes. The origins of the residual stress in CVD SiC layers have been studied. SiC layers (as thick as 30µm) with low residual stresses (on the order of several tens of MPa) have been produced by controlling CVD process parameters such as temperature and gas ratio. Wafer-level SiC planarization has been accomplished by mechanical polishing using diamond grit and bonding processes are currently under development using CVD silicon dioxide as an interlayer material. This paper reports on the work that has been done so far under the MIT micro-engine project. / Singapore-MIT Alliance (SMA)

Les nanotubes de carbone (CNTs), intégrant à la fois la structure parfaite, la géométrie unique, et des propriétés exceptionnelles, sont d'une grande importance dans le domaine des nanotechnologies. Leur association avec d'autres matériaux produit de nouvelles propriétés remarquables, et étend par conséquent leurs domaines d'applications comme charges multifonctionnelles. Cette thèse vise à développer un nouveau matériau hybride avec une structure multi-échelle à base de CNTs et de particules micrométriques d'alumine (mAl2O3) par une méthode de dépôt chimique en phase vapeur (CVD). Nos études démontrent que les structures CNTs-mAl2O3 ont une propriété exceptionnelle en matière de transport thermique dans les composites polymères. Celle-ci nous a amenée à explorer plus en profondeur les mécanismes de l'organisation des CNTs sur mAl2O3, et d'étudier la cinétique de réactions chimiques dans l'espace gazeux du réacteur CVD. Dans le premier chapitre, nous faisons une revue de l'état de l'art sur la structure, les propriétés et les applications des CNTs, ainsi que les mécanismes de croissance de CNTs par CVD. Une attention particulière est également accordée aux structures hybrides nano-micrometriques qui sont synthétisées par greffage in-situ des CNTs sur des substrats micrométriques. Dans le deuxième chapitre, nous présentons trois types de structures hybrides, qui sont classifiées selon différents modes d'organisation des CNTs sur les microsphères d'alumine. L'évolution des structures hybrides est démontrée en faisant varier le diamètre, la longueur et la densité numérique des CNTs sur mAl2O3. L'organisation specifique et la dispersion homogène des CNTs permettent de diminuer considérablement leurs résistances de contacts thermiques lorsque les matériaux hybrides CNTs-mAl2O3 sont utilisés comme charges dans les composites polymères. Une amélioration importante de la conductivité thermique des composites Epoxy/CNTs-mAl2O3, par rapport à celle des composites constitués de CNTs et de résine époxy, est obtenue à une fraction massique ultra-faible en CNTs. Dans le troisième chapitre, nous avons étudié en détail les rôles joués par les paramètres CVD et les microparticules sphériques d'alumine dans la construction de structures hybrides multiformes. En particulier, les fortes corrélations entre la température, les sources de carbone et les ratios d'hydrogène ont été discutées. Le lien entre les CNTs et les microparticules est mis en évidence, ainsi que la dynamique de croissance des CNTs. L'auto-organisation des CNTs sur mAl2O3 est expliquée par les deux mécanismes suivants. Dans un premier temps, la structure hétérogène de la surface des particules entraîne une distribution différente des particules du catalyseur, et leur arrangement cristallin spécifique détermine potentiellement l'orientation des CNTs. Dans un deuxième temps, l'auto-assemblage des CNTs est dû à l'interaction des forces faibles de Van der Waals entre CNTs voisins. Le calcul basé sur le modèle du nano-cantilever montre que l'auto-assemblage des CNTs dépend fortement de leur diamètre, de leur longueur et de leur densité numérique sur mAl2O3. Dans le quatrième chapitre, la cinétique chimique des réactions dans l'espace gazeux du réacteur CVD est numériquement analysée. Le processus non-équilibré de CVD qui contient plusieurs phénomènes physico-chimiques est simulé avec succès en combinant la cinétique des réactions chimiques avec les phénomènes de transport physique. Les champs des concentrations de chaque espèce est révélée aux températures utilisées par simulation des réactions chimiques. Les sources effectives de carbone et de fer pour la croissance des CNTs ont été éclaircies en comparant les résultats de simulation avec les observations expérimentales, y compris les mesures de spectrométrie de masse. Ces analyses sont nécessaires pour améliorer la production des hybrides avec des structures homogènes.

Thin films of refractory metals and carbides have been studied extensively over many years because of their wide range of application. The two major techniques used are Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD). These can result in the deposition of two-dimensional blanket or patterned thin films. Laser-assisted Chemical Vapour Deposition (LCVD) can provide a maskless alternative for localised deposition in two and three dimensions. This thesis describes LCVD of micrometer-sized tungsten, molybdenum and tungsten carbide rods. The kinetics, phase composition and microstructure have been studied as a function of in situ measured laser induced deposition temperature. Tungsten and molybdenum rods were deposited by hydrogen reduction of their corresponding hexafluorides, WF6 and MoF6, respectively. Single crystal and polycrystalline tungsten rods were obtained, depending on the H2/WF6 molar ratio and deposition temperature. The molybdenum rods were either single crystals or dendritic in form depending on experimental conditions. The field emission characteristics of the tungsten single crystals were investigated. The results showed LCVD to be a potential fabrication technique for field emitting cathodes. Nanocrystalline tungsten carbide rods were deposited from WF6, C2H4 and H2. TEM analysis showed that the carbide rods exhibited a layered structure in terms of phase composition and grain size as a result of the temperature gradient induced by the laser beam. With decreasing WF6/C2H4 molar ratio, the carbon content in the rods increased and the phase composition changed from W/W2C to WC/WC1-x and finally to WC1-x/C.

One important driving force in nanotechnology today is the change which can be made in the properties of a material when the dimensions of its individual building blocks are decreased below approximately 100 nm. Such small building blocks, typically nanoparticles, may induce new and unique properties compared to those of the corresponding bulk material. The challenge in nanotechnology is to make nanoparticles with a discrete particle size within the range 1-10 nm. It is also important to develop appropriate assembly methodologies in order to construct devices composed of such small building blocks. This thesis reports iron nanoparticle synthesis using laser-assisted photolysis of ferrocene. The particles were protected against oxidation by a carbon shell formed in situ during their growth. By varying the experimental conditions such as fluence, repetition rate and laser beam area, particles could be synthesized in the size range 1 to 100 nm. Their size was measured using a differential mobility analyser (DMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD). DMA was also used successfully to size-select particles to facilitate the deposition of monodisperse nanoparticle films. A theoretical "residence time approach (RTA)" model was developed to relate particle volume to the laser parameters used. The growth of these particles was studied in situ using optical emission spectroscopy; the results were compared with those from quantum mechanical calculations. The particles were characterised ex situ by TEM, convergent beam electron diffraction, XRD, X-ray photoelectron spectroscopy and Raman spectroscopy. Results from the TEM investigations revealed that the carbon shell was graphitic close to the iron core, while the outer part of the carbon shell was amorphous, indicating different growth mechanisms. Both bcc and fcc iron particles were observed.