Micromorphologic assessment of CVD (chemical vapor deposition) and conventional diamond tips and their cutting effectiveness

The aim of this study was to compare the micromorphology of CVD diamond tips coupled to ultrasound with conventional high speed diamond tips after cavity preparations, and to measure the width and depth of the cavities obtained. Two hundred bovine teeth were divided into 20 subgroups. Each of the diamond tips (10 CVD and 10 conventional) were used to prepare 10 standardized cavities, using an apparatus that controlled the time (t: 27 s), speed (5.3 mm/s) and load (0.012 KGF) of the tip against the teeth during preparation. The unused and the used (after one, five and 10 preparations) tips were analyzed by scanning electronic microscopy. The images were randomly assessed by 3 examiners with regard to the presence or absence of micromorphologic alterations. Cavity measurements were made after visualization under a stereoscopic microscope. Cavity widths and depths were analyzed by the ANOVA Factorial test (p < 0.05). The CVD diamond tips presented less wear than the conventional tips after all the cavity preparations performed, but produced shallower cavities that were equivalent in width to those made by conventional tips after the fifth preparation. CVD diamond tips may be suggested as an alternative to conventional diamond tips due to their conservative preparation and greater longevity.

Avaliação da ponta de diamante CVC (Chemical Vapor Deposition) associada ao aparelho de ultra-som

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Influence of substrate surface topography in the deposition of nanostructured diamond-like carbon films by high density plasma chemical vapor deposition

In this work, we have studied the influence of the substrate surface condition on the roughness and the structure of the nanostructured DLC films deposited by High Density Plasma Chemical Vapor Deposition. Four methods were used to modify the silicon wafers surface before starting the deposition processes of the nanostructured DLC films: micro-diamond powder dispersion, micro-graphite powder dispersion, and roughness generation by wet chemical etching and roughness generation by plasma etching. The reference wafer was only submitted to a chemical cleaning. It was possible to see that the final roughness and the sp(3) hybridization degree strongly depend on the substrate surface conditions. The surface roughness was observed by AFM and SEM and the hybridization degree of the DLC films was analyzed by Raman Spectroscopy. In these samples, the final roughness and the sp(3) hybridization quantity depend strongly on the substrate surface condition. Thus, the effects of the substrate surface on the DLC film structure were confirmed. These phenomena can be explained by the fact that the locally higher surface energy and the sharp edges may induce local defects promoting the nanostructured characteristics in the DLC films. (C) 2008 Elsevier B.V. All rights reserved.

Synthesis of anatase TiO2 supported on porous solids by chemical vapor deposition

Coating anatase TiO2 onto three different particle supports, activated carbon (AC), gamma -alumina (Al2O3) and silica gel (SiO2), by chemical vapor deposition (CVD) was studied. The effect of the CVD synthesis conditions on the loading rate of anatase TiO2 was investigated. It was found that introducing water vapor during CVD or adsorbing water before CVD was crucial to obtain anatase TiO2 on the surface of the particle supports. The evaporation temperature of precursor, deposition temperature in the reactor, flow rate of carrier gas, and the length of coating time were also important parameters to obtain more uniform and repeatable TiO2 coating. High inflow precursor concentration, high CVD reactor temperature and long coating time tended to cause block problem. Coating TiO2 onto small particles by CVD involved both chemical vapor deposition and particle deposition. It was believed that the latter was the reason for the block problem. In addition, the mechanism of CVD process in this study included two parts, pyrolysis and hydrolysis, and one of them was dominant in the CVD process under different synthesis route. Among the three types of materials, silica gel, with higher surface hydroxyl groups and macropore surface area, was found to be the most efficient support in terms of both anatase TiO2 coating and photocatalytic reaction. (C) 2001 Elsevier Science B.V. All rights reserved.

Ultrasonic Chemical Vapor Deposition-coated Tip versus High- and Low-speed Carbide Burs for Apicoectomy: Time Required for Resection and Scanning Electron Microscopy Analysis of the Root-end Surfaces

This study compared ultrasonic chemical vapor deposition (CVD)-coated tip (CVDentus #8.1117-1; Clorovale Diamantes Ind. e Com. Ltda Epp, Sao Jose dos Campos, SP, Brazil) versus high-speed (#FG700L) and low-speed (#699) carbide burs for apicoectomy, evaluating the time required for resection and analyzing the root-end surfaces by scanning electron microscopy. Thirty extracted human premolars had the canals instrumented and obturated and were randomly assigned to 3 groups (n = 10), according to the instrument used for root-end resection. The time required for resection of the apical 2 mm of each root was recorded. The resected apical segments were dried, sputter coated with gold, and examined with a scanning electron microscope at X 350 magnification. A four-point (0-3) scoring system was used to evaluate the apical surface smoothness. The results were analyzed statistically by the Kruskal-Wallis test and two-by-two comparisons analyses were performed using the Miller test. The significance level was set at 5%. Root-end resection with the high-speed bur was significantly faster (p < 0.05) compared with the low-speed bur and CVD tip. The carbide burs produced significantly smoother root-end surfaces than the CVD tip (p < 0.05). The low-speed bur produced the smoothest root-end surfaces, whereas the roughest and most irregular root ends (p < 0.05) were obtained with the CVD tip. However, no statistically significant difference (p > 0.05) was found between the high- and low-speed burs regarding the surface roughness of the resected root ends (p > 0.05). In conclusion, under the tested conditions, ultrasonic root-end resection took a longer time and resulted in rougher surfaces compared with the use of carbide burs at both high and low speed. (J Endod 2009;35:265-268)

Producción de diamante por CVD (Chemical Vapour Deposition)

A fines de la década del '60 un grupo de investigadores rusos pudo obtener diamante a partir de experimentos con reacciones en fase vapor a bajas presiones de los gases envueltos en el proceso. En 1976 Derjagin et al. mostraron que la nucleación de diamante es posible sobre sustratos de Cu y en 1982 Matsumoto demuestra que la nucleación y crecimiento continuo a bajas presiones de diamante es posible sobre diversos substratos. Las especiales propiedades del diamante (D): dureza, elevado punto de fusión, inercia química, así como elevada conductividad del calor, sonido y de señales ópticas, ubican a este material como una de las prioridades de desarrollo e investigación de grupos de excelencia en el mundo entero. (...) Objetivos Generales y Específicos: El objetivo de este proyecto se basa en la construcción de un reactor para CVD (Chemical Vapour Deposition) y de los elementos auxiliares necesarios para producir diamante sintético por este método. Determinando los parámetros que controlan el proceso: mezcla de gases adecuada, temperatura de substrato, temperatura del plasma, presión parcial de los gases, vacío necesario y otros. En la primera etapa de 2 años se priorizará la puesta a punto del método, para luego pasar al estudio de las diferentes aplicaciones tecnológicas necesarias para la región. Específicamente, en el tercer año se tratará de generar diamantes como recubrimientos para herramientas de corte, así como para trapanos de velocidad, aprovechando residuos para la industria de abrasivos. Los objetivos generales no se cirscuncriben sólo al hecho de montar un reactor en laboratorio para CVD, sino una vez encontrados los parámetros que gobiernan esta técnica, producir diamante sintético para aplicaciones en la industria de herramientas de corte, micrófonos y óptica. Otro objetivo general de importancia es la formación de recursos humanos en técnicas de vacío, ingeniería de superficie y tecnología de plasma a través del personal y de los estudiantes involucrados en el proyecto, así como los participantes en cátedras del Departamento de Mecánica. En cuanto a los objetivos específicos para los dos primeros años, es preparar, construir y poner a punto un reactor de laboratorio de filamento caliente (Hot filament) por tecnología de plasma tipo CVD para obtener diamante sintético a partir de gases.

Development of chemical vapor deposition diamond burrs using hot filament

The fabrication of boring tools (burrs) for dentistry with the use of a hot-filament chemical vapor deposition (CVD) system, to form the diamond abrading structure, is reported here. The diamond was synthesized from a methane/freon gas mixture diluted in hydrogen. Comparative drilling tests with conventional diamond burrs and the CVD diamond burrs in borosilicate glasses demonstrated a lifetime more than 20 times larger for the CVD diamond burrs. Also, heat flow experiments in dentine showed that the CVD diamond burrs induce temperature gradients of the same order as the conventional ones. These characteristics of the CVD diamond burrs are highly desirable for odontological applications where the burrs' lifetime and the low temperature processing are essential to the quality and comfort of the treatment. © 1996 American Institute of Physics.

Understanding and improving the chemical vapor deposition process for solar grade silicon production

Esta Tesis Doctoral se centra en la investigación del proceso de producción de polisilicio para aplicaciones fotovoltaicas (FV) por la vía química; mediante procesos de depósito en fase vapor (CVD). El polisilicio para la industria FV recibe el nombre de silicio de grado solar (SoG Si). Por un lado, el proceso que domina hoy en día la producción de SoG Si está basado en la síntesis, destilación y descomposición de triclorosilano (TCS) en un reactor CVD -denominado reactor Siemens-. El material obtenido mediante este proceso es de muy alta pureza, pero a costa de un elevado consumo energético. Así, para alcanzar los dos principales objetivos de la industria FV basada en silicio, bajos costes de producción y bajo tiempo de retorno de la energía invertida en su fabricación, es esencial disminuir el consumo energético de los reactores Siemens. Por otro lado, una alternativa al proceso Siemens considera la descomposición de monosilano (MS) en un reactor de lecho fluidizado (FBR). Este proceso alternativo tiene un consumo energético mucho menor que el de un reactor Siemens, si bien la calidad del material resultante es también menor; pero ésta puede ser suficiente para la industria FV. A día de hoy los FBR deben aún abordar una serie de retos para que su menor consumo energético sea una ventaja suficiente comparada con otras desventajas de estos reactores. En resumen, la investigación desarrollada se centra en el proceso de depósito de polysilicio por CVD a partir de TCS -reactor Siemens-; pero también se investiga el proceso de producción de SoG Si en los FBR exponiendo las fortalezas y debilidades de esta alternativa. Para poder profundizar en el conocimiento del proceso CVD para la producción de polisilicio es clave el conocimiento de las reacciones químicas fundamentales y cómo éstas influencian la calidad del producto resultante, al mismo tiempo que comprender los fenómenos responsables del consumo energético. Por medio de un reactor Siemens de laboratorio en el que se llevan a cabo un elevado número de experimentos de depósito de polisilicio de forma satisfactoria se adquiere el conocimiento previamente descrito. Se pone de manifiesto la complejidad de los reactores CVD y de los problemas asociados a la pérdidas de calor de estos procesos. Se identifican las contribuciones a las pérdidas de calor de los reactores CVD, éstas pérdidas de calor son debidas principalmente a los fenómenos de radiación y, conducción y convección vía gases. En el caso de los reactores Siemens el fenómeno que contribuye en mayor medida al alto consumo energético son las pérdidas de calor por radiación, mientras que en los FBRs tanto la radiación como el calor transferido por transporte másico contribuyen de forma importante. Se desarrolla un modelo teórico integral para el cálculo de las pérdidas de calor en reactores Siemens. Este modelo está formado a su vez por un modelo para la evaluación de las pérdidas de calor por radiación y modelos para la evaluación de las pérdidas de calor por conducción y convección vía gases. Se ponen de manifiesto una serie de limitaciones del modelo de pérdidas de calor por radiación, y se desarrollan una serie de modificaciones que mejoran el modelo previo. El modelo integral se valida por medio un reactor Siemens de laboratorio, y una vez validado se presenta su extrapolación a la escala industrial. El proceso de conversión de TCS y MS a polisilicio se investiga mediante modelos de fluidodinámica computacional (CFD). Se desarrollan modelados CFD para un reactor Siemens de laboratorio y para un prototipo FBR. Los resultados obtenidos mediante simulación son comparados, en ambos casos, con resultados experimentales. Los modelos desarrollados se convierten en herramientas para la identificación de aquellos parámetros que tienen mayor influencia en los procesos CVD. En el caso del reactor Siemens, ambos modelos -el modelo integral y el modelado CFD permiten el estudio de los parámetros que afectan en mayor medida al elevado consumo energético, y mediante su análisis se sugieren modificaciones para este tipo de reactores que se traducirían en un menor número de kilovatios-hora consumidos por kilogramo de silicio producido. Para el caso del FBR, el modelado CFD permite analizar el efecto de una serie de parámetros sobre la distribución de temperaturas en el lecho fluidizado; y dicha distribución de temperaturas está directamente relacionada con los principales retos de este tipo de reactores. Por último, existen nuevos conceptos de depósito de polisilicio; éstos se aprovechan de la ventaja teórica de un mayor volumen depositado por unidad de tiempo -cuando una mayor superficie de depósito está disponible- con el objetivo de reducir la energía consumida por los reactores Siemens. Estos conceptos se exploran mediante cálculos teóricos y pruebas en el reactor Siemens de laboratorio. ABSTRACT This Doctoral Thesis comprises research on polysilicon production for photovoltaic (PV) applications through the chemical route: chemical vapor deposition (CVD) process. PV polysilicon is named solar grade silicon (SoG Si). On the one hand, the besetting CVD process for SoG Si production is based on the synthesis, distillation, and decomposition of thriclorosilane (TCS) in the so called Siemens reactor; high purity silicon is obtained at the expense of high energy consumption. Thus, lowering the energy consumption of the Siemens process is essential to achieve the two wider objectives for silicon-based PV technology: low production cost and low energy payback time. On the other hand, a valuable variation of this process considers the use of monosilane (MS) in a fluidized bed reactor (FBR); lower output material quality is obtained but it may fulfil the requirements for the PV industry. FBRs demand lower energy consumption than Siemens reactors but further research is necessary to address the actual challenges of these reactors. In short, this work is centered in polysilicon CVD process from TCS -Siemens reactor-; but it also offers insights on the strengths and weaknesses of the FBR for SoG Si production. In order to aid further development in polysilicon CVD is key the understanding of the fundamental reactions and how they influence the product quality, at the same time as to comprehend the phenomena responsible for the energy consumption. Experiments conducted in a laboratory Siemens reactor prove the satisfactory operation of the prototype reactor, and allow to acquire the knowledge that has been described. Complexity of the CVD reactors is stated and the heat loss problem associated with polysilicon CVD is addressed. All contributions to the energy consumption of Siemens reactors and FBRs are put forward; these phenomena are radiation and, conduction and convection via gases heat loss. In a Siemens reactor the major contributor to the energy consumption is radiation heat loss; in case of FBRs radiation and heat transfer due to mass transport are both important contributors. Theoretical models for radiation, conduction and convection heat loss in a Siemens reactor are developed; shaping a comprehensive theoretical model for heat loss in Siemens reactors. Limitations of the radiation heat loss model are put forward, and a novel contribution to the existing model is developed. The comprehensive model for heat loss is validated through a laboratory Siemens reactor, and results are scaled to industrial reactors. The process of conversion of TCS and MS gases to solid polysilicon is investigated by means of computational fluid-dynamics models. CFD models for a laboratory Siemens reactor and a FBR prototype are developed. Simulated results for both CVD prototypes are compared with experimental data. The developed models are used as a tool to investigate the parameters that more strongly influence both processes. For the Siemens reactors, both, the comprehensive theoretical model and the CFD model allow to identify the parameters responsible for the great power consumption, and thus, suggest some modifications that could decrease the ratio kilowatts-hour per kilogram of silicon produced. For the FBR, the CFD model allows to explore the effect of a number of parameters on the thermal distribution of the fluidized bed; that is the main actual challenge of these type of reactors. Finally, there exist new deposition surface concepts that take advantage of higher volume deposited per time unit -when higher deposition area is available- trying to reduce the high energy consumption of the Siemens reactors. These novel concepts are explored by means of theoretical calculations and tests in the laboratory Siemens prototype.

Growth of ultrananocrystalline diamond film by DC Arcjet plasma enhanced chemical vapor deposition

Self-standing diamond films were grown by DC Arcjet plasma enhanced chemical vapor deposition (CVD). The feed gasses were Ar/H 2/CH 4, in which the flow ratio of CH 4 to H 2 (FCH4/FH2) was varied from 5% to 20%. Two distinct morphologies were observed by scanning electron microscope (SEM), i.e. the pineapple-like morphology and the cauliflower-like morphology. It was found that the morphologies of the as-grown films are strongly dependent on the flow ratio of CH 4 to H 2 in the feed gasses. High resolution transmission electron microscope (HRTEM) survey results revealed that there were nanocrystalline grains within the pineapple-like films whilst there were ultrananocrystalline grains within cauliflower-like films. X-ray diffraction (XRD) results suggested that (110) crystalline plane was the dominant surface in the cauliflower-like films whilst (100) crystalline plane was the dominant surface in the pineapple-like films. Raman spectroscopy revealed that nanostructured carbon features could be observed in both types of films. Plasma diagnosis was carried out in order to understand the morphology dependent growth mechanism. It could be concluded that the film morphology was strongly influenced by the density of gas phases. The gradient of C2 radical was found to be different along the growth direction under the different growth conditions. © 2012 Elsevier B.V. All rights reserved.

Mechanical and electrical properties of single-walled carbon nanotubes synthesized by chemical vapor deposition

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (Eradial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It was found that the measured Eradial decreased from 57 to 9 GPa as the diameter of the SWCNTs increased from 0.92 to 1.91 nm. The experimental results were consistent with the recently reported theoretical simulation data. The method used in this mechanical property test can be easily applied to measure the mechanical properties of other low-dimension nanostructures, such as nanowires and nanodots. The characterized sample is also an ideal platform for electrochemical tests. The electrochemical activities of redox probes Fe(CN)63-/4-, Ru(NH3) 63+, Ru(bpy)32+ and protein cytochrome c have been studied on these pristine thin films by using aligned SWCNTs as working electrodes. A simple and high performance electrochemical sensor was fabricated. Flow sensing capability of the device has been tested for detecting neurotransmitter dopamine at physiological conditions with the presence of Bovine serum albumin. Good sensitivity, fast response, high stability and anti-fouling capability were observed. Therefore, the fabricated sensor showed great potential for sensing applications in complicated solution.^

Synthesis, characterization and chemical vapor deposition of transition metal di(tert-butyl)amido compounds

Although the transition metal chemistry of many dialkylamido ligands has been well studied, the chemistry of the bulky di(tert-butyl)amido ligand has been largely overlooked. The di(tert-butyl)amido ligand is well suited for synthesizing transition metal compounds with low coordination numbers; such compounds may exhibit interesting structural, physical, and chemical properties. Di(tert-butyl)amido complexes of transition metals are expected to exhibit high volatilities and low decomposition temperatures, thus making them well suited for the chemical vapor deposition of metals and metal nitrides. Treatment of MnBr₂(THF)₂, FeI₂, CoBr₂(DME), or NiBr₂(DME) with two equivalents of LiN(t-Bu)2 in benzene affords the two-coordinate complex M[N(t-Bu)₂]₂, where M is Mn, Fe, Co, or Ni. Crystallographic studies show that the M-N distances decrease across the series: 1.9365 (Mn), 1.8790 (Fe), 1.845 (Co), 1.798 Å (Ni). The N-M- N angles are very close to linear for Mn and Fe (179.30 and 179.45°, respectively), but bent for Co and Ni (159.2 and 160.90°, respectively). As expected, the d⁵ Mn complex has a magnetic moment of 5.53 μΒ that is very close to the spin only value. The EPR spectrum is nearly axial with a low E/D ratio of 0.014. The d⁶ Fe compound has a room temperature magnetic moment of 5.55 μΒ indicative of a large orbital angular momentum contribution. It does not exhibit a Jahn-Teller distortion despite the expected doubly degenerate ground state. Applied field Mössbauer spectroscopy shows that the effective internal hyperfine field is unusually large, Hint = 105 T. The magnetic moments of Co[N(t-Bu)₂]₂ and Ni[N(t-Bu)₂]₂ are 5.24 and 3.02 μΒ respectively. Both are EPR silent at 4.2 K. Treatment of TiCl₄ with three equivalents of LiN(t-Bu)2 in pentane affords the briding imido compound Ti₂[μ-N(t-Bu)]₂Cl₂[N(t-Bu)₂]₂ via a dealkylation reaction. Rotation around the bis(tert-butyl)amido groups is hindered, with activation parameters of ΔH‡ = 12.8 ± 0.6 kcal mol-1 and ΔS‡ = -8 ± 2 cal K-1 ·mol-1, as evidenced by variable temperature 1H NMR spectroscopy. Treatment of TiCl₄ with two equivalents of HN(t-Bu)₂ affords Ti₂Cl₆[N(t-Bu)₂]₂. This complex shows a close-contact of 2.634(3) Å between Ti and the carbon atom of one of the CH₃ substituents on the tert-butyl groups. Theoretical considerations and detailed structural comparisons suggest this interaction is not agostic in nature, but rather is a consequence of interligand repulsions. Treatment of NiI₂(PPh3)₂ and PdCl₂(PPh₃)₂ with LiN(t-Bu)₂in benzene affords Ni[N(t-Bu)₂](PPh₃)I and Pd₃(μ₂-NBut₂)2(μ₂-PPh₂)Ph(PPh₃) respectively. The compound Ni[N(t-Bu)₂](PPh₃)I has distorted T-shape in geometry, whereas Pd₃(μ₂-NBut₂)₂(μ₂-PPh₂)Ph(PPh₃) contains a triangular palladium core. Manganese nitride films were grown from Mn[N(t-Bu)₂]₂ in the presence of anhydrous ammonia. The growth rate was several nanometers per minute even at the remarkably low temperature of 80⁰C. As grown, the films are carbon- and oxygen-free, and have a columnar morphology. The spacings between the columns become smaller and the films become smoother as the growth temperature is increased. The composition of the films is consistent with a stoichiometry of Mn₅N₂.

Mechanical and Electrical Properties of Single-walled Carbon Nanotubes Synthesized by Chemical Vapor Deposition

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (Eradial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It was found that the measured Eradial decreased from 57 to 9 GPa as the diameter of the SWCNTs increased from 0.92 to 1.91 nm. The experimental results were consistent with the recently reported theoretical simulation data. The method used in this mechanical property test can be easily applied to measure the mechanical properties of other low-dimension nanostructures, such as nanowires and nanodots. The characterized sample is also an ideal platform for electrochemical tests. The electrochemical activities of redox probes Fe(CN)63-/4-, Ru(NH3)63+, Ru(bpy)32+ and protein cytochrome c have been studied on these pristine thin films by using aligned SWCNTs as working electrodes. A simple and high performance electrochemical sensor was fabricated. Flow sensing capability of the device has been tested for detecting neurotransmitter dopamine at physiological conditions with the presence of Bovine serum albumin. Good sensitivity, fast response, high stability and anti-fouling capability were observed. Therefore, the fabricated sensor showed great potential for sensing applications in complicated solution.

Gas collisions and pressure quenching of the photoluminescence of silicon nanopowder grown by plasma-enhanced chemical vapor deposition

The quenching of the photoluminescence of Si nanopowder grown by plasma-enhanced chemical vapor deposition due to pressure was measured for various gases ( H2, O2, N2, He, Ne, Ar, and Kr) and at different temperatures. The characteristic pressure, P0, of the general dependence I(P) = I0¿exp(¿P/P0) is gas and temperature dependent. However, when the number of gas collisions is taken as the variable instead of pressure, then the quenching is the same within a gas family (mono- or diatomic) and it is temperature independent. So it is concluded that the effect depends on the number of gas collisions irrespective of the nature of the gas or its temperature.

Linear and nonlinear optical properties of Si nanocrystals in SiO2 deposited by plasma-enhanced chemical-vapor deposition

Linear and nonlinear optical properties of silicon suboxide SiOx films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24¿at.¿%. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80¿eV in the nanosecond regime and at 1.50¿eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index (n2 ~ ¿10¿8¿cm2/W) and nonlinear absorption coefficient (ß ~ 10¿6¿cm/W). Under femtosecond excitation conditions, a smaller nonlinear refractive index is found (n2 ~ 10¿12¿cm2/W), typical of nonlinearities arising from electronic response. The contribution per nanocrystal to the electronic third-order nonlinear susceptibility increases as the size of the Si nanoparticles is reduced, due to the appearance of electronic transitions between discrete levels induced by quantum confinement.

Properties of nitrogen doped silicon films deposited by low-pressure chemical vapor deposition from silane and ammonia

Nitrogen doped silicon (NIDOS) films have been deposited by low-pressure chemical vapor deposition from silane SiH4 and ammonia NH3 at high temperature (750°C) and the influences of the NH3/SiH4 gas ratio on the films deposition rate, refractive index, stoichiometry, microstructure, electrical conductivity, and thermomechanical stress are studied. The chemical species derived from silylene SiH2 into the gaseous phase are shown to be responsible for the deposition of NIDOS and/or (silicon rich) silicon nitride. The competition between these two deposition phenomena leads finally to very high deposition rates (100 nm/min) for low NH3/SiH4 gas ratio (R¿0.1). Moreover, complex variations of NIDOS film properties are evidenced and related to the dual behavior of the nitrogen atom into silicon, either n-type substitutional impurity or insulative intersticial impurity, according to the Si¿N atomic bound. Finally, the use of NIDOS deposition for the realization of microelectromechanical systems is investigated.