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

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Optical properties of boron carbide (B5C) thin films fabricated by plasma-enhanced chemical-vapor deposition

Variable angle of incidence spectroscopic ellipsometry was used to determine the optical constants near the band edge of boron carbide (B5C) thin films deposited on glass and n-type Si(111) via plasma-enhanced chemical-vapor deposition. The index of refraction n, the extinction coefficient k, and the absorption coefficient are reported in the photon energy spectrum between 1.24 and 4 eV. Ellipsometry analysis of B5C films on silicon indicates a graded material, while the optical constants of B5C on glass are homogeneous. Line shape analyses of absorption data for the films on glass indicate an indirect transition at approximately 0.75 eV and a direct transition at about 1.5 eV. ©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.

Metalorganic chemical vapor deposition of GaAsN epilayers: microstructures and optical properties

In this article, we investigate the parameters used in the MOCVD growth of GaAsN epilayers on GaAs substrates and some of their microstructures and optical properties. The N incorporation was found to mainly depend on the growth temperature and the fractional 1,1-dimethylhydrazine molar flow. A thin highly strained interface layer was observed between GaAsN and GaAs, which, contrary to previously published results, was not N enriched. The low-temperature (10 K) photoluminescence spectra were composed of several emissions that we attribute to a combination of interband transition and transitions involving localized defect states. (C) 2004 Elsevier B.V. All rights reserved.

Structural characteristics of GaSB / GaAs nanowire heterostructures grown by metal-organic chemical vapor deposition

Highly lattice mismatched (7.8%) GaAs/GaSb nanowire heterostructures were grown by metal-organic chemical vapor deposition and their detailed structural characteristics were determined by electron microscopy. The facts that (i) no defects have been found in GaSb and its interfaces with GaAs and (ii) the lattice mismatch between GaSb/GaAs was fully relaxed suggest that the growth of GaSb nanowires is purely governed by the thermodynamics. The authors believe that the low growth rate of GaSb nanowires leads to the equilibrium growth. (c) 2006 American Institute of Physics.

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.

Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition during the carbonization of polyacrylonitrile fibers

We used microwave plasma enhanced chemical vapor deposition (MPECVD) to carbonize an electrospun polyacrylonitrile (PAN) precursor to form carbon fibers. Scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the fibers at different evolution stages. It was found that MPECVD-carbonized PAN fibers do not exhibit any significant change in the fiber diameter, whilst conventionally carbonized PAN fibers show a 33% reduction in the fiber diameter. An additional coating of carbon nanowalls (CNWs) was formed on the surface of the carbonized PAN fibers during the MPECVD process without the assistance of any metallic catalysts. The result presented here may have a potential to develop a novel, economical, and straightforward approach towards the mass production of carbon fibrous materials containing CNWs. © 2013 American Institute of Physics.

Furnace chemical vapor deposition (FCVD) method for special optical fibers fabrication

The features of the Furnace Chemical Vapor Deposition (FCVD) method of manufacturing preforms for special optical fibers are considered. It is shown that misalignment of substrate silica tube and furnace hole axes has a negative effect on the quality of fabricated preforms, leading to angular and radial asymmetry of the refractive index profile. Ways of getting rid of this and other disadvantages of the FCVD method are described. Some advantages of the FCVD method over the MCVD method are shown. It was demonstrated that the FCVD method, despite some drawbacks, allows to manufacture high-quality fiber preforms with good symmetry of the refractive index profile, and thus it is promising for fabrication of dispersion, dispersion varying and active fibers. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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.^

Growth and characterization of III-V compound semiconductor nanostructures by metalorganic chemical vapor deposition

Planar <110> GaAs nanowires and quantum dots grown by atmospheric MOCVD have been introduced to non-standard growth conditions such as incorporating Zn and growing them on free-standing suspended films and on 10° off-cut substrates. Zn doped nanowires exhibited periodic notching along the axis of the wire that is dependent on Zn/Ga gas phase molar ratios. Planar nanowires grown on suspended thin films give insight into the mobility of the seed particle and change in growth direction. Nanowires that were grown on the off-cut sample exhibit anti-parallel growth direction changes. Quantum dots are grown on suspended thin films and show preferential growth at certain temperatures. Envisioned nanowire applications include twin-plane superlattices, axial pn-junctions, nanowire lasers, and the modulation of nanowire growth direction against an impeding barrier and varying substrate conditions.

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.

Promoting strain relaxation of Si0.72Ge0.28 film on Si (100) substrate by inserting a low-temperature Ge islands layer in UHVCVD

A flat, fully strain-relaxed Si0.72Ge0.28 thin film was grown on Si (1 0 0) substrate with a combination of thin low-temperature (LT) Ge and LT-Si0.72Ge0.28 buffer layers by ultrahigh vacuum chemical vapor deposition. The strain relaxation ratio in the Si0.72Ge0.28 film was enhanced up to 99% with the assistance of three-dimensional Ge islands and point defects introduced in the layers, which furthermore facilitated an ultra-low threading dislocation density of 5 x 10(4) cm (2) for the top SiGe film. More interestingly, no cross-hatch pattern was observed on the SiGe surface and the surface root-mean-square roughness was less than 2 nm. The temperature for the growth of LT-Ge layer was optimized to be 300 degrees C. (C) 2008 Elsevier B.V. All rights reserved.