Obtención de Si3N4 mediante SHS

En el presente trabajo se presentan los resultados obtenidos en el estudio de las reacciones de Síntesis Autopropagada a Alta Temperatura (Self-propagating High-temperature Synthesis, SHS) de Nitruro de Silicio. La síntesis autopropagada a alta temperatura consiste básicamente en la generación de reacciones altamente exotérmicas capaces de automantenerse. Se puede considerar como principal ventaja del método el ahorro energético que supone. La síntesis se realiza sobre una mezcla inicial de silicio metálico sobre la cual se realizan adiciones de diluyente y otros aditivos (sales amónicas) que afectan al desarrollo de la reacción. Se ha estudiado la influencia que en este sistema pueden tener las proporciones de las distintas incorporaciones en la mezcla, tanto en el material resultante como en las condiciones de reacción. Igualmente se ha estudiado la posibilidad de utilización de nuevos aditivos que puedan minimizar el impacto medio ambiental. Se presentan los estudios microestructurales del material obtenido, la identificación cristalográfica de las fases presentes así como los comportamientos de los parámetros que definen la propia reacción. Con la información obtenida se propone el mecanismo predominante de la síntesis del Nitruro de Silicio mediante SHS.

Dynamic range extension of SiPM detectors with the time-gated operation

The silicon photomultiplier (SiPM) is a novel detector technology that has undergone a fast development in the last few years, owing to its single-photon resolution and ultra-fast response time. However, the typical high dark count rates of the sensor may prevent the detection of low intensity radiation fluxes. In this article, the time-gated operation with short active periods in the nanosecond range is proposed as a solution to reduce the number of cells fired due to noise and thus increase the dynamic range. The technique is aimed at application fields that function under a trigger command, such as gated fluorescence lifetime imaging microscopy.

Structure of a-Si:H/a-Si1-xCx:H multilayers deposited in a reactor with automated substrate holder

This paper deals with the structural properties of a-Si:H/a-Si1-xCx: H multilayers deposited by glow-discharge decomposition of SiH4 and SiH4 and CH4 mixtures. The main feature of the rf plasma reactor is an automated substrate holder. The plasma stabilization time and its influence on the multilayer obtained is discussed. A series of a-Si:H/a-Si1-xCx: H multilayers has been deposited and characterized by secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). No asymmetry between the two types of interface has been observed. The results show that the multilayers present a very good periodicity and low roughness. The difficulty of determining the abruptness of the multilayer at the nanometer scale is discussed.

Technological solution for the automatic replacement of the catalytic filaments in HWCVD

The degradation of the catalytic filaments is the main factor limiting the industrial implementation of the hot wire chemical vapor deposition (HWCVD) technique. Up to now, no solution has been found to protect the catalytic filaments used in HWCVD without compromising their catalytic activity. Probably, the definitive solution relies on the automatic replacement of the catalytic filaments. In this work, the results of the validation tests of a new apparatus for the automatic replacement of the catalytic filaments are reported. The functionalities of the different parts have been validated using a 0.2 mm diameter tungsten filament under uc-Si:H deposition conditions.

Degradation of thin tungsten filaments at high temperature in HWCVD

The degradation of the filaments is usually studied by checking the silicidation or carbonization status of the refractory metal used as catalysts, and their effects on the structural stability of the filaments. In this paper, it will be shown that the catalytic stability of a filament heated at high temperature is much shorter than its structural lifetime. The electrical resistance of a thin tungsten filament and the deposition rate of the deposited thin film have been monitored during the filament aging. It has been found that the deposition rate drops drastically once the quantity of dissolved silicon in the tungsten reaches the solubility limit and the silicides start precipitating. This manuscript concludes that the catalytic stability is only guaranteed for a short time and that for sufficiently thick filaments it does not depend on the filament radius.

Coexistence of Two Kinds of Fluorinated Hydrogenated Micelles as Building Blocks for the Design of Bimodal Mesoporous Silica with Two Ordered Mesopore Networks

A simple and effective route has been developed for the synthesis of bimodal (3.6 and 9.4 nm) mesoporous silica materials that have two ordered interconnected pore networks. Mesostructures have been prepared through the self assembly mechanism by using a mixture of polyoxyethylene fluoroalkyl ether and triblock copolymer as building block. The investigation of the RF8(EO)9/P123/water phase diagram evidences that in the considered surfactant range of concentrations, the system is micellar (L1). DLS measurements indicate that this micellar phase is composed of two types of micelles, the size of the first one at around 7.6 nm corresponds unambiguously to the pure fluorinated micelles. The second type of micelles at higher diameter consists of fluorinated micelles which have accommodated a weak fraction of P123 molecules. Thus, in this study the bimodal mesoporous silica are really templated by two kinds of micelles.

Tailored Jeffamine molecular tools for ordering mesoporous Silica

Herein, we report the formation of organized mesoporous silica materials prepared from a novel nonionic gemini surfactant, myristoyl-end capped Jeffamine, synthesized from a polyoxyalkyleneamine (ED900). The behavior of the modified Jeffamine in water was first investigated. A direct micellar phase (L1) and a hexagonal (H1) liquid crystal were found. The structure of the micelles was investigated from the SAXS and the analysis by Generalized Indirect Fourier Transformation (GIFT), which show that the particles are globular of coreshell type. The myristoyl chains, located at the ends of the amphiphile molecule are assembled to form the core of the micelles and, as a consequence, the molecules are folded over on themselves. Mesoporous materials were then synthesized from the self-assembly mechanism. The recovered materials were characterized by SAXS measurements, nitrogen adsorptiondesorption analysis, transmission and scanning electron microscopy. The results clearly evidence that by modifying the synthesis parameters, such as the surfactant/silica precursor molar ratio and the hydrothermal conditions, one can control the size and the nanostructuring of the resulting material. It was observed that, the lower the temperature of the hydrothermal treatment, the better the mesopore ordering.

Anisotropic surface properties of micro/nanostructured a-C:H:F thin films with self-assembly applications

The singular properties of hydrogenated amorphous carbon (a-C:H) thin filmsdeposited by pulsed DC plasma enhanced chemical vapor deposition (PECVD), such as hardness and wear resistance, make it suitable as protective coating with low surface energy for self-assembly applications. In this paper, we designed fluorine-containing a-C:H (a-C:H:F) nanostructured surfaces and we characterized them for self-assembly applications. Sub-micron patterns were generated on silicon through laser lithography while contact angle measurements, nanotribometer, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the surface. a-C:H:F properties on lithographied surfaces such as hydrophobicity and friction were improved with the proper relative quantity of CH4 and CHF3 during deposition, resulting in ultrahydrophobic samples and low friction coefficients. Furthermore, these properties were enhanced along the direction of the lithographypatterns (in-plane anisotropy). Finally, self-assembly properties were tested with silicananoparticles, which were successfully assembled in linear arrays following the generated patterns. Among the main applications, these surfaces could be suitable as particle filter selector and cell colony substrate.

Síntesis de capas de SiC en substrato de Si mediante implantación iónica

En este trabajo se investiga la síntesis de estructuras SiC/Si mediante implantación iónica de carbono en Si. Las implantaciones se han realizado a energías entre 25 y 300 keV y las dosis en el rango lO^^ylO^^ cm , manteniendo el substrato a temperatura ambiente o 500°C. Algunas estructuras han sido recocidas a 1150°C. Los resultados indican que implantando a temperatura ambiente se forma una capa de SiC amorfa y de composición gradual, que recristaliza formando precipitados de ß-SiC con orientaciones aleatorias después del recocido. Además se forma un capa superficial rica en carbono, debida a la difusión del carbono hacia la superficie durante la implantación, y que desaparece con el recocido. Implantando a 500°C se forma directamente una capa con una muy alta densidad de precipitados de ß-SiC orientados preferencialmente con la matriz de silicio. Dada la estabilidad térmica y química de dicha capa se han realizado membranas de SiC mediante técnicas fotolitográficas y ataque químico selectivo, cuya rugosidad superficial es inferior a 6 nm. Estas membranas muestran unos gradientes de tensiones residuales, que prácticamente desaparecen después del recocido. Los resultados confirman la potencialidad de la implantación iónica para la formación de estructuras microme-cánicas de SiC sobre Si.

Calorimetry of hydrogen desorption from ɑ-Si nanoparticles

The process of hydrogen desorption from amorphous silicon (ɑ-Si) nanoparticles grown by plasmaenhanced chemical vapor deposition (PECVD) has been analyzed by differential scanning calorimetry (DSC), mass spectrometry, and infrared spectroscopy, with the aim of quantifying the energy exchanged. Two exothermic peaks centered at 330 and 410 °C have been detected with energies per H atom of about 50 meV. This value has been compared with the results of theoretical calculations and is found to agree with the dissociation energy of Si-H groups of about 3.25 eV per H atom, provided that the formation energy per dangling bond in ɑ-Si is about 1.15 eV. It is shown that this result is valid for ɑ-Si:H films, too