A combined FEG-SEM and TEM study of silicon nanodot assembly

Nanodots forming dense assembly on a substrate are difficult to characterize in terms of size, density, morphology and cristallinity. The present study shows how valuable information can be obtained by a combination of electron microscopy techniques. A silicon nanodots deposit has been studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) to estimate essentially the dot size and density, quantities emphasized because of their high interest for application. High resolution SEM indicates a density of 1.6 × 1012 dots/cm2 for a 5 nm to 10 nm dot size. TEM imaging using a phase retrieval treatment of a focus series gives a higher dot density (2 × 1012 dots/cm2) for a 5 nm dot size. High Resolution Transmission Electron Microscopy (HRTEM) indicates that the dots are crystalline which is confirmed by electron diffraction. According to HRTEM and electron diffraction, the dot size is about 3 nm which is significantly smaller than the SEM and TEM results. These differences are not contradictory but attributed to the fact that each technique is probing a different phenomenon. A core-shell structure for the dot is proposed which reconcile all the results. All along the study, Fourier transforms have been widely used under many aspects

High activity redox catalysts synthesized by chemical vapor impregnation

The use of precious metals in heterogeneous catalysis relies on the preparation of small nanoparticles that are stable under reaction conditions. To date, most conventional routes used to prepare noble metal nanoparticles have drawbacks related to surface contamination, particle agglomeration, and reproducibility restraints. We have prepared titania-supported palladium (Pd) and platinum (Pt) catalysts using a simplified vapor deposition technique termed chemical vapor impregnation (CVI) that can be performed in any standard chemical laboratory. These materials, composed of nanoparticles typically below 3 nm in size, show remarkable activity under mild conditions for oxidation and hydrogenation reactions of industrial importance. We demonstrate the preparation of bimetallic Pd–Pt homogeneous alloy nanoparticles by this new CVI method, which show synergistic effects in toluene oxidation. The versatility of our CVI methodology to be able to tailor the composition and morphology of supported nanoparticles in an easily accessible and scalable manner is further demonstrated by the synthesis of Pdshell–Aucore nanoparticles using CVI deposition of Pd onto preformed Au nanoparticles supported on titania (prepared by sol immobilization) in addition to the presence of monometallic Au and Pd nanoparticles

Prediction of LPCVD silicon film structure using combined experimental and numerical analyses

Experiments and numerical simulations were conducted to predict some structural features of silicon films prepared by Low Pressure Chemical Vapor Deposition. The relationships between the deposition conditions at film-gas interface and the resulting film structure were systematically investigated. A mechanism was proposed to account for the structural changes observed in the films when varying the deposition conditions. A numerical code was then developed which is able to simulate the crystalline structure of the deposited films. A few illustrative simulation examples are given and compared with the corresponding experiments

Prediction of LPCVD silicon film microstructure from local operating conditions using numerical modeling

Numerical modeling of LPCVD silicon film microstructure was performed by means of two combined numerical codes. First, a a two-dimensional kinetic reactor model was used to calculate the conditions at the gas-substrate interface. Next, these conditions were used as imput parameters of a semi-empirical two-dimensional statistical model in order to predict the cristallized content of the deposits. This model also gave an image of the space distribution of crystallites inside the layer. A lot of experimental knowledge was required in order to develop a realistic aigorithm, accounting for the most important events. Most of the coefficients of the empirical laws were obtained by fitting the calculations to various experiments. Although the capabilities of our statistical model are yet limited, numerical simulations succeeded in predicting several microstructural features of LPCVD silicon films from operating conditions

XPS study of CVD silicon thin films deposited on various substrates from SiH4 gaseous precursor

CVD silicon films were deposited from SiH4 pyrolysis on amorphous SiO2 layers heated at various temperatures in the range 560-620°C, and on amorphous SiNx or polycrystalline silicon layers at 580°C. According to the substrate temperature, the silicon films can be completely crystallized for the highest temperature, or amorphous for the lowest temperature, in the case of a-SiO2 substrates. For intermediate temperatures (570 or 580°C), the silicon films are crystallized near the a-SiO2 substrate and then amorphous up to the surface, when they are entirely amorphous on a-SiNx or c-Si substrates as shown elsewhere by TEM and SEM observations. X.P.S. valence band spectra, core levels photoelectrons and SiKL2,3L2,3 Auger transitions examinations, have shown that a lower growth rate of the silicon films on the a-SiO2 substrate at 570 or 580°C leads to the formation of nanocrystallized silicon deposits at the early stage of the deposition. For a-SiNx substrate, a higher growth rate was observed at the first stage of the deposition, at 580°C. These results can contribute to the understanding of the relationship between the structure changes of the deposit and the nature of the substrates

Correlations between stress and microstructure into LPCVD silicon films

Silicon films have been deposited by low pressure chemical vapour deposition (LPCVD) from silane SiH4 at temperatures and pressures varying respectively from 520°C to 750°C and from 100 mTorr to 300 mTorr. Films residual stresses are studied as a function of deposition parameters (temperature and total pressure). Major stress variations (from compressive to tensile values) are explained through the cumulated influences of the deposition and crystallisation phenomena, evidencing correlations with silicon microstructure (amorphous, semicrystalline, mixed crystalline or polycrystalline) characterised by transmission electron microscopy (TEM) . Finally, residual stress and silicon microstructure are directly related to the ratio between deposition and crystallisation rates, enabling the development of laws for their modelling into silicon films deposited by LPCVD

Fluid dynamic simulation of CrO2(OH)2 volatilization and gas phase evolution during the oxidation of a chromia forming alloy

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Kinetics of the initial stages of film formation during low pressure chemical vapour deposition of polysilicon by pyrolysis of silane

A kinetic model for description of the process of silicon film formation on silica by thermal decomposition of silane at reduced pressure has been proposed. The model is based on the concept of kinetic interdependence between heterogeneous catalytic chemical reaction and fundamental structure forming phenomena - nucleation and nuclei growth. A number of experimental data for deposition rates and polysilicon grains sizes have been mathematically processed in order to derive kinetic equations for the rates of nucleation and nuclei growth as functions of reactor operating conditions (pressure and temperature) as well as process duration. Furthermore, based on both the good correspondence achieved between the experimental results and the model, and the deductions of thermodynamic theory of nucleation, the kinetic equations derived were analysed in regard to the general description of silicon film structure evolution. The analysis of the model, by confïrming the general trends established between the arrival and the surface diffusion rates of silicon adspecies, contributes to clarify the mechanism of the initial stages of film microstructure formation. The results obtained show that kinetics of structure evolution can be successfully described by developing the existing CVD phenomenological kinetic models further to an atomistic level