Films of hydrogenated silicon oxycarbonitride. Part I. Chemical and phase composition

The method of preparation of hydrogenated silicon oxycarbonitride films with variable composition SiC _x N _y O _z : H by the plasma chemical vapor decomposition of a volatile organosilicon compound, 1,1,1,3,3,3-hexamethyldisilazane (enhanced to IUPAC, bis(trimethylsilyl)amine) in a gas phase containing nitrogen and oxygen in the temperature range of 373–973 K has been developed. It has been shown that nitrogen and oxygen provide the decrease in carbon content in films due to gas-phase reaction giving volatile products (CN)₂, CH₄, CO, and H₂(H). The obtained SiC _x N _y O _z : H films are nanocomposite, in the amorphous part of which the nanocrystals are distributed, which belong to the determined phases of the Si-C-N system, namely, α-Si₃N₄, α-Si_{3 ? x} C _x N₄, and graphite.     

Ion beam deposition of amorphous hydrogenated carbon films on amorphous silicon interlayer: Experiment and simulation

A thick layer of amorphous silicon (a-Si) was deposited on industrial grade crystalline n-Si < 111 > substrate by means of electron beam evaporation. On top of a-Si layer, amorphous hydrogenated carbon (a-C:H) film was grown by direct ion beam deposition from acetylene precursor gas. In order to study on atomic level the a-C:H film growth on amorphous silicon, a theoretical model was developed in a form of reaction rate (kinetic) equations. Numerical simulation using this model has revealed that the ratio of sp³/sp² content in the film is heavily influenced by relaxation rate of the carbon atoms in a sub-surface region of the film that were activated by ion irradiation. The final structure of a-C:H film does not depend much on elemental composition and structure of amorphous Si coating, provided that deposition procedure is not terminated at its initial stage but continues for more than 60 s. It became evident, therefore, that the use of a-Si interlayer with a-C:H films could be particularly beneficial when a need arises to minimize or eliminate the effect of the substrate. As one of such cases, a poor adhesion of amorphous carbon on steel and other ferrous alloys could be mentioned.     

Simulation of silicon film growth by silane decomposition at low pressures and temperatures

Silicon film deposition by silane decomposition in LPCVD (Low Pressure Chemical Vapor Deposition) process has been simulated by numerical computation of the governing transport and reaction equations, assuming that the rate of silane decomposition in the gas phase controls the overall film growth rate. The film growth rate and the film uniformity increase with the reactant flow rate when the flow rate is relatively low, but they decrease at higher flow rates due to the negative effect of the reduced reaction time in the reactor. Accordingly, the film deposition process is optimized by controlling the reactant flow rate so that the position of the maximum SiH₄-decomposition rate in the gas phase is located above the substrate region. With a larger degree of the substrate tilting, the growth rate decreases but the film uniformity is improved. The film uniformity is also improved when the pressure is low.     

Efficient visible luminescence of nanocrystalline silicon prepared from amorphous silicon films by thermal annealing and stain etching

Films of nanocrystalline silicon (nc-Si) were prepared from hydrogenated amorphous silicon (a-Si:H) by using rapid thermal annealing. The formed nc-Si films were subjected to stain etching in hydrofluoric acid solutions in order to passivate surfaces of nc-Si. The optical reflectance spectroscopy revealed the nc-Si formation as well as the high optical quality of the formed films. The Raman scattering spectroscopy was used to estimate the mean size and volume fraction of nc-Si in the annealed films, which were about 4 to 8 nm and 44 to 90%, respectively, depending on the annealing regime. In contrast to as-deposited a-Si:H films, the nc-Si films after stain etching exhibited efficient photoluminescence in the spectral range of 600 to 950 nm at room temperature. The photoluminescence intensity and lifetimes of the stain etched nc-Si films were similar to those for conventional porous Si formed by electrochemical etching. The obtained results indicate new possibilities to prepare luminescent thin films for Si-based optoelectronics.     

Simulation of silicon film growth by silane decomposition in a mercury-sensitized photo-CVD process

Deposition of silicon film by a mercury-sensitized photo-CVD process has been simulated by numerical solution of governing equations with proper boundary conditions. The results indicate that the film deposition rate is controlled by homogeneous decomposition of the reactant, silane, in the gas phase. The growth rate increases but the film uniformity decreases with the increase of reactant inlet concentration. Increase in the reactant flow rate decreases the deposition rate but gives no effect on the film uniformity. Among process variables, the light intensity and the mercurysaturator temperature are important parameters.     

The relationship between structure and mechanical properties of hydrogenated amorphous carbon films

There is increasing interest in the relations between Raman fit parameters and the mechanical properties of diamond-like carbon films. The present work describes these relations in hydrogenated diamond-like carbon films (a-C:H) deposited by an ion beam source operated at varied discharge voltages, i.e. kinetic carbon species energies. A number of highly distinct relations between Raman fit parameters and mechanical properties are identified for the a-C:H films investigated. For example the nanohardness (H) and reduced elastic modulus (E) increase almost linearly with an increase in full width at half maximum of the G-band (FWHM (G)). The film elasticity, expressed as H³/E² increases with increasing FWHM (G). In addition, H and E increase linearly with decreasing intensity ratio of the D-band and the G-band (I_D/I_G). H and E also increase with the G-band dispersion (Disp. (G)), i.e. the rate of change of the G-band position vs. excitation energy. Hydrogen contents in all films are approximately equal and range from 21.2 to 23.5 at.% over the entire set of investigated samples.     

Computational study of impact of composition,density, and temperature on thermal conductivity of amorphous silicon boron nitride

We study thermal conductivity (κ) of amorphous silicon boron nitride (a‐SiBN) for different compositions and densities as a function of temperature using density functional theory (DFT) calculations and equilibrium molecular dynamic (MD) simulations. Our library of amorphous structures consists of network models comprising 100‐200 atoms and large‐scale models with up to 57 000 atoms generated using the empirical Marian‐Gastreich two‐body potential. Crystalline structures within the Si₃N₄‐BN system are considered as well. We use 2 distinct approaches to compute thermal conductivity of a‐SiBN. To estimate κ in the high‐temperature limit we feed Clarke's phenomenological model with elasticity data obtained by DFT calculations. We further perform equilibrium MD simulations and apply the Green‐Kubo method. This approach shows decrease of κ with increasing temperature and provides results at high temperatures that agree with results derived within Clarke's model. We find that κ of a‐SiBN depends on composition and increases as the BN content in the structure increases. The effect is pronounced at low temperature but almost vanishes at high temperature. Furthermore, thermal conductivity depends on density and porosity, with a linear relation between κ and density.     

Chemical composition of subcellular particles from cultured cells of human tissue

Chemical composition of subcellular components of HeLa, KB, human heart and liver tissue-culture cell lines have been studied. The concentration of RNA, protein and phospholipid (μg/μg of DNA) of total subcellular particles was similar for all four cell lines studied. The greatest RNA concentration and lowest protein concentration is found in the microsomes as compared to the other subcellular fractions of HeLa and KB cells. The lipid P/Protein N ratio of mitochondria was greater than the other subcellular fractions from tissue-culture cell lines studied. Phosphatidyl choline and phosphatidyl ethanolamine are the major phospholipids with the former more predominant in all of the subcellular fractions of tissue-culture cells studied. Phosphatidyl inositol, phosphatidyl serine, sphingomyelin and polyglycerol phosphatide were shown to be present. Phosphatidyl choline composition (per cent of total lipid-P) is greatest in the microsomes when compared with the other subcellular fractions obtained from all of the cell lines studied except the nuclear fraction of human liver cells. Correspondingly, the mitochondrial fraction for all of the tissue culture cell lines contains the greatest composition of phosphatidyl ethanolamine except for the human liver and heart cells. The mitochondrial fraction contains the lowest amount of phosphatidyl inositol. Polyglycerol phosphatide is mainly present in the mitochondrial fraction of the tissue-culture cells. Part of a thesis submitted to the Graduate School of the University of North Dakota in partial fulfillment for the degree of Doctor of Philosophy.     

Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Numerical simulations

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used to quantitatively measure aerosol chemical compositions. The physical and chemical mechanisms affecting the ionization efficiency of evolved gas molecules are not fully understood. We have developed a numerical model for simulating the dynamics of gas molecules evolved from aerosol particles. The simulation model is composed of two main sections. The first section simulates the elastic collisions of the evolved gas molecules in a small region near the vaporization source (collision domain), where the mean free paths of the molecules are much shorter than those in the surrounding high vacuum environment. The second section simulates the free-molecular dynamics from the boundary of the first section to the ionizer. The ionization efficiencies of ammonia and hydrogen iodide molecules that evolved from ammonium iodide particles were evaluated. Our results suggest that the molecular collisions during the early stage of plume expansion and possible changes in the molecular velocities induced by these collisions could be an important mechanism affecting the observed variability in the ionization efficiency. However, the physical and chemical processes of the vaporization and ionization of aerosol particles in TDAMSs may be too complex to be quantitatively reproduced using simplified numerical models.

Copyright © 2019 American Association for Aerosol Research     

Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Laboratory experiments

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used for quantitative analysis of aerosol chemical composition, and the ionization efficiency of evolved gas molecules from aerosol particles is an important parameter for such analysis. We performed laboratory experiments using a custom-made TDAMS to investigate the key factors affecting ionization efficiency. Ammonium chloride (NH₄Cl) and ammonium iodide (NH₄I) were used as test compounds because their thermal decomposition products are expected to be simple (dominated by ammonia (NH₃) and hydrogen halide (HX)). The ion signals originating from NH₃ and HX were measured by altering the position of the ionizer relative to the vaporization point. The ratio of ion signal from NH₃ to that from HX increased with increasing divergence angle of evolved gas plumes, which suggests that the angular distribution of gas molecules could be an important factor affecting the ionization efficiency.

Copyright © 2018 American Association for Aerosol Research     

Nucleation and growth of carbon black particles during thermal decomposition of benzene

The authors determined the mechanism of carbon black formation during the thermal decomposition of benzene diluted with a stream of nitrogen. The kinetics study of nucleation and growth of particles led them to the following conclusions: The initial hydrocarbon is transformed by a gas phase reaction into macromolecules. The partial pressure of the macromolecules increases with reaction time until a supersaturation is high enough to induce condensation of macromolecules into droplets. The formation of liquid nuclei eliminates the supersaturation, and the formation of additional liquid nuclei becomes impossible. The macromolecules which continue to be formed maintain the nuclei growth. The liquid droplets are pyrolysed into a solid material. A statistical study of the distribution curves of the particle diameters indicates that the growth rate of each particle is proportional to its diameter.     

Chemical composition and mass emission factors of candle smoke particles

The aim of this study is to investigate the physical and chemical properties of particle emissions from candle burning in indoor air. Two representative types of tapered candles were studied during steady burn, sooting burn and smouldering (upon extinction) under controlled conditions in a walk-in stainless steel chamber. Steady burn emits relatively high number emissions of ultrafine particles dominated by either phosphates or alkali nitrates. The likely source of these particles is flame retardant additives to the wick. Sooting burn in addition emits larger particles mainly consisting of agglomerated elemental carbon. This burning mode is associated with the highest mass emission factors. Particles emitted during smouldering upon extinction are dominated by organic matter. A mass closure was illustrated for the total mass concentration, the summed mass concentration from chemical analysis and the size-integrated mass concentration assessed from number distribution measurements using empirically determined effective densities for the three particle types.     

Chemical composition and structural transformations of amorphous chromium coatings electrodeposited from Cr(III) electrolytes

Amorphous chromium coatings were electrodeposited from Cr(III)-based solutions containing organic (HCOONa) or phosphorus-containing (NaH₂PO₂) additives. Their structure was studied by a combination of X-ray diffraction (XRD), valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the Cr K-edge. Metalloid atoms (C or P) incorporated in electroplates structure are chemically bonded to chromium (i.e. are located in the first coordination shell). Upon annealing at elevated temperatures in vacuum, these amorphous coatings crystallize into a mixture of phases containing metallic chromium and chromium carbides or chromium phosphides. Quantitative analysis of valence-to-core XES data demonstrates that the average local structure of chromium in the amorphous coatings does not change significantly during crystallization.     

Relationship between bonding structure and mechanical properties of amorphous carbon containing silicon

Unhydrogenated amorphous carbon films with different silicon concentrations were synthesized by magnetron sputtering, and the corresponding evolution of inter-atomic bonding configurations, surface roughness and mechanical properties like hardness, modulus and stress was analyzed. Introducing silicon into amorphous carbon not only reduced the sp²-hybridized carbon bonding, it also helped to reduce residual stress. Both the hardness and elastic modulus suffered degradation when the silicon concentration was low. But these properties recovered when silicon dosage increased. Surface roughness increased when silicon concentration was low, but decreased when the silicon dosage increased. Such changes in the mechanical properties were closely related to the carbon and silicon inter-atomic interaction. The amorphous carbon network was modified by silicon, and affected by deposition kinetics. The mismatch in the atomic size and bond length, and the alteration of the carbon hybridization were determined to be the basis for the changes in the mechanical properties.     

Study of electronic structure and phase composition of porous silicon

An investigation into the electronic structure and phase composition of porous silicon surface layers with a developed structure of nanopores has been carried out using ultrasoft X-ray and X-ray photoelectron spectroscopy. Samples of porous silicon have been obtained on p-type substrates in various regimes of electrochemical etching.     

通过一水乙酰丙酮锌的热分解形成纳米级ZnO颗粒

采用X-射线粉料衍射、差热分析、傅里叶变换红外线光谱以及热场发射扫描电子显微镜,研究了在大气气氛中通过一水乙酰丙酮锌的热分解来形成ZnO颗粒.通过在≥200℃的直接加热,生成了单相ZnO.分2个步骤制备了纳米级ZnO:(a)将一水乙酰酮锌加热到150℃,对水和有机相进行蒸馏;(b)进一步加热在300℃时获得先质.     

Formation of nanosize ZnO particles by thermal decomposition of zinc acetylacetonate monohydrate

Formation of ZnO particles by thermal decomposition of zinc acetylacetonate monohydrate in air atmosphere has been investigated using XRD, DTA, FT-IR, and FE-SEM as experimental techniques. ZnO as a single phase was produced by direct heating at ≥200 °C. DTA in air showed an endothermic peak at 195 °C assigned to the ZnO formation and exothermic peaks at 260, 315 and 365 °C, with a shoulder at 395 °C. Exothermic peaks can be assigned to combustion of an acetylacetonate ligand released at 195 °C. ZnO particles prepared at 200 °C have shown no presence of organic species, as found by FT-IR spectroscopy. Particles prepared for 0.5 h at 200 °C were in the nanosize range from ∼20 to ∼40 nm with a maximum at 30 nm approximately. The crystallite size of 30 nm was estimated in the direction of the a₁ and a₂ crystal axes, and in one direction of the c-axis it was 38 nm, as found with XRD. With prolonged heating of ZnO particles at 200 °C the particle/crystallite size changed little. However, with heating temperature increased up to 500 or 600 °C the ZnO particle size increased, as shown by FE-SEM observation. Nanosize ZnO particles were also prepared in two steps: (a) by heating of zinc acetylacetonate monohydrate up to 150 °C and distillation of water and organic phase, and (b) with further heating of so obtained precursor at 300 °C.     

Crystallization of an amorphous silicon nitride powder produced in a radiofrequency thermal plasma

Crystallization behavior of an amorphous silicon nitride powder produced in an RF thermal plasma by the vapor-phase reaction of silicon tetrachloride and ammonia has been investigated. Effects of annealing conditions such as temperature and duration of heat treatment on the properties of powders were studied. Changes in the chemical and phase compositions, as well as in the morphology of powders were measured and interpreted. Annealing of the amorphous silicon nitride powder at 1450°C for 120 min resulted in a powder of about 80% crystalline phase content with an /β ratio of about 6.5. ©     

Chemical composition and corrosion protection of silane films modified with CeO2 nanoparticles

The present work aims at understanding the role of CeO₂ nanoparticles (with and without activation in cerium(III) solutions) used as fillers for hybrid silane coatings applied on galvanized steel substrates.The work reports the improved corrosion protection performance of the modified silane films and discusses the chemistry of the cerium-activated nanoparticles, the mechanisms involved in the formation of the surface coatings and its corrosion inhibition ability.The anti-corrosion performance was investigated using electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and d.c. potentiodynamic polarization. The chemical composition of silanised nanoparticles and the chemical changes of the silane solutions due to the presence of additives were studied using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance spectroscopy (NMR), respectively.The NMR and XPS data revealed that the modified silane solutions and respective coatings have enhanced cross-linking and that silane-cerium bonds are likely to occur.Electrochemical impedance spectroscopy showed that the modified coatings have improved barrier properties and the SVET measurements highlight the corrosion inhibition effect of ceria nanoparticles activated with Ce(III) ions. Potentiodynamic polarization curves demonstrate an enhanced passive domain for zinc, in the presence of nanoparticles, in solutions simulating the cathodic environment.     

Plasma-chemical synthesis of silicon carbonitride films from trimethyl(diethylamino)silane

Silicon carbonitride films of different compositions have been synthesized by plasma-enhanced chemical vapor deposition with the use of trimethyl(diethylamino)silane as the initial compound. The conditions used for the deposition of the films have been chosen from the thermodynamic modeling. The properties of the films prepared have been investigated using ellipsometry, scanning electron microscopy, X-ray photoelectron spectroscopy, IR spectroscopy, and synchrotron X-ray powder diffraction. It has been established that the synthesis temperature substantially affects the kinetics of growth and physicochemical properties of silicon carbonitride layers in the range of synthesis conditions under investigation. The films prepared consist of nanoparticles whose size increases with increasing synthesis temperature. The refractive index of the films increases from 1.6 to 2.8 with an increase in the temperature.