Si diffusion in magnetron sputtered silicon carbide films deposited on silicon and carbon substrates

Self-diffusion of silicon in magnetron sputtered silicon carbide films deposited on different substrates (crystalline silicon and glassy carbon) is investigated. Since crystallization of amorphous silicon carbide films strongly depends on the substrate, the diffusivity of silicon is expected to depend on the substrate as well. Isotope hetero-structures and secondary ion mass spectrometry were used for analysis. For amorphous samples an upper limit of the diffusivity of 1 × 10^− 21 m²/s is derived at 1100 C°. For crystallized films diffusivities between 1350 °C and 1600 °C are found to be not significantly different for the two types of substrates. For samples deposited on glassy carbon substrates an activation enthalpy ΔH_D = (8.7 ± 0.9) eV was found for the self-diffusion of Si. The consequences of our findings for crystallization are discussed.     

Kinetics and mechanism of reaction between silicon carbide and silica

The kinetics and mechanism of reaction between SiO₂ and SiC are studied. The reaction rate is shown to be limited by the carbon diffusion from the SiC bulk to the SiC/SiO₂ interface, where carbon reacts with the SiO and oxygen resulting from SiO₂ decomposition. Equations are presented which describe the time variation of SiC conversion and the temperature variation of the carbon diffusivity in SiC.     

Thermodynamic simulation of silicon deposition from the gas phase of Si-Cl-H system

Deposition of silicon from gas mixtures including trichlorosilane and silicon tetrachloride are studied with thermodynamic simulation techniques. Process outputs for a wide range of conditions (temperature, pressure, composition of the input gas phase) are determined. Conditions for the process of obtaining polycrystalline silicon and synthesis of epitaxial layers are recommended.     

Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates

SiC films were deposited on cemented carbide substrates by employing microwave plasma chemical vapor deposition method using tetramethylsilane (Si(CH₃)₄) diluted in H₂ as the precursor. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and scratching technique were used to characterize morphology, composition, phases present and adhesion of the films. Experimental results show that the deposition pressure has great influence on morphologies and phase composition of the films. In sequence, SiC films with a cauliflower-like microstructure, granular films with terrace-featured SiC particles coexisting with Co₂Si compound and clusters of nanometer SiC nanoplatelets appear as a function of the deposition pressure. In terms of plasma density and substrate temperature, this sequential appearance of microstructures of SiC films was explained. Adhesion tests showed that among the three types of films studied, the films with the terrace-featured SiC particles have relatively higher adhesion. Such knowledge will be of importance when the SiC films are used as interlayer between diamond films and cemented carbide substrates.     

Antimicrobial activity of silica coated silicon nano-tubes (SCSNT) and silica coated silicon nano-particles (SCSNP) synthesized by gas phase condensation

Silica-coated, silicon nanotubes (SCSNTs) and silica-coated, silicon nanoparticles (SCSNPs) have been synthesized by catalyst-free single-step gas phase condensation using the arc plasma process. Transmission electron microscopy and scanning tunneling microscopy showed that SCSNTs exhibited a wall thickness of less than 1 nm, with an average diameter of 14 nm and a length of several 100 nm. Both nano-structures had a high specific surface area. The present study has demonstrated cheaper, resistance-free and effective antibacterial activity in silica-coated silicon nano-structures, each for two Gram-positive and Gram-negative bacteria. The minimum inhibitory concentration (MIC) was estimated, using the optical densitometric technique, and by determining colony-forming units. The MIC was found to range in the order of micrograms, which is comparable to the reported MIC of metal oxides for these bacteria. SCSNTs were found to be more effective in limiting the growth of multidrug-resistant Staphylococcus aureus over SCSNPs at 10 μg/ml (IC 50 = 100 μg/ml).     

Morphology, electrical and optical properties of undoped ZnO layers deposited on silicon substrates by PEMOCVD

The goal of this work is to investigate the morphology, electrical and optical properties of undoped ZnO (i-ZnO) thin layers deposited on Si substrates with (100) and (111) orientations. Plasma enhanced metalorganic chemical vapor deposition (PEMOCVD) was used for the deposition of i-ZnO layers at different temperatures. Atomic force microscopy (AFM), ellipsometry and four-probe method were used for the analysis. It is found that substrate orientation and growth temperature determine the morphological (grains size, surface roughness) as well as electrical properties of ZnO films. It is shown that the refractive index value depends on the surface morphology. It is concluded that properties of i-ZnO layers deposited on different Si substrates at different conditions exhibit some trends and peculiarities, which have to be taken into account for the processing of heterojunction solar cells by the PEMOCVD method.     

Study of carbide-forming element interlayers for diamond nucleation and growth on silicon and WC-Co substrates

Diamond nucleation and growth on several typical carbide-forming elements (CFE) (Ti, Cr and W) coated Si and WC-Co substrates were studied. The ion beam sputtered CFE interlayers show an amorphous/nanocrystalline microstructure. The diamond formed on the CFE coated substrates shows higher nucleation density and rate and finer grain structure than on uncoated substrates. Consequently, nanocrystalline diamond thin films can be formed on the CFE coated substrates under conventional microcrystalline diamond growth conditions. Among the three tested CFE interlayers, diamond has the highest nucleation density and rate on W layer and the lowest on Ti layer. The diamond nucleation density and rate on CFE coated WC-Co are much higher than those on widely used metal nitride coated WC-Co.     

Effects of additives on the nitridation process of foamed materials containing silicon and silicon carbide powders

Without using any additive, the nitridation process of silicon powder was slow and the main product was α-Si₃N₄ due to the cycling production of SiO species. The addition of Al₂O₃ and Y₂O₃ could facilitate the nitridation process resulting in a higher β-Si₃N₄ content presumably due to the liquid phase formed between Al₂O₃, Y₂O₃ and surface silica on silicon powder. When a small amount of 1.76% Fe₂O₃ was added, the accelerated nitridation process was attributed to the FeSi₂ liquid phase produced by reaction Fe element with surface silica at a lower temperature of 1212 °C, but the Al₂O₃ and Y₂O₃ additives could still be active for sustaining the nitridation process at higher temperature. At a higher Fe₂O₃ concentration of 3.46%, the nitridation process was mainly controlled by the formed FeSi₂ liquid phase. This study has demonstrated the active role of using Al₂O₃ and Y₂O₃ combination and Fe₂O₃ on the nitridation process, which could be helpful for further investigation on reaction bonding of SiC and Si₃N₄ ceramics.     

High gas sensitivity of tin oxide ultrathin films deposited on glasses and alumina substrates

Tin oxide ultrathin films were deposited on optically flat Pyrex glass, optically flat quartz glass and sintered alumina substrates by the ion-beam sputtering method. Films with thickness varying from about 1 to 500 nm were annealed in air at 500 °C for 50 h. The gas sensing properties of these films were investigated at 300 °C against synthetic air containing 0.5% hydrogen. A small substrate dependence of sensor behaviour was observed; however, an overwhelmingly important thickness dependence occurred in all substrates tried. Sensitivities higher than ∼10⁴ or 10⁵ were obtained in films in a narrow thickness range of about 3 to 20 nm, and films thicker or thinner than this were relatively insensitive.     

Influence of carbide formation on the strength of carbon fibres on which silicon and titanium have been deposited

Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10^–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation _m d ^–1/2, where d is the thickness of the TiC layer.     

Quantitative investigation into the nucleation mechanism of lead and silver films vacuum deposited onto SiO,carbon and mica substrates

The origin of the occurrence of nuclei was quantitatively investigated using a microbalance and an electron microscope. The completion time of preferred nucleation and the time variation of the number density of stable nuclei were expressed in terms of measurable quantities such as the sticking coefficient of the incident vapour beam and the surface diffusion distance of adatoms. An analysis of the experimental results showed that the sum of the calculated values of the completion time of preferred nucleation and the appearance time of nuclei ranged from 2 to 60 s, whereas the time at which nucleus saturation occurred was found to range from 100 to 6000 s. It was concluded that the observed increase in the density of nuclei is caused by random nucleation. The occurrence of nucleus saturation can also be explained on the basis of random nucleation and growth coalescence.     

Influence of interface compounds on interface bonding characteristics of aluminium and silicon carbide

The interface plays an important role in improving the mechanical properties of metal matrix composites. Hence, it is essential to evaluate interface bonding of Aluminium/Silicon carbide. The interface bonding of Aluminum/Silicon carbide samples were prepared by various processing temperatures at constant holding time. The interface compounds at the interface were evaluated by an energy dispersive spectroscope and diffusion length of compounds was calculated by Arrhenius equation. The interface structure was analyzed by a scanning electron microscope. The interface characteristics were evaluated by tensile test and microhardness test.     

Influence of filament-to-substrate distance on the spectroscopic,structural and optical properties of silicon carbide thin films deposited by HWCVD technique

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition (HWCVD) technique from pure silane and methane gas mixture. The effect of filament distance to the substrate on the structural and optical properties of the films was investigated. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman scattering spectroscopy and UV–Vis–NIR spectroscopy were carried out to characterize SiC films. XRD patterns of the films indicated that the film deposited under highest filament-to-substrate distance were amorphous in structure, while the decrease in distance led to formation and subsequent enhancement of crystallinity. The Si–C bond density in the film structure obtained from FTIR data, showed significant increment with transition from amorphous to nano-crystalline structure. However, it remained almost unchanged with further improvement in crystalline volume fraction. From Raman data it was observed that the presence of amorphous silicon phase and sp ² bonded carbon clusters increased with the decrease in distance. This reflected in deterioration of structural order and narrowing the optical band gap of SiC films. It was found that filament-to-substrate distance is a key parameter in HWCVD system which influences on the reactions kinetics as well as structural and optical properties of the deposited films.     

微波等离子体化学气相沉积法在硅片上同时生长碳化硅和金刚石

研究了衬底温度、核化密度、衬底表而预处理等工艺参数对微波等离子体化学气相沉积法在硅片上同时生长碳化硅和金刚石的影响.采用扫描电镜、X-射线衍射、喇曼光谱和红外光谱对样品进行了表征.结果表明:从高核化密度生长的金刚石膜中探测不到碳化硅;不论对硅衬底进行抛光预处理还是未抛光预处理,从低核化密度牛长的金刚石厚膜中总能探测到碳化硅.碳化硅生长在硅衬底上未被金刚石覆盖的地方,或者是在金刚石晶核之间的空洞处.碳化硅形成和金刚石生长是同时发生的两个竞争过程.此研究结果为制备金刚石和碳化砟复合材料提供了一种新的方法.     

Investigation of physico-chemical properties of conductive Ga-doped ZnO thin films deposited on glass and silicon wafers by RF magnetron sputtering

We report on the physico-chemical properties of Undoped and Ga-doped ZnO films fabricated on glass and p-Silicon wafers at room temperature by RF magnetron sputtering using a ZnO and Ga₂O₃ mixture raw powder target without sintering procedure. X-ray diffraction (XRD) and energy dispersion spectroscopy (EDS), scanning electronic microscopy, Raman scattering, ultraviolet–visible spectroscopy, photoluminescence (PL), Hall effect and impedance spectroscopy technique have been applied for the comparative study of ZnO and ZnO:Ga thin films. XRD and Raman studies have shown that the deposited films have a preferred orientation growth with Ga atoms both in substitutional and interstitial positions. EDS analyses have allowed to show that the metallic Ga atoms have been incorporated in the ZnO films. Doping by gallium resulted in a slight increase in the optical band gap energy of the films while the optical transmittance remains about 80 %. The PL analysis at room temperature revealed violet, blue, green and red emissions. Room temperature Hall measurements show that the lowest resistivity was 3.40 × 10^?4 Ω cm with an electron mobility of 18.56 cm²/V.s for an optimum Ga concentration of 4 wt%. Impedance spectroscopy study showed that σ_ac obeys the relation \(\sigma_{ac} = A\omega^{s}\). The exponent ‘‘s’’ was found to decrease with increasing the temperature. It is found that, the AC conductivity of all samples follow the correlated barrier hopping model. The Nyquist plots showed a single semicircle, indicating an equivalent circuit with a single parallel resistor R and capacitance C network. The values of the activation energy E _a deduced from both DC conductivity and relaxation frequency for all the studied samples ranged from 0.51 to 0.73 eV and the results are explained on the basis of the induced defects due to the addition of Ga into the ZnO films.     

Optical,structural and electrical characteristics of aluminum oxynitride thin films deposited in an Ar-N gas mixture RF-sputtering system

The optical, structural and electrical characteristics of aluminum oxynitride thin films deposited on silicon by rf-sputtering under a fixed oxygen flow and two different Ar and N gas flows are reported. The stoichiometry of the films was studied by EDS as a function of the deposition parameters. In general, the relative oxygen content within the films was higher for a high N/Ar (5/1) gas flow ratio, these films presented refractive indexes in the range of 1.5–2.0, with deposition rates close to 4.0 nm/min, and surface roughness of approximately 13 Å. Films deposited with a low N/Ar (1/5) flow ratio presented refractive indexes in the range of 1.7 to 2.0, deposition rates of 7 nm/min and surface roughness of 26 Å. IR spectroscopy measurements on these films presented an absorption band spreading from 500 to 900 cm^?1. The width and peak of this band depends on the rf power and correlates with the oxygen content in the films. Films with the best electrical characteristics present an average dielectric constant of 7.2 and 8.7 standing electric fields up to 4.5 and 2 MV/cm without observing destructive dielectric breakdown for high and low N/Ar gas ratios respectively.     

Influence of polymer infiltration and pyrolysis process on mechanical strength of polycarbosilane-derived silicon carbide ceramics

In this article, strength evaluation of silicon carbide (Si–C) ceramics fabricated from polycarbosilane (PCS) precursor is described. Si–C ceramics was prepared by firing a green body made of the mixture of Si–C nano-powders and a PCS solution at 1,273 K in N₂ gas for an hour. To obtain dense Si–C, the solution was infiltrated into the produced body, and then it was fired again. The polymer infiltration and pyrolysis (PIP) process was conducted up to 12 cycles. Si–C ceramics was diced to be rectangle shape measuring 1.0 mm × 3.0 mm × 0.5 mm, and was subjected to the three-point bending test for measurement of the Young’s modulus and bending strength. Si–C specimens fabricated through PIP processes less than 2 cycles showed non-linear force–displacement curves like a polymer, whereas those through the processes more than 3 cycles showed linear relations and fractured in a brittle manner. The Young’s modulus of 12-cycles-PIPs specimen was found to be 56 GPa on average, which was approximately 22-fold of non-PIP specimen. The bending strength was also increased with an increase in the number of PIP process. The maximum value was found to be 157 MPa. The cause of the influence of PIP process on the mechanical characteristics is discussed using a PCS-derived Si–C model.     

Formation process of mixed silicon and tungsten carbide from copyrolysis of polysilane and metallic tungsten: Part I

Copyrolysis of polysilane with metallic tungsten addition in the range of 0–12 at % W was performed. A significant decrease of the mass loss during the polysilane pyrolysis was observed. Also the specific surface area, gas evolution and occuring phase formation changed. The tungsten forms silicides and carbides depending on the applied pyrolysis temperature. The reactions are controlled by transport phenomena of carbon and tungsten. In the end of the process the silicon carbide (SiC) and tungsten carbide (WC) are stable. The obtained powders are sintered to porous ceramics with a significant pore gradient and a variation of the hardness in a micrometer range.     

Formation process of mixed silicon and titanium carbide from copyrolysis of polysilane and metallic titanium: Part II

In part I we reported a copyrolysis of polysilane with metallic tungsten [1]. Now copyrolysis of chlorine containing polysilane with metallic titanium addition in the range of 0–10 at % Ti was performed. A significant decrease of the mass loss during the polysilane pyrolysis was observed. Also the specific surface area, gas evolution and occuring phase formation changes. The titanium forms preferently carbides and temporary silicides depending on the applied pyrolysis temperature. The reactions are controlled by transport phenomena of carbon and titanium. In the end of the process silicon carbide (SiC) and titanium carbide (TiC_x) are stable.