Initial stages in the growth of polycrystalline diamond on silicon

Polycrystalline diamond films prepared in a hot filament chemical vapour deposition reactor were investigated with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to identify chemically and structurally distinguishable phases during nucleation and early stages of diamond growth. This was achieved by investigating a series of films grown under identical conditions for 5 min to 4 h. In addition, the interface between a solid diamond film and its silicon substrate was studied after the film had been removed from the substrate. Carbon deposition commences initially with the simultaneous growth of diamond crystallites along scratches and a layer of microcrystalline graphite covering the remainder of the substrate. Small amounts of SiC could also be identified during the first 100 min of deposition. Once the individual diamond crystallites have grown together to form a continuous layer, the graphitic phase in the spectra is replaced by an amorphous carbon phase which we attribute to the grain boundaries between the crystals. Inspection of the film backside revealed that the amorphous carbon had merely overgrown the microcrystalline graphite which was still present as the major component. Only after prolonged growth times (24 h) did the Raman and XPS spectra exhibit the characteristic diamond features free from any other contributions. On the basis of these observations a model for the initial stages of diamond growth on Si is developed.     

Diamond nucleation and growth under very low-pressure conditions

The synthesis of thin diamond films using various chemical vapor deposition methods has received significant attention in recent years due to the unique characteristic of diamond, which make it an attractive candidate for a wide range of applications. In order to grow diamond epitaxially, the proper control of diamond nucleation on mirror-polished Si is essential. Adding the negative bias voltage to the substrate is the most popular method. This paper has proposed a new method to greatly enhance the nuclear density. Under very low pressure (1 torr), the high-density nucleation of diamond is achieved on mirror-polished silicon in a hot-filament chemical vapor deposition (HFCVD). Scanning electron microscopy has demonstrated that the nuclear density can be as high as 10¹⁰–10¹¹ cm⁻². Raman spectra of the sample have shown a dominant diamond characteristic peak at 1332 cm⁻¹. The pressure effect has been discussed in detail and it has been shown that the very low pressure is a very effective means to nucleate and grow diamond films on mirror-polished silicon. Extraordinary pure hydrogen (purity=99.9999%) was used as the source. Compared with the highly pure hydrogen (purity=99.99%), we found that the density of nucleation was greatly increased. The residual oxygen in the hydrogen displayed a very obvious negative effect on the nucleation of diamond, although it can accelerate the growth of diamond. Based on these results, it was suggested that the enhanced nucleation at very low pressure should be attributed to an increased mean free path, which induced a high density of atomic hydrogen and hydrocarbon radicals near the silicon surface. Atomic hydrogen can effectively etch the oxide layer on the surface of silicon and so greatly enhance the nucleation density.     

Early stages of growth of gold layers sputter deposited on glass and silicon substrates

Extremely thin gold layers were sputter deposited on glass and silicon substrates, and their thickness and morphology were studied by Rutherford backscattering (RBS) and atomic force microscopy (AFM) methods. The deposited layers change from discontinuous to continuous ones for longer deposition times. While the deposition rate on the silicon substrate is constant, nearly independent on the layer thickness, the rate on the glass substrate increases with increasing layer thickness. The observed dependence can be explained by a simple kinetic model, taking into account different sticking probabilities of gold atoms on a bare glass substrate and regions with gold coverage. Detailed analysis of the shape of the RBS gold signal shows that in the initial stages of the deposition, the gold layers on the glass substrate consist of gold islands with significantly different thicknesses. These findings were confirmed by AFM measurements, too. Gold coverage of the silicon substrate is rather homogeneous, consisting of tiny gold grains, but a pronounced worm-like structure is formed for the layer thickness at electrical continuity threshold. On the glass substrate, the gold clusters of different sizes are clearly observed. For later deposition stages, a clear tendency of the gold atoms to aggregate into larger clusters of approximately the same size is observed. At later deposition stages, gold clusters of up to 100 nm in diameter are formed.     

Impact of AFM-induced nano-pits in a-Si:H films on silicon crystal growth

Conductive tips in atomic force microscopy (AFM) can be used to localize field-enhanced metal-induced solid-phase crystallization (FE-MISPC) of amorphous silicon (a-Si:H) at room temperature down to nanoscale dimensions. In this article, the authors show that such local modifications can be used to selectively induce further localized growth of silicon nanocrystals. First, a-Si:H films by plasma-enhanced chemical vapor deposition on nickel/glass substrates are prepared. After the FE-MISPC process, yielding both conductive and non-conductive nano-pits in the films, the second silicon layer at the boundary condition of amorphous and microcrystalline growth is deposited. Comparing AFM morphology and current-sensing AFM data on the first and second layers, it is observed that the second deposition changes the morphology and increases the local conductivity of FE-MISPC-induced pits by up to an order of magnitude irrespective of their prior conductivity. This is attributed to the silicon nanocrystals (<100 nm) that tend to nucleate and grow inside the pits. This is also supported by micro-Raman spectroscopy.     

Electrocrystallization of lead dioxide: Analysis of the early stages of nucleation and growth

The kinetics of the early stages of the electrocrystallization of lead dioxide onto a vitreous carbon electrode is studied via the analysis of the early rising portion of the current-time transients (CTT's). The CTT's are recorded when the rate of charge-transfer across the electrode/electrolyte interface is the sole controlling mechanism for crystal growth. Induction times for the nucleation of the 3D growth centres are determined as a function of the applied potential. A possible controlling mechanism for the onset of the nucleation of the 3D growth centres is thus considered to be the need for the formation of at least a monolayer of deposit and/or adsorbed layers.It has been shown that CTT's that are recorded during the charge-transfer controlled growth processes allows the possibility of observing the formation of two distinct phases of a deposit, a phenomenon that would not be observed if CTT's are recorded when diffusion is the controlling mechanism for growth. The electrocrystallization of lead dioxide is shown to proceed, at least initially, via the formation of two distinct phases of PbO₂.     

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp³ and sp² network and the quantitative analysis were done by Fourier transform infrared spectroscopy.     

Diamond growth on Ir/CaF2/Si substrates

A new multi-layered structure of heteroepitaxial (1 0 0) and (1 1 1) Ir grown on CaF₂-buffered (0 0 1) and (1 1 1) Si wafers by UHV electron-beam evaporation was prepared for the deposition of diamond films. A two-step process of bias-enhanced nucleation and a subsequent growth by controlling the α growth parameter was performed to deposit (0 0 1) and (1 1 1) diamond films by chemical vapor deposition, respectively. Scratching or seeding by fine diamond powders was also attempted on the (1 1 1) substrates to enhance the diamond nucleation density. Raman spectroscopy, X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy were used to characterize the Ir/CaF₂/Si substrates as well as the diamond films grown on top of iridium layer. Heteroepitaxial relationship between the deposited diamond grains and (0 0 1) substrates has been observed.     

STM and XPS studies of early stages of carbon nanotube growth by surface decomposition of 6H–SiC(000-1) under various oxygen pressures

The effects of oxygen on the early stage of carbon nanocap and nanotube growth by surface decomposition of 6H–SiC(000-1) were investigated, using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Our results showed that the presence of oxygen enhances the decomposition of the SiC surface, promoting carbon nanocap and nanotube growth. We also observed active-to-passive oxidation transition below 10^− 1 Pa, which is consistent with the pressure–temperature phase diagram previously estimated from experimental results above 10^− 1 Pa.     

Hydrogen-induced CO2 formation from ethylene deposits on Pt during consecutive O2 and H2 exposures

The oxidation of C₂H₄ deposits on polycrystalline Pt when exposed to consecutive O₂ and H₂ pulses at room temperature has been investigated in a long (L = 36 mm), shallow (d = 600–700 nm) micromachined glass–SiO₂–Pt channel. Hydrogen-induced CO₂ formation from species accumulated on the Pt surface was observed. Frequent switching of the O₂/H₂ exposure pulses was found to increase the efficiency of the oxidation of the carbonaceous deposits markedly. The observations may be of general interest for the regeneration of contaminated catalysts.     

Transmission electron microscopy study of the very early stages of diamond growth on iridium

The diamond nuclei generated during bias enhanced nucleation (BEN) on iridium were not detected so far by high resolution transmission electron microscopy (HRTEM). The aim of the present work was to investigate their earliest appearance after BEN by applying very short growth steps, ranging from 5 s to 1 min. On all the samples with growth step crystalline diamond could be identified unequivocally by HRTEM and reflection high energy electron diffraction (RHEED). After 5 s the former nuclei have evolved into crystallites of 2 nm thickness and about 10 nm width. At that time the carbon precursor phase which appears amorphous in HRTEM and which was formed by the ion bombardment has largely disappeared. After 10 s no residues are left, which proves that the 1–2 nm-thick amorphous carbon layer is only stable under biasing conditions. The rapid etching of the precursor phase and the simultaneous slow increase in volume of the tiny diamond crystals results in a minimum in carbon coverage several seconds after termination of the BEN process.     

Chemical stability of silicon nitride coatings used in the crystallization of photovoltaic silicon ingots. Part II: Stability under argon flow

Processing of photovoltaic silicon by solidification is currently carried out under argon flow in silica crucibles coated with an oxidized silicon nitride powder. A series of experiments was performed to study the reactions between coating components under argon flow by varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of gaseous reaction species from the inside of the porous coating to the flowing argon. The conclusions drawn are used to discuss different practical aspects of the photovoltaic silicon crystallization process.     

Chemical and structural transformations of silicon submitted to H2 or H2/CH4 microwave plasmas

Silicon substrates are often used to synthesize polycrystalline diamond films by microwave plasma assisted chemical vapour deposition technique (MPCVD). In the case of highly oriented diamond films, several steps are employed to carefully prepare the silicon surface (pre-treatment steps), to nucleate diamond crystals (nucleation step) and to thick the film (growth step). In this study, we characterize {100} silicon substrates and diamond released from its silicon substrate by electronic microscopies (TEM and SEM), by Atomic Force Microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), to follow the substrate transformations after each step, particularly the formation and the evolution of the silicon carbide and to characterise the diamond films grown on the carburised silicon. We show that according to the experimental conditions and the level of surface/gas contamination by carbon and silicon species, isolated islands or continuous β-SiC compound are formed over the silicon surface and can generate defects such as voids or strip structures that influence the subsequent diamond nucleation and growth.     

The insulating properties of a-C:H on silicon and metal substrates

Amorphous carbon has many important applications. In electronic terms, its use as a dielectric is receiving greater attention. This is particularly important for applications in magnetic head devices as a reader gap insulation layer. Results are presented for resistivity and breakdown fields for hydrogenated amorphous carbon on silicon, undoped and doped with nitrogen, using an atomic flux source. Current–voltage characteristics were analysed using a numerical algorithm to determine trap densities. The results indicated that such films can meet the breakdown specifications, on silicon, and that nitrogen doping improves their characteristics. Thickness trends indicate improvements are likely as gaps are scaled. The density of states determination indicated that high breakdown was correlated, in the undoped case, with high DOS but this was not so for the doped films. The DOS was found to increase as the thickness decreased. On substrates other than silicon, the films were observed to have increased roughness, poorer adhesion and a more polymer-like quality. These changes were reflected in a reduction in the observed breakdown field.     

A geometric model of growth for cubic crystals: Diamond

A mathematical and software implementation of a geometrical model of the morphology of growth in a cubic crystal system, such as diamond, is presented based on the relative growth velocities of four low index crystal planes: {100}, {110}, {111}, and {113}. The model starts from a seed crystal of arbitrary shape bounded by {100}, {110}, {111} and/or {113} planes, or a vicinal (off axis) surface of any of these planes. The model allows for adjustable growth rates, times, and seed crystal sizes. A second implementation of the model nucleates a twinned crystal on a {100} surface and follows the evolution of its morphology. New conditions for the stability of penetration twins on {100} and {111} surfaces in terms of the alpha, beta, and gamma growth parameters are presented.     

Fabrication and photocatalytic properties of silicon nanowires by metal-assisted chemical etching: effect of H2O2 concentration

In the current study, monocrystalline silicon nanowire arrays (SiNWs) were prepared through a metal-assisted chemical etching method of silicon wafers in an etching solution composed of HF and H₂O₂. Photoelectric properties of the monocrystalline SiNWs are improved greatly with the formation of the nanostructure on the silicon wafers. By controlling the hydrogen peroxide concentration in the etching solution, SiNWs with different morphologies and surface characteristics are obtained. A reasonable mechanism of the etching process was proposed. Photocatalytic experiment shows that SiNWs prepared by 20% H₂O₂ etching solution exhibit the best activity in the decomposition of the target organic pollutant, Rhodamine B (RhB), under Xe arc lamp irradiation for its appropriate Si nanowire density with the effect of Si content and contact area of photocatalyst and RhB optimized.     

H2O-CO2、H2O-N2、H2O-He水平管外自然对流凝结换热特性研究

不凝性气体制约换热设备安全和系统效率,为研究不凝性气体-蒸气于水平管外自然对流凝结换热机理和特性,实验测量了不凝性气体He、N₂、CO₂质量分数分别为1.16%~18.18%、7.56%~60.86%、11.39%~70.95%,壁面过冷度为5~25 K,总压力为5~101 kPa的H₂O-He、H₂O-N₂、H₂O-CO₂自然对流条件下水平管外凝结换热特性,对比分析了H₂O-He、H₂O-N₂、H₂O-CO₂的不凝性气体质量含量、壁面过冷度以及压力因素的影响。压力和壁面过冷度一定,相同质量分数时,实验凝结传热系数与Nusselt理论解的比值(Q/Q_Nu)由大到小依次为：H₂O-CO₂、H₂O-N₂、H₂O-He;相同摩尔分数时,Q/Q_Nu由大到小依次为：H₂O-He、H₂O-N₂、H₂O-CO₂。相同总压力和不凝性气体质量分数时,H₂O-He的Q/Q_Nu随着壁面过冷度的增加下降最为缓慢。相同不凝性气体质量分数和壁面过冷度时,H₂O-He的Q/Q_Nu值最小,其受压力影响最为显著。     

H2O-CO2、H2O-N2、H2O-He水平管外自然对流凝结换热特性研究

不凝性气体制约换热设备安全和系统效率,为研究不凝性气体-蒸气于水平管外自然对流凝结换热机理和特性,实验测量了不凝性气体He、N₂、CO₂质量分数分别为1.16%~18.18%、7.56%~60.86%、11.39%~70.95%,壁面过冷度为5~25 K,总压力为5~101 kPa的H₂O-He、H₂O-N₂、H₂O-CO₂自然对流条件下水平管外凝结换热特性,对比分析了H₂O-He、H₂O-N₂、H₂O-CO₂的不凝性气体质量含量、壁面过冷度以及压力因素的影响。压力和壁面过冷度一定,相同质量分数时,实验凝结传热系数与Nusselt理论解的比值(Q/Q_Nu)由大到小依次为：H₂O-CO₂、H₂O-N₂、H₂O-He;相同摩尔分数时,Q/Q_Nu由大到小依次为：H₂O-He、H₂O-N₂、H₂O-CO₂。相同总压力和不凝性气体质量分数时,H₂O-He的Q/Q_Nu随着壁面过冷度的增加下降最为缓慢。相同不凝性气体质量分数和壁面过冷度时,H₂O-He的Q/Q_Nu值最小,其受压力影响最为显著。     

Mechanism of diamond epitaxial growth on silicon

According to this study and to our previously reported nucleation model, we can explain the mechanism of diamond heteroepitaxial growth as follows: microscopic nucleation sites, i.e. etching scars, bunching atomic steps, and grooves formed by growing SiC, are formed on the substrate surface at the beginning of the bias application and are related to the crystal orientation of the substrate. Carbon atoms that reach the substrate surface are trapped at the nucleation sites and form clusters. Since these clusters are in the embryonic form during ion irradiation, deformation, rotation and migration can easily take place to form clusters with shapes that correspond to the shapes of these sites. At the same time, the conversion from sp² to sp³ progresses, and diamond nuclei of critical sizes are thought to be formed. This phenomenon strongly suggests that the heteroepitaxial growth of diamond involves a graphoepitaxial process in the formation of a critical nucleus size. In this study, we examined in detail the effect of bias on substrates to clarify the mechanism of heteroepitaxial growth of diamond.     

Electrochemical corrosion of silicon carbide ceramics in H2SO4

Sintered silicon carbide materials have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on a SSiC material in H₂SO₄ at different voltages and subsequent detailed investigation of the formed surfaces was carried out. The first time a systematic local measurement of the thickness of the oxide layers was carried out. The measurements revealed the formation of SiO₂ surface layers with thickness up to 125 μm. The measured values also showed a strong deviation from grain to grain. The thickness of the layers does not correlate with the crystallographic orientation of the grains or the SiC-polytypes. The data indicate that the behaviour is caused by the variation of the resistivity of the grain boundaries. The measured thicknesses as a function of the electrical charge transferred indicate that the electrochemical oxidation results in the SiO₂ and carbon dioxide.