The influence of crystallographic orientation on the wetting of silicon on quartz single crystals

Dynamic hexagonal spreading patterns of small silicon droplets on the basal plane (001) of quartz were observed by video microscopy. A detailed analysis of the hexagonal triple line demonstrates that the patterns show slight chiral distortions that can be attributed to the screw axis of the substrate crystal. This article reveals the detailed influence of crystal symmetry on the anisotropy of reactive wetting. In this context, a first discussion about the interplay of wetting and etching of a crystal is provided.     

The wetting of silicon nitride by chromium-containing alloys

The wetting of ceramic materials by metallic melts is the most important characteristic of brazing alloys. The effect of chromium additions to copper-base alloys on the wetting of silicon nitride was investigated. Wetting experiments were carried out on Si₃N₄ using liquid Cu-Cr, Cu-Ni-Cr, Cu-Si-Cr and Cu-Ni-Si-Cr alloys. The addition of chromium to liquid copper up to its solubility limit promoted wetting on Si₃N₄. Improved wetting with a higher chromium content was achieved by the addition of nickel to the Cu-Cr alloys. The formation of an interfacial reaction layer, which is detrimental for brazing ceramics, was suppressed by the addition of silicon to the chromium-containing brazing alloys.     

The wetting,reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si₃N₄ by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si₃N₄ to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si₃N₄ was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si₃N₄(s) + 4Ti (1 ? sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si₃N₄, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si₃N₄ to Si₃N₄ was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.     

Geometrically tuned and chemically switched wetting properties of silicon nanotips

The wetting properties of silicon nanotips (SiNTs) are discussed. SiNTs were prepared by single step dry etching of silicon wafers in an electron cyclotron resonance plasma of silane, methane, argon and hydrogen and water contact angles were measured as a function of their aspect ratio (α) and the inter-tip distance. The hydrophilic nature of the SiNTs is tunable with α and the inter-tip distance. Super-hydrophilicity with water contact angles close to 2° was observed with α>12 (length ～1500?nm). Upon coating a 1500?nm long SiNT with TiO(2), the water contact angle jumped from 2° to ～140°, demonstrating a switchover from super-hydrophilic to hydrophobic surface properties.     

The role of silicon in wetting and pressureless infiltration of SiCp preforms by aluminum alloys

Silicon plays an important role in the production of Al/SiC metal matrix composites. As an alloying element in aluminum, silicon retards the kinetics of the chemical reactions that result in the formation of the unwanted intermetallics Al₄C₃ and Al₄SiC₄. As a thin coating on silicon carbide, silicon becomes an active participant in a thermally activated chemical reaction that enhances wetting of silicon carbide by aluminum alloys. Consequently, Al/SiC composites made with siliconized silicon carbide and silicon rich aluminum alloys show mechanical properties that are significantly different from those of similar composites produced with unsiliconized silicon carbide or with aluminum alloys that do not contain silicon. It is shown that a silicon coating on SiC significantly enhances wetting of SiC particles by aluminum alloys, reduces porosity, does not affect the modulus of elasticity, but decreases the modulus of rupture of Al/SiC metal matrix composites.     

The effect of the wetting droplets size on power consumption during drum granulation

Changes of torque during fine material (foundry dolomite) granulation in a horizontal drum granulator at changing wetting parameters were studied. The variable parameters were droplet diameter and wetting of granular material. The bed of loose material was sprayed during feeding, at a constant liquid flow rate V_l = 0.012 m³/h. The size of wetting liquid droplets was changed using different rates of air flow through pneumatic spraying nozzles in the range from V_p = 1.0 to 3.0 m³/h and applying a sprinkler which supplied (drop-wise) the liquid uniformly along the entire drum length. In each test, instantaneous values of torque on the granulator shaft were measured in 1 s time intervals. The effect of droplet size and moisture content of the bed of granular material on torque in the whole granulation cycle was estimated. It was found that bed wetting conditions had a significant influence on the nucleation and growth of agglomerates. A minimum (boundary) size of liquid droplets, at which the bed is not transformed into granulated material, depends on the total amount of liquid supplied at the wetting stage. Changes of torque during the drum granulation are a good representation of the phenomena related to the transformation of wetted feed into granulated product and can be an easy indicator of the process.The work was carried out within the project no. 4 T09C 023 22. financed by the State Committee for Scientific Research in the years 2002–2005.     

Localized-surface-plasmon enhanced emission from porous silicon by gold nanoparticles

The porous silicon (PS) samples, decorated by Au nanoparticles (NPs) possessing localized-surface-plasmon (LSP) resonance, are prepared by the conventional anodization method. Photoluminescence (PL) is studied systematically, in particular, its dependence on the excitation power. It is found that undecorated PS samples exhibit a saturation behavior in PL intensity with increasing the pumping laser power, while the luminescence of Au-decorated PS hybrid samples have a purely linear dependence on the excitation power. In the linear response region of PS samples, addition of metal NPs layer moderately suppresses the emission while, in the saturation region, the net emission is enhanced by approximately up to 4-fold. Several possible mechanisms are discussed. We believe that the observed PL enhancement in saturation region is dominantly due to the resonant coupling between the LSP of Au NPs and the electronic excitation of PS, which inhibits the nonradiative Auger recombination process at high excitation power. These results indicate that the plasmon effect could be useful for designing even more efficient optoelectronic devices such as super bright light emitting devices and solar cells with high efficiencies. Despite many challenges, Au NPs can potentially be applied to introduce LSP resonance for the future silicon-based optoelectronics or photonics.     

Control of plasma process instabilities during thin silicon film deposition

Fourier transform infrared absorption spectroscopy (FTIR), optical emission spectroscopy (OES), self-bias voltage and plasma impedance controls were applied as in situ process diagnostics during the deposition of amorphous silicon thin-films. The diagnostic abilities of OES and FTIR are compared. The FTIR in-situ direct measurement of silane concentration in exhaust line is more precise than OES control. All in situ process diagnostics clearly indicates the inconsistency of plasma properties and therefore of deposition conditions. The drifts are comparable with the film deposition time. The FTIR measurement of reactant concentration in the process chamber evidence that the strong silane concentration drop (about 50%) in a plasma is the cause of the short-term drift of OES signals (SiH? emission), plasma impedance and self-bias voltage signals. The influences of the deposition chamber geometry and technological parameters on process drifts are considered. The decrease of the gas residence time in the reactor leads to a decrease of Initial Transient State phenomena. Finally, the improvement of solar cell performance based on thin silicon films is demonstrated when drifts are reduced.     

Isotope-modified silicon layers obtained by plasma enhanced chemical vapor deposition from gaseous silicon tetrafluoride

Layers of silicon with an isotope composition that differs from the native composition have been obtained by the method of plasma enhanced chemical vapor deposition from gaseous silicon tetrafluoride. The deposited layers possess an amorphous structure and high oxygen content. The Raman spectra and the spectra of optical transmission in the IR range are presented. The in-depth distribution of Si isotopes has been determined using the second ion mass spectrometry techniques.     

Nonlinear response of a silicon waveguide enhanced by a metal grating

We theoretically demonstrate that nonlinear optical response in a thin silicon waveguide within a wide wavelength regime can be enhanced by a metal grating. Numerical simulation indicates that the enhancement factor of the four-wave mixing signal varies with the position. The largest enhancement factor of the four-wave mixing is more than 10⁴ at a certain position in the IR spectrum with proper geometric parameters. More importantly, the wavelength of four-wave mixing with the same enhancement factor can be controlled dynamically within a wide wavelength regime.     

Zinc oxide nanorod growth on gold islands prepared by microsphere lithography on silicon and quartz

Gold islands, vapor deposited on silicon and quartz by microsphere lithography patterning, are used to nucleate arrays of ZnO nanorods. ZnO is grown on approximately 0.32 microm2 Au islands by carbothermal reduction in a tube furnace. Scanning electron microscopy (SEM) and energy dispersive atomic X-ray spectroscopy (EDS) confirm that the gold effectively controls the sites of nucleation of ZnO. Atomic force microscopy (AFM) shows that approximately 30 nm diameter nanorods grow horizontally, along the surface. Alloy droplets that are characteristic of the vapor-liquid-solid (VLS) mechanism are observed at the tips of the nanorods. The spatial growth direction of VLS catalyzed ZnO nanorods is along the substrate when they nucleate from gold islands on silicon and quartz. The energy of adhesion of the VLS droplet to the surface can account for the horizontal growth.     

LiF enhanced nucleation of the low temperature microcrystalline silicon prepared by plasma enhanced chemical vapour deposition

A 15-nm lithium fluoride (LiF) thin film evaporated on glass substrate is shown to enhance the nucleation of microcrystalline Si grown by plasma enhanced chemical vapour deposition at the amorphous/microcrystalline boundary conditions. The effect is more pronounced at low substrate temperatures, nucleation density being 10 times higher at ∼ 80 °C. The effect is ascribed to the ionic chemical nature of LiF, the low work function material used in organic electronic devices, and we propose its use for micro patterning crystalline Si regions in otherwise amorphous Si film.     

Spreading of liquid lead droplets on metallic iron covered by silicon oxide particles or films

As well known, the spreading of a liquid metal droplet on a solid metal is very sensitive to the presence of chemical heterogeneities on the solid metal. In this study, wetting experiments with liquid lead on heterogeneous surfaces composed of iron and silicon oxide particles or films were performed using the dispensed drop technique. High purity iron and binary iron–silicon substrates with different silicon contents were studied. Before the wetting experiments, the substrates are annealed at 850 °C in a N₂–H₂ atmosphere in order to reduce iron oxides and to form silicon oxide particles or films on the surface. The liquid lead droplet is then released onto the metallic substrate partly or wholly covered by the oxides. The spreading of the liquid metal droplet strongly depends on the surface area fraction covered by the oxides.     

Direct bonding of Cu to oxidized silicon nitride by wetting of molten Cu and Cu(O)

When pressureless sintered silicon nitride with the main additives Y₂O₃ and Al₂O₃, having a thermal conductivity K = 20 W/m K, was oxidized at 1240–1360 °C in still air, the resulting surface oxide layer easily bonded to a copper plate in the temperature region between 1065 and 1083 °C, and in the oxygen concentration range of 0.008–0.39 wt%, as shown in a Cu–O phase diagram. The oxide on the silicon nitride was characterized as Y₂O₃·2SiO₂ and mixed silicate glass with additives and impurities that diffused through the grain boundary. The bonding strength of Cu/Si₃N₄ depends on the amount or layer thickness of silicate glass and reaches as high as 100 MPa by shear at room temperature. Detailed analysis of the oxidation layer and the peeled-off surfaces of directly bonded Si₃N₄/Cu reveal that the main mechanism of bonding is wetting to glassy silicate phase by mixtures of molten Cu and α-solid solution Cu(O), which solidify to α + Cu₂O below 1065 °C by a eutectic reaction. The direct reactive wetting of molten Cu, supplied from the grain boundary of a Cu plate, on the glassy phase enables very tight chemical bonding via oxygen atoms.     

A model for the quartz crystal microbalance frequency response to wetting characteristics of corrugated surfaces

We consider the effect of surface roughness, and its unique wetting behavior, on the response of a quartz crystal microbalance (QCM) resonator operating in contact with a fluid. The rough surface is modeled as sinusoidally corrugated particular to the case of a fixed relationship between amplitude and periodicity, as would arise from polishing with monodisperse spherical particles. The penetration of fluid into the troughs of the corrugations and the resulting meniscus are determined as a competition between surface tension and compression of the trapped gas. Liquid contained below the corrugation peaks, but above the gas/liquid meniscus, is trapped and behaves as an ideal mass layer, contributing a frequency shift that adds to that arising from liquid entrainment. This model allows QCM responses on rough surfaces to be described as a function of liquid properties and contact angle. This permits responses on hydrophobic surfaces to be understood in terms of incomplete surface wetting.     

Optical and structural properties of silicon oxynitride deposited by plasma enhanced chemical vapor deposition

We present an overview of the properties of silicon oxynitride material (SiON) deposited by plasma enhanced chemical vapor deposition (PECVD) for photovoltaic applications. SiON films were deposited using silane (SiH₄), ammonia (NH₃) and nitrogen protoxide (N₂O) as precursor gases in a low frequency PECVD. Varying the gas flow mixture leads to a whole range of SiON layers starting from the silicon oxide to the silicon nitride with unique stoichiometries and properties. Thanks to spectroscopic ellipsometry measurements we have confirmed the suitability of SiON for antireflection coating layers due to the range of the refractive indexes attainable. SiON structure was analyzed by X-ray photo-electron spectroscopy. We have thus highlighted the critical role of oxygen behavior on the SiON network and the progressive replacement of nitrogen by oxygen atoms when the oxygen precursor increases. The type of chemical bonds present in SiON layers was also investigated by infrared spectroscopy. The SiON layers also contain a non-negligible amount of hydrogen which might be useful for passivation applications. The behavior of hydrogen content was thus analyzed by elastic recoil decay analysis and desorption characterization. A typical rapid thermal annealing was performed on the SiON samples in order to simulate the solar cells contact annealing and to investigate its impact on the dielectric film properties. It was found that hydrogen becomes weakly bonded to the films and strongly decreases in quantity with the annealing. The surface passivation effect is presented in the last part of this paper. The trend before and after a rapid thermal annealing showed opposite results which could be explained by the high porosity of the layers and the formation of Si-O bonds.     

Crystallization of β-FeSi2 droplets on silicon substrates by room-temperature pulsed laser deposition

This paper reports the fabrication process of β-FeSi₂ droplets on silicon substrates at room temperature by ArF excimer pulsed laser deposition (PLD). The chemical treatment of substrate could compensate the thermal treatment of the deposited droplets. Observations with the transmission electron microscopy revealed that the crystallization of droplet began from the surface of droplet rather than from the interface between the melt and the substrate.     

Growth of silicon nanoclusters on different substrates by plasma enhanced chemical vapor deposition

We report an atomic force microscopy study of the early stages of growth of silicon nanoclusters formed on different substrates by plasma-enhanced chemical vapor deposition, using dichlorosilane (SiH2CI2) and hydrogen (H2) as reactive gases. (100) n-type single crystalline silicon, fused silica, amorphous silicon nitride and corning glass, were used as substrates for the growth of the nanoclusters, which were formed at low substrate temperature (200 degrees C). The diameter, height and number density of the clusters were controlled by the deposition time and pressure. It was found that not only the plasma conditions but also the surface characteristics of the substrate influence the cluster density, shape, and size. For the ordered silicon surface and the amorphous fused silica, the nanoclusters result oval in shape and exhibit preferential growth along the surface. When deposited over amorphous silicon nitride and corning glass, the density of nanoclusters increases and there is a tendency toward columnar growth since the diameter of the nanoclusters tends to decrease. We conclude that although the specific features of the nanoclusters originate from the chlorine chemistry introduced by the SiHxCly deposition precursor and the chemical stability of chlorine-terminated surfaces under hydrogen plasma, the surface quality and roughness also plays an important role on the nucleation and mobility of the species. The combination of both effects gives rise to the different nanostructured growths observed.     

纳米多孔硅的电化学制备及性能研究

纳米多孔硅是一种潜在的化学和生物传感材料,本文采用电化学腐蚀法制备纳米多孔硅。采用SEM技术分析多孔硅的表面形貌,研究了腐蚀条件对多孔硅的孔隙率、厚度、I-V特性的影响。结果表明,多孔硅的孔隙率随着腐蚀电流密度和腐蚀时间的增加而呈线性增大趋势;其厚度随着腐蚀电流密度的增加而近似呈线性增大趋势,随腐蚀时间的成倍增加而显著增大;其I-V特性表现出非整流的欧姆接触。