Growth chemistry of amorphous silicon and amorphous silicon–germanium alloys

It is generally found that the optimum growth of a-Si:H and a-(Si,Ge):H films and devices depends critically upon the particular growth technique, and the particular growth parameters used to grow the film. Different techniques give very different results, which are sometimes contradictory. The standard model for growth assumes that the Si surface is mostly covered with H bonds, and that growth takes place primarily from silyl radicals. The model assumes that excess surface H is eliminated by a silyl radical, and that the adjacent or wing bonded H atoms are eliminated by a spontaneous bond breaking and H₂ formation. In this paper, we show that this model is thermodynamically incorrect, and that it does not explain many of the experimental data, such as why bombardment with inert gases reduces H content, and why material grown at higher growth rates is more unstable. Rather, we suggest that the fundamental limitation to the growth of a-Si:H and a-(Si,Ge):H is the elimination of excess H, both from the surface and from the bulk. The excess H is not eliminated by spontaneous reactions, nor by interactions with the silyl molecule. Rather, it is eliminated by interactions with free H radicals and ions. Inert ions, such as He and Ar, can accelerate the desorption of H from the surface. Atomic and ionic H can diffuse into the material, and also remove subsurface excess H, reforming the Si microstructure. We also show that the influence of ion bombardment is critical for growing high quality a-(Si,Ge):H alloys, and that deposition conditions that lead to low ion bombardment flux can produce poor materials.     

Different strategies for the synthesis of silicon carbide–silicon nitride composites from preceramic polymers

Three strategies for the synthesis of silicon carbide–silicon nitride composites from organosilicon preceramic polymers were investigated. Firstly, polymeric precursors with reactive groups for silicon carbide and silicon nitride were synthesized, blended and pyrolyzed. Secondly, a polymeric precursor for silicon carbide was mixed with silicon powder which acts as a reactive filler and the resulting mixtures were pyrolyzed. Thirdly, a co-polycyclodisilazane–silane single polymeric precursor was synthesized and pyrolyzed. The precursors and the various stages of processing were studied using gel permeation chromatography, Fourier transform-infrared spectroscopy, thermo-mechanical analysis, thermogravimetry and X-ray diffraction. In each instance silicon carbide–silicon nitride composites were prepared successfully.     

A sound barrier for silicon?

For the first time in thirty five years, the clockspeed of the fastest commercial computer chips has not increased. Is the semiconductor industry just pausing for breath or about to suffer a fate similar to that of aerospace?     

Sol–gel synthesis of silicon carbide on silicon pyramids: a promising candidate for supercapacitor electrodes

In this study, Si porous pyramids nanostructures were synthesized by the metal-assisted chemical etching technique. Different KOH concentrations were used to develop high surface area Si porous pyramids for application as supercapacitor electrodes. Field-emission scanning electron microscope (FE-SEM) studies showed that 5% KOH solution will lead to high surface area Si pyramids with a specific capacitance of 90.3 F/cm². Silicon carbide (SiC) thin film was coated on Si pyramids (SiC@Si) using a facile sol–gel method followed by a carbothermal reduction process. Tetraethylorthosilicate and sugar were used as carbon sources. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and FE-SEM analysis were used to characterize the developed SiC@Si samples. The developed SiC@ Si electrode exhibited a high specific capacitance of 135.5 F/cm² at a scan rate of 10 mV/s (in 1 M NaOH electrolyte). The supercapacitor capability of this SiC@Si structure is significantly higher than classical materials. Because of its facile, controllable and efficient synthesis technique, this novel SiC@Si can be considered a very promising candidate for power sources applications.

     

The α/β silicon nitride phase transformation

The literature on the / silicon nitride transformation is reviewed briefly. Data are presented on the kinetics of the tranformation of 1600° C on low and high purity silicon nitride powders. The addition of magnesia increased the rate of transformation while the addition of yttria had no effect. Scanning electron photomicrographs show clearly the morphology changes that accompany the transformation. It is concluded that the transformation occurs via a solution-precipitation mechanism and that and are probably low and high temperature forms of silicon nitride.     

Gas pressure sintering of silicon nitride — current status

Gas pressure sintering of silicon nitride is now common for obtaining a dense as well as tough product. A review of earlier works has revealed that there are still controversies in prediction of mechanism of the sintering process. Analysis of the kinetics of the process obtained by integration of a dilatometer inside the furnace has been presented. The mechanism of sintering silicon nitride powder compacts in presence of hyperbaric nitrogen atmosphere has been discussed. It has been observed that intermediate stage densification kinetics is very much susceptible to sintering atmosphere and time of pressurisation. The microstructure of the final product has been found to be dependent primarily on the method of incorporation of additive in the starting silicon nitride powders.     

Gallium nitride heterostructures on 3D structured silicon

We investigated GaN-based heterostructures grown on three-dimensionally patterned Si(111) substrates by metal organic vapour phase epitaxy, with the goal of fabricating well controlled high quality, defect reduced GaN-based nanoLEDs. The high aspect ratios of such pillars minimize the influence of the lattice mismatched substrate and improve the material quality. In contrast to other approaches, we employed deep etched silicon substrates to achieve a controlled pillar growth. For that a special low temperature inductively coupled plasma etching process has been developed. InGaN/GaN multi-quantum-well structures have been incorporated into the pillars. We found a pronounced dependence of the morphology of the GaN structures on the size and pitch of the pillars. Spatially resolved optical properties of the structures are analysed by cathodoluminescence.     

Modification of eutectic silicon in Al–Si alloys

The mechanical properties of Al–Si alloys are strongly related to the size, shape and distribution of eutectic silicon present in the microstructure In order to improve mechanical properties, these alloys are generally subjected to modification melt treatment, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Improper melt treatment procedures, fading and poisoning of modifiers often result in the structure which is far from the desired one. Hence it is essential to assess the effectiveness of melt treatment before pouring. A much investigated reliable thermal analysis technique is generally used for this purpose. The deviation from the standard curve in thermal analysis helps in assessing the level of refinement of the Si structure. In the present review an attempt is made to discuss various aspects of modification, including mechanism, interaction of defects and non-destructive assessment by thermal analysis.     

Investigation of optical properties of core–shell silicon nanowires

The ability to control the size, orientation, composition and morphology of silicon nanowires (SiNWs) presents an ideal platform for exploring a wide range of potential technological applications. In this work, we demonstrated the detail study of optical properties of highly disordered core–shell SiNWs that were grown by atmospheric pressure chemical vapor deposition. The microstructure of SiNWs was characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The TEM study shows that the SiNWs consists of crystalline core silicon surrounded by thick amorphous silicon oxide. The total diameter including the outer SiO₂ sheath was 60–80 nm. The reflection and absorption of a-SiO₂/c-SiNWs were affected by process parameter like silane flow rate and hydrogen dilution. The optical reflection of SiNWs decreased with increasing photon energy across the visible and near the ultraviolet range, approaching moth's eye antireflection. Specifically, a minimum reflection of 2–3% was observed at 400 nm. The band gap is estimated at ∼1.32 eV by quasi-direct band Tauc's plot. The sum of localized states at the band edge is ∼0.53 eV. Straight SiNWs have lower reflection than those of nanoparticles mixed SiNWs and coil mixed SiNWs. The reflection and absorption of SiO₂/SiNWs were confirmed to respond strongly to infrared with increasing H₂ flow rate.     

A grain‐boundary trapping model of polycrystalline silicon film

Abstract

A grain‐boundary trapping model, which modifies the trapping theories Proposed by Seto and Baccarani et al., is proposed to explain the electrical transport properties of polycrystalline silicon films. The trapping capability of grain‐boundary traps, the average carrier concentration, and the width of depleted‐layer in the space‐charge region have been carefully considered and so the theory can be applied to polycrystalline silicon films of both small and large grain‐size. When three sets of experimental data with different grain sizes are compared to the theoretical curves, a good agreement is obtained.     

Effect of solidification cooling rate and phosphorus inoculation on number per unit volume of primary silicon particles in hypereutectic aluminium—silicon alloys

Collected data on derived number per unit volume of primary silicon particles in hypereutectic Al-Si alloys show a power relationship with solidification cooling rate of the form where typically and mm^-3 (K/s)^-1 in the absence of phosphorus and mm^-3 (K/s)^-1 in its prescence. Significantly lower apparent values of from one set of results appear to stem from measurement of a mean long dimension rather than diameter of particle sections as well as lower measured undercoolings than in Bridgman experiments at similar .     

Carbon reaction with levitated silicon – Experimental and thermodynamic approaches

Metallurgical grade silicon (MG-Si) has become a new source of raw material for the photovoltaic industry. The use of this material as an alternative feed stock has however introduced phenomena that are detrimental to both the yield of the manufacturing process and the performance of the photovoltaic cells produced. This is mainly related to the presence of carbon, which precipitates to silicon carbide (SiC) in the ingot.     

Kinetics of silicon–metal alloy infiltration into porous carbon

The kinetics of infiltration of Si and its alloys into porous carbon has been investigated theoretically using a modified Washburn model. The effect of alloy composition and temperature on infiltration has been quantified. The resulting characteristics depend on the type of alloy and the composition of the secondary metallic phase. Aluminum generally suppresses infiltration rate while the influence of Cu depends on composition. Infiltration is enhanced for low Cu concentration below a threshold value, and attenuated for higher concentration. Increasing the temperature enhances both the infiltration capacity and reaction rate.     

Phosphorus δ-doped silicon: mixed-atom pseudopotentials and dopant disorder effects

Within a full density functional theory framework we calculate the band structure and doping potential for phosphorus δ-doped silicon. We compare two different representations of the dopant plane; pseudo-atoms in which the nuclear charge is fractional between silicon and phosphorus, and explicit arrangements employing distinct silicon and phosphorus atoms. While the pseudo-atom approach offers several computational advantages, the explicit model calculations differ in a number of key points, including the valley splitting, the Fermi level and the width of the doping potential. These findings have implications for parameters used in device modelling.     

Chemical-assisted laser parallel nanostructuring of silicon in optical near fields

The authors present a simple and efficient technique for producing hexagonal arrays of nanostructures on silicon surfaces in chemical solutions. It utilizes the effect of optical near-field enhancement by self-assembled particle-lens arrays and a thermally induced chemical reaction with an alkaline solution. About 10(8) features can be produced simultaneously by one single laser pulse. Furthermore, the shape of the structures was found to be controllable, from concave holes to convex bumps, by means of a post-etching process, in the same chemical solution.     

Mechanical testing of titanium/aluminium–silicon interfaces by push-out

Mechanical properties of Ti/Al–7Si assemblies produced by insert moulding were studied with a classical push-out test and a variant that is the circular-bending test. Special care has been taken for controlling both the reactivity at the Ti/Al–7Si interface and the metallurgical health of the Al–7Si matrix. Mechanical tests until complete debonding have been completed with interrupted tests, metallographic characterizations and FEM analysis of elastic stress state. A mean shear strength of the interface of about 120 MPa was obtained. When the Ti insert is solely fretted in the matrix, without chemical interaction between Ti and the Al–7Si alloy, the mean shear strength is significantly lower (48 MPa). This result clearly shows that chemical interaction at the interface (formation of a thin TiSi layer at the Ti side and a thick Al₃Ti(Si) layer at the Al–7Si alloy side) improves the mechanical properties of the assembly. It is also shown that the failure sequence is characterized both by crack propagation from bottom to top and matrix yielding from top to bottom. Actually, interface damaging begins by crack initiation at the specimen bottom face (not at the top face and under the indenter) in a nearly pure mode I solicitation at a radial tensile stress of about 100 MPa.     

Dissolution kinetics of primary silicon in hypereutectic Al–Si melt

The dissolution process of primary silicon particles in Al–18%wt silicon alloy was studied both by a melt overheating experiment and by theoretical analysis. A dissolution model of primary silicon in the melt was established based on atomic diffusion and taking account to interface reaction and curvature of particles. The results show that the theoretical curve agrees with the experimental curve at an overheating temperature of 1100°C. However, there was some deviation at 700°C due to retained silicon clusters in the melt at lower temperature. Therefore, the model is in accord with experiment when not considering the influence of retained silicon clusters.