Reducing the effects of mismatch between zinc oxide and silicon by silane plasma modification

We investigated the mismatch between zinc oxide (ZnO) and silicon (Si) upon reduction by silane plasma modification in a plasma-enhanced chemical vapor deposition system. This plasma treatment was only carried out for 10?s and the Si–H bonds that were provided by the silane plasma modification as dangling bonds on the Si wafer in addition to functioning as a conjunction layer to reduce the defects. The X-ray diffraction analysis of the ZnO/p-type silicon structure produced by silane plasma modification has a slightly lower full width at the half maximum, which improved the ZnO film’s crystalline properties. After the silane plasma modification ZnO/Si diode is produced, the measured current–voltage characteristics gave favorable rectifying properties and reverse bias had a low leakage current. The ZnO/Si diode under illumination increased the short-circuit current (I_sc) from 7.32 to 19.75?mA/cm², which is an improvement compared with a conventional bare ZnO/Si diode because of the reduced ZnO/Si interface states. Therefore, the silane plasma modification diminishes the effects of the interface and improves the ZnO/Si diode performance.     

Characterization of Al-Si-alloys rapidly quenched from the melt

Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 10⁶ and 10⁷ K sec^–1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.     

Effect of calcium–silicon ratio on microstructure and nanostructure of calcium silicate hydrate synthesized by reaction of fumed silica and calcium oxide at room temperature

A C–S–H series with calcium–silicon ratio 0.6–3.0 was synthesized by pozzolanic reaction. Phase composition, nanostructural and morphological characteristics were determined using XRD, XRF, SEM and ²⁹Si NMR. Most of the samples were phase-pure, poorly crystalline C–S–H. Significant changes in the nanostructure of the C–S–H samples were observed when the calcium–silicon ratio reached values of 0.8, 1.0 and 1.5. At calcium–silicon ratio 0.8 the basal XRD peak began to develop, crosslinking between layers was seen below this ratio but not above, and there was a substantial decrease in mean silica chain length at this ratio. At calcium–silicon ratio 1.0 there was a pronounced microstructural change from granular to reticular and another substantial decrease in mean chain length (indicated by an abrupt increase in the Q¹ peak intensity and decrease in the Q² peak intensity). At calcium–silicon ratio 1.5 the basal XRD peak began to diminish again, the mean silica chain length decreased further, and isolated tetrahedra (Q⁰) were observed.     

Deposition and electrical characteristics of S-doped boron nitride thin films

cBN/Si n–p heterojunctions have been fabricated and characterized. n-type cBN films were grown on p-type Si wafers using RF reactive sputter, and the n-type cBN films were obtained by adding S (sulfur) into working gas. The I–V (current–voltage) characteristics have obvious rectification. The fitting results show that the current transporting model for the cBN/Si n–p heterojunctions is the same as Anderson’s transporting model. The C–V (capacitance–voltage) characteristics are close to that of ideal heterojunctions. The built-in potential in the cBN/Si n–p heterojunctions was determined from C–V measurements at 4.71 V. The dopant concentration in the n-type cBN films was also determined from the C–V measurements at 6.50 × 10¹⁴/cm³.     

Thermodynamic and kinetic properties of indium-implanted silicon II: High temperature diffusion in an inert atmosphere

Impurity depth profiles were obtained by differential sheet resistivity and Hall coefficient measurements on Si(100) implanted with indium to doses of 10¹³ and 10¹⁴ ions cm^-2 at an energy of 190 keV.After recovery of the implant damage the samples were diffused in an inert atmosphere at temperatures ranging from 1000 to 1200 °C.The data show that the carrier concentration is nearly independent of the implanted dose, while the indium diffusivity in silicon is higher for the samples implanted at 10¹⁴ ions cm^-2. The solid solubility of indium in silicon is around 10¹⁷ cm^-3. The hole mobility is about half that of boron-doped silicon and, in the range considered, the main contribution to the resistivity is from ionized impurity scattering.A model is proposed to account for the peculiar behaviour observed in indium-implanted silicon.     

Sources of Carbon Impurities in the Preparation of High-Purity Monoisotopic <Superscript>28</Superscript>Si by a Hydride Method

This paper examines sources of carbon impurities in polycrystalline monoisotopic ²⁸Si prepared by a hydride method. Analytical data on the concentrations of carbon-containing impurities in volatile silicon compounds (²⁸SiH₄ and ²⁸SiH₄), process gases (Ar and H₂), and polycrystalline ²⁸Si are used to identify the major sources of carbon in the polycrystalline ²⁸Si prepared by the hydride method. These are the starting ²⁸SiH₄ and calcium hydride used in ²⁸SiH₄ conversion into ²⁸SiH₄. The rate of carbon intake into polycrystalline silicon from the apparatus material during the monosilane pyrolysis process does not exceed 9 × 10¹¹ cm^–2 h^–1. Polycrystalline silicon has been precipitated from monosilane with different concentrations of hydrocarbon impurities. At hydrocarbon concentrations in the range 10^–4 to 10^–3 mol %, the carbon concentration in the monosilane correlates with that in the silicon obtained from it. High-purity monosilane has been used to prepare polycrystalline ²⁸Si samples with concentrations of carbon impurities in the range (0.8–2.3) × 10¹⁵ cm^–3. Based on calculations of the carbon impurity distribution along the length of a zone-refined ingot, we examine the effect of the initial carbon concentration in the starting polycrystal on the yield of single-crystal monoisotopic ²⁸Si. Requirements are formulated for the carbon concentration in polycrystalline ²⁸Si which ensure a high yield of single crystals with parameters suitable for metrological applications.     

Carrier mobility in undoped SiC layers grown on silicon by a new epitaxial technique

The first results of studying the electrical properties of thin silicon carbide (SiC) layers grown on silicon using a new method of solid-phase epitaxy are presented. The type of carriers in these SiC/Si films is determined, and their concentration and mobility are measured. SiC films grown by the proposed method on Si substrates possess n-type conductivity. The concentration of majority carriers (electrons) in undoped SiC layers amounts on average to n ～ 10¹⁸ cm^?3, and their mobility varies within μ = 27–85 cm²/(V s), depending on the regime of synthesis.     

Production of submicron SiC particles by d.c. thermal plasma: a systematic approach based on injection parameters

Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl₄ feeding rate, C/Si and H₂/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental set-up allows the production of β-SiC powders at a rate of 200 g h⁻¹ with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen (1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl₄ feeding rate and the H₂/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling the particle size.     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.     

Silicide formation by Ar+ ion bombardment of Pd/Si

Palladium films, 45 nm thick, evaporated on to Si(111) were irradiated to various doses with 78 keV Ar⁺ ions to promote silicide formation. Rutherford backscattering spectroscopy (RBS) shows that intermixing has occurred across the Pd/Si interface at room temperature. The mixing behaviour is increased with dose which coincides well with the theoretical model of cascade mixing. The absence of deep RBS tails for palladium and the small area of this for silicon spectra indicate that short-range mixing occurs. From the calculated damage profiles computed with TRIM code, the dominant diffusion species is found to be silicon atoms in the Pd/Si system. It is also found that the initial compound formed by Ar⁺ irradiation is Pd₂Si which increases with dose. At a dose of 1×10¹⁶ Ar⁺ cm^–2, a 48 nm thickness of Pd₂Si was formed by ion-beam mixing at room temperature.     

A new type of nanostructure in Si/C composite electrodes for lithium-ion batteries

The composition and structure of thin-film Si/C composite anodes produced by alternately depositing controlled amounts of silicon and carbon using magnetron plasma sputtering have been determined by atomic force microscopy, x-ray diffraction, and optical spectroscopy (Raman and UV through IR specular reflectance spectra). The silicon-to-carbon volume ratio in the films was varied from 39.5: 60.5 to 87: 13, and their thickness ranged from 100 to 480 nm. The surface of the films was found to have a nanogranular structure, which had not been reported previously for Si/C composites. This morphology is atypical of structureless silicon layers deposited under the same conditions but is similar to the nanostructure of a thin carbon film consisting of grains uniform in shape and size (D _av = 20–25 nm). Reducing the carbon content of the composites from 60 to 36% increases the grain size from 25 to 45–50 nm. At high silicon contents (near 80%), the nanostructure of the composites is less homogeneous: in addition to nanograins, there are structureless silicon zones. The homogeneity of the nanostructure depends on the Si: C ratio and the sequence and thicknesses of the deposited Si and C layers. Thin (104–173 nm) films containing more than 30% carbon (they have isolated silicon clusters) reveal the highest activity for the lithium intercalation-deintercalation process. Their Raman spectra show strong luminescence characteristic of silicon nanoparticles less than 5–6 nm in size. This effect is missing in the thicker films, in which the silicon forms an infinite cluster and which have a stronger tendency to degrade.     

Synthesis of Si nanocones using rf microplasma at atmospheric pressure

We report the synthesis of silicon nanocones using the rf microplasma discharge at atmospheric pressure. The products formed underneath the tube electrode on Fe-coated crystalline silicon were constituted mainly of silicon and silicon oxide despite the use of a methane-argon mixture. Carbon nanotubes and silicon nanowires were also formed around the silicon nanocones. The number density and average size of silicon nanocones increased with the plasma exposure time accompanied by the enlargement of their surface distribution. The growth mechanism of silicon nanocones is discussed in terms of the catalytic growth via diffusion of silicon through FeSi_x nanoclusters with nanocrystalline Si particle, and Si oxidation due to the plasma heating.     

Electrical properties of bulk silicon dioxide and SiO2/Si interface formed by tetraethylorthosilicate (TEOS)-oxygen plasma enhanced chemical vapor deposition

The electrical properties of bulk silicon dioxide and the SiO₂/Si interface formed by TEOS/O₂ PECVD were investigated. Additionally, the gas phase in the glow discharge was investigated using OES analysis under various experimental conditions. Changes of TEOS/O₂ ratio and the deposition temperature influenced the electrical properties of silicon oxide films. With decreasing TEOS/O₂, ratio, the electrical properties of bulk silicon dioxide and the SiO₂/Si interface were improved. This is thought to be due to the decrease of carbon impurity in the growing oxide film. At higher deposition temperatures, the oxide films had good electrical properties, which is thought to be due to the change of structure in the oxide film. From C–V analysis for all experimental conditions, the P_b center defects were observed near E_v+0.25eV and E_v+0.73eV in the Si band gap. The magnitude was influenced by process parameters such as the TEOS/O₂ ratio and the deposition temperature. From OES analysis, the main emission peaks observed in the glow discharge were from CO, CO ₂ ⁺ , CH, and C. With decreasing TEOS/O₂ ratio, the emission intensity of CH decreased and that of CO increased.     

Simple Construction of Multistage Stable Silicon–Graphite Hybrid Granules for Lithium-Ion Batteries

Because of its high specific capacity, the silicon–graphite composite (SGC) is regarded as a promising anode for new-generation lithium-ion batteries. However, the frequently employed two-section preparation process, including the modification of silicon seed and followed mixture with graphite, cannot ensure the uniform dispersion of silicon in the graphite matrix, resulting in a stress concentration of aggregated silicon domains and cracks in composite electrodes during cycling. Herein, inspired by powder engineering, the two independent sections are integrated to construct multistage stable silicon–graphite hybrid granules (SGHGs) through wet granulation and carbonization. This method assembles silicon nanoparticles (Si NPs) and graphite and improves compatibility between them, addressing the issue of severe stress concentration caused by uncombined residue of Si NPs. The optimal SGHG prepared with 20% pitch content exhibits a highly reversible specific capacity of 560.0 mAh g⁻¹ at a current density of 200 mA g⁻¹ and a considerable stability retention of 86.1% after 1000 cycles at 1 A g⁻¹. Moreover, as a practical application, the full cell delivers an outstanding capacity retention of 85.7% after 400 cycles at 2 C. The multistage stable structure constructed by simple wet granulation and carbonization provides theoretical guidance for the preparation of commercial SGC anodes.     

Surface passivation of silicon by rf magnetron-sputtered silicon nitride films

Si-rich silicon nitride (SiN_x(:H)) films are deposited on single crystalline p-type silicon to investigate their properties as a passivation and antireflection coating for solar cells. The SiN_x(:H) films were reactively sputtered from an intrinsic Si-target in an Ar/N₂/H₂ rf (13.56 MHz) magnetron plasma at substrate temperatures from 150°C to 350°C. The optical band gap of Si-rich SiN_x(:H) becomes lower than 3 eV which was determined from spectral data of the complex refractive index. Infrared spectra show a strong Si–H band in Si-rich films. The effective surface recombination velocity (SRV) was calculated from the effective life time in SiN_x(:H) covered p-Si wafers by the microwave detected photoconductivity decay (MW PCD) technique. Very low values for the effective SRV of about 60 cm/s were determined. The low values of the effective SRV are due to field-effect passivation. The field-effect passivation of the SiN_x(:H)/Si contact is explained with the model of a hetero junction.     

Microstructure and wear behaviour of silicon doped Cr–N nanocomposite coatings

Hard Cr–N and silicon doped Cr–Si–N nanocomposite coatings were deposited using closed unbalanced magnetron sputtering ion plating system. Coatings doped with various Si contents were synthesized by changing the power applied on Si targets. Composition of the films was analyzed using glow discharge optical emission spectrometry (GDOES). Microstructure and properties of the coatings were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and nano-indentation. The harnesses and the elastic modulus of Cr–Si–N coatings gradually increased with rising of silicon content and exhibited a maximum at silicon content of 4.1 at.% and 5.5 at.%. The maximum hardness and elastic modulus of the Cr–Si–N nanocomposite coatings were approximately 30 GPa and 352 GPa, respectively. Further increase in the silicon content resulted in a decrease in the hardness and the elastic modulus of the coatings. Results from XRD analyses of CrN coatings indicated that strongly preferred orientations of (111) were detected. The diffraction patterns of Cr–Si–N coatings showed a clear (220) with weak (200) and (311) preferred orientations, but the peak of CrN (111) was decreased with the increase of Si concentration. The XRD data of single-phase Si₃N₄ was free of peak. The peaks of CrN (111) and (220) were shifted slightly and broadened with the increase of silicon content. SEM observations of the sections of Cr–Si–N coatings with different silicon concentrations showed a typical columnar structure. It was evident from TEM observation that nanocomposite Cr–Si–N coatings exhibited nano-scale grain size. Friction coefficient and specific wear rate (SWR) of silicon doped Cr–N coatings from pin-on-disk test were significantly lower in comparison to that of CrN coatings.     

Optical properties of nano-silicon

We investigated the optical properties of silicon clusters and Si nanocrystallites using photolumine-scence (PL) and Raman scattering technique. Broad luminescence band in the red region was observed from Si-doped SiO₂ thin films deposited by co-sputtering of Si and SiO₂ onp-type Si (100) substrates, annealed in Ar and O₂ atmosphere. Nanocrystalline Si particles fabricated by pulsed plasma processing technique showed infrared luminescence from as grown film at room temperature. Raman spectra from these films consisted of broad band superimposed on a sharp line near 516 cm⁻¹ whose intensity, frequency, and width depend on the particle sizes arising from the phonon confinement in the nanocrystalline silicon. We also performed PL, Raman and resonantly excited PL measurements on porous silicon film to compare the optical properties of Si nanostructures grown by different techniques. An extensive computer simulation using empirical pseudo-potential method was carried out for 5–18 atoms Si clusters and the calculated gap energies were close to our PL data. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Effect of nanoporous silicon coating on silicon solar cell performance

The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells. A screen-printed solar cell technology is used to fabricate n⁺-p silicon solar cell. Nanoporous silicon (PS) layer on n⁺-type Si wafers or on the frontal surface of (n⁺-p)Si solar cell was formed by electrochemical etching in HF-containing solution. The surface morphology, porosity, spectra of photoluminescence and reflectance of PS layers were analyzed. The photovoltaic characteristics of two silicon solar cell type with and without PS layer (PS/(n⁺-p)Si and (n⁺-p)Si cell) were measured and compared. The spectra of photosensitivity of cells were measured in the wavelength range of 300-1100 nm. An average reflection of the porous silicon layer, fabricated on a polished silicon surface, is decreased to 4%. A remarkable increment of the conversion efficiency by 20% have been achieved for PS/(n⁺-p)Si solar cell comparing to (n⁺-p)Si solar cell without PS layer. The results, related with improving of the performance of PS/(n⁺-p)Si solar cell, have been attributed to the effective antireflection and the wide-gap window role of nanoporous silicon on the silicon solar cell.     

Numerical study of antireflection coatings in waveguide structures with silicon nanoparticles interlayer

In this paper, the reflection properties of a multilayer structure containing silicon nanoparticles (Si-NPs), silicon nitride (Si₃N₄), and silicon (Si) is investigated theoretically and numerically. The structure is arranged and its main parameters are defined. The required equations for the propagation of electromagnetic plane waves are derived in detail to obtain the reflection coefficients in a closed form. The reflected power of the structure is determined using these coefficients. In the numerical results, the mentioned power is computed and illustrated as a function of wavelength of the incident radiation, angle of incidence, and Si-NPs parameters. Optimal Si-NPs parameters (particle’s diameter, space between particles, and layer thickness) for low reflection are proposed. These theoretical parameters could effectively be used to design new solar cells.     

High-purity silicon isotopes <Superscript>28</Superscript>Si, <Superscript>29</Superscript>Si,and <Superscript>30</Superscript>Si

The current status of the problem of obtaining high-purity silicon isotopes ²⁸Si, ²⁹Si, and ³⁰Si is analyzed. The scheme of obtaining monoisotopic silicon includes the stages of isotope separation in the form SiF₄, synthesis and deep purification of isotopically enriched silane, obtaining polycrystalline silicon-28,-29, and-30, and growing monocrystals. The basic problems and methods of their solution in the synthesis and deep purification of silane and obtaining poly-and monocrystals of isotopically enriched silicon are discussed. Data characterizing the achieved level of chemical and isotopic purity of high-purity monocrystals of silicon-28 with a main isotope content of more than 99.99% and silicon-29 and silicon-30 with isotopic purity higher than 99% are presented. In monocrystalline ²⁸Si, the boron content was 4.5 × 10¹³, the phosphorus content was 5 × 10¹¹, the carbon and oxygen contents were <1 × 10¹⁶ at/cm³, and the specific resistance was 800 Ω cm. The results of investigation of heat capacity, heat conduction, photoluminescence, and electron paramagnetic resonance spectra for monoisotopic silicon-28 are presented. The heat conduction of monoisotopic silicon is increased considerably owing to the reduced photon scattering on isotopic inhomogeneities. In the region of 20–30 K, the heat conduction of silicon-28 with an isotopic purity of 99.98% is higher by a factor of 8 than the heat conduction of natural silicon. Investigations of photoluminescence spectra in the magnetic field in the low-temperature region demonstrated the capability of optical detection of nuclear spin states of a phosphorus admixture in high-purity silicon-28. p ]Topical questions for further investigations and possible fields of practical application of high-purity isotopically enriched silicon are discussed.