Silicon-rich Carbide Research Articles

In-situ growth mechanism and structural evolution of silicon quantum dots embedded in Si-rich silicon carbide matrix prepared by RF-PECVD

Amorphous silicon-rich silicon carbide (a-SixC1-x) thin films that contain Si quantum dots (QDs) were prepared using radio-frequency plasma-assisted chemical vapor deposition (RF-PECVD) from reactive silane and methane precursor gases at a substrate temperature of 300 °C. The effect of the silane-to-methane flow ratio on the structural properties of the Si QDs/a-SixC1-x nanocomposite films, is systematically studied by means of high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The obtained results confirm the in-situ formation of Si QDs within the amorphous SiC matrix. Moreover, it is found that the Si crystalline fraction increases from 60.7 to 78.5% with increasing the gas flow ratio. Photoluminescence analyses reveal broader emission spectra which are explained in terms of quantum confinement-related emission corresponding to different QD sizes. Based on the obtained results, the growth mechanisms of Si QDs during the deposition of the a-SixC1-x film and the amorphous-to-crystalline transition are discussed.

Si diffusion induced adhesion and corrosion resistance in annealed RF sputtered SiC films on graphite substrate

Silicon carbide (SiC) is one of the promising candidates for graphite protection in different applications involving high temperatures and a highly corrosive environment. An ideal Silicon carbide coating should withstand a corrosive environment without compromising its adhesion. Herein, RF magnetron sputtered silicon-rich SiC thin films were deposited on a graphite substrate followed by annealing at 1000 °C, 1200 °C, and 1400 °C in an inert atmosphere. The impact of annealing temperature on microstructure, adhesion and chemical stability of SiC thin films was demonstrated. Different analytical techniques like Scanning electron microscopy (SEM), X-Ray Diffraction (XRD), Fourier's Transform Infrared (FTIR) spectroscopy and nano-indentation were used to study microstructural evaluation and mechanical characteristics. Moreover, the electrochemical analysis (Tafel and Electrochemical impedance spectroscopy) was performed in 3.5% NaCl solution. The microstructural analysis revealed that the amorphous SiC thin film turned into a crystalline and dense film upon annealing. Scanning electron micrographs showed that silicon-rich regions at SiC film surface started to disappear as Si diffuses into graphite matrix at elevated temperatures. Both these factors contributed to improvement in the adhesion of SiC coating with graphite substrate as annealing temperature increased. In addition, the nano-indentation hardness of 5.2 GPa was obtained for as grown sample, which decreased at 1000 °C, and then increased at 1200 °C and 1400 °C. Furthermore, the electrochemical analysis confirmed the enhancement in corrosion resistance upon annealing at a temperature of 1200 °C and 1400 °C due to Si diffusion and film compactness in these samples.

Crystal structures and the electronic properties of silicon-rich silicon carbide materials by first principle calculations

Silicon carbide has been used in a variety of applications including solar cells due to its high stability. The high bandgap of pristine SiC, necessitates nonstoichiometric silicon carbide materials to be considered to tune the band gap for efficient solar light absorptions. In this regards, thermodynamically stable Si-rich SixC1-x materials can be used in solar cell applications without requiring the expensive pure grade silicon or pure grade silicon carbide. In this work, we have used density functional theory (DFT) to examine the stability of various polymorphs of silicon carbide such as 2H–SiC, 4H–SiC, 6H–SiC, 8H–SiC, 10H–SiC, wurtzite, naquite, and diamond structures to produce stable structures of Si-rich SixC1-x. We have systematically replaced the carbon atoms by silicon to lower the band gap and found that the configurations of these excess silicon atoms play a significant role in the stability of Si-rich SixC1-x. Hence, we have investigated different configurations of silicon and carbon atoms in these silicon carbide structures to obtain suitable SixC1-x materials with tailored band gaps. The results indicate that 6H-SixC1-x is thermodynamically the most favorable structure within the scope of this study. In addition, Si substitution for C sites in 6H–SiC enhances the solar absorption, as well as shifts the absorption spectra toward the lower photon energy region. In addition, in the visible range the absorption coefficients are much higher than the pristine SiC.

Stability and Electronic Properties of Silicon-Rich Silicon Carbide Structures

The high stability of silicon carbide make it an attractive materials to be used in different applications such as solar cells. Silicon-rich silicon carbide (Si-rich SiC) materials are necessary to tune the band gap for efficient solar light absorptions. Si-rich SiC materials which are thermodynamically stable can be used in solar cell applications without requiring the expensive pure grade silicon or pure grade silicon carbide. Density functional theory (DFT) calculations were used in order to examine different phases of Si-rich SiC to predict stable structures. Since the configurations play a significant role in getting stable results, different configurations of silicon and carbon atoms in Si-rich SiC were considered. The electronic structures were studied, and the total energies were calculated as well as the formation energies. We will present that higher-order hexagonal-phases are more favorable structures than other Si-rich SiC structures due to their more covalent nature of bonding compared to the cubic counterpart.

A ternary–3D analysis of the optical properties of amorphous hydrogenated silicon–rich carbide

Abstract We introduce a novel method for the analysis of the dependence of the optical parameters of Hydrogenated Silicon–rich Carbide (SRC:H) on the alloy composition by using a ternary–3D diagram. The SRC:H samples were fabricated using the very high frequency (VHF) plasma enhanced chemical vapour deposition technique. The composition is given as Si%+C%+H% = 100, with the carbon fraction Cf = C%/(C%+Si%) varying between 0.18 and 0.72. The actual composition was determined by Rutherford Back Scattering analysis. The optical constants are determined by means of Reflectance and Transmittance UV–visible spectroscopy, and are modelled by means of the Jellison–Tauc–Lorentz–with Gaussian Band Tail model. The unique properties of the ternary–3D diagram give unexpected insight on the origin of the dependence of optical properties on composition. It is shown that the wavelength position of the Lorentz oscillator is governed by Cf with virtually no influence of H%. Conversely, the refractive index is strongly affected by H%, besides Cf. We show that using the ternary–3D diagram approach it is possible to separate the contributions of the different components, and derive the analytical dependence between optical parameters and composition. In the second part of the paper such findings are used for a reversed analysis, that is, we employ the obtained analytical formulations in order to retrieve the SRC:H composition within better than ±4% absolute error for all components.

Silicon-Rich Silicon Carbide Hole-Selective Rear Contacts for Crystalline-Silicon-Based Solar Cells.

The use of passivating contacts compatible with typical homojunction thermal processes is one of the most promising approaches to realizing high-efficiency silicon solar cells. In this work, we investigate an alternative rear-passivating contact targeting facile implementation to industrial p-type solar cells. The contact structure consists of a chemically grown thin silicon oxide layer, which is capped with a boron-doped silicon-rich silicon carbide [SiCx(p)] layer and then annealed at 800-900 °C. Transmission electron microscopy reveals that the thin chemical oxide layer disappears upon thermal annealing up to 900 °C, leading to degraded surface passivation. We interpret this in terms of a chemical reaction between carbon atoms in the SiCx(p) layer and the adjacent chemical oxide layer. To prevent this reaction, an intrinsic silicon interlayer was introduced between the chemical oxide and the SiCx(p) layer. We show that this intrinsic silicon interlayer is beneficial for surface passivation. Optimized passivation is obtained with a 10-nm-thick intrinsic silicon interlayer, yielding an emitter saturation current density of 17 fA cm-2 on p-type wafers, which translates into an implied open-circuit voltage of 708 mV. The potential of the developed contact at the rear side is further investigated by realizing a proof-of-concept hybrid solar cell, featuring a heterojunction front-side contact made of intrinsic amorphous silicon and phosphorus-doped amorphous silicon. Even though the presented cells are limited by front-side reflection and front-side parasitic absorption, the obtained cell with a Voc of 694.7 mV, a FF of 79.1%, and an efficiency of 20.44% demonstrates the potential of the p+/p-wafer full-side-passivated rear-side scheme shown here.

In Situ Cleaning Process of Silicon Carbide Epitaxial Reactor

In order to develop the in situ cleaning process using chlorine trifluoride gas for a silicon carbide epitaxial reactor, the etching conditions and process were studied for removing the silicon carbide film formed on the susceptor. The formed silicon carbide film consisted of stacked layers of a polycrystalline 4H-like silicon carbide film, a polycrystalline 3C-silicon carbide film and a silicon-rich silicon carbide film. The average etching rate of the formed film was about four times higher than the silicon carbide coating film of the carbon susceptor. By adjusting the etching temperature to less than 330°C, the formed silicon carbide films could be removed without significant damage to the susceptor.

Tail absorption in the determination of optical constants of silicon rich carbides

We propose a method to analyze reflectance and transmittance spectra of hydrogenated amorphous silicon-rich carbide alloys, which allows the determination of n–k spectra with very good accuracy. The method is based on the Tauc–Lorentz formulation proposed by Jellison and Modine, in which we introduce an absorption band to take into account the tail states. The method is used to retrieve the n–k spectra of a set of silicon-rich carbide films fabricated in a variety of conditions. The results on oscillator parameters are discussed. We show that by introducing this absorption band we achieve a better accuracy in the determination of the oscillator parameters. The resulting oscillator frequency appears to be well correlated with compositional data.

Influence of boron doping and hydrogen passivation on recombination of photoexcited charge carriers in silicon nanocrystal/SiC multilayers

The influence of boron (B)-doping and remote plasma hydrogen passivation on the photoexcited charge carrier recombination in silicon nanocrystal/SiC multilayers was investigated in detail. The samples were prepared by high temperature annealing of amorphous (intrinsic and B-doped) Si1−xCx/SiC superlattices. The photoluminescence (PL) intensity of samples with B-doped silicon rich carbide layers was found to be up to two orders of magnitude larger and spectrally red shifted in comparison with that of the other samples. Hydrogen passivation leads to an additional increase in PL intensities. The PL decay can be described well by a mono-exponential function with a characteristic decay time of a few microseconds. This behavior agrees well with the picture of localized PL centers (surface states) together with the passivation of non-radiative defects by boron. The samples with B-doped SiC layers exhibit an additional PL band in the green spectral region that is quenched by hydrogen passivation. Its origin is attributed to defects due to suppression of crystallization of amorphous SiC layers as a result of B-doping. Measurement of ultrafast transient transmission allowed us to study the initial (picosecond) carrier dynamics. It was found to be dependent of pump intensity and interpreted in terms of multiparticle electron-hole recombination.

Identification and tackling of a parasitic surface compound in SiC and Si-rich carbide films

Abstract Silicon carbide and silicon rich carbide (SiC and SRC) thin films were prepared by PECVD and annealed at 1100 °C. Such a treatment, when applied to SiC/SRC multilayers, aimed at the formation of silicon nanocrystals, that have attracted considerable attention as tunable band-gap materials for photovoltaic applications. Optical and structural techniques (X-ray photoelectron spectroscopy, Reflectance and Transmittance, Fourier Transformed Infrared Spectroscopy) were used to evidence the formation, during the annealing treatment in nominally inert atmosphere, of a parasitic ternary SiO x C y surface compound, that consumed part of the originally deposited material and behaved as a preferential conductive path with respect to the nanocrystal layer in horizontal electrical conductivity measurements. The SiO x C y compound was HF-resistant, with composition dependent on the underlying matrix. It gave rise to a Si-O related vibration in FTIR analysis, that may be misinterpreted as due to silicon oxide. The compound, if neglected, can affect the structural and electrical characterization of the material. To overcome this problem, a procedure is analyzed, based on the deposition of a sacrificial capping a-Si:H layer that partially oxidizes, and is removed by tetra methyl ammonium hydroxide (TMAH) after annealing. XPS analysis revealed that the resulting surface is mainly made up of SiC regardless of the composition of the underlying SRC layer. Subsequent SF 6 :O 2 dry etching results in a porous SiC-rich surface layer. The proposed method is effective in controlling the SRC surface configuration, and allows the performance of reliable optical and electrical characterization.

Optical And Electrical Properties Of Si Nanocrystals Embedded In SiC Matrix

Silicon nanocrystals (Si NCs) embedded in a dielectric matrix showing tunable band gap properties have recently emerged as attracting top absorbers in silicon based high efficiency multijunction devices. This paper presents optical and electrical characterization of Si NCs in SiC matrix resulting from annealing at 1100 °C of silicon-rich carbide (SRC)/SiC multilayers produced by Plasma Enhanced Chemical Vapour Deposition (PECVD), varying either the Si content in the SRC or the SiC thickness. Simulation of Reflectance and Transmittance spectra in the UV-Vis revealed that 1) the Si crystallization increases with increasing Si content; 2) a severe shrinkage of the multilayers occurs upon annealing due to the release of hydrogen and to crystallisation; 3) the growth of nanocrystals is affected by atomic environment and diffusivity of involved atoms at the investigated temperature. Temperature dependent conductivity measurements are performed on multlayers and on reference layers. The results show evidence of defect state conduction in the SiC matrix. Copyright © 2012 VBRI Press.

Self-aggregated Si quantum dots in amorphous Si-rich SiC

Amorphous silicon quantum dots (Si-QDs) self-aggregated in silicon-rich silicon carbide are synthesized by growing with plasma-enhanced chemical vapor deposition on (100)-oriented Si substrate. Under the environment of Argon (Ar)-diluted Silane (SiH4) and pure methane (CH4), the substrate temperature and RF power are set as 350°C and 120W, respectively, to provide the Si-rich SiC with changing fluence ratio (R=[CH4 ]/[SiH4]+[CH4]). By tuning the fluence ratio from 50% to 70%, the composition ratio x of Si-rich Si1−xCx film is varied from 0.27 to 0.34 as characterized by X-ray photoelectron spectroscopy (XPS), which reveals the component of Si2p decreasing from 66.3 to 59.5%, and the component of C1s increasing from 23.9% to 31% to confirm the formation of Si-rich SiC matrix. Annealing of the SiC sample from 650°C to 1050°C at 200°C increment for 30min induces the very tiny shift on the wavenumber of the crystalline Si (c-Si) related peak due to the precipitation of Si-QDs within the SiC matrix, and the Raman scattering spectra indicate a broadened Raman peak ranging from 410 to 520cm−1 related to the amorphous Si accompanied with the significant enhancement for SiC bond related peak at 980cm−1. From the high resolution transmission electron microscopy images, the critical temperature for Si-QD precipitation is found to be 850°C. The self-assembly of the crystallized Si-QDs with the size of 3±0.5nm and the volume density of (3±1)×1018 (#/cm3) in Si-rich SiC film with R=70% are observed after annealing at higher temperature.

Finite Silicon Atom Diffusion Induced Size Limitation on Self-Assembled Silicon Quantum Dots in Silicon-Rich Silicon Carbide

Finite Si atom diffusion induced size limitation of self-assembled Si quantum dots (Si-QDs) in silicon-rich silicon carbide (SiC) is demonstrated. After annealing, the Si-QDs with a size of 3 ± 1 nm are precipitated in the matrix of SiC0.51 deposited by low-temperature plasma-enhanced chemical vapor deposition with Argon-diluted silane and methane mixture. The amorphous-Si dependent Raman scattering peak at ∼470 cm−1 is narrowing with increasing temperature, and the Si-CH3 rocking-mode absorption line is shifted by dehydrogenation after high-temperature treatment. The self-assembled Si-QDs in SiC0.51 with a volume density of 4.4 × 1018 cm−3 transfer from amorphous to crystalline phase by increasing annealing temperature from 850°C to 1050°C. The calculated Si atom diffusion coefficient of 3-4 × 10−4 nm2 s−1 in Si-rich SiC0.51 is 7 orders of magnitude larger than that in pure SiC, which coincides with the linear extrapolation from pure Si and SiC and reveals nonlinear proportionality with C/Si composition ratio and Si-QD size.

Si-rich a-SiC:H thin films: Structural and optical transformations during thermal annealing

Amorphous hydrogenated silicon-rich silicon carbide (a-Si 0.8C 0.2:H) thin films were prepared by plasma enhanced chemical vapour deposition and were thermally annealed in a conventional resistance heated furnace at annealing temperatures up to 1100 °C. The annealing temperatures were varied and the samples were characterised with Auger electron spectroscopy, glancing incidence X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy. As-deposited a-Si 0.8C 0.2:H thin films contain a large amount of hydrogen and are amorphous. When annealing the films, the onset of Si crystallisation appears at 700 °C. For higher annealing temperatures, we observed SiC crystallites in addition to the Si nanocrystals (NCs). The crystallisation of SiC correlates with the occurrence of a strong PL band, which is strongly reduced after hydrogen passivation. Thus PL signal originates from the SiC matrix. Si NCs exhibit no PL yield due to their inhomogeneous size distribution.

Decarbonization mechanisms of polycarbosilane during pyrolysis in hydrogen for preparation of silicon carbide fibers

Polycarbosilane (PCS) fibers are cured by electron beam irradiation in helium. Then, the cured fibers are pyrolyzed under hydrogen. The mechanisms of carbon removal during pyrolysis are investigated using chemical elemental analysis, FTIR, Raman, and AES analysis. The development of microstructure and phase is examined by SEM, TEM, and XRD. The results show that the thermal cleavage of relatively weak Si–H and Si–CH3 bonds takes place first during pyrolysis in hydrogen, generating free radicals. The free radicals then react with C–H bonds or with each other to form Si–CH2–Si groups, releasing hydrogen and methane. As temperature increases, the Si–CH2–CH2–Si groups in PCS begin to dissociate and react with hydrogen to form methane, resulting in the further removal of carbon and giving silicon-rich silicon carbide fibers (i.e. C/Si <1).

Electrical Properties of Silicon-Rich Silicon Carbide Films Prepared by Using Catalytic Chemical Vapor Deposition

We studied electrical properties of the silicon rich silicon carbide (SRSC) films deposited by catalytic chemical vapor deposition (Cat-CVD) technique. A mixture of SiH4, CH4 and H2 gas was employed as the source gas in deposition process. Atomic composition of SRSC films and formation of Si QDs in SRSC films depended on control of CH4/SiH4 mixture ratios. The each different tunneling behavior was observed in current-voltage (I-V) curve of two samples prepared at different CH4/SiH4 mixture ratios. The PL spectra of two samples were also each different. These results were analyzed with relating formation of Si QDs in SRSC films.

Electrical Properties of Silicon-Rich Silicon Carbide Films Prepared by Using Catalytic Chemical Vapor Deposition

not Available.

Paper derived SiC–Si 3N 4 ceramics for high temperature applications

Biomorphic Si 3N 4–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N 2 or N 2/H 2), in the temperature range of 1300–1450 °C SiC–Si 3N 4 ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si 3N 4 and/or β-Si 3N 4 phases depending on the nitridation conditions. The SiC–Si 3N 4 ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.

Variable liquid crystal pretilt angles on various compositions of alignment layers

The authors introduce variable liquid crystal (LC) pretilt angles via ion beam (IB) irradiation of silicon carbide (SiC) layers of various compositions. To control the composition of the SiC layer, the authors altered the rf power ratio between the graphite target and silicon target. The pretilt angle of the silicon-rich SiC layer was constant regardless of IB irradiation angle; however, the carbon-rich SiC layer showed variable pretilt angles, depending on IB irradiation angle. The authors attribute variable pretilt angle to the competition between van der Waals interactions, favoring the vertical alignment, and pi-pi interactions, favoring the LC alignment parallel to IB direction.

Practical aspects of deposition of CVD SiC and boron silicon carbide onto high temperature composites

The application of a uniform chemical vapor deposited coating of silicon carbide (RT42 ™), silicon-rich silicon carbide (RT42A ™) and boron—silicon carbide (RT43 ™) onto high temperature composites require establishment of custom coating conditions for each component. These shapes include nozzles, rectangular panels, structural tubes, blades, vanes and other irregularly shaped components. Conditions of coating need to be tailored to suit the component shape as well as coating chemistry desired. This attention to fixturing and parameter control is necessary when the deposition process is carried out in a large hot walled CVD reactor with temperature and pressure limitations. Successful coating requires understanding component geometry, holding techniques, gas flow, system pressure, and containment techniques to form uniform deposits with reproducible stoichiometry, structure and thickness. Some additional precautions include shielding of the metallic retort vessel to reduce destructive siliciding, vacuum/pressure control for operation at 675–725 Torr for stoichiometric SiC and 100 Torr for Si-rich (non-stoichiometric) SiC and an internal graphite coating chamber designed to direct the gases across and through the components being coated. By designing the coating process around system limitations, a uniform deposit can be obtained by “rotating” parts over a series of coating cycles where each typical application is 37.5–62.5 u.