Amorphous Silicon Carbide Matrix Research Articles

Hydrogen evolution in hydrogenated microcrystalline silicon carbide thin films

Evolved gas analysis (EGA), infrared attenuated reflection (ATR), and Raman spectroscopy experiments are used to study hydrogen evolution in hydrogenated microcrystalline silicon carbide (μc-Si1−xCx:H) films prepared by plasma-enhanced chemical vapor deposition. The results are compared with microcrystalline silicon (μc-Si:H). The effused hydrogen and carbon-hydride groups (CH, CH2, and CH3) are measured up to 800 °C. Their EGA curves have a peak at 410 °C, attributed to the methyl groups incorporated in the amorphous matrix during the deposition process. Moreover, hydrogen evolution curves show narrow and sharp peaks centered at 425 and 520 °C, corresponding to hydrogen desorbing from silicon hydrides at grain boundaries. While its content is more important than hydrogen bonded to silicon in the amorphous and denser crystalline regions of μc-Si1−xCx:H, but remains lower than in the μc-Si:H film. Raman and ATR data indicate that the μc-Si1−xCx:H film is composed of small size silicon crystallites embedded in a hydrogenated amorphous silicon carbide matrix and confirmed that carbon is incorporated in the amorphous matrix as methyl groups (CH3), inducing a decrease in SiHx groups compared to the μc-Si:H film.

Structure, optical property, and reaction process of Si quantum dots embedded in an amorphous silicon carbide matrix

Silicon quantum dots (QDs) embedded in an amorphous silicon carbide matrix were prepared using silane, methane, and hydrogen as reactive gases at a low substrate temperature by radio-frequency plasma enhanced chemical vapor deposition followed by thermal annealing at 1000 °C in a N2 atmosphere in the quartz furnace. The structure, optical properties, and reaction process of the synthesized Si QDs embedded in an amorphous silicon carbide matrix under different discharge powers are investigated. With the increase in the discharge power from 40 to 100 W, the experimental result measured by X-ray diffraction, Raman spectroscopy, UV-Visible spectroscopy, and field emission scanning electron microscopy reveals that the average size and the crystal volume fraction of Si QDs decrease from 4.4 to 3.4 nm and from 61.1% to 46.3%, respectively, while the optical bandgap and the deposition rate increase from 1.75 to 1.96 eV and from 15.5 to 16.5 nm/min, respectively. Moreover, the real-time diagnosis of plasma by optical emission spectroscopy (OES) is used to determine the chemical species and excitation temperature under the glow discharge of silane, methane, and hydrogen gas mixtures. Based on the OES measurement, the chemical reactions happening in the plasma and growth mechanism for the synthesis of Si QDs embedded in an amorphous silicon carbide matrix have been proposed. This work plays a significant role in preparation of the Si QDs embedded in an amorphous silicon carbide matrix for third-generation photovoltaic solar cells.

Grain growth of nanocrystalline 3C-SiC under Au ion irradiation at elevated temperatures

Nanocrystalline silicon carbide (SiC) represents an excellent model system for a fundamental study of interfacial (grain boundary) processes under nuclear radiation, which are critical to the understanding of the response of nanostructured materials to high-dose irradiation. This study reports on a comparison of irradiation effects in cubic phase SiC (3C-SiC) grains of a few nanometres in size and single-crystal 3C-SiC films under identical Au ion irradiation to a range of doses at 700 K. In contrast to the latter, in which the lattice disorder is accumulated to a saturation level without full amorphization, the average grain size of the former increases with dose following a power-law trend. In addition to coalescence, the grain grows through atomic jumps and mass transport, where irradiation-induced vacancies at grain boundaries assist the processes. It is found that a higher irradiation temperature leads to slower grain growth and a faster approach to a saturation size of SiC nanograins. This unusual behaviour could be associated with irradiation-induced grain nucleation and growth in amorphous SiC matrix in which the 3C-SiC grains are embedded. The results could potentially have a positive impact on structural components of advanced nuclear energy systems.

Silicon quantum dots embedded in amorphous SiC matrix for third-generation solar cells: Microstructure control by RF discharge power

A low-frequency (460 kHz), low-pressure, thermally non-equilibrium, high-density inductively coupled plasma (ICP) has been used to synthesize a novel, advanced photovoltaic material suitable for fabrication of third-generation solar cells. Silicon quantum dots (SQDs) embedded in an amorphous silicon carbide matrix were prepared at a very low substrate temperature of approximately 200°C without any hydrogen dilution. The effect of the radio-frequency (RF) power of the plasma discharge on the morphology and structure of the embedded quantum dots was studied. A brief discussion on the possible mechanisms of the quantum dot formation in the ICP is presented. This study is relevant to third-generation photovoltaic solar cells.

Growth of Wide-Bandgap Nanocrystalline Silicon Carbide Films by HWCVD: Influence of Filament Temperature on Structural and Optoelectronic Properties

Silicon carbide (SiC) thin films have been deposited using a hot-wire chemical vapor deposition technique on quartz substrates with a mixture of silane, methane, and hydrogen gases as precursors at a reasonably high deposition rate of approximately 15 nm/min to 50 nm/min. The influence of the filament temperature (TF) on the structural, optical, and electrical properties of the SiC film has been investigated using x-ray diffraction, Raman scattering, Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet–visible–near infrared transmission spectroscopy, and dark conductivity (σd) studies. Films deposited at low TF (1800°C to 1900°C) are amorphous in nature with high density of Si–Si bonds, whereas high-TF (≥2000°C) films are nanocrystalline embedded in an amorphous SiC matrix with higher concentration of Si–C bonds and negligible concentration of Si–Si bonds. The bandgap (Eg) varies from 2.5 eV to 3.1 eV and σd (50°C) from ∼10−9 Ω−1 cm−1 to 10−1 Ω−1 cm−1 as TF is increased from 1900°C to 2200°C. This increase in Eg and σd is due to microstructural changes and unintentional oxygen doping of the films.

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 1019 cm-3 to 3.7 × 1017 cm-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.

Improvement of solar cells performance by boron doped amorphous silicon carbide/nanocrystalline silicon hybrid window layers

Boron doped amorphous silicon carbide/nanocrystalline silicon (p-type a-SiC/nc-Si) hybrid window layer has a great potential to improve the solar cells performance for its high optical band gap, carrier mobility and doping efficiency. Thin (<30nm) p-type a-SiC/nc-Si hybrid films have been successfully deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) from silane–methane gas mixtures at a low temperature of 150°C. Raman, FT-IR and HRTEM measurements were employed to investigate the structural characteristics of hybrid structure films. High quality p-type a-SiC/nc-Si hybrid window layer with controllable nano-sized silicon crystals embedded in amorphous silicon carbide matrix was obtained. Hybrid structure films with high values of optical band gap (∼2.2eV) and σd (∼10−3S/cm) were achieved in high hydrogen dilution conditions with optimized doping concentrations. Applying high quality p-type window layers and intrinsic a-Si layers, both high open circuit voltage (>0.96V) and high quantum efficiency values at short wavelengths were achieved for n–i–p type a-Si single junction solar cell. Meanwhile, the efficiency of the amorphous silicon and silicon–germanium (a-Si/a-SiGe) double junction solar cell was significantly improved to 11.8% by incorporating p-type a-SiC/nc-Si hybrid structure window layer.

Shock-wave propagation through pristine a-SiC and carbon-nanotube-reinforced a-SiC matrix composites

We report on the results of a large-scale molecular dynamics simulation study of shock-wave propagation in pristine amorphous silicon carbide and carbon-nanotube-reinforced amorphous silicon carbide matrix composites. We seek to understand the effects of ensembles of aligned nanotubes, both transversely and longitudinally oriented, on the shock-wave structure and dynamics and structural rearrangements taking place in the shock-loaded composite materials. It is found that the presence of aligned nanotubes in amorphous silicon carbide matrix leads to a reduction of shock-wave velocity and modifies the shock-wave front structure in a wide range of impact velocities. The temporal evolution of density profiles behind the shock-wave front is studied and conclusions are drawn regarding the effects of carbon nanotubes on the structural rearrangements in the shock-loaded composite materials. The mechanisms of carbon nanotube failure under shock loadings and their implications for energy dissipation rates in composite material systems are discussed for both considered cases of carbon nanotube alignments.

Room temperature visible photoluminescence of silicon nanocrystallites embedded in amorphous silicon carbide matrix

The nanocrystalline silicon embedded in amorphous silicon carbide matrix was prepared by varying rf power in high vacuum plasma enhanced chemical vapor deposition system using silane methane gas mixture highly diluted in hydrogen. In this paper, we have studied the evolution of the structural, optical, and electrical properties of this material as a function of rf power. We have observed visible photoluminescence at room temperature and also have discussed the role played by the Si nanocrystallites and the amorphous silicon carbide matrix. The decrease of the nanocrystalline size, responsible for quantum confinement effect, facilitated by the amorphous silicon carbide matrix, is shown to be the primary cause for the increase in the PL intensity, blueshift of the PL peak position, decrease of the PL width (full width at half maximum) as well as the increase of the optical band gap and the decrease of the dark conductivity.

Highly conductive boron-doped nanocrystalline silicon-carbide film prepared by low-hydrogen-dilution photo-CVD method using ethylene as a carbon source

Boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films with a low-concentration of hydrogen-dilution were grown by a mercury-sensitized photo-chemical vapor deposition method using silane (SiH 4), hydrogen (H 2), diborane (B 2H 6), and ethylene (C 2H 4) as a carbon source. From the Raman and FTIR spectrum measurements, the p-nc-SiC:H film is composed of nanosize crystal silicon embedded in a hydrogenated amorphous silicon-carbide matrix. A dark conductivity as high as 1.7×10 −1 S/cm, with an optical bandgap is 2.0 eV, and a crystal volume fraction of 50%, were obtained. We tested these films as window material for amorphous silicon solar cells, obtaining an initial conversion efficiency of 10.4% without using any back reflectors.

Hydrogen plasma-enhanced chemical vapour deposition of silicon carbide thin films from dodecamethylcyclohexasilane

(Me 2 Si) 6 was used as a single-source precursor to grow silicon carbide thin films on silicon substrates by hydrogen PECVD. Smooth thin films with good adhesion to the substrates were obtained at temperatures of 1223-1423K. The films deposited at temperatures above 1273K were polycrystalline β-SiC, whereas the films grown at 1223K were composed of microcrystals of β-SiC in an amorphous silicon carbide matrix.