Papers by Keyword: Chemical Vapor Deposition (CVD)

Abstract: Yield of carbon nanotubes (CNTs) depends on numerous process parameters such as temperature of synthesis, type of catalyst, type of precursor, time of precursor flow and partial pressure of precursor gas as well as carrier gas, etc. Experiments were performed in order to find the optimum temperature of synthesis for varying time of precursor flow. The yield was evaluated in terms of mass of crystalline CNTs per gram of substrate and/or catalyst. The CNTs were grown on a calcium carbonate (CaCO3) substrate, with iron-cobalt (Fe-Co) as a catalyst, using acetylene (C2H2) as a precursor gas and argon (Ar) as a carrier gas. A three-stage purification process, incorporating two acid treatment steps and one annealing step, was used for purification which ensures high grade purity of CNTs. The highest yield of 21.4 g of CNTs per g of catalyst was achieved at 700oC for 60 min of synthesis. The CNTs were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Raman, Thermo-gravimetric analysis (TGA), and Gas chromatography (GC).

Abstract: The development of new power generation plants firing fossil fuel is aiming at achieving higher thermal efficiencies of the energy conversion process. The major factors affecting the efficiency of the conventional steam power plants are the temperature and, to a lesser extent, the pressure of the steam entering the turbine. The increased operating temperature and pressure require new materials that have major oxidation resistance. Due to this problem, in the last years numerous studies have been conducted in order to develop new coatings to enhance the resistance of steels with chromium contents between 9 and 12% wt against steam oxidation in order to allow operation of steam turbines at 650 0C. In this study, Si protective coatings were deposited by CVD-FBR on ferritic steel P-91. These type of coatings have shown to be protective at 650 0C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). The results show a substantial increase of steam oxidation protection afforded by Si coating by CVD-FBR process.

Abstract: Multi-walled carbon nanotubes (MWCNTs) have been grown, simultaneously at different temperatures (600 °C, 700 °C, 725 °C, and 750 °C), by thermal chemical vapour deposition (CVD) of acetylene (C2H2) gas, in ammonia (NH3) atmosphere. The samples have been characterized morphologically by scanning electron microscopy (SEM) and a structurally by Raman spectroscopy. These analyses show that, when the temperature of substrate increases, the CNT average diameter and the density distribution decrease, and the CNTs are less defective. However there is a temperature limit (725 °C) for CNT growth. We have grown CNTs onto Si3N4/Si substrates with Pt electrodes, in order to realize the gas sensing devices. The CNTs grown at 725 °C, exhibit a fast response and a high sensitivity to NO2 gas.

Abstract: Silicon nitride coatings were prepared by chemical vapor deposition (CVD) method. The effect of raw precursor materials (SiH4 and NH3) ratio, gas flow amount, reaction temperature, and reaction pressure and deposition time to the microstructure, chemical compositions and crystal structures of the silicon nitride coatings are investigated. The results suggest that when the SiH4 and NH3 are used as the precursors, Ar is used as the protect gas, H2 is used as the carrier gas, the flow amount of SiH4 and NH3 is 200 sccm and 1000 sccm, the reaction pressure is 100 Pa, the reaction temperature is 900 oC and the deposition time is 30 min, the silicon nitride coatings with high deposition rate (~ 85 Å/min), small grain size (~ 0.2 µm) and high density are obtained. XRD results of the obtained silicon nitride coatings suggest their phase structure is amorphous.

Abstract: Crystallographic quality of the epitaxial layers depends on the process temperature, partial pressures of active components and the surface polarity and also on the crystallographic quality of the subsurface layer resulting from the preparation of the substrate. The polishing etching in hydrogen-propane atmosphere of 4H-SiC substrate of different orientations and polarity was studied. The optimization of the polishing etching has been achieved with respect to the flow of C3H8, the duration and the temperature of the process. The investigation of the surface of SiC substrate before and after in situ polishing-etching in H2+C3H8 atmosphere was carried out by Nomarski interference contrast microscopy (DIC) and atomic force microscope (AFM).

Abstract: Conversion of basal plane dislocations (BPD) to threading edge dislocations (TED) in 8° off-cut 4H-SiC within an n+ buffer layer would eliminate the nucleation site for Shockley-type stacking faults in active device regions grown on such buffer layers. To that end, the propagation and conversion of BPDs through in situ growth interrupts is monitored using ultraviolet photoluminescence (UVPL) wafer mapping. The optimized growth interrupt scheme lasts for 45 minutes with a propane mass flow of 10 sccm at growth temperature. This scheme has shown a conversion efficiency of up to 99% for samples with electron (hole) concentrations < 5x1014 cm-3 (8x1015 cm-3). Samples subjected to a 45 or 90 minute interrupt under 10 sccm of propane, regardless of conversion efficiency, exhibit a “slit” in the surface morphology associated with each BPD and oriented perpendicular to the off-cut and BPD propagation direction. Repetition of the optimal interrupt sequence with a 5 μm epilayer spacer grown between the two interrupts resulted in the same conversion efficiency as a single optimal growth interrupt. Incorporation of the optimal interrupt into an n+ layer is more complicated as attempts to do so in layers doped with nitrogen to 2x1018, 2x1017 and 2x1016 cm-3 resulted in conversion efficiencies of ~6%.

Abstract: In this work we report synthesis and characterization of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by plasma chemical vapor deposition (P-CVD) method at 200 0C on glass substrates. Film properties are carefully and systematically investigated as a function of argon (Ar) flow rate. Characterization of these films with Raman spectroscopy revealed that the addition of Ar into SiH4-H2 plasma endorses the growth of crystallinity in the films. The Fourier transform infrared (FTIR) spectroscopic analysis showed that with increasing Ar flow rate the hydrogen bonding in the films shifts from mono-hydride (Si-H) to di-hydride (Si-H2) and (Si-H2)n complexes. The hydrogen content in the films was found < 7 at. % over the entire range of studied Ar flow rate. The band gap of nc-Si:H films was found to be higher than hydrogenated amorphous silicon (a-Si:H) films (> 2 eV). The nc-Si:H films with dark conductivity 1.3x10-7 S/cm having deposition rate as high as 2.5 Å/s and of crystalline fraction 98 % have been obtained.

Abstract: . Carbon nanofibers have been attracted many attentions for their potential applications in nanocomposites and electromagnetic wave-absorbing materials due to their remarkable mechanical, electrical and other properties. Ethanol as carbon source possesses low toxicity, easier storage and transportation. In this paper, we report ethanol catalytic chemical vapor deposition (ECCVD) for synthesizing carbon nanofibers. We utilized ferrocene as catalyst precursor and use ethanol as carbon source to synthesize carbon nanofibers by ethanol chemical vapor deposition. The deposits were characterized by employed scanning electron microscopy, transmission electron microscope and Raman spectroscopy.

Abstract: Chemical vapor deposition, Catalyst, Carbon nanocoils, High efficient growth Abstract: Carbon nanocoils (CNCs) were prepared by thermal chemical vapor deposition (CVD) using a Fe and Sn containing solution as the catalyst predecessor. The solutions of Fe2(SO4)3/SnCl2, FeCl3/SnCl2 and Fe(NO3)3/SnCl2 with the mol ratios of 3:0.1 to 3:1 were used as catalysts. Comparing the catalysts in different composition ratios with the grown deposits after CVD, we found that the optimum mol ratio between Fe and Sn is 6:0.1. It is noted that the catalyst combination of Fe2(SO4)3 /SnCl2 obviously increases the quantity of the grown carbon deposits indicating that it has the largest catalytic activity among the three kinds of combinations. Large surface area of catalyst films formed by release of SO3 from the decomposition of Fe2(SO4)3 over the temperature of 480 °C is very good for the carbon nanocoils growth and the introduction of sulfide impurities are the key factors leading to the high efficient growth for carbon nanocoils. It is known that Fe-additions lead to the growth of carbon nanotubes or nanofibers, while Sn induces their helical growth and a little sulfur impurities may induce the efficient growth of carbon nanocoils.

Abstract: The physical background and present status of the application of metal-insulator-silicon structures with semiconductor nanocrystals embedded in the insulator layer for memory purposes is breafly summarized.