Uniform design and regression analysis of LPCVD boron carbide from BCl3-CH4-H2 system

Boron carbide was prepared by low pressure chemical vapor deposition (LPCVD) from BCl₃-CH₄-H₂ system. The deposition process conditions were optimized through using a uniform design method and regression analysis. The regression model of the deposition rate was established. The influences of deposition temperature (T), deposition time (t), inlet BCl₃/CH₄ gas ratio (δ), and inlet H₂/CH₄ gas ratio (θ) on deposition rate and microstructure of the coatings were investigated. The optimized deposition parameters were obtained theoretically. The morphologies, phases, microstructure and composition of deposits were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman micro-spectroscopy, transmission electron microscopy (TEM), energy dispersive spectra (EDS), and Auger electron spectra (AES), the results showed that different boron carbides were produced by three kinds of deposition mechanisms.     

The influence of treatment temperature on the acidity of MWCNT oxidized by HNO3 or a mixture of HNO3/H2SO4

The nature of multi-walled carbon nanotubes (MWCNTs) varies with the change in oxidation conditions. In this work, the effect of treatment temperatures on the acidity of MWCNTs was studied. Oxidation was performed by refluxing the MWCNTs in nitric acid or mixtures of sulfuric acid and nitric acid at different temperatures. After oxidative treatment, a quantitative characterization of o-MWCNTs has been performed using acid-base titrations which show that the number of surface acidic functional groups increased by increasing the treatment temperatures. Energy dispersive X-ray (EDX) measurements show that the oxygen content increased with increasing treatment temperatures. Fourier transform infrared absorption spectroscopy (FTIR) was used for qualitative characterization. It has been demonstrated that the acidity is a function of the type of oxidizing agent used and the treatment temperatures. Due to the importance in attachment strategies and functionalization, this study adds to the global discussion of the possibility of controlling the MWCNTs’ surface chemistry which plays a crucial role in determining its reactivity.     

Modified growth of Ge quantum dots using C2H4 mediation by ultra-high vacuum chemical vapor deposition

C₂H₄ mediations were used to modify the Stranski-Krastanow growth mode of Ge dots on Si(0 0 1) at 550 °C by ultra-high vacuum chemical vapor deposition. With appropriate C₂H₄-mediation to modify the Si surface, the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition at the core. These C₂H₄-mediated Ge dots, almost bounded by {1 1 3} facets, have an average diameter and height of 55 and 9 nm, respectively. We propose two major mechanisms to depict the formation of these C₂H₄-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful scheme to tune the topography of Ge dots in an UHV/CVD condition for possible optoelectronic applications.     

Effect of H2 dilution on a-CN:H films deposited by hot-wire chemical vapor deposition

Hydrogenated amorphous carbon nitride (a-CN:H) thin films were deposited by hot-wire chemical vapor deposition (HWCVD) using the gas mixture of CH₄, NH₃ and H₂ precursor gases. The structural and electronic environments studies of H₂ diluted a-CN:H films were carried out by Raman spectroscopy and X-ray photoelectron spectroscopy. The nitrogen content increases while the total carbon contents decreases with increase in H₂ flow rate from 0 sccm to 20 sccm in the a-CN:H films. Moreover, the detail analysis of the carbon core orbital, valence band and hole states of a-CN:H were discussed with different H₂ flow rate.     

Preparation of photocatalytic Fe2O3-TiO2 coatings in one step by metal organic chemical vapor deposition

There are two major difficulties in the TiO₂ liquid-solid photocatalytic system: effective immobilization of the TiO₂ particles; and improving the catalytic activity under visible light. To simultaneously solve these two problems, Fe₂O₃-TiO₂ coatings supported on activated carbon fiber (ACF), have been prepared in one step by a convenient and efficient method—metal organic chemical vapor deposition (MOCVD). XRD results revealed that Fe₂O₃-TiO₂ coatings mainly composed of anatase TiO₂, α-Fe₂O₃ phases and little Fe₂Ti₃O₉. The pore structure of ACF was preserved well after loading with Fe₂O₃-TiO₂ coatings. UV-vis diffuse reflectance spectra showed a slight shift to longer wavelengths and an enhancement of the absorption in the visible region for Fe₂O₃-TiO₂ coatings, compared to the pure TiO₂ sample. A moderate Fe₂O₃-TiO₂ loading (13.7 wt%) was beneficial to mineralizing wastewater because the intermediates could be adsorbed onto the surface of photocatalyst following decomposition. The stable performance revealed that the Fe₂O₃-TiO₂ coatings were strongly adhered to the ACF surface, and the as prepared catalysts could be reused showing potential application for wastewater treatment.     

Deposition and characterization of carbon nitride films from hexamethylenetetramine/N2 by microwave plasma-enhanced chemical vapor deposition

Carbon nitride thin films were deposited on Si(1 0 0) substrate by microwave plasma-enhanced chemical vapor deposition (PECVD). Hexamethylenetetramine (HMTA) was used as carbon and nitrogen source while N₂ gas was used as both nitrogen source and carrier gas. The sp³-bonded C---N structure in HMTA was considered significantly in the precursor selection. X-ray diffraction analysis indicated that the film was a mixture of crystalline - and β-C₃N₄ as well as graphitic-C₃N₄ and β-Si₃N₄ which were not easily distinguished. Raman spectroscopy also suggested the existence of - and β-C₃N₄ in the films. X-ray photoelectron spectroscopy study indicated the presence of sp²- and sp³-bonded C---N structures in the films while sp³C---N bonding structure predominated to the sp² C---N bonding structure in the bulk composition of the films. N was also found to be bound to Si atoms in the films. The product was, therefore, described as CN_x:Si, where x depends on the film depth, with some evidences of crystalline C₃N₄ formation.     

Effect of processing parameters on the deposition rate of Si3N4/Si2N2O by chemical vapor infiltration and the in situ thermal decomposition of Na2SiF6

Silicon nitride (Si₃N₄) and oxynitride (Si₂N₂O) were deposited by chemical vapor infiltration (CVI) through a novel route involving the in-situ thermal decomposition of Na₂SiF₆ in commercial nitrogen precursors containing impurity oxygen. In addition, the quantitative effect of processing time (30, 60, 90, 120 min), temperature (1000, 1100, 1200 and 1300 °C), nitrogen precursor (N₂ or N₂-5%NH₃) and gas flow rate (46.5, 93, 120 and 240 cm³/min) on phase percentage and deposition rate of Si₃N₄ and Si₂N₂O was investigated. Analysis of variance shows that the parameter that most significantly impacts the total amount of deposited phase is the processing temperature, followed by processing time and nitrogen precursor. Regardless of the nitrogen precursor, at 1300 °C, Si₃N₄ and Si₂N₂O depositions follow an S-like and parabolic behavior, respectively. The incubation period shown by Si₃N₄ in N₂-5%NH₃ is associated to a decrease in the O₂ partial pressure during Si₂N₂O formation while the rapid increase at long processing times is attributed to the enhanced effect of hydrogen. PACS 81.15.Gh; 81.05.Je; 81.15.-z; 81.05.Rm; 47.85.L-     

Effect of argon ion activity on the properties of Y2O3 thin films deposited by low pressure PACVD

Yttrium oxide thin films are deposited by microwave electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapour deposition process using an indegeneously developed Y(thd)₃ {(2,2,6,6-tetramethyl-3,5-heptanedionate)yttrium} precursor. Depositions were carried out at two different argon gas flow rates keeping precursor and oxygen gas flow rate constant. The deposited coatings are characterized by X-ray photoelectron spectroscopy (XPS), glancing angle X-ray diffraction (GIXRD) and infrared spectroscopy. Optical properties of the films are studied by spectroscopic ellipsometry. Hardness and elastic modulus of the films are measured by load depth sensing nanoindentation technique. Stability of the film and its adhesion with the substrate is inferred from the nanoscratch test.It is shown here that, the change in the argon gas flow rates changes the ionization of the gas in the microwave ECR plasma and imposes a drastic change in the characteristics like composition, structure as well as mechanical properties of the deposited film.     

Buffers for high temperature superconductor coatings. Low temperature growth of CeO2 films by metal–organic chemical vapor deposition and their implementation as buffers

Smooth, epitaxial cerium dioxide thin films have been grown in-situ in the 450–650°C temperature range on (001) yttria-stabilized zirconia (YSZ) substrates by metal–organic chemical vapor deposition (MOCVD) using a new fluorine-free liquid Ce precursor. As assessed by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM), the epitaxial films exhibit a columnar microstructure with atomically abrupt film-substrate interfaces and with only minor bending of the crystal plane parallel to the substrate surface near the interface and at the column boundaries. With fixed precursor temperature and gas flow rate, the CeO₂ growth rate decreases from 10 Å/min at 450°C to 6.5 Å/min at 540°C. The root-mean-square roughness of the films also decreases from 15.5 Å at 450°C to 4.3 Å at 540°C. High-quality, epitaxial YBa₂C₃O_7−x films have been successfully deposited on these MOCVD-derived CeO₂ films grown at temperatures as low as 540°C. They exhibit T_c=86.5 K and J_c=1.08×10⁶ A/cm² at 77.4 K.     

The low temperature synthesis of Euand Dy activated Sr3Al2O6 nanophosphors by microwave method

The Eu²⁺and Dy³⁺ activated Sr₃Al₂O₆ (S3A2O-ED) nanophosphors were synthesized by a new microwave method. The S3A2O-ED sample calcined in microwave oven at around 650 °C for 20 min possesses a cubic Sr₃Al₂O₆ single phase. The sample showed small size (80–100 nm) and spherical shape. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Comparing with conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature and time. However, the brightness of S3A2O-ED nanophosphors was reduced. The change of luminescent intensity in S3A2O-ED nanophosphors could be attributed to the effect of surface energy.     

Structural and optical properties of nearly stress-free m-plane ZnO film on (1 0 0) γ-LiAlO2 with a GaN buffer layer by metal-organic chemical vapor deposition

m-plane ZnO film was epitaxially deposited on (1 0 0) γ-LiAlO₂ by metal-organic chemical vapor deposition at 600 °C with a GaN buffer layer. The epitaxial relationships between ZnO and GaN, GaN and (1 0 0) γ-LiAlO₂ were determined by X-ray diffraction Φ-scans. There exhibits very small decrease for the E₂ mode shift (0.3 cm⁻¹) of ZnO in the Raman spectrum, which indicates the epitaxial ZnO film was under a slight tensile stress (5.77 × 10⁷ Pa). Unlike the highly strained a-plane ZnO, temperature dependent photoluminescence spectra show that the free A exiton emission was observed with the temperature ≤138 K.