Q.F. Huang

Y-junction carbon nanotubes grown by in situ evaporated copper catalyst

Y-junction carbon nanotubes have been grown by hot-filament chemical vapor deposition for which a gas mixture of acetone and hydrogen was fed and in situ evaporated copper was supplied. Transmission electron microscopy images reveal that, two of three branching angles around the Y-junction are obtuse (>90°) while the other is sharp (<90°). The sharp branching angles ranging between 50° and 80° are nearly twice the bending angles of simple bend junctions. This indicates that the Y-junction can be presented in structure as a combination of two bend junctions. An atomic configuration involving six heptagons on the three saddle surfaces is proposed to understand the topological structure of the observed Y-junction carbon nanotubes.

Branching carbon nanotubes deposited in HFCVD system

Abstract Branching carbon nanotubes were deposited as a by-product of diamond thick films in a hot filament chemical vapor deposition (HFCVD) system using acetone and hydrogen. The branching nanotubes were observed to be uniformly ‘Y’ shaped. Electron diffraction pattern (EDP) and Raman scattering analysis showed that the nanotubes were well graphitized. A broadened graphite (0002) diffraction spot in EDP from the nanotube ‘Y’ junction was also observed as a result of negative curvature surfaces. Energy dispersive X-ray (EDX) analysis indicated that the nanotubes contained a small amount of Cu, which originated from the parts near the hot filaments. The Cu is believed to have acted as a catalyst for the large production of the branching carbon nanotubes.

Conduction mechanism in molybdenum-containing diamond-like carbon deposited using electron cyclotron resonance chemical vapor deposition

The conduction mechanism of molybdenum-containing (Mo) diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition was investigated. It is found that there is a conductivity turning point at around 115 K, above which the conductivity is strongly temperature dependent. This indicates that two types of conduction mechanisms, thermal activation and tunneling coexist in the films, and they dominate the conduction behavior in the high and low temperature regimes, respectively. Within the temperature range investigated, the Poole–Frenkel effect is to be expected for thermal activation. However, due to the low concentration of Mo in the films, this effect was not observable. Tunneling is thought to occur between the Mo clusters or the sp2 clusters. A conductivity model, based on the thermal activation and tunneling, is proposed, and showed good agreement with the results obtained at low field. The conduction behavior at high field is also discussed and some possible mechanism...

Investigation of molybdenum-carbon films (Mo–C:H) deposited using an electron cyclotron resonance chemical vapor deposition system

We have recently proposed a technique for depositing metal incorporated carbon films (Me–C:H) based on an electron cyclotron resonance chemical vapor deposition (ECR) process. This technique employs an ECR plasma derived from the excitation of source gases CH4 and Ar, together with two grids embedded within the chamber that serve as the source of the metal. It has been successfully applied for the deposition of tungsten–carbon films (W–C:H) which have been shown to exhibit a wide range of electrical, optical, and microstructural properties. These properties can be controlled through varying the deposition conditions such as the bias voltages at the grids and the substrate holder, and the flow ratio of CH4/Ar. In this work, we report on the growth and characterization of molybdenum–carbon (Mo–C:H) films deposited using the above technique incorporating two pure Mo grids. The effect of radio-frequency induced direct-current (dc) bias at the substrates was investigated. It was found that the resistivity of t...

Effect of microwave power on diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition

Abstract Diamond-like carbon (DLC) films have been deposited using electron cyclotron resonance chemical vapor deposition (ECR-CVD) under various microwave power conditions. Langmuir probe measurement and optical emission spectroscopy (OES) were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness and optical gap measurements. It was found that the ion density and all signal peaks in the optical emission (OE) spectra increased monotonously following the increase in microwave power. Raman spectra and optical gap measurements indicate that the films become more graphitic with lower content of sp3-hybridized carbon atoms as the microwave power was increased. IR and hardness measurements indicate a reduction in hydrogen content and decrease in hardness for the film produced at relatively high microwave powers. A deposition mechanism is described which involved the ion bombardment of film surfaces and hydrogen–surface interactions. The deposition rate of DLC film is correlated to the ion density and CH3 density.

Metal-containing amorphous carbon film development using electron cyclotron resonance CVD

Abstract A technique for depositing metal–carbon (Me-C:H) thin films is demonstrated based on two metal screen grids embedded within an electron cyclotron resonance chemical vapour deposition (ECR-CVD) system. The grids are negatively biased and supported at adjustable distances above the substrate holder in the deposition chamber. With source gases of methane and argon, sputtering of the metal grids by Ar + results in the incorporation of metal in the growing carbon films. The amount of metal in the films can be very well controlled over a wide range by varying the bias voltage at the grids, the separation of the grids from the substrate holder and the ratio of CH 4 /Ar. Furthermore, by separately biasing the substrate holder, the properties of the films can be varied resulting in the formation of a great variety of Me-C:H films with very different mechanical and structural properties. Tungsten (W-C:H) and molybdenum (Mo-C:H) incorporated carbon films were deposited using this technique, with the metal fractions controlled by varying the flow ratio of CH 4 /Ar and the bias at the substrates. The films were characterised using Rutherford backscattering, X-ray diffraction and Raman scattering measurements, and also in terms of their conductivity, optical absorption and hardness. Large changes are observed in the conductivity and optical gap of the films even at low fraction of metal incorporated. Metal carbides formation was observed for films deposited under bias. The results suggest that the substrate bias has a crucial effect on the incorporation of metal into the a-C:H films and their resulting microstructures.

X-RAY PHOTOELECTRON SPECTROSCOPY OF MOLYBDENUM-CONTAINING CARBON FILMS

Results from x-ray photoelectron spectroscopy (XPS) measurements of molybdenum-containing carbon films (Mo–C:H) deposited using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system are reported in this article. The Mo–C:H films were deposited using a technique with two Mo screen grids incorporated inside the ECR-CVD chamber. The versatility of this technique arises from the ability to control the degree of plasma ionization, sputtering rate of the metal grids, and energy of the impinging ions. Variation of the (CH4/Ar) gas flow ratio results in a change of the Mo fraction within the range of 0.32–15.11 at. %. For large amounts of Mo, the C 1s peak was split into four components with binding energies of 283.05, 284.67, 286.22, and 288.17 eV. These were identified as carbidic (metallic), polymeric, and oxidic (single- and double-bond) carbon, respectively. The presence of oxygen was detected in the films, due possibly to free-radical absorption at the film surface during deposition, or...

Preparation and electron field emission properties of nano-diamond films

Abstract Nano-diamond films with a mean grain size of 20 nm have been successfully prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technique using gas mixture of nitrogen–methane–hydrogen. The structure and surface morphology of the films are examined using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Field emission results show that good crystal quality nano-diamond film has a threshold electric field of 3 V/μm, much lower than the typical threshold electric field of diamond films grown using conventional gas mixture of hydrogen–methane, indicating that nano-diamond film is a good candidate for electron emitter material.

Investigation of tungsten incorporated amorphous carbon film

Metal incorporated carbon films (Me-C:H) were deposited using a new technique with two W screen grids incorporated inside an electron cyclotron resonance chemical vapour deposition (ECR-CVD) chamber. This technique is versatile when applied to the deposition of Me-C:H films as the degree of plasma ionisation, the sputtering rate of the metal grids and the energy of the impinging ions can be independently controlled. In this work, the proposed technique is demonstrated through W-C:H deposition at different flow ratios of CH4 to Ar. A DC bias of 2 330V was applied to the upper and lower grids with either the substrate floating or under direct DC bias. The films were characterised in terms of their conductivity, atomic concentration (RBS), atomic configuration(XPS and XRD), hardness and optical absorption. The resistivities and the optical gaps of the films were noted to decrease drastically upon incorporation of several atomic percentage of W. WC formation is only observed for films deposited with the substrate under direct DC bias. q 1999 Elsevier Science S.A. All rights reserved.

CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders

Abstract Effects of ultrasonic pretreatment on chemical vapor deposition (CVD) diamond nucleation on Si substrates were systematically studied. Pure 1.5–40 μm-diamond powder and mixtures of 1.5–5 μm-diamond as well as 5–20 μm-Tantalum Carbide (TaC) powder were used in ultrasonic pretreatment. The root-mean-square (Rms) surface roughness of the pretreated substrates, residual diamond and TaC powders left on the substrates were examined using atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction (XRD), respectively. It was observed that the Rms surface roughness increases with increasing diamond or TaC powder size, and there is some diamond or TaC powder left on the substrates after ultrasonic pretreatment. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD) technique and characterized by field emission scanning electron microscopy (FE-SEM). It was found that CVD diamond nucleation density strongly depends on particle size of diamond or TaC powder used, the nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances CVD diamond nucleation much more significantly than that of pure diamond powder. A mixture of 1.5 μm-diamond and 20 μm-TaC powders has an equivalent nucleation enhancement efficiency, which could be caused by pure 40 μm-diamond powder.