Takayuki Nagano

Diamond Synthesis by Microwave-Plasma Chemical Vapor Deposition Using CH3Cl and CH2Cl2 as Carbon Source

Polycrystalline diamond films have been synthesized from gas mixture systems of CH2Cl2-H2 and CH3Cl-H2 by the microwave-plasma chemical vapor deposition (CVD) technique. The gas reaction speed was promoted by using CH3Cl and CH2Cl2, although the deposition rate decreased, due probably to surface etching by the chlorine in the case of high-concentration CH2Cl2. A broad band centered at about 1500 cm-1 in the Raman spectra decreased considerably in the films compared with those synthesized from the CH4-H2 gas mixture. Very little residual chlorine was detected in the deposited films. The use of chlorine-permuted methane as a carbon source was found to be advantageous for high-purity diamond deposition at low temperature and high concentration.

Effects of Surface Oxides of SiC on Carbon Nanotube Formation by Surface Decomposition

The effects of chemical treatment on carbon nanotube (CNT) formation by surface decomposition of 6H–SiC were investigated. In the case of 6H–SiC cleaned with CH2Cl2, CNTs were formed only on the C-(0001) face. On the other hand, in the case of 6H–SiC etched with HF solution, CNTs were formed on both C-(0001) and Si-(0001) faces. X-ray photoelectron spectroscopy (XPS) analysis detected SiOxCy on the C-(0001) face, and SiO2 and SiOvCw on the Si-(0001) face of SiC cleaned with CH2Cl2. The existence of SiO2 on the SiC surface prevented CNT formation. The length of CNTs on the Si-(0001) face was about 65% of that on the C-(0001) face. The length of CNTs on the C-(0001) face was not affected by chemical treatment. The difference of CNT length between the C and the Si faces originated from the anisotropy of thermal oxidation.

Preparation of Silicon-on-Insulator Substrate on Large Free-Standing Carbon Nanotube Film Formation by Surface Decomposition of SiC Film.

Epitaxial 3C–SiC films were grown on silicon-on-insulator (SOI) substrates with [111] and [100] orientations by a chemical vapor deposition (CVD) method using an alternating gas supply. The SiC films were removed from the SOI substrates by chemical etching treatments. The sizes of the free-standing SiC (111) films, which were restricted by crack formation during the etching process, depended on the thickness and crystal quality of the silicon overlayer (SOL) on SOI substrates, while that of the free-standing SiC (100) film was maintained even after the etching. Carbon nanotubes (CNTs) were formed on both C and Si faces of the SiC (111) films by surface decomposition at 1973 K at 1.33×10-2 Pa. The growth rate of CNTs on the C face was three times faster than that of CNTs on the Si face. A free-standing CNT film was formed after complete surface decomposition of the SiC film. The area of the free-standing CNT film was about 80 mm2. On the other hand, graphite was formed on the SiC (100) film surface which was removed from the SOI (100) substrate.

Production of Highly Oriented Carbon Nanotube Film by Surface Decomposition of Silicon Carbide Polycrystalline Film

3C-SiC polycrystalline films with preferred [111] orientation were grown on graphite and glassy carbon substrates by a chemical vapor deposition (CVD) method using an alternating gas supply. The surface morphology of the SiC (111) film was not affected by the crystal quality or the surface roughness of the substrates. The SiC (111) films on graphite and glassy carbon substrates were heated at 1973 K for 0.5 ~4 h at 1.33×10-2 Pa. Carbon nanotubes (CNTs) were oriented perpendicular to the graphite and glassy carbon substrates. The degree of CNT orientation was hardly affected by the crystal quality or the surface roughness of the substrates. We can produce highly oriented CNTs on amorphous, polycrystalline or conductive substrates without a catalyst by this fabrication process.

Creation of Highly Oriented Freestanding Carbon Nanotube Film by Sublimating Decomposition of Silicon Carbide Film

Freestanding carbon nanotube films were created by sublimating decomposition of silicon carbide films, which were grown on thin silicon-on-insulator (SOI) substrates by chemical vapor deposition with alternating gas supply of SiH2Cl2 and C2H2. The sublimating decomposition of silicon carbide is performed by annealing the SiC film at 1600°C in vacuum (1×10-2 Pa). The carbon nanotubes were highly oriented perpendicular to the single-crystal SiC(111) film.

Surface Etching of 6H–SiC(0001) by Annealing in Vacuum for Obtaining an Atomically Flat Surface

In order to obtain a flat SiC surface, both the Si(0001) and the C(000) faces of 6H–SiC were annealed in vacuum at temperatures above 1700°C for 2 h. After removing graphite films grown on the Si face, scratches due to polishing vanished, and a smooth surface with atomically flat crystal terraces of width ranging from 200 to 400 nm and step height ranging from 0.8 to 2.2 nm appeared. On the other hand, a rough surface with many holes and particles was observed on the C face when using the same procedures as for the Si face. The etching technique for the Si face is useful for preparing SiC substrates for epitaxial growth, although there were some small pits on the surface due to the native defects in the crystal.

Etching to obtain atomically flat 6H-SiC (000-1) surface after annealing in nitrogen ambient

The etching process of the 6H-SiC(000-1) surface was investigated to obtain a flat surface by removing scratches due to polishing. An atomically flat surface with a step-terrace structure without scratches has been obtained by removing the graphite layer grown on the substrate after annealing in N 2 at a temperature of 1900 °C for 6 h. The step height corresponds to that of a 6H-SiC unit cell. In contrast, a rough surface was observed on the Si face, 6H-SiC (0001), using the same process as for the C face.