Jae-Kap Lee

Growth of carbon nanotubes by chemical vapor deposition

Abstract The growth behavior of carbon nanotubes (CNT) deposited from C2H2 by thermal CVD method was investigated. Nickel particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N2, H2, Ar, NH3 and their mixtures to investigate the effect of the environment on the CNT growth behavior. The deposition was performed at 850°C in atmospheric pressure. In pure N2 environment, thick carbon layer deposition occurred on the substrate without CNT growth. The Ni particles encapsulated by the carbon deposition could not work as the catalyst in this condition. However, the growth of CNT was enhanced as the H2 concentration increased in the mixture of N2 and H2 environment. In pure H2 environment, randomly tangled CNTs could be obtained. The growth of CNT was much enhanced when using NH3 as the environment gas. Vertically aligned CNTs could be deposited in NH3 environment, whereas the CNT growth could not be obtained in the mixture of N2 and H2 environment of the same ratio of N/H. These results were discussed in terms of the passivation of the catalyst caused by the excessive deposition of carbon on the catalyst surface. For the deposition of the CNT, the decomposition rate of C2H2 should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface by excessive carbon deposition. The present work showed that the composition of environment gas significantly affects the reaction kinetics in the CNT growth. It is also noted that nitride surface layer formation on Ni catalyst in NH3 environment can affect the CNT growth behavior.

Growth of γ-Al2O3 thin films on silicon by low pressure metal-organic chemical vapour deposition

Abstract Metal-organic chemical vapour deposition of Al2O3 using aluminium isopropoxide (Al(OC3H7)3) and nitrous oxide (N2O) via thermal pyrolysis was investigated with the goal of producing high quality Al2O3p-Si(100) interfaces. From the X-ray diffraction analysis, the film was found to be a γ-Al2O3 heteroepitaxial film. The stoichiometry of the grown Al2O3 film was similar to that of sapphire observed from Auger electron spectroscopy. Room temperature capacitance-voltage measurements clearly reveal metal-insulator-semiconductor behaviour for samples with the Al2O3 insulator gate, and the interface state densities at the Al2O3p-Si heterointerface were approximately 1011 eV−1 cm−2, at levels centred in the silicon energy gap.

The large area deposition of diamond by the multi-cathode direct current plasma assisted chemical vapor deposition (DC PACVD) method

Abstract Recently, considerable progress in direct current plasma assisted chemical vapor deposition (DC PACVD) of diamond materials has been achieved by adopting a multi-cathode system with cathode temperatures maintained above 2000 °C. This paper presents an overview of the multi-cathode DC plasma CVD method. The plasma operates in the transition region between abnormal glow and arc discharges on a current–voltage curve. Its gas temperature, as estimated by optical emission spectroscopy, is approximately 3000 K. The standard apparatus composed of seven cathodes enables the deposition of diamond films with various film attributes including good optical transparency on substrates of 3–4 inches in diameter.

Large size plasma generation using multicathode direct current geometry for diamond deposition

The deposition area of diamond film is increased by applying a geometry of multiple cathodes and a single anode in direct current (dc) plasma assisted chemical vapor deposition (PACVD). Each cathode is made of Ta and connected independently to its own dc power supply. The operating pressure is 1.3 × 10 4 Pa (100 Torr), and methane-hydrogen mixed gas is used as reaction gas. The voltage and the current applied to each cathode are 650 V and 4 A, respectively. The transition from a diffuse glow to an arc is prevented by maintaining cathode temperatures above 2000 °C, which inhibits carbon deposition on the cathodes. Translucent diamond film of 3 in. diameter, thicker than 200 μm, is grown using seven cathodes with 3% CH 4 –H 2 mixed gas for 110 h. The deposition area can be increased further by increasing the number of cathodes.

Structure of multi-wall carbon nanotubes: AA′ stacked graphene helices

The structure of multi-wall carbon nanotubes has been attributed previously to disordered stacking of the graphene planes. Evidence is presented that the nanotubes analyzed in this paper occur with stacked graphene layers in the sequence of AA′, where alternate graphene planes are translated by half the hexagon width. We further present proof that the crystalline materials comprise graphene helices (∼5 nm in width), rather than in the form of a perfect tube. We also show that the structural model proposed here may be a common structure for multi-wall carbon nanotubes.

Effect of additional gas on diamond deposition by DC PACVD

Abstract The effect of the addition of Ar and nitrogen gas on the deposition of diamond using a direct current (DC) plasma of CH 4 -H 2 gas mixture is investigated. The DC plasma is generated by applying a voltage between 820 and 900 V and the resulting current is between 5.5 and 6 A. The addition of Ar makes the plasma unstable and is limited to 5% owing to the plasma extinction at higher Ar concentrations. Growth rate and non-diamond carbon content in the diamond film increase with the Ar amount slightly irrespective of deposition temperature. An optical emission spectrum shows the increase of emission of C 2 and CH while showing a constant average electron energy of the plasma. On the contrary, by adding nitrogen, the morphology is changed to a ball-like diamond shape as well as growth rate of diamond film is decreased abruptly. The optical emission intensities of C 2 and hydrogen also show an abrupt drop with the nitrogen addition. The role of plasma species in the deposition behaviour of diamond is also discussed.

Properties of diamond films deposited by multi-cathode direct current plasma assisted CVD method

Abstract Four diamond films were prepared by the multi-cathode direct current plasma-assisted chemical vapor deposition (DC-PACVD) method and optical and thermal properties were characterized. Optical transmission and thermal conductivity were strongly dependent on the power density and the methane concentration. Impurities such as, H, Na, Al, Si, K, Ca and Ta were detected by SIMS analysis. The Ta concentration in diamond films was found to be around 300 ppm by RBS measurement and Ta inclusion originated from the Ta cathode kept above 2100°C. Optical and thermal properties of the diamond film deposited with a growth rate of 4 μm/h at 0.37 kW/cm2 (17 kW on φ76 mm substrate) and 5% CH4 were similar to that of the type IIa natural diamond.

Effect of the buoyancy force on diamond formation during synthesis under a high pressure

Abstract In diamond synthesis, the effect of the buoyancy force has been observed. As diamond particles grow, some of the diamonds formed on the bottom surface of each layer float up through the liquid solvent to the top surface owing to the difference between the specific gravities of the diamond and liquid solvent. This makes a difference between the numbers and sizes of the diamond particles formed on the top and the bottom surfaces of each solvent layer. Additionally it is seen that the temperature gradient in the specimen affects their formation only at the beginning of synthesis.

Effects of gravity and temperature gradient on the diamond formation during synthesis at 4.4 GPa and 1300°C

Abstract In diamond synthesis, it is observed that the number of diamond particles on each solvent surface are determined by the coupled effects of gravity and temperature gradient in the specimens. At the beginning of the diamond synthesis, the effect of temperature gradient is relatively stronger than that of gravity, which makes more diamond particles form on the bottom surface of the lowest solvent layer in the bottom cool end. As the synthesis proceeds, the diamond particles grow and float up through the liquid solvent from the bottom surface to the top surface. Buoyancy due to the difference in the specific gravities of diamond and liquid solvent causes a difference in the number of the diamond particles formed on the top and bottom surfaces of each solvent layer. More diamond particles are always observed on the top surfaces of the solvent layers than on the bottom surfaces in the respective solvent layers, except for the lowest solvent layer at the beginning of the synthesis.

Dependence of Gas Pressure on Cr Oxide Thin Film Growth Using a Plasma Focus Device

Chromium oxide thin films have been deposited on silicon substrates using a tabletop 9kJ mathertyped plasma focus (PF) device. Before deposition, pinch behavior with gas pressure was observed. Strength of pinches was increased with increasing working pressure. Deposition was performed at room temperature as a function of working pressure between 50 and 1000 mTorr. Composition and surface morphology of the films were analyzed by Auger Electron Spectroscopy and Scanning Electron Microscope, respectively. Growth rates of the films were decreased with pressure. The oxide films were polycrystalline containing some impurities, Cu, Fe, C and revealed finer grain structure at lower pressure.