Takeshi Inaoka

Carbon nanofibers synthesized by decomposition of alcohol at atmospheric pressure

In the present study, we fabricated the carbon nanofibers (CNFs) by decomposition of methyl alcohol at atmospheric pressure. The CNFs were grown on Ni/Si substrates using simplified hot-filament chemical vapor deposition equipment. The deposits mainly consist of the semicrystalline CNFs, in which a few of carbon nanotubes are included. On the 30-nm-thick Ni/Si substrates, the mean length of the CNFs is 2–3 μm, and their average diameter is less than 100 nm. The as-deposited CNFs were evaluated by both scanning and transmission electron microscopes. The field-electron-emission properties of CNFs were characterized as well.

Synergy Effect of Particle Radiation and Ultraviolet Radiation from Capacitively Coupled Radio Frequency Argon Plasmas on n-GaN Etching Damage

The change in the morphology of n-GaN surfaces etched by capacitively coupled RF Ar plasmas has been studied from the viewpoint of a synergy effect of particle radiation and UV radiation from the RF plasmas. The particle radiation (in particular, the energy of Ar+ impinging on n-GaN) is intensified with decreasing gas pressure from 200 to 10 mTorr, whereas the intensity of the UV radiation (whose peak wavelength corresponds to the GaN band-gap energy) is significantly weakened. The reverse result occurs when the gas pressure increases. Each type of radiation brings about a smooth surface morphology similar to that of the as-grown surface. However, at 50 or 100 mTorr, at which both types of radiation are expected to coexist, the surface morphology shows various types of pits (defects or dislocations), which seem to be induced by the synergy effect.

Structural characteristics and field electron emission properties of nano-diamond/carbon films

Nano-diamond/carbon (NDC) films were fabricated by the microwave plasma-enhanced chemical-vapor-deposition method. It is found that such NDC films exhibit much better electron emission properties. The turn-on field is only ≈2.5 V/μm, which is smaller by a factor of eight than that of the conventional microcrystalline diamond films. Raman spectroscopy and curve fitting technique were employed to investigate the bonding structure of the NDC films. The films are confirmed to consist of sp2–sp3 bonded amorphous matrix in which the high-density diamond crystallites are embedded. Transmission electron microscopy observation indicates that the average size of the diamond crystallites is less than 5 nm. The good field emission properties of NDC films are discussed relating to film microstructure and electrical conductivity.

Effects of Capacitively Coupled Radio Frequency Krypton and Argon Plasmas on Gallium Nitride Etching Damage

GaN etching damage characteristics by capacitively coupled radio frequency Kr and Ar plasmas have been found to differ significantly, on the basis of experimental and simulation results. The morphology of a GaN surface etched by Kr plasma is as smooth as that of the as-grown surface, and is independent of gas pressure and etching time. The agreement between the experimental and simulated etching depths for the Kr plasma, which are lower than those for the Ar plasma, indicates a significant contribution to the GaN damage of the physical etching effect. In contrast, Ar plasma etching produces a rough surface, which is dependent on gas pressure and etching time, and appears to be due to a chemical effect. The difference in the GaN surface morphologies etched by the Kr and Ar plasmas may be attributed to the different depths etched for these two plasmas. Moreover, the simulation shows that, for the Kr plasma, Ga is preferentially etched from GaN, whereas the preferential etching of N occurs for the Ar plasma. The difference in preferential etching between the Kr and Ar plasmas may be related to the difference between the GaN surface morphologies etched by these two plasmas.

Damage Analysis of Plasma-Etched n-GaN Crystal Surface by Nitrogen K Near-Edge X-ray Absorption Fine Structure Spectroscopy

The surface of an n-GaN crystal etched with an Ar, Kr, or Xe plasma was analyzed by nitrogen K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectroscopy was carried out with the total electron yield (TEY) method in a sample current mode and the total fluorescence yield (TFY) method by measuring the amount of fluorescence using a photodiode. The shapes of the spectra of Ar plasma-etched samples obtained by the TEY method became smooth (blunt) with increasing Ar pressure from 10 to 200 mTorr. However, those obtained by the TFY method did not change with pressure. These results indicate that the etching damage was restricted in the shallow region of less than a few nm from the surface. A change in the NEXAFS spectral shape of Kr or Xe plasma-etched samples was not observed even when measured by the TEY method. This result indicates that the surface damage in Kr or Xe plasma-etched samples was less pronounced than that in Ar plasma-etched samples.

Damage Characteristics of TiO2 Thin Film Surfaces Etched by Capacitively Coupled Radio Frequency Helium Plasmas

Damage characteristics of TiO2 thin film surfaces etched by capacitively coupled RF He plasmas are found to be dependent on gas pressure and etch time. At a low gas pressure (10 mTorr), the morphology of TiO2 surface etched for 5 min is smooth like the as-grown surface. When the etch time lengthens to 60 min, the surface morphology is smoother. However, the atomic O concentration at the surface is lower than that of the as-grown surface. On the other hand, at a high gas pressure (50–100 mTorr), the He plasma etch causes a rough surface morphology (surface defects) when the etch time lengthens to 60 min.

Damage Analysis of n-GaN Crystal Etched with He and N2 Plasmas

To understand the details of etching-induced damage on a GaN surface, n-GaN crystals were plasma-etched with He and N2 gases. The etched surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) methods. The composition of the surface etched with He plasma changed significantly to being Ga-rich with the N/Ga ratio nearly equaling 0.4–0.5. The ratio of the surface etched with N2 plasma was about 0.6. The shape of the near-edge X-ray absorption fine structure (NEXAFS) of the N-K edge deformed with increasing gas pressure and processing time. The deformation can be explained by the increase in the band widths of a number of peaks in the NEXAFS spectra owing to the increase in the degree of structural disorder in the crystal. The increase in band width for the surface etched with N2 plasma was larger than that for the surface etched with He plasma. The above results can be explained with the model of the elastic energy transfer ratio of He+ and N2+ ions incident on the solid surface.

Characteristics of TiO2 Thin Film Surfaces Treated by Helium and Air Dielectric Barrier Discharge Plasmas

The characteristics of TiO2 thin film surfaces treated with He and air dielectric barrier discharge (DBD) plasmas at different gas pressures are investigated. There is a difference between the two DBD plasma characteristics: for He-DBD, which is an atmospheric pressure glow discharge (APGD), the breakdown voltage and discharge current hardly change with increasing gas pressure, whereas for air-DBD, which is basically a filamentary discharge, they increase with increasing gas pressure. There is also a difference between the characteristics of TiO2 surfaces treated with the two DBDs. The surface roughness for He-DBD is lower than the roughness of the as-grown surface, whereas that for air-DBD is higher. The surface hydrophilicity for He-DBD is more enhanced than the hydrophilicity of the as-grown surface regardless of UV irradiation. The hydrophilicity for air-DBD is dependent on UV irradiation. It is more enhanced with UV irradiation; it is not improved adequately without UV irradiation.

Effect of Dielectric Barrier Discharge Air Plasma Treatment on TiO2 Thin Film Surfaces

Surface treatment effect on TiO2 thin films with the anatase phase by dielectric barrier discharge (DBD) air plasmas has been investigated for a variety of gas pressures and treatment times. At a low gas pressure (100 hPa) at which a glow-like discharge plasma occurs, hydrophilicities of TiO2 thin films treated at 5 and 30 min are enhanced compared with that of the as-grown thin film. For the 5 min treatment, this trend is more pronounced probably due to oxygen absorbed on the surface from the air plasma. For the 30 min treatment, the enhanced hydrophilicity is probably due to oxygen vacancy created on the surface by a high fluence of the plasma. When the gas pressure increases to 400 hPa at which a streamer discharge plasma occurs, the hydrophilicity is more weakened than that of the as-grown thin film: the plasma-induced damage occurs regardless of the treatment time. This result would probably result from the higher discharge current and UV light intensity caused by the higher breakdown voltage based on Paschens law.

Characterization of N-doped diamond films by transmission electron microscopy

The N-doped diamond films have been investigated by transmission electron microscopy (TEM). TEM images clearly reveal that both the density and distribution of the planar defects are greatly affected by the N/C ratio (the ratio of N atoms to C atoms in the source gas). As the N/C ratio increases, much more planar defects are formed. Moreover, the phenomenon that a heavily nitrogen doping leads to the generation of the amorphous domains in diamond lattice is directly confirmed by TEM images for the first time, and the average size of these amorphous domains is on the order of nanometers. Based on the TEM observation results, the nitrogen doping effects on the crystallinity variation of chemical-vapor-deposited diamonds are discussed.