Shuichi Ogawa

Effect of Carrier Gas (Ar and He) on the Crystallographic Quality of Networked Nanographite Grown on Si Substrates by Photoemission-Assisted Plasma-Enhanced Chemical Vapor Deposition

In this study, networked nanographite is formed on a Si substrate without any metal catalysts by photoemission-assisted plasma-enhanced chemical vapor deposition. We investigated the carrier gas dependence of the crystallographic quality of networked nanographite when Ar and He are used as the carrier gases. When Ar is the carrier gas, Raman spectroscopy and grazing incident X-ray diffraction show that the crystallographic quality deteriorates makedly with decreasing growth temperature, indicating that amorphous carbon is deposited at low temperatures (below ~500 °C). On the other hand, when He gas is used as a carrier gas, a high quality nanographite can be grown even at temperatures as low as room temperature. Thus, there is a significant difference in the temperature dependence of crystallographic quality for the two carrier gases.

Si(001) Surface Layer-by-Layer Oxidation Studied by Real-Time Photoelectron Spectroscopy using Synchrotron Radiation

High-resolution O 1s and Si 2p photoelectron spectroscopy using synchrotron radiation was employed to clarify a layer-by-layer oxidation reaction mechanism on a Si(001) surface from the viewpoint of point defect generation due to an oxidation-induced strain at a SiO2/Si interface. The Siβ and Siα components in Si 2p3/2 spectra, which are assigned to the first and second strained Si layers, respectively, below the transition layer composed of suboxides, Si1+, Si2+, and Si3+, significantly decrease during the step-by-step temperature increase-enhanced growth of the second oxide layer. Because of the corresponding band bending changes measured using the O 1s peak position, which are caused by defect-related band gap states, the observed decreases in Siβ and Siα components, indicating a decrease in interfacial strain, are induced not only by the structural relaxation of a SiO2 network due to a thermal annealing effect, but also due to the generation of point defects at the SiO2/Si interface. Continuous band bending changes with the growth of the third oxide layer also suggest that the point defects are generated during oxide growth, whereas the Siβ and Siα components are maintained almost constant. On the basis of the observed interfacial strain and point defect generation changes, the layer-by-layer growth kinetics of the first, second and third oxide layers is discussed using a unified Si oxidation reaction model mediated by point defect generation at the SiO2/Si interface [S. Ogawa and Y. Takakuwa: Jpn. J. Appl. Phys. 45 (2006) 7063].

Development of an Amorphous Selenium-Based Photodetector Driven by a Diamond Cold Cathode

Amorphous-selenium (a-Se) based photodetectors are promising candidates for imaging devices, due to their high spatial resolution and response speed, as well as extremely high sensitivity enhanced by an internal carrier multiplication. In addition, a-Se is reported to show sensitivity against wide variety of wavelengths, including visible, UV and X-ray, where a-Se based flat-panel X-ray detector was proposed. In order to develop an ultra high-sensitivity photodetector with a wide detectable wavelength range, a photodetector was fabricated using a-Se photoconductor and a nitrogen-doped diamond cold cathode. In the study, a prototype photodetector has been developed, and its response to visible and ultraviolet light are characterized.

Temperature Dependence of Oxidation-Induced Changes of Work Function on Si(001)2×1 Surface Studied by Real-Time Ultraviolet Photoelectron Spectroscopy

At the initial stage of oxidation on a Si(001)2×1 surface, real-time ultraviolet photoelectron spectroscopy revealed that the O2 dosage dependences of band bending and work function due to a surface dipole layer show a distinct change with increasing temperature from 300 to 600°C in a Langmuir-type adsorption region, while oxygen uptake curves are almost the same at all temperatures examined. In constant to a dual-oxide-species (DOS) model in which the surface migration of adsorbed oxygen is not considered for Langmuir-type adsorption, the observed changes in work function due to the surface dipole layer mean that adsorbed oxygen can migrate on the surface more frequently with increasing temperature, leading to a decrease in the number of adsorbed oxygen atoms bonded at dimer backbond centers and furthermore a significant structural change of the oxide layer.

Rate-Limiting Reactions of Growth and Decomposition Kinetics of Very Thin Oxides on Si(001) Surfaces Studied by Reflection High-Energy Electron Diffraction Combined with Auger Electron Spectroscopy

The growth and decomposition kinetics of very thin oxides on Si(001) surfaces were investigated by reflection high-energy electron diffraction combined with Auger electron spectroscopy (RHEED–AES) to monitor the reaction rates of oxide growth and decomposition in real time, and changes in surface structure and interface morphology simultaneously. The oxides prepared by stopping the second oxide layer growth at various coverages following the first oxide layer growth by Langmuir-type adsorption at 500 °C under an O2 pressure of 2.6×10-4 Pa were isothermally annealed by increasing temperature from 500 to 666 °C at the same time that O2 gas was quickly evacuated. It was observed that (1) oxide thickness continued to decrease significantly as measured on the basis of changes in O KLL Auger electron intensity ΔIO-KLL before voids appeared at a nucleation time tN as recognized by the appearance of half-order spots in RHEED patterns, (2) the SiO2/Si interface morphology was considerably roughened corresponding to ΔIO-KLL during void nucleation as observed by the evolution of the RHEED intensity of bulk diffraction spots ΔIbulk, (3) the ratio of ΔIbulk to ΔIO-KLL was almost constant, and (4) oxide decomposition rate during void nucleation, which was approximately given by ΔIO-KLL/tN, showed a good linear correlation with the second oxide layer growth rate α immediately before starting the decomposition reaction. The good correlation between ΔIO-KLL/tN and α clearly indicates that the rate-limiting reaction of the second oxide layer growth is closely related to that of the oxide decomposition during void nucleation. All the above-mentioned observations can be comprehensively interpreted using a reaction model proposed for the rate-limiting reactions of oxide growth and decomposition, in which the point defect generation (emitted Si atoms + vacancies) caused by the strain due to the volume expansion of oxidation plays a crucial role because of the high reactivity of the emitted Si atoms and vacancies with dangling bonds. Under an O2 atmosphere, both emitted Si atoms and vacancies are the preferential adsorption sites of O2 molecules in the oxide and at the SiO2/Si interface, respectively. The oxide can be decomposed by the emitted Si atoms to produce SiO molecules, which desorb from the surface, leading to oxide removal in vacuum with SiO2/Si interface morphology roughening, but may be oxidized within the oxide under an O2 atmosphere.

Graphene Growth and Carbon Diffusion Process during Vacuum Heating on Cu(111)/Al2O3 Substrates

In this study, the behavior of carbon atoms in the annealing/cooling process of graphene/Cu(111) substrates is investigated using photoelectron spectroscopy and secondary ion mass spectroscopy. After the growth of graphene on Cu(111) surfaces, Cu2O was formed at the graphene/Cu interface during transportation through air atmosphere. The Cu2O layer completely disappeared by vacuum annealing at 500 °C. Graphene was decomposed and carbon atoms diffused into the Cu substrate by further elevation of annealing temperature to 950 °C. When the sample was cooled down, the carbon atoms did not segregate on the surface and remained in the Cu substrate. This result indicates the carbon atoms easily diffuse into Cu substrates in vacuum annealing while the amount of diffused carbon atoms in the thermal chemical vapor deposition (CVD) process is smaller, suggesting that the barrier layer, which prevents the diffusion of C atoms, exists on Cu surfaces in the graphene CVD growth.

Formation of Diamond-Like Carbon Films by Photoemission-Assisted Plasma-Enhanced Chemical Vapor Deposition

Diamond-like carbon (DLC) films, which are an amorphous carbonaceous allotrope composed of sp2 carbon, sp3 carbon, and hydrogen, were prepared by photoemission-assisted plasma-enhanced chemical vapor deposition (PA-PECVD). The electrical behavior during film growth monotonically depended on the methane source gas concentration. Raman analysis of the films suggests that a DLC film grown at a high methane concentration condition contains a small number of graphitic domains, decreasing amorphicity of the film. In contrast, at a low concentration, the methane molecules were transformed into sufficiently fragmented radicals, forming a lot of graphitic nuclei and increasing the amorphicity. However, the variations of the relative dielectric constant, breakdown strength, and optical bandgap exhibited extreme values at an intermediate methane concentration. Thus, the two growth modes give different DLC films with varying combinations of electrical and optical characteristics.

Vacuum Annealing Formation of Graphene on Diamond C(111) Surfaces Studied by Real-Time Photoelectron Spectroscopy

To clarify the graphene formation process on a diamond C(111) surface, changes in the chemical bonding state caused by annealing in vacuum were investigated by photoelectron spectroscopy using synchrotron radiation. It was difficult to study the formation of sp2-bonded carbon atoms on a diamond C(111) surface using photoelectron spectroscopy because the peak of the sp2 component overlaps the peak of the surface sp3 component as a result of the 2×1 reconstruction. Therefore, we focused on the shift in the C 1s photoelectron spectra and energy loss spectra caused by band bending depending on the temperature. As a result, we found that graphitization on the diamond C(111) surface began at approximately 1120 K, which was lower than that for a SiC substrate. The obtained photoelectron spectra indicated that a buffer layer composed of sp2-bonded carbon atoms existed at the interface between graphene and the diamond C(111) surface.

Relation Between Oxidation Rate and Oxidation-Induced Strain at SiO2/Si(001) Interfaces during Thermal Oxidation

To experimentally verify the Si oxidation reaction model mediated by point defect (emitted Si atoms and their vacancies) generation due to oxidation-induced strain, real-time photoelectron spectroscopy using synchrotron radiation was employed to simultaneously evaluate the amount of oxidation-induced strained Si atoms at the SiO2/Si interface, oxidation state, and oxidation rate during oxidation on n-type Si(001) surfaces with O2 gas. It is found that both the oxidation rate and the amount of strained Si atoms at the completion of the first-oxide-layer growth decrease gradually with increasing temperature from 300 to 550 °C, where the oxide grows in the Langmuir-type adsorption manner. It is found that the interface strain and oxidation rate have a strong correlation. We discuss the reason for the oxide coverage and oxidation temperature dependences of interfacial strain from the viewpoint of the behavior of adsorbed oxygen during the first-oxide-layer growth.

Low-temperature synthesis of diamond films by photoemission-assisted plasma-enhanced chemical vapor deposition

Photoemission-assisted plasma-enhanced chemical vapor deposition (PA-PECVD), a process in which photoelectrons emitted from a substrate irradiated with ultraviolet light are utilized as a trigger for DC discharge, was investigated in this study; specifically, the DC discharge characteristics of PA-PECVD were examined for an Si substrate deposited in advance through hot-filament chemical vapor deposition with a nitrogen-doped diamond layer of thickness ∼1 μm. Using a commercially available Xe excimer lamp (hν = 7.2 eV) to illuminate the diamond surface with and without hydrogen termination, the photocurrents were found to be 3.17 × 1012 and 2.11 × 1011 electrons/cm2/s, respectively. The 15-fold increase in photocurrent was ascribed to negative electron affinity (NEA) caused by hydrogen termination on the diamond surfaces. The DC discharge characteristics revealed that a transition bias voltage from a Townsend-to-glow discharge was considerably decreased because of NEA (from 490 to 373 V for H2 gas and from...