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This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO

Gold nanoparticles have been commonly made by a wet process from chloroauric acid or dry process using vacuum evaporation to solid surfaces. Here, we have shown a method to make gold particle from ...

Graphene gas-barrier performance holds great interest from both scientific and technological perspectives. Using in situ synchrotron X-ray photoelectron spectroscopy, we demonstrate that chemical vapor-deposited monolayer graphene loses its gas-barrier performance almost completely when oxygen molecules are imparted with sub-electronvolt kinetic energy but retains its gas-barrier performance when the molecules are not energized. The permeation process is nondestructive. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.

In this study, we tried to decrease the hydrogen content in diamond-like carbon (DLC) grown by photoemission-assisted plasma enhanced chemical vapor deposition (PA-PECVD) using Ar/CH4 mixed with CO2. When the CO2 flux was changed from 0 to 10 sccm with the Ar and CH4 fluxes maintained at 50 and 10 sccm, respectively, the growth rate decreased from 11 to 3 μm/h. Secondary mass spectroscopy measurements confirmed that the amount of O mixed into the DLC was increased through incorporation of CO2 into feed gas flow. The O concentration in the DLC was quantitatively evaluated by X-ray photoelectron spectroscopy (XPS) to be 0.6 atomic % at a CO2 flow ratio of 14%. Raman spectroscopy and XPS revealed that the amount of H trapped in the DLC decreased as the CO2 flow ratio was increased and the sp3/sp2 ratio remained almost unchanged. These results were interpreted by a model involving O radicals acting on the DLC surface associated with CO/CO2 and H2O, resulting in a decrease of the growth rate and H content. A portion of the O radicals also became incorporated into the DLC as C O C bonds.

The surface growth mechanisms of diamondlike carbon (DLC) films has been clarified. DLC films were synthesized in atmospheres with a fixed methane-to-argon ratio at different temperatures up to 700 °C by the photoemission-assisted glow discharge of photoemission-assisted plasma-enhanced chemical vapor deposition. The electrical resistivity of the films decreased logarithmically as the synthesis temperature was increased. Conversely, the dielectric constant of the films increased and became divergent at high temperature. However, the very high electrical resistivity of the film synthesized at 150 °C was retained even after post-annealing treatments at temperatures up to 500 °C, and divergence of the dielectric constant was not observed. Such films exhibited excellent thermal stability and retained large amounts of hydrogen, even after post-annealing treatments. These results suggest that numerous hydrogen atoms were incorporated into the DLC films during synthesis at low temperatures. Hydrogen atoms terminate carbon dangling bonds in the films to restrict π-conjugated growth. During synthesis at high temperature, hydrogen was desorbed from the interior of the growing films and π-conjugated conductive films were formed. Moreover, hydrogen radicals were chemisorbed by carbon atoms at the growing DLC surface, leading to removal of carbon atoms from the surface as methane gas. The methane molecules decomposed into hydrocarbons and hydrogen radicals through the attack of electrons above the surface. Hydrogen radicals contributed to the etching reaction cycle of the film; the hydrocarbon radicals were polymerized by reacting with other radicals and the methane source. The polymer radicals remained above the film, preventing the supply of the methane source and disrupting the action of argon ions. At high temperatures, the resultant DLC films were rough and thin.

The chemical structure of diamondlike carbon (DLC) films, synthesized by photoemission-assisted glow discharge, has been analyzed by Raman spectroscopy. Raman analysis in conjunction with the sp 2 cluster model clarified the film structure. The sp 2 clusters in DLC films synthesized at low temperature preferred various aliphatic structures. Sufficient argon-ion assist allowed for formation of less strained DLC films containing large amounts of hydrogen. As the synthesis temperature was increased, thermal desorption of hydrogen left carbon dangling bonds with active unpaired electrons in the films, and the reactions that followed created strained films containing aromatic sp 2 clusters. In parallel, the desorption of methane molecules from the growing surface by chemisorption of hydrogen radicals prevented the action of argon ions, promoting internal strain of the films. However, in synthesis at very high temperature, where sp 2 clusters are sufficiently dominant, the strain was dissolved gradually. In contrast, the DLC films synthesized at low temperature were more stable than other films synthesized at the same temperature because of stable hydrogen–carbon bonds in the films.

The relationship between the chemical structure and electrical characteristics of diamondlike carbon (DLC) films has been clarified. The DLC films were formed in atmospheres with different ratios of methane to argon under the photoemission-assisted Townsend discharge. The dependence of the dielectric constant of the films on methane concentration in the synthesis was arch-like with a maximum. The breakdown strength was constant irrespective of the synthesis atmosphere and was approximately the same for both electrical polarizations. Raman spectra of the films were deconvoluted into five active bands. Raman analysis in conjunction with the sp2 cluster model elucidated the DLC structure. The sp2 cluster model comprises sp2 clusters floating in a dielectric matrix sea. The sp2 clusters were rather aliphatic for films formed in low methane concentration. The clusters grew to become aromatic with increasing methane concentration. Defects or dangling bonds increased similarly, but they were terminated with hydrogen for films formed in high methane concentration. The hydrogen-terminated bonds occupied large amounts of space in the DLC films, causing internal strain. The dielectric constant of the whole DLC film was determined by the size of sp2 clusters, dielectric constant of the matrix sea material, and space volume induced by the hydrogen–carbon bonds. The breakdown strength was determined by the balance between the size of sp2 clusters and density of dangling bonds in the matrix sea. However, because the dependences of these factors on methane concentration were opposing, the breakdown strength was approximately constant irrespective of methane concentration.

Potassium (K) doping of multilayer graphene was demonstrated by means of a wet chemical process using potassium hydroxide (KOH) aqueous solution (KOH treatment). The presence of K atoms along the stacking direction was confirmed from depth profiles of 41K+ ions obtained by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The intensity images of 41K+ ions obtained by TOF-SIMS suggested that the K atoms existed throughout the whole area. For the KOH-treated multilayer graphene, no peak due to K intercalation between graphene layers was obtained by x-ray diffraction (XRD); in Raman spectra, splitting of the G-band peak and disappearance of the 2D-band peak were not observed. A graphite intercalation compound structure was not determined by either XRD or Raman results. However, the up-shift in the G-band peak position in the Raman spectra suggested that K atoms were doped in the graphene. X-ray photoelectron spectroscopy carbon 1s spectra implied that KOH treatment resulted in K-termination at the edges and/or domains of graphene. In addition, a C1s shoulder peak appeared at 1 eV higher binding energy compared to the C1s peak of pristine graphene. Temperature-dependent conductivity measurement results indicated that the conductivity of multilayer graphene was increased by KOH treatment. In addition, the conductivity increased with increasing temperature, which could be explained by band overlap.

The dependence of features of diamondlike carbon (DLC) films on their synthesis methods were clarified by Raman spectroscopy. The DLC films were synthesized by three different methods: photoemission-assisted glow discharge (PAGD), ionized deposition (ID), and unbalanced magnetron sputtering (UBMS). The main difference among these methods was the influence of ions on film formation. Raman analysis in conjunction with the sp 2 cluster model led to the elucidation of each film structure. The DLC film synthesized in PAGD with efficient ion assist consisted of dispersed, small, and unstrained sp 2 clusters surrounded by a dielectric matrix of sp 3 carbon. The clusters were aliphatic, which reflects the methane gas source; however, hydrogen atoms were etched away to some extent because of bond rearrangement caused by the ion assist. The sp 2 clusters synthesized by ID were well-grown aromatic clusters, which reflect the benzene gas source; however, the clusters contained certain amounts of defects, and sufficient electrical isolation occurred among the clusters. The well-grown sp 2 clusters caused weak internal strain of the film. In UBMS without the ion assist, strongly strained sp 2 clusters that included numerous defects from damaged graphite (polyaromatic) radicals by sputtering were created. The density of the clusters was high, and the film was conductive. The defects were removed by introducing methane as a second hydrocarbon source during the synthesis. However, the strain was enhanced further in conjunction with the defect removal.

We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 oC. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 oC leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart., Comment: 9 pages, 4 figures in physica status solidi (a) 2016

In order to improve the efficiency of the surface flattening process using photoemission-assisted plasma ion source, current-bias voltage characteristic and Ar+ ions/Ar atoms ratio in the plasma was investigated. The glow discharge starting voltage decreases by UV irradiation and a great number of Ar+ ions were irradiated to the substrate. On the other hand in PA Townsend discharge, the number of Ar+ ions reached at the substrate is smaller than that in glow discharge due to space charges near the cathode substrate.

Controlled oxygen doping into a diamond-like carbon (DLC) film was succeeded under a current regulation by the photoemission-assisted Townsend (PAT) discharge plasma with the aid of UV-excited photoelectrons. An oxygen-doped DLC layer was formed on a non-doped DLC underlayer in the methane/argon plasma containing a certain amount of carbon dioxide as an oxygen dopant source. Oxygen-related radicals generated in the doped layer synthesis did not etch and penetrate the underlayer. However, such controlled doping failed in the photoemission-assisted glow (PAG) discharge where high-energy oxygen-related radicals etched the underlayer. A non-doped DLC layer finally capped the doped layer to build an oxygen box-doped DLC film. The dielectric constant of the film was unchanged irrespective of the amount of oxygen incorporated in the doped layer. However, very high breakdown strength was obtained, especially, over 10 MV/cm in negative electrical polarization when the doped layer contained large amount of oxygen. This result suggested that the interfaces between the doped layer and the non-doped side layers exhibited an n-type barrier. The voltage characteristics in the layer syntheses under the same current regulation suggested that the UV photons passed through the doped layer and excited photoelectrons in the non-doped underlayer. The photoelectrons went back through the doped layer and emitted from the surface to cause reactions.

Energies of emitted electrons from n-type diamond NEA surface and graphene/n-type diamond junction were measured by combined field emission spectroscopy/ultraviolet photoelectrons spectroscopy (FES/UPS) system. For n-type diamond NEA surface, the energies of emitted electrons indicated that the electrons emitted from conduction band of the diamond. For graphene/diamond junction, electron emission from both valance bands of diamond and graphene are confirmed.

In the present study, high-sensitivity photodetection has been demonstrated using an amorphous selenium (a-Se) based photodetector driven by nitrogen-doped diamond cold cathode. The emission current – applied voltage characteristics are compared between different lighting conditions, and their sensitivities are evaluated in terms of nominal quantum efficiency. The estimated nominal quantum efficiency was 10–40 for visible and up to 1000 for ultraviolet, proving a successful photodetection utilizing carrier multiplication in a-Se.

Oxidation at the oxide/Si(111) interface at room temperature (RT) and 550 °C has been investigated by real-time X-ray photoelectron spectroscopy. Self-accelerating interface oxidation is observed in oxidation at RT, but not in oxidation at 550 °C. During self-acceleration at RT, the component corresponding to strained Si atoms in the second layer below the oxide/Si(111) interface, Siβ, increases significantly. It is suggested that the interface oxidation rate is strongly correlated with the generation of strain at the oxide/Si(111) interface. Furthermore, a self-accelerating interface oxidation model of Si(111)7 × 7 surfaces that includes the point defect (emitted Si atoms + vacancies) generation is proposed. Copyright © 2014 John Wiley & Sons, Ltd.

The Auger intensity of Si-LVV from the Si(001)2 × 1 surface has been measured at room temperature while changing the glancing angle of the incident electron beam of reflection high-energy electron diffraction (RHEED). Such a beam-rocking Auger electron spectroscopy (BRAES) profile shows intensity anomalies under surface wave resonance conditions. The surface structure of Si(001)2 × 1 has been analyzed by rocking curves of diffraction spots within the 0th Laue zone. Since RHEED intensities limited to the 0th Laue zone can be determined by the projected potential of Si atomic rows along the incident azimuth, the phase of asymmetric dimers with flip-flop motion can be neglected. According to the Si(001)2 × 1 surface structure model which was confirmed by the rocking curve analysis, it was found that the calculated wave-field intensity profile for Si atoms reveals similar intensity anomalies to those of the experimental BRAES profile. The background profile is also discussed in terms of the beam irradiation area and its transmissivity. Copyright © 2014 John Wiley & Sons, Ltd.

The surface structure of InP(111)A has been investigated using reflection high-energy electron diffraction (RHEED) rocking curves and first-principles calculations. We used Ar ion sputtering and low-temperature annealing at about 200 °C as a cleaning method, which yields a (1 × 1) surface structure. RHEED rocking curves reveal that the interatomic distance of the top surface bilayer is compressive. First-principles calculations were performed based on a (2 × 2) unit mesh to obtain a stable structure. The calculated results suggest that a surface compressive bilayer is necessary when In atoms are removed from the surface. We conclude that the InP(111)A-(1 × 1) surface structure comprises about 20%–40% In defects, which exist randomly on the surface, and the compressed surface bilayer has an interval of 0.1 A.

Carbon-based materials, such as carbon nanotubes and graphene nanoribbons, have been studied as interconnect materials because of their lower resistivity, higher thermal conductivity, and intrinsically higher current carrying capacity compared to Cu. We have succeeded in growing networked-nanographite (NNG) films on dielectric layers without any metal catalysts, and fabricated NNG wires as the first step toward multi-layer graphene interconnects. In this study, we investigated the electrical properties and reliability, such as current tolerance and thermal stability, of these NNG wires. In addition, we investigated the mechanism of resistivity decreases in NNG. Our results show that the NNG wires can sustain a higher current density than Cu wires. However, their resistivity gradually decreases due to thermal stress. Raman and X-ray photoelectron spectroscopy revealed that the decrease in resistivity can be attributed to an increase in sp2 bonding corresponding to the formation of a graphene sheet. It is important to clarify the mechanism of the decrease in resistivity, not only to improve thermal stability but also to obtain lower resistivity in carbon interconnects.

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.

High-sensitivity ultraviolet (UV) photodetection has been attempted by utilizing the carrier multiplication effect in amorphous selenium. The prototype photodetector presented in this Letter showed an extremely high sensitivity to UV light, so that up to 1000 carriers are generated per incident photon. This result should lead to the development of an ultrahigh-sensitivity photodetector that can be used for spectrometric applications covering the visible to UV region. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Diamond-like carbon (DLC) films grown by photoemission-assisted plasma-enhanced chemical vapor deposition (PA-PECVD) have attracted attention as a gate insulator for graphene-channel field effect transistors (GFETs). In this study, the possibility of using PA-PECVD to grow insulating DLC films for GFETs is explored by focusing on the growth rate and uniformity of DLC films on Si substrates. Initially, the DLC films were formed at a constant rate but the growth rate decreased rapidly when the thickness reached approximately 400 nm. This is because of a decrease in photoelectron emissions from the Si substrates as they are covered by DLC films which absorb UV photons. However, the DLC films formed uniformly at thicknesses less than 16%. This result indicates that PA-PECVD is a promising method for growing DLC films as the gate dielectric layer of GFETs.

A carbonaceous field effect transistor, designated as a ‘DLC-GFET’, consisting of a graphene channel and a diamondlike carbon (DLC) top-gate dielectric film was fabricated. A DLC film was formed ‘directly’ onto the graphene channel using our original photoemission-assisted plasma-enhanced chemical vapor deposition (PA-CVD) without forming complex passivation interlayers. The DLC-GFET exhibits clear ambipolar characteristics with a slightly positive shift of the neutral points (Dirac voltages). The Raman analysis suggests that the electrical characteristics are due to unintentional hole doping from the DLC film into the graphene channel. The maximum transconductances per unit device width in the n-channel and the p-channel modes (gmn,max and gmp,max) are 14.6 mS/mm and 8.8 mS/mm, respectively, with a gate thickness of 48 nm and a gate length of 5 μm. Formation of the DLC top-gate does not cause obvious damage to the graphene channel. Hence, the PA-CVD method is suitable to form gate dielectrics onto graphene.