Wakana Takeuchi

Electrical conduction control of carbon nanowalls

The electrical conduction behavior of carbon nanowalls (CNWs) has been evaluated by Hall measurement. CNWs, which comprise stacks of graphene sheets standing on the substrate, are fabricated by fluorocarbon/hydrogen plasma enhanced chemical vapor deposition. We have investigated the effect of N2 addition to C2F6∕H2 system on the electrical properties of CNWs. The CNWs grown with the C2F6∕H2 plasma exhibit p-type conduction. As a result of the nitrogen inclusion in the CNWs, the conduction type of the CNWs changes to n type. The carrier concentration is controllable by changing the flow rate of the additional N2 during the CNW growth process.

Initial growth process of carbon nanowalls synthesized by radical injection plasma-enhanced chemical vapor deposition

We synthesized carbon nanowalls (CNWs) using radical injection plasma-enhanced chemical vapor deposition. The initial growth process of CNWs was investigated with and without O2 gas addition to a C2F6 capacitively coupled plasma with H radical injection. In the case of the CNW synthesis without the addition of O2 gas, scanning electron microscopy (SEM), transmission electron microscopy, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy revealed that a 10-nm-thick interface layer composed of nanoislands was formed on a Si substrate approximately 1 min prior to CNW formation. In contrast, with O2 gas addition, SEM and XPS revealed that an interface layer was not formed and that CNWs were grown directly from nanoislands. Moreover, Raman spectroscopy suggested that the interface layer was composed of amorphous carbon and that O2 gas addition during CNW growth is effective for achieving a high graphitization of CNWs. Therefore, O2 gas addition has the effect of reducing the amorphicity and disorde...

Growth and applications of GeSn-related group-IV semiconductor materials

Abstract We review the technology of Ge1−xSnx-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge1−xSnx-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge1−xSnx-related material thin films and the studies of the electronic properties of thin films, metals/Ge1−xSnx, and insulators/Ge1−xSnx interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge1−xSnx-related materials, as well as the reported performances of electronic devices using Ge1−xSnx related materials.

Electron field emission enhancement of carbon nanowalls by plasma surface nitridation

Carbon nanowalls (CNWs) are two-dimensional carbon nanostructures consisting of stacked graphene sheets standing vertically on the substrate. The sharp edges of CNWs provide us with opportunities for applications as electron field emitter arrays. The effects of nitrogen plasma (NP) treatment on the surface of CNWs have been investigated in order to improve the electron field emission properties. The electron emission current from the edges of CNWs was drastically increased by the NP treatment. Morphological and chemical changes in the CNWs after the NP treatment were characterized using scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy.

Development of measurement technique for carbon atoms employing vacuum ultraviolet absorption spectroscopy with a microdischarge hollow-cathode lamp and its application to diagnostics of nanographene sheet material formation plasmas

This study describes the development of a compact measurement technique for absolute carbon (C) atom density in processing plasmas, using vacuum ultraviolet absorption spectroscopy (VUVAS) employing a high-pressure CO2 microdischarge hollow-cathode lamp (C-MHCL) as the light source. The characteristics of the C-MHCL as a resonance line source of C atoms at 165.7 nm for VUVAS measurements of the absolute C atom density are reported. The emission line profile of the C-MHCL under typical operating conditions was estimated to be the Voigt profile with a ΔνL/ΔνD value of 2.5, where ΔνL is the Lorentz width and ΔνD is the Doppler width. In order to investigate the behavior of C and H atoms in the processing plasma used for the fabrication of two-dimensional nanographene sheet material, measurements of the atom densities were carried out using the VUVAS technique. The H atom density increased with increasing pressure, while the C atom density was almost constant at 5×1012 cm−3. The density ratio of C to H atoms ...

Synthesis of Platinum Nanoparticles on Two-Dimensional Carbon Nanostructures with an Ultrahigh Aspect Ratio Employing Supercritical Fluid Chemical Vapor Deposition Process

Carbon nanowalls (CNWs), two-dimensional carbon nanostructures comprising plane graphene layers standing vertically on a substrate, have attracted considerable attention for several applications, because of their ultrahigh aspect ratio and large surface area. We developed a formation method of platinum (Pt) nanoparticles using the supercritical fluid-chemical vapor deposition (SCF-CVD), and demonstrate the synthesis of dispersed Pt nanoparticles of 2 nm diameter on the entire surface of CNWs, by using SCF-CVD employing metal–organic compound. The SCF-CVD process has proved quite effective for the synthesis of Pt nanoparticles on the intricate surface of carbon nanostructures with narrow interspaces.

Interface properties of Al2O3/Ge structures with thin Ge oxide interfacial layer formed by pulsed metal organic chemical vapor deposition

We have examined the formation of a thin GeO2 layer on a Ge substrate by pulsed metal organic chemical vapor deposition (MOCVD) using tetraethoxygermanium (TEOG) and H2O to precisely control GeO2 layer thickness. Also, we have investigated the feasibility of the use of a thin GeO2 layer formed by pulsed MOCVD at the Al2O3/Ge interface. Pulsed MOCVD enables thin GeO2 layer formation with thickness control by the self-limited adsorption of TEOG. For the growth of a thick GeO2 layer, it is a key to allow TEOG and H2O molecules to sufficiently react. Furthermore, we found that the MOCVD-GeO2 layer has a high etching tolerance to Al2O3 deposition and can reduce the interface state density of the Al2O3/Ge structure. Therefore, GeO2 formation by pulsed MOCVD using TEOG and H2O is a candidate method for realizing high-quality high-k/Ge gate stacks.

Understanding of interface structures and reaction mechanisms induced by Ge or GeO diffusion in Al2O3/Ge structure

The reaction mechanisms at Al2O3/Ge interfaces with thermal oxidation through the Al2O3 layer have been investigated. X-ray photoelectron spectroscopy reveals that an Al6Ge2O13 layer is formed near the interface, and a GeO2 layer is formed on the Al2O3 surface, suggesting Ge or GeO diffusion from the Ge surface. It is also clarified that the Al6Ge2O13 layer is formed by the different mechanism with a small activation energy of 0.2 eV, compared with the GeO2 formation limited by oxygen diffusion. Formation of Al-O-Ge bonds due to the AlGeO formation could lead appropriate interface structures with high interface qualities.

Robustness of Sn precipitation during thermal oxidation of Ge1−xSnx on Ge(001)

The thermal robustness of Sn segregation and precipitation in epitaxial Ge1−xSnx layers on Ge(001) substrates with a Sn content greater than the equilibrium solubility limit has been investigated for applications of Ge1−xSnx in high-performance metal–oxide–semiconductor field-effect transistors (MOSFETs). Sn segregation and precipitation occur on the Ge1−xSnx surface after epitaxial growth of the Ge1−xSnx layer at 150 °C. After the thermal oxidation of the Ge1−xSnx layer below 500 °C, there are no significant decreases in the average Sn content in the Ge1−xSnx layer and no additional Sn segregation on the Ge1−xSnx surface. However, Sn precipitation occurs at the Ge1−xSnx surface during the thermal oxidation of the Ge1−xSnx layer with an average Sn content as high as 8.7% at 600 °C, causing a decrease in the Sn content in the Ge1−xSnx layer. The Sn content in the Ge1−xSnx oxide is 1.5 times greater than that observed near the Ge1−xSnx surface for the sample with a Sn content of 8.7% after the thermal oxidation at 400 to 500 °C. The capacitance–voltage characteristics of the Al/Al2O3/Ge1−xSnx/Ge MOS capacitors treated with thermal oxidation at 400 °C indicate that the slow state density increases with the Sn content. Meanwhile, the small interface state density could be achieved via thermal oxidation of the Ge1−xSnx layer, even with a high Sn content.

Interfacial Reaction Mechanisms in Al2O3/Ge Structure by Oxygen Radical Process

We have investigated the impacts of the oxygen radical process on the interfacial structures and electrical properties of Al2O3/Ge structures to clarify the interfacial reaction mechanisms. At a low process temperature, the oxygen radical process can introduce oxygen atoms to the Al2O3/Ge interface without a thermally activated process in spite of the high barrier property of the oxygen diffusion for the Al2O3 layers. In addition, the oxygen radical process at a low process temperature can relatively suppress the diffusion of Ge atoms from the Ge substrate or GeO molecules from the Al2O3/Ge interface to the surface of the Al2O3 layer. However, at a high process temperature, Ge atoms and/or GeO molecules actively diffuse into the Al2O3 layer during the oxygen radical process as well as the O2 thermal annealing, and the diffusion changes the depth distribution of Ge oxides in the Al2O3/Ge structure. From the analysis of the electrical properties of MOS capacitors, the interface state density (Dit) of the Al2O3/Ge structure decreases not with increasing thickness of the Ge oxide interlayer but with the amount of Ge oxide near the Al2O3/Ge interface. The increase in the amount of the Ge oxide distributed in the Al2O3 layer induces the increase in the capacitance equivalent thickness (CET). The diffusion of Ge into the Al2O3 layer with a high process temperature causes the unexpected increase in CET. Therefore, the oxygen radical process at low temperature effectively decreases Dit of Al/Al2O3/Ge MOS capacitors without increasing CET.