Kazuo Tsugawa

Low-temperature synthesis of large-area graphene-based transparent conductive films using surface wave plasma chemical vapor deposition

We present a low-temperature (300–400 °C), large-area (23 cm×20 cm) and efficient synthesis method for graphene-based transparent conductive films using surface wave plasma chemical vapor deposition. The films consist of few-layer graphene sheets. Their transparency and conductivity characteristics make them suitable for practical electrical and optoelectronic applications, which have been demonstrated by the proper operation of a touch panel fabricated using the films. The results confirm that our method could be suitable for the industrial mass production of macroscopic-scale graphene-based films.

Characterization of hydrogen-terminated CVD diamond surfaces and their contact properties

Abstract Metal-diamond contact properties have been investigated on diamond (001) 2 × 1 surfaces. The 2 × 1 reconstruction has been examined by atomic-scale scanning tunnelling microscopy. From ab initio molecular orbital calculations, the stablest structure of the 2 × 1 unit is a monohydride carbon dimer. On the surfaces the highest quality Schottky diodes have been fabricated using metals with lower electronegativities. The Schottky barrier heights depend on the metal electronegativity. Moreover ohmic contacts have been obtained in metals with higher electronegativities. These results indicate that the surface states are effectively reduced by the 2 × 1 surface reconstruction.

High-Performance Diamond Metal-Semiconductor Field-Effect Transistor with 1 µm Gate Length

High-performance metal-semiconductor field-effect transistors (MESFETs) using the p-type surface conductive layer on homoepitaxial diamond are demonstrated. The maximum transconductance is 110 mS/mm, which is the highest value ever reported in diamond FETs. This value exceeds the normal transconductance of a Si–metal-oxide semiconductor field-effect transistors (MOSFET) with equivalent gate length. The transconductance of the present diamond FETs is proportional to the reciprocal of gate length. Accordingly, the characteristics can be improved by the refinement of gate length. By using an appropriate FET fabrication process, it is expected that the transconductance of a diamond MESFET exceeds 500 mS/mm at gate lengths less than 0.2 µm.

Large-area surface wave plasmas using microwave multi-slot antennas for nanocrystalline diamond film deposition

An advanced slot antenna was developed and the enlargement of surface wave plasma was experimentally investigated using the array configuration of slot antennas for large-area nanocrystalline diamond (NCD) film depositions. An original-geometry slot antenna with an annular shape, designed in this work, permitted broad microwave radiations and vast extent of plasma propagations. Uniform plasmas were produced over a deposition area of 600 × 360 mm2 with densities of ~1017 m−3 using hydrogen gas by an array configuration of five slot antennas and four rows of the array. A uniform (thickness variation: 10%) and smooth (Ra: 11 nm) NCD film deposition on a 300 × 300 mm2 borosilicate glass plate was achieved using the plasma and a mixed gas of H2/CH4/CO2.

Field emission and structure of aligned carbon nanofibers deposited by ECR-CVD plasma method

Abstract Aligned carbon nanofibers and hollow carbon nanofibers were grown by MW ECR-CVD method using methane and argon mixture gas at a temperature of 550°C. The carbon nanofibers and the hollow carbon nanofibers were deposited perpendicularly on Si substrates and on Si substrates coated with Ni catalyst, respectively. From TEM analysis the diameter and length of the nanofibers are approximately 60 nm and 15 μm, respectively. Raman spectra of these aligned carbon nanofibers showed new bands of 1340 and 1612 cm−1 of the first-order Raman scattering and 2660, 2940 and 3220 cm−1 of the second-order Raman scattering. The second-order Raman scattering bands were assigned to two overtone and one combination bands on the basis of a similar assignment of micro-crystal graphite by Nemanich and Solin. By the measurement of XPS C1s band energies of 284.6 eV for the carbon nanofiber and 284.7 eV for the hollow carbon nanofiber indicate mainly sp2 carbon component in the inclusion of a small amount (

Cu/CaF2/Diamond Metal-Insulator-Semiconductor Field-Effect Transistor Utilizing Self-Aligned Gate Fabrication Process

High-performance metal-insulator-semiconductor field-effect transistors (MISFET) on hydrogen-terminated homoepitaxial diamond films are demonstrated. The gate insulator is evaporated CaF2 which does not cause interface states. This is the first study of a CaF2/diamond MISFET fabricated by a self-aligned gate fabrication process by which the gate length and the source gate spacing are effectively reduced. The maximum transconductance is 86 mS/mm, which is the highest value in diamond MISFETs at present.

Electrical characterization of graphene films synthesized by low-temperature microwave plasma chemical vapor deposition

In this Letter, we discuss the results of Hall effect measurements to examine the electrical properties of the graphene films synthesized by low-temperature microwave plasma chemical vapor deposition. Van der Pauw devices with sizes of 50–100 μm were fabricated, for which we observed p-type conduction and mobility from 10 to 100 cm2/V s. To investigate the mobility dispersion, we performed Raman mapping to quantify the number of defects and the disorder in graphene films. The results suggest that the D-band/G-band intensity ratio is correlated with the mobility. Moreover, we discuss the factors controlling the mobility and how to improve the quality of the graphene films by reducing the number of defects.

First demonstration of heat dissipation improvement in CMOS technology using Silicon-On-Diamond (SOD) substrates

We have fabricated Silicon-On-Diamond (SOD) substrates on which, for the first time, we integrated n and p Fully Depleted MOSFETs high-K/metal gate down to 200nm gate length. The devices show excellent electrical characteristics and a 57% improvement of the thermal resistance compared to the co-processed one on standard SOI.

Improvement of multilayer graphene synthesis on copper substrate by microwave plasma process using helium at low temperatures

A pretreatment method for copper foil substrates for graphene synthesis with microwave plasma has been developed using a helium and hydrogen gas mixture. Contaminants on the surface of the substrate, particularly copper oxides, were effectively removed by this method, which was confirmed by X-ray photoelectron spectroscopy (XPS). Graphene was grown by microwave-plasma-assisted chemical vapor deposition (MWCVD) subsequent to the pretreatment of the substrate in the same apparatus. Using the hydrogen and helium gas mixtures as the diluent gas for methane, the contamination in the deposited graphene by undesirable impurities from the ambient in the reaction apparatus, particularly silicon etched from the quartz window by microwave plasma, was successfully suppressed. The multilayer graphene films synthesized at low temperatures on the pretreated copper foil substrates exhibited higher crystallinity, as shown in transmission electron microscopy (TEM) images and Raman scattering spectra, than those on the untreated substrates.

Enhanced thermal performances of silicon-on-diamond wafers incorporating ultrathin nanocrystalline diamond and silicon layers: Raman and micro-Raman analysis

Silicon-on-diamond (SOD) wafers potentially present thermal advantages over standard silicon-on-insulator (SOI) counterparts based on SiO2 as the buried insulating layer. This work reports the fabrication of high quality SOD wafers by the bond and etch back SOI process. One key parameter in the CVD diamond growth process is the substrate temperature. We focused here on two processes based on either high or low processing temperature to produce nanocrystalline diamond (NCD) on silicon. Both type of NCD films have been analyzed via Raman spectroscopy. Results have been correlated with scanning electron microscopy observations. In a second part, SOD wafers are fabricated and the crystalline quality of the active silicon layer is assessed via Raman measurements and high resolution transmission electron microscopy imaging. It is shown that the growth of diamond did not induce any structural defect or strain in the thin top silicon layer. Eventually, we demonstrate the efficiency of diamond integration thanks t...