Naoshi Sakuma

Synthesis of a Closely Packed Carbon Nanotube Forest by a Multi-Step Growth Method Using Plasma-Based Chemical Vapor Deposition

We report a synthesis of a closely packed multi-walled carbon nanotube (MWCNT) forest by a multi-step growth method, including a new approach to immobilize catalytic nanoparticles, using plasma-based chemical vapor deposition. The CNT packing density reaches one-half of the theoretical value, where the space of 30–40% is filled with MWCNTs. This value is approximately one order of magnitude larger than that of as-grown CNT forest synthesized using conventional methods. The method is applicable even at a spatially restricted region, for example, in trench or via hole, and is available at the growth temperature as low as 450 °C.

NANOSTRUCTURAL CONDUCTIVITY AND SURFACE-POTENTIAL STUDY OF LOW-FIELD-EMISSION CARBON FILMS WITH CONDUCTIVE SCANNING PROBE MICROSCOPY

Simultaneous surface topography and conductivity/potential measurements were carried out on low-field-emission (1 V/μm) carbon films by combining conductive atomic force microscopy and Kelvin probe force microscopy. The current image showed that highly conducting sites and nonconducting regions coexisted on a micro- and/or nanoscale. Further, in situ I–V characteristics of both regions demonstrated that the conducting sites have an Ohmic property, whereas nonconducting regions have a degenerated Schottky property. When combined with the current image, the contact potential difference image showed that the conducting sites have a highest contact potential difference of 0.5 V, which implies the existence of a graphite phase. It is revealed that the conducting channels play an important role in the low-field-emission process. It is also suggested that the combination of conductivity and surface-potential measurements is an effective method for investigating complex-phase nanostructural surfaces.

A 2-million-pixel CCD image sensor overlaid with an amorphous silicon photoconversion layer

A highly sensitive 2-million-pixel high-definition charge-coupled device (CCD) image sensor was developed that features an overlaid amorphous silicon photoconversion layer on an interline transfer-type CCD scanner. The device is adapted to the 16:9 aspect ratio. 1125 scanning lines and 2:1 interlace high-definition TV system. A dual-channel horizontal CCD register is used to reduce the operating frequency to one half of the 74.25-MHz readout frequency. A horizontal period signal storage memory (1H line memory) is provided between the vertical CCD register and the horizontal CCD register to provide the signal distribution from the vertical CCD to the horizontal CCD register during the 3.77- mu s short horizontal blanking interval. This device realized a 1000 TV line horizontal limiting resolution 210 nA/1x high sensitivity. Total random noise was found to be 52 electrons RMS and a 72-dB dynamic range was achieved. >

Electrical characterization of phosphorus-doped n-type homoepitaxial diamond layers by Schottky barrier diodes

Temperature-dependent current–voltage (I–V), capacitance–voltage (C–V) measurements, and frequency-dependent C–V measurements have been carried out to investigate electrical properties of phosphorus (P)-doped n-type homoepitaxial diamond layers. We have fabricated lateral dot-and-plane (with ring-shaped-gap) Schottky barrier diodes. Frequency-dependent capacitance measurements revealed the existence of a deep donor level. C–V measurements deduced that the net donor concentration was 6.2×1017 cm−3 and the corresponding built-in potential was 4.0 eV, when the P concentration was 8.3×1017 cm−3. Phosphorus electrical activity was 0.75 in the P-doped diamond layer. The carrier thermal activation energy (the donor level) was evaluated to be 0.6 eV from the relation between the net donor concentration and the carrier concentration.

Low operation voltage field emitter arrays using low work function materials fabricated by transfer mold technique

Extremely sharp, uniform, and low operation voltage field emitter arrays (FEAs) using low work function materials such as LaB/sub 6/ and TiN have been developed by the Transfer Mold emitter fabrication technique to realize high efficient and high reliable devices for the first time. Because of the sharpening effect on the tips by thermally oxidized SiO/sub 2/ layer of the molds, emitter tip radii are as small as less than 10 nm (2.5-5 nm). The turn-on voltages of LaB/sub 6/ and TiN FEA are 110-130 V lower than that of conventional Mo FEA. That of the gated LaB/sub 6/ FEA is as low as 28 V even without high vacuum baking treatment. Transfer Mold technique provides superior uniformity, sharpness and easiness of selecting low work function materials including diamond which might have negative electron affinity.

R&D of diamond films in the Frontier Carbon Technology Project and related topics

R&D activities on diamond chemical vapor deposition (CVD) and field emission in the Frontier Carbon Technology Project are presented. The topics are (1) morphology control of diamond films grown by a 60-kW, 915-MHz microwave plasma CVD reactor, (2) growth technology of large single crystal diamond with a low density of defects, (3) heteroepitaxial growth technology of diamond films on Pt, (4) fabrication of sharp emitter tips on single crystal diamond, (5) field emission study from diamond particles, and (6) intense field emission from ion implanted homoepitaxial diamond layer. Research results of field emission obtained by Kyoto University and North Carolina State University are also described.

Carbon Nanotube Vias Fabricated by Remote Plasma-Enhanced Chemical Vapor Deposition

Multiwalled carbon nanotubes (CNTs) have been grown by remote plasma-enhanced chemical vapor deposition at temperatures as low as 400 °C. In via formation, the selective growth of CNT bundles in via holes at 430 °C and chemical mechanical polishing for planarization have been performed. The electrical evaluation of CNT single vias with various diameters reveals that the via resistance is inversely proportional to the via area. This result indicates that the CNTs are grown with uniform quality and density in the via holes with various diameters and stable contact formations are obtained. Moreover, the resistances of single vias are approximately equivalent to the via resistances estimated from the resistances of via chains, demonstrating the via-to-via uniformity of the CNT vias obtained by the remote plasma-enhanced chemical vapor deposition.

High-quality carbon nanotube growth at low temperature by pulse-excited remote plasma chemical vapor deposition

Carbon nanotube (CNT) growth at temperatures below 400 °C by pulse-excited remote plasma chemical vapor deposition was demonstrated. Reduction of plasma power was carried out in order to decrease the amount of all particles, ions, electrons, and radicals. In addition, a biased plate-type screening electrode was introduced to removal of charged particles, ions, and electrons. The negative bias below 50 V was most effective for growth rate. High-quality CNT growth with the growth rate of 98 nm/min was successfully obtained at 400 °C. The results suggest that both removal of charged particles and control of the amount of radicals are important for high-quality CNT growth at temperatures below 400 °C.

Low-Temperature Graphene Growth Originating at Crystalline Facets of Catalytic Metal

We explored the characteristic behavior of low-temperature graphene growth on catalytic metal films. The results suggested that graphene growth originates from the crystalline facets with specific angles with respect to the crystalline orientation of the catalytic metals at low temperatures, which is different from the conventional growth models. The G/D ratio of the Raman spectrum of the graphene film was affected by both the number of specific facets and the width of the terrace. Because of this behavior, it is important to prepare the surface conditions with a smaller number of facets and a wider terrace for high-quality graphene growth at low temperatures.

Fabrication of 70-nm-diameter carbon nanotube via interconnects by remote plasma-enhanced chemical vapor deposition and their electrical properties

We have succeeded in fabricating ultrafine carbon nanotube (CNT) via interconnects with SiOC interlayer dielectrics. High-quality multiwalled CNTs are grown in via holes with a diameter of 70 nm using pulse-exited remote plasma-enhanced chemical vapor deposition at 450 °C. The resistance of a 70-nm-diameter CNT via is 52 Ω, which is the lowest ever reported for CNT via interconnects. The CNT via interconnect has the capability to sustain current density as high as 1×108 A/cm2.