Itaru Gunjishima

Ultrahigh-quality silicon carbide single crystals

Silicon carbide (SiC) has a range of useful physical, mechanical and electronic properties that make it a promising material for next-generation electronic devices. Careful consideration of the thermal conditions in which SiC {0001} is grown has resulted in improvements in crystal diameter and quality: the quantity of macroscopic defects such as hollow core dislocations (micropipes), inclusions, small-angle boundaries and long-range lattice warp has been reduced. But some macroscopic defects (about 1–10 cm-2) and a large density of elementary dislocations (∼ 104 cm-2), such as edge, basal plane and screw dislocations, remain within the crystal, and have so far prevented the realization of high-efficiency, reliable electronic devices in SiC (refs 12–16). Here we report a method, inspired by the dislocation structure of SiC grown perpendicular to the c-axis (a-face growth), to reduce the number of dislocations in SiC single crystals by two to three orders of magnitude, rendering them virtually dislocation-free. These substrates will promote the development of high-power SiC devices and reduce energy losses of the resulting electrical systems.

Improved Diameter Control of Carbon Nanotubes Using Fe–V–O Nanoparticles as the Catalyst

Fe–O and Fe–V–O nanoparticles (NPs) containing 4.0, 8.1, 12.3, 16.4, and 20.6 at. % V were synthesized by a liquid phase process. Carbon nanotubes (CNTs) were grown from each NP using acetylene as the carbon source, in order to investigate the dependence of CNT diameter on V concentration. The results show that the diameters of the CNTs are larger than those of the NPs, but tend to approach that of the NP as V concentration increases. This suggests that the diameter control of CNTs can be improved by adding of V to Fe when Fe–O NPs are used as a catalyst of CNT growth.

TSD Reduction by RAF (Repeated a-Face) Growth Method

A reduction in threading screw dislocation (TSD) density in 4H-SiC (silicon carbide) crystal is required for SiC power devices. In this study, TSD’s transformation by the RAF (repeated a-face) growth method [1] is observed by transmission X-ray topography (g=0004) of the cross-section of the crystal. Increasing the number of repetitions of a-face growth and offsetting c-face growth to an angle of several degrees reduce TSDs. TSD density is reduced to 1.3 TSD/cm2. The RAF growth method is very effective towards growing high quality SiC crystals.

Growth of Vertically Aligned Carbon Nanotubes from Highly Active Fe–Ti–O Nanoparticles Prepared by Liquid-Phase Synthesis

Monodispersed Fe–Ti–O nanoparticles and Fe–O nanoparticles were prepared by liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs were obtained in the case of Fe–Ti–O nanoparticles, whereas few CNTs were grown when Fe–O particles were used. CNTs with a diameter of 4.3±0.7 nm were grown from Fe–10 at. % Ti nanoparticles with a diameter of 3.8±0.6 nm. These CNTs mainly consist of double- and triple-walled CNTs. The catalytic activity of nanoparticles peaks at around 20–25 at. % Ti. The diameter controllability of Fe–20 at. % Ti nanoparticles was lower than that of Fe–10 at. % Ti nanoparticles, although a 200-µm-thick CNT forest was grown within 10 min. The energy-dispersive X-ray spectroscopy (EDS) analysis of the Fe–Ti–O nanoparticles before and after CNT growth revealed the depletion of Ti in the nanoparticles during the CNT growth, indicating the extraction of Ti from the nanoparticles.

In situ Optical Imaging of Carbon Nanotube Growth

In situ optical imaging of vertically aligned carbon nanotube (CNT) growth has been performed. The thickness and growth rate of CNTs were accurately measured in real time using an image processor. A thickness variation ranging from approximately 7 to 250 µm was observed. As growth proceeded, growth rate degradation occurred with relaxing its gradient. However, our in situ observations revealed that growth rate recovered upon varying the growth parameters, such as temperature, pressure, and C2H2/H2 flow rate ratio, suggesting that in situ growth rate control is applicable to CNT growth.

Development of RAF Quality 150mm 4H-SiC Wafer

We have developed RAF (Repeated a-face) growth method which is high quality bulk crystal growth technology [1, 2]. A block crystal more than 150 mm square size was produced by the RAF growth method. Since c-face growth crystal was produced on the seed obtained from the block crystal, high quality 150mm 4H-SiC wafer was achieved. This paper reports the results of the quality evaluation.

Acetylene-Treated Titania Nanotube Arrays (TNAs) as Support for Oxygen Reduction Reaction (ORR) Platinum Thin Film Catalysts

AbstractPlatinum layers show higher specific oxygen reduction reaction (ORR) activities than nanoparticles, and smooth monolayers of platinum on polycrystalline gold have been achieved by electrodeposition from CO-saturated solutions. Since Pt monolayers are interesting catalytic systems, this methodology was attempted on acetylene-treated titania nanotubes (TNAs) with high conductivity. However, the investigation of the as-treated TNAs found that probably nanotubes with an oxygen-containing graphitic overlayer were formed. It was observed that deposition after partial oxidative removal of the overlayer led to very low ORR activities while deposition on the intact overlayer gave rise to the highest activities obtained in our research so far. This is attributed to a “tie-layer” effect, in which the carbon layer screens the negative effects of the underlying TiO2 layer. The interesting effects of the graphitic overlayer on the ORR activity of the Pt deposits on acetylene-treated TNAs offer a strategy to mitigate the unfavorable interactions of the Pt/TiO2 interface. However, the carbon layer in this study was found not to be stable upon potential cycling. Graphical AbstractOnly Gold Lets Platinum be Platinum: Monolayer amounts of Pt, electrodeposited under CO termination, so far, only show specific activities comparable to polycrystalline platinum if deposited on polycrystalline gold. TiO2 modifications as supports facilitate higher activities only at enhanced conductivities, which usually sacrifice their stability.

Growth of Diameter-Controlled Carbon Nanotubes from Fe-V-O Nanoparticles Size-Classified by Ligand-Exchanged Fractional Precipitation

Colloidal Fe-V-O nanoparticles prepared as carbon nanotube (CNT) growth catalysts were precisely size-classified by fractional precipitation. Furthermore, the classification ability was improved by the fractional precipitation after ligand exchange process, which allowed us to obtain narrower size distributions of nanoparticles. CNTs were grown from the nanoparticles in order to investigate the dependence of diameter distribution of CNTs on that of nanoparticles. The results show that the diameter distribution of CNTs grown from classified nanoparticles was narrower than that of CNTs grown from as-prepared nanoparticles.

In situ growth rate control of carbon nanotubes by optical imaging method

Feedback control of the growth rate of carbon nanotubes (CNTs) was performed. By measuring the CNTs thickness using an optical microscope and an image processor, and instantaneous calculation of growth rate, continuous data of CNT thickness/growth rate versus growth time curves were obtained. The growth rate for the constant growth condition degraded as time progressed. In contrast, growth rate degradation was suppressed by optimizing the growth conditions by changing the growth parameters such as growth temperature, pressure, and gas flow rate. As a result, a larger thickness was obtained under the varied growth condition in the measurement time range.

Alignment-retainable nitrogenation of cylindrical carbon nanotubes by thermal reaction with ammonia following UV oxidation: chemical alteration effects on electrical conductivity

Cylindrical carbon nanotubes (CNTs) pretreated by UV irradiation were able to react with NH(3) to give nitrogen-containing CNTs without destroying their vertically aligned morphology. This process provided incorporation of nitrogen mostly at pyridinic and pyrrolic sites and promoted disordering, which was correlated with decreased electrical conductivity of CNT yarns.