Aki Tominaga

Synthesis method for ultrananocrystalline diamond in powder employing a coaxial arc plasma gun

A new method that enables us to synthesize ultrananocrystalline diamond (UNCD) in powder is proposed. Highly energetic carbon species ejected from a graphite cathode of a coaxial arc plasma gun were provided on a quartz plate at a high density by repeated arc discharge in a compact vacuum chamber, and resultant films automatically peeled from the plate were aggregated and powdered. The grain size was easily controlled from 2.4 to 15.0 nm by changing the arc discharge energy. It was experimentally demonstrated that the proposed method is a new and promising method that enables us to synthesize UNCD in powder easily and controllably.

Room-temperature hard coating of ultrananocrystalline diamond/nonhydrogenated amorphous carbon composite films on tungsten carbide by coaxial arc plasma deposition

Ultrananocrystalline diamond (UNCD)/nonhydrogenated amorphous carbon (a-C) composite films were deposited on unheated WC containing Co by coaxial arc plasma deposition. The hardness of the film is 51.3 GPa, which is comparable with the highest values of hard a-C films deposited on nonbiased substrates. The deposited film is approximately 3 µm thick, which is one order larger than that of hard a-C films. The internal compressive stress is 4.5 GPa, which is evidently smaller than that of comparably hard a-C films. The existence of a large number of grain boundaries in the UNCD/a-C film might play a role in the release of the internal stress.

Fabrication of ultrananocrystalline diamond/nonhydrogenated amorphous carbon composite films for hard coating by coaxial arc plasma deposition

Ultrananocrystalline diamond (UNCD)/nonhydrogenated amorphous carbon (a-C) composite (UNCD/a-C) films were deposited on cemented carbide (WC-Co) substrates by coaxial arc plasma deposition (CAPD). To suppress the graphitization induced by Co in the WC-Co, the film deposition was carried out on unheated substrates. The hardness and Young’s modulus were 51.3 GPa and 520.2 GPa, respectively. These values are comparable or rather larger than those of UNCD/a-C films deposited on other substrates such as Si, which implies that the graphitization of UNCD/a-C hardly occurs. Surprisingly, UNCD/a-C films could be deposited at the maximum film thickness of approximately 3 μm in spite of the room temperature growth. The internal compress-stress of the film is approximately 4.5 GPa, which is evidently smaller than that of comparably hard a-C films. The existence of a large number of grain boundaries in the films, which is structural specific to UNCD/aC, might play an important role in the release of an internal stress in the film. It was experimentally demonstrated that UNCD/a-C films prepared by CAPD are potential hard coating materials for WCCo.

Si and Cr Doping Effects on Growth and Mechanical Properties of Ultrananocrystalline Diamond/Amorphous Carbon Composite Films Deposited on Cemented Carbide Substrates by Coaxial Arc Plasma Deposition

Si and Cr doped ultrananocrystalline diamond/amorphous carbon composite films were deposited on cemented carbide (WC-Co) substrates by using coaxial arc plasma deposition with Si and Cr blended graphite targets. The undoped films deposited at room temperature and a repetition rate of arc discharges of 1 Hz have the maximum hardness of 51 GPa and Young’s modulus of 520 GPa. With increasing substrate temperature and repetition rate, the hardness and modulus are degraded, which might be because the growth of sp2 bonds is thermally enhanced. The doping of Cr and Si degrades the hardness and modulus. From energy-dispersive X-ray spectroscopic measurements, the diffusion of Co atoms from the substrates into the films were observed for the Sidoped films. Since the Co diffusion induce the graphitization due to the catalytic effects, the degraded hardness and modulus of the Si doped films should be attributable to the catalytic effects of Co. For the Cr-doped films, the degraded hardness and modulus might be because of the Co catalytic effects being enhanced by the bombardment of Cr atoms whose atomic weight is much larger than that of C and the formation of chromium carbide. It was found that the doping of Si and Cr for the deposition of UNCD/a-C films deposited on WC-Co by CAPD is not effective for the improvement of the hardness and modulus

Heteroepitaxial growth of β-AlN films on sapphire (0001) in nitrogen atmospheres by pulse laser deposition

β-AlN thin films were heteroepitaxially grown on sapphire (0001) substrates with a smooth surface showing steps in nitrogen atmospheres by pulsed laser deposition using sintered AlN targets, and their films were structurally studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Horizontally oriented growth of β-AlN(111) was confirmed by XRD in 2θ–θ scan. The epitaxial relationship between β-AlN and sapphire was derived to be β-AlN(111) ∥ sapphire (0001) from the TEM measurement. The existence of dislocation defects was implied from the electron diffraction pattern of the film. The heteroepitaxial growth probably occurs in domain match epitaxy, accompanied by the generation of dislocations defects at the interface.

Epitaxial growth of n-type β-FeSi thin films on p-type Si(111) substrates by radio-frequency magnetron sputtering and rectifying action of heterojunctions

Tarek M. Mostafa, Motoki Takahara, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban*, Nathaporn Promros**, Aki Tominaga, Maiko Nishibori, and Tsuyoshi Yoshitake*** 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand 4Research Center for Synchrotron Light Applications, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe3Si/FeSi2 artificial lattices with various cross-sectional areas

Ken-ichiro Sakai*, Yūki Asai, Yūta Noda, Takeshi Daio, Aki Tominaga, Kaoru Takeda, and Tsuyoshi Yoshitake* 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Control and Information Systems Engineering, Kurume National College of Technology, Kurume, Fukuoka 830-8555, Japan 3International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 8190395, Japan 4Department of Electrical Engineering, Fukuoka Institute of Technology, Fukuoka 8110295, Japan

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe3Si/FeSi2 artificial lattices

Current-perpendicular-to-plane (CPP) junctions of Fe3Si/FeSi2 were fabricated from Fe3Si/FeSi2 artificial lattice films, which were prepared by facing-target direct-current sputtering, by employing a focused ion beam (FIB) technique. CPP structurization was confirmed by scanning electron microscopy. The CPP junctions, in which antiferromagnetic interlayer coupling is induced between the Fe3Si layers, exhibited a clear hysteresis loop in the electrical resistance for current injection, which is probably due to current-induced magnetization switching. The critical current density for it is approximately 3.3 × 101 A/cm2, which is at least four orders smaller than the values that have ever been reported.

Crystalline-structural evaluations of cubic AlN thin films heteroepitaxially grown on sapphire (0001) by pulsed laser deposition

Cubic β-AlN thin films with different thicknesses were grown on sapphire (0001) in nitrogen atmosphere by pulsed laser deposition with sintered AlN targets, and their film structures were evaluated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was found that β-AlN layers with a lattice constant of 7.89 A are epitaxially grown on sapphire (0001) with a relationship of βAlN(111)[11]∥Al2O3(0001)[100] at film thicknesses of less than 20 nm, and at larger thicknesses, polycrystalline β-AlN grains are grown on the epitaxial β-AlN layers in the Stranski–Krastanov (SK) mode.