Masao Kamiko

Superhardness effects of heterostructure NbN/TaN nanostructured multilayers

Although superhardness effects have been extensively investigated for epitaxial ceramic nanomultilayer films with the same crystal structures in the last decade, those for multilayers with different crystal structures have been seldom studied. In this article, NbN/TaN nanomultilayers have been designed and deposited by reactive magnetron sputtering. The results showed that the crystal structures of NbN and TaN are face-centered cubic and hexagonal in superlattice films, respectively, and the lattice plane (111) of NbN is coherent with the (110) of TaN, i.e., {111}fcc-NbN∥{110}h-TaN. The results of microhardness measurement showed that the superhardness effects of NbN/TaN multilayers exist in a wide range of modulation period from 2.3 to 17.0 nm. This phenomenon is different from that of epitaxial ceramic multilayers where the maximum hardness usually takes place at a modulation period of 5.0–10.0 nm. It is proposed that the coherent stresses and the structural barriers (fcc/hexagonal) to dislocation motio...

Structure transformations and superhardness effects in V/Ti nanostructured multilayers

An anomalous enhancement of hardness has been found in the V/Ti nanostructured multilayer films, in which the constituent materials have the same elastic modulus. The high-resolution transmission electron microscopy results showed that the V layers take bcc structure at all modulation periods from 2.8 to 150.0 nm. The Ti layers are bcc structured at modulation periods less than 3.0 nm. The Ti layers are hcp structured at modulation periods of more than 5.9 nm. There is a Burgers orientation relation between the bcc V and the hcp Ti. Coherent stress hardening between the interfaces of bcc V and hcp Ti layers and the different crystal structures appear to be one likely reason for the hardness enhancement in V/Ti nanostructured multilayers.

Structure, hardness, and elastic modulus of Pd/Ti nanostructured multilayer films

The structure, hardness, and elastic modulus of Pd/Ti multilayers deposited by radio-frequency magnetron sputtering were investigated by x-ray diffraction, high-resolution transmission electron microscopy, and nanoindentation. Both the Ti and Pd layers were face-centered-cubic structures in all modulation periods from 2.8 nm to 90.0 nm in Pd/Ti multilayers. There are stacking faults in Ti layers at large modulation periods, where the crystal structure is hexagonal close packed. An anomalous hardness enhancement was observed. The hardness values of Pd/Ti multilayers are three times and two times the values measured in Pd films and as calculated by the rule of mixture for Pd and Ti films, respectively. The modulus values of Pd/Ti multilayers are between those of constituent single layer films at a larger modulation period, and increase slightly at a smaller modulation period. The elastic modulus difference model cannot explain this hardness enhancement, since the elastic modulus is almost the same for the c...

Electron field emission from undoped polycrystalline diamond particles synthesized by microwave-plasma chemical vapor deposition

Electron emission from polycrystalline diamond particles (PDPs) was obtained at low electric fields in the absence of intentional doping. The PDPs were synthesized on a silicon substrate using microwave-plasma chemical vapor deposition accompanied by bias-enhanced nucleation. Polycrystalline diamond particles of two different sizes, i.e., ∼500 nm and 2 to 5 μm, were obtained, the surfaces of which were covered with small crystal grains composed of fine facets. Electron emission from the PDPs was characterized by Fowler-Nordheim tunneling with low turn-on-field values (0.8 – 2.0 V/μm) and a low barrier height of 0.02 eV. An emission current greater than 5 μA was maintained for over 24 h in a cathode based on the developed PDPs. In contrast, single-crystalline diamond particles prepared for comparative purposes exhibited no emission up to 2.5 V/μm. Auger electron spectroscopy revealed that the surface oxygen content modified by annealing in air did not affect the emission properties. The macroscopic spatial...

Enhancement in layer-by-layer growth in heteroepitaxial growth of Co on Au(111) surface by Bi surfactant

The surfactant effect of Bi on the heteroepitaxial growth of Co on the Au(111) surface has been studied. With the predeposition of submonolayer Bi on Au(111) prior to evaporation of Co, more long-lasting reflection high-energy electron diffraction intensity oscillations were observed at room temperature. This implied that Bi enhanced the layer-by-layer growth of Co on the Au(111) surface. The dependence of the Co film growth on the thickness of the Bi surfactant layer suggested that there existed a suitable amount of Bi surfactant that enhanced a smoother layer-by-layer growth. The Auger electron spectra revealed that Bi was segregated at the top of the surface. Therefore, Bi was concluded to be an effective surfactant to enhance the layer-by-layer growth of Co on Au(111).

Epitaxial growth of fcc-Ag(0 0 1) nanodots on MgO(0 0 1) substrates via Ti seed layer-assisted agglomeration

We have analysed the influence of Ti seed layer (2.0 nm thick) on the agglomeration of Ag films (4.0 nm thick) grown onto MgO(0 0 1) single crystal substrates by RF magnetron sputtering. The samples were deposited at room temperature and post-annealed at 200–450 °C for 4 h while still maintaining the chamber vacuum condition. The surface profile of the sample, as analysed using atomic force microscopy, confirms that the insertion of a Ti seed layer between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot. Furthermore, the atomic concentration depth profile of the Ag/Ti/MgO film, as estimated by using angle-resolved x-ray photoelectron spectroscopy, suggests that the nanodot surface mainly consists of Ag. Moreover, x-ray diffraction studies prove that the initial deposition of the Ti seed layer onto MgO(0 0 1) prior to the Ag deposition yields high-quality face-centred cubic (fcc)-Ag(0 0 1) oriented epitaxial nanodots. Based on these results, it can be concluded that the Ti thin film acts as a seed layer, assisting the epitaxial growth of the Ag nanodot onto the MgO substrate.

EFFECT OF Bi SURFACTANT IN THE HETEROEPITAXIAL GROWTH OF Co ON Cu SURFACES

We have investigated the effect of Bi on the heteroepitaxial growth of Co on Cu by reflection high-energy electron diffraction (RHEED) measurements. It was found that Bi enhanced the layer-by-layer growth of Co on the Cu(111) surfaces at 100°C. The dependence of the growth on Bi layer thickness suggested that there existed a suitable amount of Bi surfactant layer that enhanced smoother layered growth. On the contrary, for the case of Co growth on Cu(100), Bi depressed the layer-by-layer growth of Co on Cu(100). The surface segregation effect of Bi was also studied by Auger electron spectroscopy (AES).

Perpendicular magnetic anisotropy and magneto-optical properties of (Co–Tb)/Pd multilayers

Abstract The perpendicular magnetic anisotropy (PMA) and magneto-optical properties of sputtered RE–TM (Co–Tb)/Pd multilayered films were studied. Thick CoTb layers with high Tb concentration were observed to be amorphous. Like in the crystalline Co/Pd multilayered films, perpendicular magnetic anisotropy was observed only for (Co–Tb)/Pd multilayers with thin magnetic layer. The magnetic layer thickness range, over which the easy magnetization axis is perpendicular to the film plane, is much broader for (Co–Tb)/Pd multilayers than that for Co/Pd multilayered films. Kerr rotation angles were found to decrease with increasing Tb concentration.

High Transparency and Electrical Conductivity of SnO2:Nb Thin Films Formed through (001)-Oriented SnO:Nb on Glass Substrate

Nb-doped (101)-oriented SnO2 (SnO2:Nb) thin films were fabricated through postdeposition thermal annealing of (001)-oriented Nb-doped SnO films grown on glass substrates using pulsed laser deposition. Nb doping resulted in an anisotropic lattice deformation explained by Nb in Sn site. Resistivity, electron concentration, and mobility values of 3.7×10-3 Ω cm, 6.5×1019 cm-3, and 26 cm V-1 s-1, respectively, were achieved in the films at room temperature for a dopant concentration of 1 at. %. The optical band gap increased from 3.9 to 4.1 eV with Nb doping ≥1 at. %.

Bi-enhanced Heteroepitaxial Layered Growth of Cr on Fe(100)-c(2× 2)O Reconstruction Surfaces

We have investigated the effect of Bi on the homoepitaxial growth of Cr on Fe(100) by reflection high-energy electron diffraction (RHEED) measurements. It was found that Bi enhances the layered growth of Cr on Fe(100)-c(2× 2)O reconstruction surfaces. The dependence of the growth on Bi layer thickness suggests that there exists a suitable amount of Bi surfactant layer that enhances smoother layered growth. The surface segregation effect of Bi was studied by Auger electron spectroscopy.