Yoshiko Tsuji

Initial growth and texture formation during reactive magnetron sputtering of TiN on Si(111).

The initial growth and texture formation mechanism of titanium nitride (TiN) films were investigated by depositing TiN films on (111) silicon substrates by using reactive magnetron sputtering of a Ti metallic target under a N2/Ar atmosphere, and then analyzing the films in detail by using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Two power sources for the sputtering, dc and rf, were compared. At the initial growth stage, a continuous amorphous film containing randomly oriented nuclei was observed when the film thickness was about 3 nm. The nuclei grew and formed a polycrystalline layer when the film thickness was about 6 nm. As the film grew further, its orientation changed depending on the deposition conditions. For dc sputtering, the appearance of (111) or (200)-preferred orientations depended on the N2 partial pressure, and the intensity of the preferred orientation increased with increasing film thickness. For rf sputtering, however, when the film thickness was small (for ins...

Combinatorial method to prepare metal nanoparticles that catalyze the growth of single-walled carbon nanotubes

Enhanced surface diffusion at the growth temperature of single-walled carbon nanotubes (SWNTs) can cause coarsening of metal catalysts. By balancing the nominal thickness and surface diffusion length of metals, metal nanoparticles of desirable size are expected to form spontaneously under the SWNTs growth conditions. Our combinatorial method, using a library of nominally 0.001 to 1 nm thick sputter-deposited cobalt patterns, identified in a single experimental run that cobalt nanoparticles from submonolayers can catalyze the growth of high-quality SWNTs.

Effect of interfacial interactions on the initial growth of Cu on clean SiO2 and 3-mercaptopropyltrimethoxysilane-modified SiO2 substrates

The effect of interfacial interactions on the initial growth of Cu on clean SiO2 and 3-mercaptopropyltrimethoxysilane (MPTMS)-modified SiO2 substrates by sputter deposition was studied using transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. Plasma damage during sputter deposition makes surfaces of MPTMS-modified SiO2 substrates consist of small MPTMS islands several tens of nanometers in diameter and bare SiO2 areas. These MPTMS islands are composed of disordered multilayer MPTMS aggregates. The initial growth behavior of Cu on MPTMS-modified SiO2 substrates differs from that on clean SiO2 substrates, although Cu grows in three-dimensional-island mode on both of them. After a 2.5-monolayer Cu deposition on clean SiO2 substrates, spherical Cu particles were formed at a low number density of 1.3×1016 /m2 and at a long interparticle distance of 5 nm. In contrast, after the same amount of deposition on MPTMS-modified SiO2 substrates, Cu particles pre...

Field emission properties of single-walled carbon nanotubes with a variety of emitter morphologies

Field emission properties of single-walled carbon nanotubes (SWCNTs), which were prepared through alcohol catalytic chemical vapor deposition for 10–60 s, were characterized in a diode configuration. Protrusive bundles at the top surface of samples act selectively as emission sites. The number of emission sites was controlled by emitter morphologies combined with texturing of Si substrates. SWCNTs grown on a textured Si substrate exhibited a turn-on field as low as 2.4 V/µm at a field emission current density of 1 µA/cm2. Uniform spatial luminescence (0.5 cm2) from the rear surface of the anode was revealed for SWCNTs prepared on the textured Si substrate. Deterioration of field emission properties through repetitive measurements was reduced for the textured samples in comparison with vertically aligned SWCNTs and a random network of SWCNTs prepared on flat Si substrates. Emitter morphology resulting in improved field emission properties is a crucial factor for the fabrication of SWCNT-electron sources. Morphologically controlled SWCNTs with promising emitter performance are expected to be practical electron sources.

Preferred orientation and film structure of TaN films deposited by reactive magnetron sputtering

The structural evolution of tantalum nitride (TaN) films deposited by reactive rf magnetron sputtering were investigated in detail by using transmission electron microscopy (TEM) and x-ray diffractometry (XRD) for a wide range of thickness from 2 nm to 2 μm under various N2/Ar flow ratios from 0 to 20 vol % on both amorphous SiO2 (a-SiO2) and randomly oriented polycrystalline fcc TaN (poly-fcc-TaN) substrates. Although the films had various crystalline structures [including tetragonal Ta, bcc Ta(N), and fcc TaN] of different preferred orientation (PO) and had amorphous phases depending on deposition conditions, the formation mechanism of these structures was systematically explained by mapping them on 2D graphs of film thickness vs N2/Ar flow ratio. The texture map of films deposited on a-SiO2 substrates reflected both nucleation and growth stages, whereas that of films deposited on poly-fcc-TaN substrates reflected mainly the growth stage. Comparison of these two maps allowed the nucleation and growth pr...

Nanostructure and magnetic properties of c-axis oriented L10-FePt nanoparticles and nanocrystalline films on polycrystalline TiN underlayers

We performed a systematic study of the nanostructure and magnetic properties of FePt on templates of either (200)-oriented polycrystalline TiN underlayers with in-plane grain sizes from 5.8 to 10 nm (poly-TiN) or highly (200)-textured TiN underlayers epitaxially grown on single-crystalline MgO (100) substrates (epi-TiN). For small nominal FePt thicknesses (0.7–8.0 nm), FePt forms particulate films with the magnetic easy axis perpendicular to the film plane on every template TiN underlayer. In addition, the coercivity of nominally 1.4-nm-thick FePt at 300 K in the out-of-plane direction increases from 5.3 kOe for 5.8-nm-sized poly-TiN to 12.9 kOe for 10-nm-sized poly-TiN and reaches 16.3 kOe for epi-TiN, which shows that the coercivity strongly depends on the degree of the c-axis orientation. For larger FePt nominal thicknesses (16–64 nm), FePt particles percolate and form continuous films, and the direction of the easy magnetic easy axis becomes random. The coercivity of nominally 64-nm-thick FePt at 300 ...

Efficient field emission from triode-type 1D arrays of carbon nanotubes

Carbon nanotube (CNT) emitters were formed on line-patterned cathodes in microtrenches through a thermal CVD process. Single-walled carbon nanotubes (SWCNTs) self-organized along the trench lines with a submicron inter-CNT spacing. Excellent field emission (FE) properties were obtained: current densities at the anode (J(a)) of 1 microA cm(-2), 10 mA cm(-2) and 100 mA cm(-2) were recorded at gate voltages (V(g)) of 16, 25 and 36 V, respectively. The required voltage difference to gain a 1:10 000 contrast of the anode current was as low as 9 V, indicating that a very low operating voltage is possible for these devices. Not only a large number of emission sites but also the optimal combination of trench structure and emitter morphology are crucial to achieve the full FE potential of thin CNTs with a practical lifetime. The FE properties of 1D arrays of CNT emitters and their optimal design are discussed. Self-organization of thin CNTs is an attractive prospect to tailor preferable emitter designs in FE devices.

Structure and magnetic property of c-axis oriented L10-FePt nanoparticles on TiN/a-Si underlayers

L10-FePt is a promising material for high-density perpendicular magnetic recording media. The authors previously reported that c-axis oriented L10-FePt nanoparticle monolayers can be formed on (200)-oriented polycrystalline template TiN underlayers on SiO2 by using a conventional sputtering method. In this study, TiN nanostructures, such as the degree of (200) orientation, were improved by first depositing a buffer layer, such as amorphous Si onto SiO2, and the grain size could be controlled by adjusting either the deposition temperature or TiN thickness. When FePt nanoparticles were formed on a template TiN underlayer with a buffer layer of amorphous Si, both their degree of c-axis orientation and their magnetic properties were improved; FePt nanoparticles with nominal thickness of 1.4 nm had coercivity of 12.9 kOe in the out-of-plane direction at 300 K.

c-Axis Oriented Face-Centered-Tetragonal-FePt Nanoparticle Monolayer Formed on a Polycrystalline TiN Seed Layer

c-Axis oriented face-centered-tetragonal (fct)-FePt magnetic nanoparticles are a promising candidate for high density perpendicular magnetic recording media. In this study, TiN was investigated as a seed layer to achieve c-axis orientation of fct-FePt nanoparticles. First, a (200)-oriented, polycrystalline TiN layer with grain size around 10 nm was prepared by reactive sputter-deposition at 873 K on SiO2, and then FePt was sputter-deposited at 973 K on it. Both in-plane and out-of-plane X-ray diffraction revealed that FePt had fct structure with c-axis orientation. Plan-view field emission scanning electron microscopy showed that FePt formed well-isolated nanoparticles. The particle diameter increased with increasing nominal thickness of FePt, and it was similar to the size of the TiN grains when nominal thickness was 1.4 nm. Cross-sectional transmission electron microscope images indicated that single FePt nanoparticles grew on single TiN grains, namely, one nanoparticle per grain, with an epitaxial relationship. Superconducting quantum inference device measurement at 300 K revealed that the FePt nanoparticles had coercivity of 6.2 and 0.8 kOe for the out-of-plane and in-plane directions, respectively. The FePt nanoparticle monolayer sputter-deposited on polycrystalline TiN seed layer is a promising candidate for perpendicular magnetic recording media.

Observation of Phase Transition from Dissolved Organic Semiconducting Materials to Solid Film during Drying

We studied the formation dynamics of low-molecular-weight organic semiconducting molecules of N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenylbenzidine (DNTPD) during solvent evaporation. We dropped the DNTPD–toluene solution on a glass substrate and performed in situ measurements of mass, temperature, scattered light intensity, and photoluminescence (PL). We found that both light scattering and PL measurements were able to detect the onset of solid film formation from dissolved organic semiconducting materials. When a solid thin film forms during drying, sudden changes in scattered light and PL are observed. Furthermore, we also found that a period of time after the onset was necessary for completion of thin solid film formation. Observations and in situ PL measurement during thin-film formation by spin coating revealed that this period of time affected the optical properties of the film. This result indicates that quantitative information on the film formation process is quite important to obtain thin films with desired properties by coating and drying. Our in situ measurements were simple and practical approaches to monitor the formation dynamics of organic thin films during drying.