Yukiko Yamada-Takamura

Characterization of α-phase aluminum oxide films deposited by filtered vacuum arc

Abstract Aluminum oxide (Al2O3) films were deposited by the filtered vacuum arc method, using a highly pure aluminum cathode and oxygen gas. Substrate temperature as high as 780°C was necessary for the formation of corundum structured α-Al2O3. Applying RF power and thus negative bias voltage to the substrate enables to increase and control the energy of substrate bombarding ions. With a negative bias voltage of 200 V, it was possible to deposit films containing α-Al2O3 with pre-heating the substrate to a temperature lower than 500°C. Increasing the voltage, thus increasing the ion energy, resulted in lowering of the critical pre-heating substrate temperature for formation of α-phase. Additionally, there was a clear difference in crystal orientation of α-Al2O3 between the films grown with and without substrate bias voltage, which was confirmed by infrared spectroscopy. Structure and phase evolution of the film were also studied by cross-sectional transmission electron microscopy.

Crystal structure characterisation of filtered arc deposited alumina coatings: temperature and bias voltage

Using a filtered vacuum arc deposition device, stoichiometric aluminum oxide (Al2O3) films, with thickness ranging from 20 nm to several microns, were produced under various substrate bias voltages and temperatures. Analysis of the resulting alumina crystal structures was performed with transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. Depending on the negative substrate bias voltage, the deposition temperature required to form α-Al2O3 could be reduced. A crystal phase diagram showing the effect of bias and temperature is presented. Also, preliminary hydrogen permeation measurements of these coatings deposited on thin palladium foil show a good barrier performance as compared with uncoated samples.

Tuning of silicene-substrate interactions with potassium adsorption

The evolution of the electronic structure and the structural stability of epitaxial silicene on ZrB2(0001) thin films exposed to K atoms has been studied by angle-resolved photoelectron spectroscopy and low-energy electron diffraction. Potassium adsorption leads to charge donation to the silicene lattice, which is accompanied by the partial filling of a formerly unoccupied π* band and by the increasing hybridization between the diboride surface state and the lower branch of the back-folded π band. The results allow an identification of silicene-derived π electronic states and confirm that before K adsorption, the interactions at the silicene-substrate interface are rather weak.

Development of a metal–tip cantilever for noncontact atomic force microscopy

We report on a focused-ion-beam fabrication of a metal–tip cantilever for noncontact atomic force microscopy (AFM) and demonstrate its superior performance by observing atomically resolved AFM images of the Si(111)7×7 surface. Characterization of the tip apex by transmission electron microscope revealed that the tip radius is less than 5nm. Detrimental changes in the resonance frequency and the Q factor of the cantilever due to the attachment of the metal tip are small and do not affect the performance of the AFM imaging. Since the fabrication technique is applicable to any materials, various functional probes can be developed with this method.

Progress in the materials science of silicene

Abstract In its freestanding, yet hypothetical form, the Si counterpart of graphene called silicene is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Such interesting electronic properties are not realized in two-dimensional (2D) Si honeycomb lattices prepared recently on metallic substrates where the crystal and hybrid electronic structures of these ‘epitaxial silicene’ phases are strongly influenced by the substrate, and thus different from those predicted for isolated 2D structures. While the realization of such low-dimensional Si π materials has hardly been imagined previously, it is evident that the materials science behind silicene remains challenging. In this contribution, we will review our recent results that lead to an enhanced understanding of epitaxial silicene formed on diboride thin films, and discuss the remaining challenges that must be addressed in order to turn Si 2D nanostructures into technologically interesting nanoelectronic materials.

Hydrogen permeation barrier performance characterization of vapor deposited amorphous aluminum oxide films using coloration of tungsten oxide

Abstract Low hydrogen diffusivity and solubility of aluminum oxide make a vapor deposited aluminum oxide (Al 2 O 3 ) film an anticipated hydrogen permeation barrier coating. In this paper, amorphous Al 2 O 3 film was deposited using filtered vacuum arc method. As a substrate, vapor deposited amorphous tungsten oxide (WO 3 ) film was used in order to characterize hydrogen permeation barrier performance of the Al 2 O 3 film utilizing coloration of WO 3 when it forms H x WO 3 . The samples were exposed to the flux and angular quantified atomic hydrogen beam, and the degree of coloration was characterized by visible to near infrared range transmission spectroscopy. Using this method, a half-micron thick amorphous Al 2 O 3 film reducing atomic hydrogen reaching the underlying WO 3 film to 3×10 −4 was measured. Furthermore, Al 2 O 3 film as thin as 20 nm showed atomic hydrogen reduction of 4×10 −3 . The results indicate effectiveness of thin vapor deposited Al 2 O 3 film as a permeation barrier against atomic hydrogen.

Mechanism of nucleation and growth of cubic boron nitride thin films

Abstract The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp2-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal +h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp2-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.

Surface electronic structure of ZrB2 buffer layers for GaN growth on Si wafers

The electronic structure of epitaxial, predominantly single-crystalline thin films of zirconium diboride (ZrB2), a lattice-matching, conductive ceramic to GaN, grown on Si(111) was studied using angle-resolved ultraviolet photoelectron spectroscopy. The existence of Zr-derived surface states dispersing along the Γ¯-M¯ direction indicates a metallic character provided by a two-dimensional Zr-layer at the surface. Together with the measured work function, the results demonstrate that the surface electronic properties of such thin ZrB2(0001) buffer layers are comparable to those of the single crystals promising excellent conduction between nitride layers and the substrate in vertical light-emitting diodes on economic substrates.

Unfolding method for first-principles LCAO electronic structure calculations

Unfolding the band structure of a supercell to a normal cell enables us to investigate how symmetry breakers such as surfaces and impurities perturb the band structure of the normal cell. We generalize the unfolding method, originally developed based on Wannier functions, to the linear combination of atomic orbitals (LCAO) method, and present a general formula to calculate the unfolded spectral weight. The LCAO basis set is ideal for the unfolding method because the basis functions allocated to each atomic species are invariant regardless of the existence of surface and impurity. The unfolded spectral weight is well defined by the property of the LCAO basis functions. In exchange for the property, the non-orthogonality of the LCAO basis functions has to be taken into account. We show how the non-orthogonality can be properly incorporated in the general formula. As an illustration of the method, we calculate the dispersive quantized spectral weight of a ZrB2 slab and show strong spectral broadening in the out-of-plane direction, demonstrating the usefulness of the unfolding method.

Diverse forms of bonding in two-dimensional Si allotropes: Nematic orbitals in the MoS 2 structure

The interplay of