Akira Otsuki

Synthesis of near net shape high density TiB/Ti composite

TiB/Ti composite were produced by SHS (self propagating high temperature synthesis) under conditions of PHIP (pseudo-hot isostatic pressing). The synthesis conditions were optimized to produce highly dense materials conforming a high volume fraction of TiB. Self-propagating reaction of the Ti and B powders occurred above the α- to β-Ti transformation temperature (1155 K). The composites consist of Ti matrices with dispersed TiB reaction products of 2–30 μm size. Near fully dense composites with a TiB/Ti volume ratio of 33–44 vol.% were synthesized for B/Ti atomic ratios of 0.3–0.4 at pseudo isostatic pressures of 100 MPa. The Vickers hardness and fracture stress of the composites are 5.6–7.1 and 2.1–2.3 GPa, respectively, and the wear of the composite is approximately 7–9% of the Ti and Ti–6Al–4V.

Structural change in p-type porous silicon by thermal annealing

The morphological change of p-type porous silicon during annealing has been investigated. The x-ray diffraction (XRD) pattern was composed of a sharp Bragg reflection peak and a diffuse scattering. The diffuse scattering is not related to the presence of the amorphous phase. The shape of the XRD pattern started to change at an annealing temperature as low as 400 °C, and the 2θ angle of the sharp peak varied at a temperature as low as 350 °C. These changes at low temperatures seem to be closely related to the desorption of hydrogen and the resultant change of the dangling bond density in porous silicon. The molecular orbital calculations also support the participation of dangling bonds in the structural reorganization in the surface region.

Carrier transport mechanism of Ohmic contact to p-type diamond

The carrier transport mechanism through the p-diamond/metal interface was studied by measuring specific contact resistances (ρc) using a transmission line method for Ti, Mo, and Cr (carbide forming metals) and Pd and Co (carbon soluble metals) metals contacting to the boron-doped polycrystalline diamond films. Schottky barrier heights (φB) of around 0.5 eV were measured for the annealed contacts. The present result indicates that formation of thermally stable graphite layers at the diamond/metal interfaces upon annealing would pin the Fermi level of the p-diamond. This model led to the preparation of in situ Ohmic contacts by depositing a thin diamondlike carbon on the p-diamond surface prior to metal deposition, and also to excellent Schottky contacts with breakdown voltages higher than 900 V. The present experiment concluded that the existence of a graphite layer at the diamond/metal interface controlled the electrical properties through the p-diamond/metal interface.

Formation of nanoscale Fe/Ag multilayer by repeated press-rolling and its layer thickness dependence of magnetoresistance

Abstract We have successfully fabricated Fe/Ag multilayers that simultaneously possess the layer thickness of 4 nm and the layer number of above 10 000 by applying the repetition of pressing and rolling directly from a macroscopic stack of metal sheets. The clear correlation between the Ag layer thickness and MR ratio is observed, indicating that the critical layer thickness exhibiting magnetoresistance is 100 nm.

Formation of ohmic contacts to p‐type diamond using carbide forming metals

The measurement of the specific contact resistance, ρC, and microstructural analysis at the metal/diamond interface were carried out for diamond with various acceptor concentrations, NA, in order to understand the carrier transport mechanism at the metal/diamond interface. The ρC measurements were carried out for polycrystalline boron‐doped semiconducting diamonds which were prepared by the microwave plasma chemical vapor deposition. The acceptor concentrations, estimated by the boron concentrations measured by secondary ion mass spectroscopy, ranged from 3×1018 to 3×1020 cm−3. Ti and Mo films, which form carbides with diamond, were deposited on the diamonds using the electron‐beam evaporation technique. The ρC values were measured by the circular transmission line method before and after annealing at temperatures in the range of 400–600 °C. The dependence of the ρC values on the acceptor concentrations suggested that the dominant transport mechanism was the field‐emission for the diamond with NA around 1...

Bulk metallic multilayers produced by repeated press-rolling and their perpendicular magnetoresistance

Bulk Fe/Ag multilayers with layer thickness of about 10 nm have been successfully fabricated by repeated press-rolling directly from a macroscopic stack of metal sheets. The press-rolled multilayers exhibited giant magnetoresistance of 13% in the current perpendicular to the plane geometry and 4% in the current in the plane geometry at 5 K.

NiGe‐based ohmic contacts to n‐type GaAs. I. Effects of In addition

Contact resistances of NiGe ohmic contacts, which had been previously developed in our laboratory, were reduced significantly by adding a small amount of In to the NiGe contacts without deteriorating the thermal stability, the surface smoothness, and the shallow diffusion depth. The optimum layer thicknesses to prepare the low resistance ohmic contacts were determined to be 60 nm for Ni, 100 nm for Ge, and 3 nm for In, and the contact resistances (Rc) less than 0.3 Ω mm were obtained after annealing at temperatures in the range between 600 and 700 °C. Microstructural analysis at the GaAs/metal interface of the contact with low Rc showed formation of ‘‘regrown’’ GaAs and InxGa1−xAs layers between the GaAs substrate and high melting point NiGe compounds. Based on the present electrical measurements and microstructural analysis, a model for the current transport of the NiGe‐based ohmic contacts was proposed, which explained well the dependencies of the contact resistances on the microstructure at the GaAs/me...

NIGE-BASED OHMIC CONTACTS TO N-TYPE GAAS. II: EFFECTS OF AU ADDITION

Our efforts have been continued to improve the electrical properties of NiGe ohmic contacts by adding a small amount of a third element to the NiGe contacts without deteriorating thermal stability and surface smoothness. In the present study, Au was chosen as the third element, and the optimum conditions to produce thermally stable, low resistance ohmic contacts were determined by preparing a variety of contacts with different thickness ratios of the Ni, Ge, and Au layers. The best ohmic contact was prepared by depositing sequentially Ni (40 nm), Au (5 nm), and Ge (100 nm) onto the n‐type GaAs substrate, and annealing at 450 °C for 5 s. This contact provided the contact resistance of about 0.2 Ω mm, which is lower than that of the NiGe(In) contacts. The present contact had smooth surface after contact formation and showed excellent thermal stability during isothermal annealing at 400 °C. The cross‐sectional observation using high‐resolution electron microscopy indicated that the GaAs/metal interface was u...

Dependence of contact resistance on metal electronegativity for B‐doped diamond films

The contact resistance (Rc) measurements and microstructural analysis at the metal/diamond interfaces were carried out by depositing a variety of metals on the polycrystalline B‐doped diamonds which were prepared by microwave plasma chemical‐vapor deposition. The Rc values, which were measured by the transmission line method, were found to be dependent on the electronegativities of the metals, and the lower Rc values were obtained for the metals with higher electronegativity. The Rc values of the Ti and Mo contacts decreased upon annealing at elevated temperatures, and the TiC and Mo2C compounds were observed at the metal/diamond interfaces by cross‐sectional transmission electron microscopy and x‐ray diffraction. The reduction of the Rc values is believed to be due to creation of crystal defects induced by formation of the carbides. The present experiment suggests that the electronegativities of the metals would provide guidance to select Schottky and ohmic contact materials of future diamond semiconduct...

Carrier transport mechanisms through the metal/p-type diamond semiconductor interface

Abstract Carrier transport mechanisms at p-diamond/metal interfaces were studied by analyzing the dependencies of the specific contact resistivities ( ρ c ) on the measurement temperature and the acceptor concentration ( N A ). A variety of metals, such as Ti, Mo, Cr (carbide forming metals), Pd and Co (carbon soluble metals), were deposited on boron-doped polycrystalline diamond films, and the ρ c values were measured by a transmission line method. A constant Schottky barrier height (SBH) of around 0.5 eV was measured for these annealed contacts, and the reason was believed to be due to phase transformation from metastable diamond to a stable conductive graphite layer in the vicinity of the diamond/metal interface. By controlling the crystal structure at the diamond/metal interface, non-annealed ohmic and high-voltage Schottky contacts were successfully developed.