Y. Oikawa

Ni Dot Array Formation on Scratches of Si(111) by Electroless Deposition

Arrays of Ni dots with well defined size and interspace of 10 nm order were deposited by the electroless reaction in the NiSO46H2O aqueous solution, along the scratches formed on n-Si(111) surface. The mechanism of the alignment of Ni dots are discussed in terms of a mobility of the dots and the repulsive force acting between them.

Direct formation of β-FeSi2 in evacuated fused ampoule

Abstract A simple preparation method for β-FeSi 2 was proposed to activate the interfacial reaction between Fe powder and Si powder through a heat process in a sealed ampoule. The mechanism of the reaction process was investigated by using sputtered Fe films on Si substrates and sapphire substrates. It was found that Si vapor in the ampoule reacts with solid Fe and is saturated to form e-FeSi. Once e-FeSi is formed, it initiates the reaction with solid Si to form β-FeSi 2 .

Co-dot-array formation along scratches on Si(111) surface by electroless deposition

Abstract Co-dot-array formation is investigated along scratches on n-Si(111) surface in the aqueous solution of NH 4 F and CoSO 4 ·7H 2 O. It was found that removal of oxygen from the solution by Ar-sparge on the surface of aqueous solution is effective for the formation of Co-dot-array. Some increase in the concentration of NH 4 F in the solution also tended to form the dot-array. The condition on the scratch for the formation of Co-dot-array is still left to be investigated.

Electronic structure of VO2 near phase transition by tunnelling spectroscopy

Tunnelling spectroscopy has been performed on W-doped VO2 single crystal near the metal–insulator transition temperature. A remarkable change was observed in the electronic density of states of the conduction band just below the transition temperature. Based on the observed results, a model is proposed for the onset of the metal–insulator transition in VO2 in relation to the band diagram by the local density approximation plus Hubbard U calculation. The tunnelling energy gap observed was in good agreement with band calculations and optical measurements near the transition temperature.

Effect of Fe–Si Mixed Powder on β-FeSi2 Formation from Fe–Si Mixture Film by Isothermal Process in Encapsulated Space

The isothermal process in evacuated and sealed ampoule at 930°C for 20 h for synthesizing β-FeSi2 was investigated using sputtered Fe–Si mixture film and Fe–Si mixed powder. It was found that the silicidation of the Fe–Si mixture film to β-FeSi2 was activated by Si vapor, accompanied with Fe vapor, generated and transported from the mixed powder. By this isothermal process with appropriate condition for Fe–Si mixture film preparation, a uniform and continuous single-phase β-FeSi2 film was synthesized.

Effects of Mn and/or Ni substitutions for Fe on thermoelectric properties of magnetite prepared by sintering

Effects of Mn and/or Ni substitutions for Fe were investigated on thermoelectric properties in sintered magnetite (Fe/sub 3/O/sub 4/) for temperature range 200-700/spl deg/C. Substitutions were performed in 4 series; (1) Mn/sub x/Fe/sub 3-x/O/sub 4/ (0/spl les/x/spl les/ 0.8), (2) Ni/sub x/Fe/sub 3-x/O/sub 4/ (0/spl les/x/spl les/ 0.8), (3) (Mn/sub 1/2/Ni/sub 1/2/)/sub x/Fe/sub 3-x/O/sub 4/ (0/spl les/ x/spl les/1.2) and (4) (Mn/sub 2/3/Ni/sub 1/3/)/sub x/Fe/sub 3-x/O/sub 4/ (0/spl les/ x/spl les/ 1.2). The values of electrical resistivity (/spl rho/) and Seebeck coefficient (S) for series (1) and (2) were found to be similar to the previously reported results for magnetite single crystal. Common to the 4 series, /spl rho/ slightly decreased with temperature, and S and thermoelectric power factor S/sup 2///spl rho/ increased with temperature. For series (4) with x=0.4, there was a case the thermoelectric power factor exceeded that of magnetite.

Thermoelectric properties of mixed layered compounds TiS/sub 2-x/Se/sub x/ (0/spl les//spl times//spl les/2)

Seebeck coefficients (S) were measured in TiS/sub 2-x/Se/sub x/ for 0/spl les//spl times//spl les/2 in the temperature range 77-285 K. For TiS/sub 2/ (x=0), S was negative and its absolute value increased linearly with temperature, attaining to 170/spl sim/250/spl mu/ V/K, depending on individual samples, at room temperature. With increasing x, the absolute value of temperature coefficient (TC) of S decreased in the temperature region T /spl ges/ 170K. At x /spl ap/ 1.5, TC was reduced to zero and changed its sign for larger x. This behavior is explained by the fact that the valence band of this system consists of S 3p and/or Se 4p bands and shifts upward with increasing x. For TiSe/sub 2/ (x=2), S became positive for T /spl ges/ 270K due to the increase in the effect of valence band with increasing temperature. The charge density wave (CDW) transition at 200K in TiSe/sub 2/ opens the CDW gap and pushes down the lower edge of CDW gap with decreasing temperature. Below 150K, S is controlled by the upper CDW band consisting of Ti 3d band. Thermoelectric behavior of this system can be totally explained by the above band picture. The present experiments suggest a possibility of raising S in TiS/sub 2/ by a substitution for Ti and by an itercalation to modulate the 3d band and to control the Fermi level.

XPS study on the preferential site occupations of Bi, Sr and Ca ions in Bi2Sr2CaCu2O8+d superconductors prepared by LPE method

Abstract XPS measurements were performed on the Bi 2 Sr 2 CaCu 2 O 8+ d system prepared by the LPE method using melts with compositions Bi:Sr:Ca:Cu = (2 − x ) : 2 : (1 + x ) : 2 for 0 ≤ x ≤ 0.2. The peak decompositions of Sr3d and Ca2p XPS spectra, together with the EPMA results, elucidated the occupancies of Bi, Sr and Ca atoms on the BiO-, SrO- and Ca-layers. The x dependences of the lattice parameter c and the superconducting critical temperature T c are explained by the changes of the excess oxygens in the BiO layer.

Tunneling Spectroscopy of Magnetic Impurities (=Mn,Fe,Co) in ZnO

The first observations have been made on the electronic density of states of Mn, Co and Fe-doped ZnO by tunneling spectroscopy. For Mn- and Co-doped ZnO, rising of DOS were observed around 2.8eV and 3.2eV below the bottom of the conduction band, respectively. In these cases, the Fermi level lay at the bottom of the conduction band. For Fe-doped ZnO, the DOS with about 1.2eV width was observed around the Fermi level in the mid-gap. The result reproduced the recent theoretical calculation.

Thermoelectric Properties of Sintered (MnyCo1-y) Fe2O4

Thermoelectric properties were studied on the sintered cubic spinel (Mn_yCo_1-y)Fe₂O₄ with y = 0, 0.5 and 1, for several sintering conditions. For y = 0 (CoFe₂O₄), the thermoelectric properties were widely changed by the sintering condition, and became less sensitive to the sintering condition with increasing y. The XRF manifested that the low resistivity was closely connected with oxygen deficiency