Yoshihiko Yano

Fabrication of transparent p–n heterojunction thin film diodes based entirely on oxide semiconductors

All oxide-based, transparent polycrystalline p–n heterojunctions on a glass substrate were fabricated. The structure of the diode was n+-ZnO electrode/n-ZnO/p-SrCu2O2/In2−xSnxO3 electrode on the substrate. The contact between the n- and p-type semiconducting oxides was found to be rectifying. The ratio of forward current to the reverse current exceeded 80 within the range of applied voltages of −1.5 to +1.5 V and the estimated diode factor (n value) was 1.62. The diode structure was fabricated on a glass plate with the total thickness of 1.3 μm and possessed an optical transmission of 70%–80% in the visible region.

Growth and characterization of 10-nm-thick c-axis oriented epitaxial PbZr0.25Ti0.75O3 thin films on (100)Si substrate

A 10-nm-thick PbZr0.25Ti0.75O3 thin film is epitaxially grown on a SrRuO3/BaTiO3/ZrO2/Si heterostructure substrate by reactive evaporation. Structural and electrical properties of the film are investigated. It is concluded that the film is ferroelectric and retains a native uniform upward polarization. Artificial downward polarization domains, whose average diameter is 24 nm, can be formed in the film.

Structural Study of Epitaxial BaTiO3 Crystals

Epitaxial crystals of BaTiO 3 with the thicknesses of 67 A, 230 A, 600 A and 1500 A were grown on SrTiO 3 substrates by activated reactive evaporation. The tetragonality of the crystal decreases with increasing thickness and remains in the temperature range of 15 K to 670 K. The epitaxial effect is discussed in terms of a critical thickness, as well as of a surface layer of fine particles.

Applicability of sodium sulfate as a solid electrolyte for a sulfur oxides sensor

Abstract The electromotive force (EMF) has been measured on SO2O2SO3 concentration cells using undoped and 4 at.% yttrium-doped Na2SO4 as solid electrolytes at temperatures between 920 and 1120 K. Two types of electrolytes prepared by sintering and infiltration have proven to work basically well above 973 K. The observed EMFs are consistent with the values calculated from the Nernst equation in the wide concentration range of SOx, showing that the ionic transference number is unity both in the undoped and yttrium-doped Na2SO4. A large difference in SOx pressures between the anode and the cathode leads to the lowering of the EMF because of the permeation of gases through the macroscopic defects such as pores and cracks in the electrolyte. The experimental results suggest that improvement of the sinterability and the stabilization of the high temperature phase are the indispensable problems to solve for the practical application of Na2SO4 as a solid electrolyte to an SOx sensor.

INTERFACE STATES IN ZNO VARISTOR WITH MN, CO, AND CU IMPURITIES

The interface states in ZnO with impurities of transition-metals, Mn, Co, and Cu, were investigated by the DLTS (deep-level transient spectroscopy) measurements in ZnO/PrCoO x /ZnO junctions as model systems of ZnO ceramic varistors and by the SCF-Xα-SW molecular orbital calculations using simplified cluster models. The DLTS signals, correlated to the doping of Mn and Co, are obtained with ZnO/PrCo x /ZnO junctions. The signals correspond to the interface states due to the transition-metal doping. Xα calculations indicate that the interface states attributed to the doping of transition-metals, Mn, Co, and Cu, in ZnO are created between the valence band and the conduction band, which consist of transition-metals 3 d character. The impurities of transition-metals affect interface states as well as the adsorbed excess oxygen.

Ferroelectric phase transition in BaTiO3 films

Abstract Epitaxial BaTiO 3 single crystals were grown on Pt/MgO (Pt for electrodes, MgO for substrates) by means of the activated reactive evaporation technique. X-ray diffraction patterns were measured to clarify the properties such as the temperature dependence of lattice parameters. It was found that the temperature dependence of lattice parameters and integrated intensities of Bragg reflections showed different behavior depending on the film thickness. The relative dielectric constant ϵ r was measured in order to investigate the dielectric properties of the films. Film crystals of BaTiO 3 showed the phase transitions which are different from those in bulk state.

Deep-Level Transient Spectroscopy of Interface States in ZnO/PrCoOx/ZnO Thin-Film Junctions

Deep-level transient spectroscopy is successfully applied to detect interface states in ZnO/PrCoOx/ZnO thin-film junctions. The interface state is measured at 0.70 eV below the conduction-band edge. The junctions are equivalent to a model of a single grain boundary in a ceramic varistor.