Papers by Author: Naoki Ohashi

Abstract: The effect of the surface preparation in samarium doped semiconducting barium titanate [(Ba_1-xSm_x)TiO₃] ceramics with (Ba, Sm)/Ti ratio of 1.000 was studied by means of isotope tracer technique using a secondary ion mass spectrometer. The surfaces of specimens were prepared with the chemical mechanical polishing (CMP) with colloidal silica slurry or the mechanical polishing (MP) with diamond paste. The oxygen diffusion coefficients obtained in the CMP samples were small compared to those in the mechanical polished samples. This fact suggests that the surface prepared with CMP has less oxygen defect concentration. Moreover, it was also indicated that high temperature treatment over 1000 °C is required for annihilation of defects formed by MP. The oxygen diffusion study used CMP sample brings the useful information on the oxygen defect chemistry in Sm doped BaTiO₃.

Abstract: The a-axis oriented ZnO thin films deposited on sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (D_b) in as-deposited and pre-annealed thin films were determined to be D_b [cm²/ = 9.2x10² exp (- 405 [kJ/mo / RT) and D_b [cm²/ = 1.8x10³ exp (- 418 [kJ/mo / RT), respectively. On basis of these results, the crystal orientation on D_b and the mechanism for oxygen diffusion were discussed.

Abstract: ZnO films with controlled microstructures and crystal orientations were fabricated on nonseeded substrates by a spin-spray method employing tri-sodium citrate as a structure-directing agent at 90°C. The microstructure of the films changed from a rod array to a dense film by addition of the tri-sodium citrate in the solution. The FT-IR spectra of the films prepared with citrate contained organic molecules including carboxyl groups, being attributed to citric ion. Surface analysis by XPS indicated that the adsorbed citric ions were estimated to form zinc-citrate complex including ammonium ion or ammonium citrate.

Abstract: The processes for polishing a ZnO surface were investigated with the aim of establishing a process for obtaining an atomically flat surface with high crystalline quality. The defects in a layer undergoing mechanical polishing were monitored through photoluminescence measurements, and the purity of the polished surface was characterized by SIMS. An atomicallyfishing process.

Abstract: The resistivity of transparent conductive thin film formed by sputtering exhibits a dependence on film thickness. In this study, an analysis by electromagnetic field simulation of resistance effect on the transmission characteristics was carried out. The overall resistance of the signal waveguide governs the transmission characteristics, and variation of the resistance at the interface between the substrate and the thin film has no significant effect. We evaluated a structure in which fine metal wiring that is not visible to the eye is placed on transparent conductors to reduce the resistance of the transmitting waveguide. Electromagnetic field simulations suggest that this structure improves the transmission characteristics while keeping high transparency.

Abstract: To determine the effect of the annealing atmosphere on oxygen diffusion through Ba_0.95La_0.05FeO_3-d pellets, ¹⁸O₂ tracer diffusion and high-resolution secondary ion mapping were performed. When annealing in air, the ¹⁸O concentration around the surface up to a depth of 40 µm was almost constant. On the other hand, when annealing in vacuum, the ¹⁸O concentration obviously decreased. High-resolution secondary ion mapping indicated that the ¹⁸O concentration around the grain boundary was reduced. These results suggested that the grain boundary of BLF annealed in vacuum prevents oxygen diffusion.

Abstract: The relationship between Al and Li during diffusion was studied using Al-implanted ZnO. The Al donor in ZnO acts to increase the concentration of Li contamination from the atmosphere during the annealing. It is difficult to decompose the relationship formed by diffusion between Al and Li during high-temperature annealing. The most effective method to decompose the relationship is to anneal the as-implanted ZnO at a pressure of 5×10-3 torr. This annealing increases the Al solubility limit because the ZnO surface evaporates.

Abstract: In this study, we successfully fabricated the Schottky junctions consisting of Pt electrode and high concentration Nb-doped (0.5 wt%) SrTiO3 (001) single crystal by sputtering process. The carrier concentrations of Nb-0.5wt%-doped SrTiO3 were determined as 1020 /cm3 order by Hall effect measurement. The electrical properties of junctions were investigated by measuring their current-voltage (I-V), capacitance-voltage (C-V) characteristics at temperature range from 80K to 400K. The hysteresis feature was observed that indicating the alteration of barrier height in junctions especially at lower temperature. The donor concentration and built-in potentials calculated from C-2-V data showed large discrepancy from Hall effect measurement indicating that the junctions deviate from the ideal Schottky diode model.

Abstract: The behavior of hydrogen in (Ba,Sr)TiO3 (BST) thin film capacitors under electric fields was investigated by performing secondary ion mass spectroscopy (SIMS) analyses. It was clearly observed that the ingress of atmospheric hydrogen into BST thin film capacitors occurred through the anode and that it diffused toward the cathode under electric fields. In addition, it was found that the deterioration of the I-V properties of the BST thin film capacitors can be interpreted in terms of the distribution of hydrogen concentration in the BST thin films.

Abstract: The relationship between the defect structure and luminescence property of ZnO ceramics implanted with Ar of 2×1015 – 60×1015 ions/cm2 was studied. After annealing, the heavy dose-implanted sample (Ar ≥ 30×1015 ions/cm2) was characterized by a luminescence peak at the 730-nm wavelength. Defects in the implanted region formed voids during post-annealing. Oxygen tracer experiments indicated that grain boundary diffusion in the implanted region was enhanced significantly.