Papers by Author: Takeshi Ohgaki

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: 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: Zinc isotopic heterostructured zinc oxide thin films of 64ZnO/68ZnO/64ZnO were synthesized using pulsed laser deposition. The pulsed laser was first irradiated onto a polycrystalline target of 64ZnO to deposit the 64ZnO layer, then onto the 68ZnO target to prepare the 68ZnO layer and finally, the 64ZnO target was used again. The 64ZnO/68ZnO/64ZnO layered thin film was thus obtained. The thin films were annealed at various diffusion annealing temperatures. Diffusion profiles of the zinc isotopes due to the annealing were evaluated using secondary ion mass spectrometry (SIMS). The diffusion coefficients were slightly higher near the interface between the thin film and the substrate (the inner region) compared to the near surface (the outer region).

Abstract: The defect structure of undoped ZnO and (Zn1-x,Mgx)O solid-solution films were deposited on YSZ substrate with pulsed laser deposition (PLD) to investigate defect equilibria in those films. In particular, the effects of thermal treatment on the structures and prosperities of (Zn1-x,Mgx)O
solid-solution films were examined. The films with high MgO concentration (x>0.12) decomposed to the wurtzite-type and rock-salt-type phase after thermal treatment, indicating that the solubility limit of Mg was about x=0.12 and the wurtzite-type (Zn,Mg)O films with x>0.12 were indicated to be non-equilibrium ones. The subsequent analyses of oxygen diffusivity in those films revealed that the films under non-equilibrium state, i.e., wurtzite-type (Zn1-x,Mgx)O with x>0.12, contained significantly high concentration of anion defects.

Abstract: Gallium nitride (GaN) and indium nitride (InN) films were grown on a zinc oxide (ZnO) single crystalline substrate with a (0001) orientation using molecular beam epitaxy. The interfacial structure and relaxation mechanism of the lattice mismatch at the nitride/oxide interface were investigated, particularly the effects of an (In,Ga)N alloy buffer layer on the interfacial structure of the GaN films. This layer significantly improved the crystallinity of the GaN films by gradually relaxing the lattice mismatch between the GaN and ZnO. In spite of the large lattice mismatch between InN and ZnO, InN films with high crystallinity were grown without an (In,Ga)N buffer layer. Structural analysis revealed that an InN layer with low crystallinity formed spontaneously during the initial growth stage, and this amorphous-like layer likely contributed to relaxation of the interfacial stress caused by the lattice mismatch.

Abstract: The defect structure of undoped and Al-doped ZnO films deposited by pulse laser deposition was investigated to understand the charge compensation mechanism in those films. Particularly, the effect of oxidation assist, i.e., O2 gas or oxygen radicals, on the defect structure of the resultant films was examined. The examination indicated that the defect structure of undoped ZnO was not affected by the oxidation assist, whereas the properties of Al-doped ZnO obviously varied with the method of oxidation assist. An analyses of oxygen diffusion in these films revealed that Al-doping enhanced formation of oxygen defects in Al-doped ZnO.