Papers by Author: Haruki Ryoken

Abstract: (Mg,Zn)O films were grown on Zn- and O-face ZnO single crystal substrates by pulsed laser deposition. The surface morphologies of the films grown on the Zn- and O-face substrates were quite different, indicating that no domain inversion occurred in both films. The films showed markedly different features for valence band spectra obtained by hard X-ray photoemission spectroscopy. This suggests that the effect of film polarity should be considered in X-ray photoemission spectroscopy.

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: Diffusion behavior at the interface of (001)-epitaxially grown (Ba,Sr)TiO3(BST)/electrode/buffer layer/Si thin films was examined by use of secondary ion-microprobe mass spectrometer (SIMS) and transmission electron microscope (TEM) attached with energy dispersive X-ray fluorescence spectrometer (EDX). As the (001)-epitaxially grown film, following three kinds of structure was grown; (1)BST/(La,Sr)CoO3(LSCO)/CeO2/yttria-stabilized zirconia(YSZ)/Si, (2)BST/PLD-deposited Pt/SrTiO3(ST)/LSCO/CeO2/YSZ/Si and (3)BST/sputter-deposited Pt/ST/LSCO/CeO2/YSZ/Si. For sample (1), uphill diffusion of Sr and Ti was observed at the interface of YSZ and SiO2. Diffusion of Co into CeO2 layer was also detected. These tendencies of diffusion were also observed for samples (2) and (3). In addition to these tendencies, apparent uphill diffusion of Co at the Pt layer was observed for sample (2). However, this diffusion was not observed for sample (3). It was also observed that oxygen diffusion was prevented for sputter-deposited Pt. On the other hand, oxygen diffusion was observed for PLD-deposited Pt.

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.