Papers by Author: Naoki Ohashi

Abstract: Diffusion behaviors of aluminum (Al) in zinc-oxide (ZnO) single crystals were measured by means of ion implantation technique and SIMS depth profiling. It was found that Al concentration profile had a peculiar shape with a constant-concentration region and a steep tail, which was also found in profiles of other donors such as Ga and In. Detailed analysis of the profiles revealed that the diffusivity of Al was proportional to the square of Al concentration and its intrinsic diffusivity was much smaller than previously reported values. Oxygen diffusion experiments were also performed and the implantation of Al ions enhanced the oxygen diffusion coefficients by about 20 times. This result indicates that oxygen interstitial diffusion occurs in n-type ZnO.

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 relationship between the distribution of hydrogen and the electrical properties of (Ba, Sr)TiO3 (BST) thin film capacitors was investigated using secondary ion mass spectroscopy (SIMS) analyses. It has been clearly shown that there is a close relationship between the hydrogen distribution in BST thin film and the frequency dependence of the complex impedance of the BST thin film capacitors. It was confirmed that protons, the interstitial hydrogen impurity dissolved in BST thin films annealed at 400oC, were produced by a thermal equilibrium reaction between the hydrogen in the annealing atmosphere and the BST thin films.

Abstract: The effect of ion implantation leading to contamination and diffusion of lithium impurity in ZnO ceramics substrates was investigated. The diffusion coefficients of Li in the implanted ZnO annealed at 1000 and 850°C were in good agreement with those in the non-implanted ZnO. At 700°C, Li diffusion in the implanted ZnO was strongly enhanced. Our results show that the defects introduced by the implantation enhance the impurity diffusion at low temperature annealing.

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: We have obtained Er-doped ZnO thin film in a micropattern of reverse trapezoids processed on Si substrate by sputtering and ultrafine polishing techniques. Near-infrared light emission was detected successfully from the thin film filling a single micropit with 10 μm square. Transmission electron microscopy (TEM) observation showed epitaxial growth of ZnO crystals along the curvature of the micropit.

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: Fine powder of CaZrO3 (CZ) was synthesized by using ultra-fine and highly dispersed CaCO3 as a precursor. The results showed that the calcination temperature could be reduced to inhibit grain growth of CZ. Mean grain size of the CZ fine powder was 80 nm. Moreover, the CZ powder was used to synthesize a solid solution of (Ba,Ca)(Zr,Ti)O3 (BCZT) to examine the effect of the powder’s grain size on the reactivity of BT and CZ. It was found that the fine CZ powder enhanced the BT and CZ reaction.

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.