Shunya Yamamoto

Postgrowth annealing of defects in ZnO studied by positron annihilation, x-ray diffraction, Rutherford backscattering, cathodoluminescence, and Hall measurements

Defects in hydrothermal grown ZnO single crystals are studied as a function of annealing temperature using positron annihilation, x-ray diffraction, Rutherford backscattering, Hall, and cathodoluminescence measurements. Positron lifetime measurements reveal the existence of Zn vacancy related defects in the as-grown state. The positron lifetime decreases upon annealing above 600 °C, which implies the disappearance of Zn vacancy related defects, and then remains constant up to 900 °C. The Rutherford backscattering and x-ray rocking curve measurements show the improvement of crystal quality due to annealing above 600 °C. Although the crystal quality monitored by x-ray diffraction measurements is further improved after annealing at above 1000 °C, the positron lifetime starts to increase. This is due to either the formation of Zn vacancy related defects, or the change of the Zn vacancy charge state occupancy as a result of the Fermi level movement. The electron concentration increases continuously with increa...

Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin films

Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile films was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied field. According to the measurements, the quantum efficiency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap.

Photo-induced surface charge separation in Cr-implanted TiO2 thin film

Abstract Surface charge separation efficiencies of uniformly Cr-distributed rutile grown by pulse laser deposition (PLD) using Cr/TiO 2 multi-targets and rutile with Cr concentration gradient produced by ion implantation with the appropriate post-annealing process were characterized by photo-induced transient charge separation (PITCS) measurement. Cr depth profiles were obtained by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) was used to determine that both surface Cr concentrations were the same. PITCS can be measured without direct contacts and without an externally applied field, and demonstrate the inherent property of charge separation without disturbing spontaneously formed surface band bending. There was a large charge separation for implanted rutile with Cr gradient, even in the visible light region (∼520 nm), which was more remarkable than that of uniform rutile and epitaxial anatase films.

Amorphization in aluminum oxide induced by ion irradiation

Abstract Amorphization in sapphire induced by irradiation has been observed for years but it is still in discussion whether displacements or implanted ions have an important role for the amorphization. In this work, we perform irradiation with high-energy heavy ions in thin film alpha-alumina below room temperature, so that the effects of atomic displacements, especially of damage cascades, can be revealed. Amorphization was detected at temperatures from 90 K to 230 K under irradiation with 600–900 keV xenon and krypton ions, while no amorphization was detected under irradiation with argon nor oxygen ions. The critical energy density to induce amorphization is estimated. Four stages on the crystalline-to-amorphous transformation was observed from the temperature dependence of the dose-to-amorphization. The effect of damage cascades on the irradiation-induced amorphization and defect kinetics resulting in an increase of the dose-to-amorphization will be discussed.

Pattern formation induced by co-deposition of Ni and C60 on MgO(100)

We report on the unusual phenomenon of pattern formation in the thin film of Ni+C60 mixture deposited on the MgO(100) substrate. Under certain deposition kinetics a periodic system of stripe domains was formed. The domains consist of sub-half-micrometer large droplets encompassed with a thin C60—based rind with a possible polymeric structure. The stripes are embedded in the double-layer platform falling into the epitaxial Ni and amorphous-C (a-C) thin films. During co-deposition the C60 molecules partly disintegrate and transform towards a-C. Due to the accumulation of a-C (with limited solubility in Ni) thermodynamic instability arises in the system and reaching a certain level, it triggers spontaneous partitioning (and self-organization) of the deposited material. As a principal mechanism of the pattern formation, a sequential drift and coordinate release of the thermodynamic instability is proposed.

Formation mechanisms for carbon onions and nanocapsules in C+-ion implanted copper

Copper substrates were implanted with carbon ions at temperature ranging from 570 to 973 K. Implantation microstructures were investigated using transmission electron microscopy and high-resolution electron microscopy. Carbon onions and nanocapsules were observed together with amorphous carbon layers. Most of the nanocapsules were found to be hollow and rarely included copper nanoparticles. The encapsulating of Cu nanoparticles with graphene layers, the gradual shrinkage of the encapsulated clusters, and finally the disappearance of the clusters (leaving behind hollow nanocapsules) were observed under electron irradiation at 783 K. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nanocapsules. One process involves the formation of graphene layers on grain boundaries to encapsulate copper particles. The other process is the nucleation of graphene cages, probably fullerenes, due to high co...

Effects of annealing and quenching treatments on reconstruction of rutile thin films on sapphire substrates

Morphology change in rutile TiO2 thin films on sapphire substrates prepared by pulsed laser deposition under reduced oxygen environment was investigated as a function of film thickness, temperature and cooling treatments with atomic force microscopy, X-ray diffraction and scanning electron microscopy equipped with X-ray spectroscopy (SEM/EDX). The deposited TiO2 was determined as epitaxially grown rutile films whose crystallographic correlation with substrates was (1 0 0)rutile//(0 0 0 1)sapphire. As increasing thickness of TiO2 films, smooth surface changed to island structure. In addition, the morphology of TiO2 film on α-Al2O3(0 0 0 1) varied drastically by annealing treatment from 973 to 1123 K. In case of ∼5 nm thickness films, morphology strongly depended on annealing and cooling treatments. We found interesting order structure of TiO2 islands at annealing temperature (∼1073 K) and subsequent quenching (∼1.3 K/s). Formation process of TiO2 particles on α-Al2O3(0 0 0 1) substrates is modeled based on instability of substrate at elevated temperatures.

Charge states of fast heavy ions in glancing collisions with aligned atoms in single crystals

Abstract Spectroscopy of ion-induced secondary electrons emitted from single crystals allows observation of high-energy shadowing which is the initial stage of ion channeling. The high-energy shadowing effect is sensitive to short-range (∼ 0.1 A) screening of the ions nuclear charge by inner-shell electrons and, accordingly, reflects the charge states of the ions moving in the crystal. This article reports recent progress in the study of charge states of MeV/u ions in glancing collisions with crystal atoms.

Energy Spectra of Electrons Induced by MeV Atom Clusters

The first observation of the energy distribution of electrons emitted from solids bombarded by MeV atom clusters is reported. In the backward direction, using graphite and Si bombarded by Cn+ and Aln+ (n ≤8), an appreciable suppression of electron emission has been observed at electron energies lower than ~10 eV. Electron yield per atom decreases with increasing n, and becomes less than 50% at n ≥3, relative to the case of n=1. The experimental results cannot be explained in terms of projectile stopping cross sections nor by the clearing-the-way effect. It is probable that the suppressed electron emission is a result of the suppression of the transport or surface transmission of the produced low-energy electrons, rather than of the suppression of ionization.

RBS and SEM analysis of the nickel-fullerene hybrid systems

Abstract Surprising structural variability of the Ni/C 60 /Ni thin film packaging was observed after annealing at elevated temperatures. The thermal processing of the Ni/C 60 /Ni multilayer led to a gradual disruption of the C 60 layer and to the formation of micron-sized octagonal ‘pits’ and rod-type particles. The unusual thermal response points out the complex physiochemical processes incited in the hybrid system by heat treatment. Here, based on the SEM and RBS analysis, the first insight into the microstructural evolution of the Ni/C 60 /Ni multilayer is presented.