Housei Akazawa

Nippon Telegraph and Telephone, Edo, Tōkyō, Japan

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Publications (71)141.71 Total impact

  • Housei Akazawa
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    ABSTRACT: By irradiating undoped ZnO and Ga-doped ZnO (GZO) films with electron cyclotron resonance Ar plasma, their electrical and optical properties have been modified. Both the carrier concentration and the Hall mobility increased with continuous irradiation in conductive ZnO films with a defect-rich crystalline lattice. Improved crystallinity was verified by intensifying the ZnO(002) x-ray diffraction peak and increasing the optical transmission level in the visible wavelength. This observation suggests network rearrangement through Zn atoms at interstitials or grain boundaries being oxidized or accommodated into the crystalline network. For insulating ZnO films that contain a sufficient number of oxygen atoms, the changes were better scaled with sheet resistance rather than resistivity. The interference fringes redshifted without lowering the transmittance level. Here, Ar ion impact will create oxygen vacancy donors in the near-surface region or desorb interstitial oxygen atoms that act as donor killers. In contrast, GZO films with resistivities in the 10-4 Ω cm range revealed little changes because there were very few imperfections in the crystalline lattice. The reduced amount of resistivity corresponded to the creation of oxygen vacancies in more resistive GZO films in the 10-3 Ω cm range.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2011; 29(3):031304-031304-8. · 1.43 Impact Factor
  • Housei Akazawa
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    ABSTRACT: The simultaneous sputtering of ZnO and Ga <sub>2</sub> O <sub>3</sub> by electron cyclotron resonance and rf magnetron plasma sources produced Ga-doped ZnO (GZO) films with continuously varying Ga concentration over the substrate surface. Combinatorial evaluation of electrical and optical properties of GZO film grown on silica glass substrate without heater annealing enabled identification of minimum resistivity (0.5 m Ω  cm ) at a Ga <sub>2</sub> O <sub>3</sub> content of 5.5 wt % with an optical transmittance of 90% in the visible wavelength. The monotonically decreasing mobility that was associated with increasing carrier concentration as Ga <sub>2</sub> O <sub>3</sub> content was increased indicated that conduction was governed by ionized impurity scattering. Above the critical Ga <sub>2</sub> O <sub>3</sub> content (6 wt %), carrier concentration decreased since excess Ga atoms that were incorporated beyond the solubility limit at Zn sites hindered large crystalline domains from forming. The ZnO (002) x-ray diffraction peak was suppressed and peaks assigned to Ga <sub>2</sub> O <sub>3</sub> were observed at high Ga <sub>2</sub> O <sub>3</sub> content. The optimum Ga <sub>2</sub> O <sub>3</sub> content shifted to 3.5 wt % at a deposition temperature of 200 ° C and 2.5 wt % at 300 ° C , and the minimum resistivity was further decreased to 0.28 m Ω  cm at 200 °<roman- - >C . However, the resistivities at these elevated temperatures were incredibly high both at the lower and higher side of the optimum Ga <sub>2</sub> O <sub>3</sub> content.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 04/2010; · 1.43 Impact Factor
  • Housei Akazawa
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    ABSTRACT: Pt films directly deposited on SiO2 by electron-cyclotron-resonance (ECR) plasma sputtering and DC-magnetron sputtering have been compared in terms of their performance as electrodes. The DC-magnetron sputtered Pt film consisted of sharply (111) oriented crystallites, which was reflected in hexagonal crystallites observed in atomic force microscopy images. While ECR-sputtered Pt film was also (111) oriented, the X-ray diffraction rocking curve of the (111) peak was broader than that of the DC-magnetron sputtered film. The surface image revealed fine grains, thus having a flatter surface. A scratch test revealed that ECR-sputtered films had an adhesive strength about twice that of DC-magnetron-sputtered films, which was consistent with our tape-test results. Possible reasons for the different adhesion characteristics are discussed.
    Surface & Coatings Technology - SURF COAT TECH. 01/2010; 204(11):1836-1841.
  • Housei Akazawa
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    ABSTRACT: Real-time spectroscopic ellipsometry was applied to the in situ monitoring of soft x-ray excited chemical vapor deposition of boron films on a Si(001) substrate. While B2H6 molecules cannot reactively stick to a Si clean surface below 400 °C, the decomposition of the precursor molecule through electronic excitation enables boron atoms to secure themselves on the surface. After the incubation period had ended, boron islands began to grow at isolated nucleation centers, exhibiting the Volmer–Weber-type growth. The activation energy for creating the nucleation centers was 2.9 kcal/mol, which presumably corresponded to the clustering of deposited boron atoms. The following growth stage was scaled with an activation energy of 10.3 kcal/mol. The thermally activated process corresponded to the networking of surface-terminating hydrides while releasing H2. As further growth continued, boron islands coalesced, which eventually became two-dimensional boron film. The amorphous boron films could be transformed into a crystallite phase through postannealing at 1000 °C and they were characterized in terms of their optical properties.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2010; 28(3). · 1.43 Impact Factor
  • Housei Akazawa
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    ABSTRACT: The irradiation of undoped ZnO films with electron-cyclotron-resonance argon plasma was found to improve its transparent conductive properties. While both carrier concentration and Hall mobility increased with the irradiation time for all ZnO films, reduction in resistivity was more remarkable for films having higher resistivities. Optical transmittance in the visible wavelength continuously improved and the interference fringe for thick films was red shifted. These observations can consistently be explained by assuming that collision-induced desorption or low-energy sputtering by argon ions removed oxygen atoms within ZnO crystallites, which produced electron carriers and enlarged crystallites.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2009; 2.
  • Housei Akazawa
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    ABSTRACT: The crystallographic orientation of epitaxial LiNbO3 films on the Al2O3(1120) substrate can be controlled either toward the c-axis or a-axis depending on the situation at crystallization. Crystallization during high-rate sputter deposition at elevated temperatures produced c-axis-oriented film with 60°-rotated twin domains. Minimizing the surface energy when the growing surface was exposed to abundant ion flux selected (0001) terminated mosaic crystal. In contrast, solid-phase crystallization or as crystallization during a low-rate sputter deposition yielded a-axis-oriented film coexisting with 180° polarization twin domains. The enhanced migration of atoms in low-density amorphous film as well as pseudomorphic lattice matching growth under thermal equilibrated conditions led to the self-organization of relaxed crystalline domains.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2009; 27(1). · 1.43 Impact Factor
  • Housei Akazawa
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    ABSTRACT: The crystallinity of LiNbO3 (LN) films sputtered from LN single crystal target was shown to be better than those from sintered target, confirming that the target quality is an important factor. In the particular case, Li2CO3 precipitated and crystallized to form a c-axis oriented overlayer on the surface of amorphous LiNbO3 film below 400 °C at an early stage of using an as-sintered target. This phenomenon reflects highly mobile nature of Li ions, resulting in phase-separation in the Li2O–Nb2O5 system under the plasma irradiation, which behavior represents Li-based electrolyte.
    Thin Solid Films 01/2009; · 1.87 Impact Factor
  • Housei Akazawa
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    ABSTRACT: The electrical and optical properties of undoped ZnO films deposited by electron cyclotron resonance (ECR) plasma sputtering at room temperature were characterized. The lowest resistivity we achieved was 2.6×10−3 Ωcm with optical transmittance at visible wavelengths higher than 85%. The X-ray diffraction (002) peak was weak and the rocking curve was asymmetrical, indicating that oxygen vacancies prevented large crystalline domains from forming. At low argon-sputtering-gas pressure, carrier concentration and Hall mobility increased with increasing argon pressure. When the optimum pressure (40 mPa) was exceeded, however, Hall mobility and optical transmittance were severely reduced, which indicated that excess Zn atoms were populated at the interstitials of the network. Admitting only 0.67 mPa of O2 gas during deposition deteriorated resistivity over 1 MΩcm due to high excitation efficiency in the ECR plasma. Deposition under a higher magnetic field produced lower resistivities.
    Thin Solid Films 01/2009; 518(1):22-26. · 1.87 Impact Factor
  • Housei Akazawa, Masaru Shimada
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    ABSTRACT: Distinct channels for relaxing strain were identified in epitaxial LiNbO thin films depending on whether the film was crystallized during or after sputter-deposition on an AlO(0001) substrate. When LiNbO continuously crystallizes during deposition, pseudomorphic growth continues with a build up of strain as the thickness increases. The primary process for relieving strain is achieved by dividing the crystal into small mosaic grains. Crystallization by postannealing amorphous films, in contrast, produces nearly dislocation-free, relaxed crystals with a wider domain of 100-200 nm. Introducing lattice disorder at antiphase positions that correspond to matching between 12 lattice units of LiNbO and 13 lattice units of AlO in the a-direction limits the chemical interaction with the AlO lattice within a short range and efficiently accommodates misfit strain. This results in atomic-scale grapho-epitaxy sustaining disordered lattice regions at the interface.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2008; 26(2):281-287. · 1.43 Impact Factor
  • H. Akazawa
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    ABSTRACT: Electric transport in sputtered LiNbO<sub>3</sub> (LN) films sandwiched between TiN and Si electrodes revealed Schottky emission at low electric field and Poole-Frenkel conduction at higher electric field without any hysteresis. The Poole-Frenkel conduction region was brought into the reproducible potential barrier-controlled conduction region by inserting a thin SiO<sub>2</sub> layer between LN and TiN, where current hysteresis appeared. Trapping of electrons at SiO<sub>2</sub>/LN interface states when TiN is negatively biased produces the highest potential barrier, resulting in a high resistance state, whereas releasing the trapped electrons at positive bias leads to a low resistant state. When electric contact to the TiN electrode was taken using Ag paste, another current hysteresis resulting from the connection and disconnection of Ag filaments was observed after time-dependent soft dielectric breakdown.
    Applications of Ferroelectrics, 2007. ISAF 2007. Sixteenth IEEE International Symposium on; 06/2007
  • H. Akazawa, M. Shimada
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    ABSTRACT: The kinetics of second phase formation during the thermal annealing of crystalline and amorphous LiNbO3(LN) thin films was investigated. When c-axis-oriented LN texture film was annealed at temperatures higher than 700 °C, Li2O desorbed from the surface and grain boundaries, which induced epitaxial precipitation of the LiNb3O8phase on the LN (0001) plane. While the LiNb3O8volume exhibited saturation behavior against annealing time, oxygen atoms continued to diffuse out from the inner part of the film. Arrhenius plots of the rate of LiNb3O8formation and oxygen loss were scaled with common activation energy of 30.5 kcal/mol. In terms of phase formation from amorphous LN, annealing at 700 °C in a vacuum induced rapid crystallization into mixtures of LN and LiNb3O8, the volume ratio of which can be explained by the phase separation model. Annealing in an O2atmosphere, however, produced an incubation time before crystallization and a larger volume fraction of LiNb3O8after crystallization. Oxygen molecules that penetrated into the film will hinder the migration of atoms and decelerate crystallization. Vacancies produced as a result of Li2O loss will facilitate the nucleation of LN and LiNb3O8pairs while maintaining the epitaxial relation.
    Journal of Materials Research. 05/2007; 22(06):1726 - 1736.
  • Housei Akazawa, Masaru Shimada
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    ABSTRACT: LiNbO3 (LN) films deposited on SiO2 substrate by electron cyclotron resonance plasma sputtering and thermal treatment were structurally and optically characterized. Birefringence between 60 and 70% for single crystal was confirmed for c-axis oriented films on both Si and SiO2, whereas non-annealed amorphous film was optically isotropic. Crystallization during sputtering produced less than a 10% volume fraction of c-axis oriented spherical domains associated with the interface roughness layer. This sample had a large propagation loss of 32 dB/cm at 632.8 nm due to light scattering at the grain boundaries. LN films subjected to solid phase crystallization in a vacuum produced highly c-axis oriented columnar texture domains, which yielded propagation losses between 5 and 19 dB/cm, depending on the thickness (0.64-1.35 mum) and the mode order. The bulk and surface origins for waveguiding losses are discussed based on the film structure.
    Japanese Journal of Applied Physics 01/2007; 46:1543-1548. · 1.07 Impact Factor
  • Housei Akazawa
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    ABSTRACT: We present a system in which the choice of electrode material drastically affects the resistance switching mechanism. LiNbO3 films sandwiched between TiN and Si electrodes with a thin SiO2 interlayer revealed reproducible potential barrier-controlled current hysteresis. Trapping electrons at SiO2/LiNbO3 interface states when TiN is negatively biased produces the highest potential barrier, resulting in a high resistance state, whereas releasing the trapped electrons at positive bias leads to a low resistant state. When electric contact to the TiN electrode was established using Ag paste, in contrast, conductive-bridge-type current hysteresis with six orders of magnitude higher current density was observed.
    Japanese Journal of Applied Physics 01/2007; 46. · 1.07 Impact Factor
  • Housei Akazawa
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    ABSTRACT: When the core electronic states of atoms in solids are excited by intense high-energy photons (hν>100 eV), a situation where the excited atoms are at energetically high levels while the surroundings are frozen is realized. Irradiation by a synchrotron radiation beam places a material in such a state, which leads to extensive movement of the constituent atoms. In the first half of this paper, results on how synchrotron radiation illumination causes the modification of several silicon-based materials, i.e., a-SiO2, a-Si3N4, a-SiNx:H, a-Si:H, and a-Si are described. A means for distinction between purely photolytic and thermally assisted mechanisms is presented. Factors affecting the consequence include chemical potentials in the network's topological structure, ionicity of the chemical bonds, and types and numbers of vacancies contained in the network. In the second half, synchrotron radiation-excited epitaxy of short-period Si/Si1−xGex multiple quantum wells and SinGem strained-layer superlattices is demonstrated. The atomically abrupt interfaces, similar to those obtained by solid-source molecular beam epitaxy, and the atomic-scale controllability of thickness are illustrated by real-time characterization of structural parameters through spectroscopic ellipsometry.
    Thin Solid Films 01/2007; 515(11):4670-4680. · 1.87 Impact Factor
  • H. Akazawa, M. Shimada
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    ABSTRACT: We clarified critical factors affecting the crystallographic orientation of LiNbO3 (LN) thin films deposited by electron cyclotron resonance plasma sputtering on transparent conductive oxides, i.e., TiN and indium tin oxide (ITO). When LN films were crystallized during sputtering on amorphous TiN, insufficient crystallization (400 °C), a roughened TiN surface by crystallization (460 and 530 °C), and oxidization of TiN (600 °C) interfered with the production of c-axis oriented textured film. Solid-phase crystallization of amorphous LN film on amorphous TiN through rapid thermal annealing provided a solution to disorientation factors. Because of the low crystallization temperature of ITO, both crystallization during sputtering and solid-phase crystallization by postannealing yielded polycrystalline LN for LN film on amorphous ITO, but with a substantial number of c-axis oriented domains. For both amorphous LN film on polycrystalline TiN and polycrystalline ITO, however, nucleation assisted by seeds at the interface resulted in a completely random orientation.
    Journal of Applied Physics 06/2006; 99(12):124103-124103-8. · 2.21 Impact Factor
  • Housei Akazawa
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    ABSTRACT: The kinetics of roughening and smoothing in Ge(001) surface morphology during synchrotron-radiation-excited GeH4-source homoepitaxy has been investigated. At high GeH4 pressures and low substrate temperatures, two-dimensionally flat surface morphology is maintained while passivating the epitaxially growing surface with a Ge hydride layer. After exceeding the critical temperature (330 °C), the delivery of GeHx species to the partially dangling bond terminated surface and the accelerated migration of Ge atoms result in roughening from the initial stage of growth, the onset of which can be detected in terms of a small loop in real-time ψ-Δ trajectories of ellipsometric monitoring. The roughening tendency is enhanced with increasing synchroton radiation beam intensity and substrate temperature and with decreasing GeH4 pressure. Submicrometer tall islands consisting of a (001) top surface surrounded by (113) sidewalls are produced. Roughening is terminated when all dangling bonds are tied up with H atoms, i.e., when the surface free energy is minimized. Then, preferential embedding into valleys between islands yields sinusoidal-like ripple morphology, which is eventually converted into a flat Ge epilayer with no inner defects.
    Journal of Applied Physics 05/2006; 99(10):103505-103505-9. · 2.21 Impact Factor
  • H. Akazawa, M. Shimada
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    ABSTRACT: The pyrochlore (LiNb3O8) phase in lithium niobate crystal does not contribute to ferroelectric or electro-optic properties. We investigated a way to suppress the precipitation of this phase that was formed by thermally annealing amorphous LiNbO3 films. The volume fraction of LiNb3O8 with respect to LiNbO3 becomes higher during longer annealing periods and at higher annealing temperatures because of the evaporation of Li2O units from the surface. Annealing in an O2 atmosphere promotes the formation of the pyrochlore phase more than in a vacuum. We propose rapid thermal annealing of oxygen-rich LiNbO3 film in a vacuum to provide the best solution to obtaining transparent, single LiNbO3 phase film.
    physica status solidi (a) 01/2006; 203(11):2823-2827. · 1.21 Impact Factor
  • H. Akazawa, M. Shimada
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    ABSTRACT: We investigated the orientation of domains in LiNbO3 (LN) thin films grown by electron–cyclotron resonance plasma sputtering on TiN films with various crystalline states. Deposition at 400°C on an amorphous TiN produced partially crystallized and apparently c-axis-oriented LN. When TiN crystallized at 460°C to become polycrystalline grains, the roughened surface randomized the orientation of LN. At 600°C, the reaction of TiN with oxygen atoms supplied from the plasma created a TiOx layer. Rapid thermal annealing of amorphous LN films at 460°C was the best solution for removing these disorientation factors, but annealing of amorphous LN on poly-crystalline TiN yielded no c-axis-oriented domains.
    Vacuum. 01/2006; 80(7):704-707.
  • Housei Akazawa, Masaru Shimada
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    ABSTRACT: We present a simple method to determine the linear electro-optic coefficient of thin films with spectroscopic ellipsometry. This technique allows noncontact and nondestructive measurement of the as-grown state of films without the need to fabricate a waveguide. The principle behind this is that the electro-optic effect is regarded as perturbation to the optical parameters. Comparing differences in tan Ψ spectra in terms of applied voltage and wavelength yields a change in the effective wavelength of light propagating through a dielectric medium, which is equivalent to electro-optic-induced phase shift. We demonstrate the feasibility of this procedure to determine the Pockels coefficient (r33) of a preferentially c-axis-oriented LiNbO3 film on a Si(100) substrate. Dispersion of effective r33 values at wavelengths between 0.3 and 0.8 μm was obtained.
    Journal of Applied Physics 11/2005; 98(11):113501-113501-5. · 2.21 Impact Factor
  • Housei Akazawa, Masaru Shimada
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    ABSTRACT: We developed a spectroscopic–ellipsometric approach to evaluate the electro-optic coefficient of highly c-axis oriented LiNbO3 films on an Si(100) substrate grown by electron cyclotron resonance plasma sputtering. Applying an electric field between the TiN transparent top electrode and Si substrate, the interference fringe appearing in the tanΨ spectrum was slightly modulated by phase retardation in the wavelength domain. The change in effective wavelength was converted to refractive index change, yielding dispersion in the Pockels coefficient (r33) between 0.3 and 0.8μm. At 633nm, we obtained an r33 that was 57% of the bulk LN crystal value.
    Materials Science and Engineering B-advanced Functional Solid-state Materials. 01/2005; 120(1):50-54.