Housei Akazawa

Nippon Telegraph and Telephone, Edo, Tokyo, Japan

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Publications (104)215.85 Total impact

  • Housei Akazawa
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    ABSTRACT: The crystal phases and orientations of Ga2O3 thin films were investigated by RF magnetron sputtering on sapphire C-planes, sapphire A-planes, and Si(100) substrates either in an O2 or H2O vapor ambient. When deposited under O2 gas flow, (2¯01)-oriented β-Ga2O3 grew on sapphire C-planes at 300 °C for both as-crystallization and solid-phase crystallization. In contrast, (110)-oriented α-Ga2O3 grew on sapphire A-planes above 600 °C. Randomly-oriented polycrystalline β-Ga2O3 was formed on Si(100) substrates. These experimental results indicate that while a high temperature is needed to form β-Ga2O3, the activation barrier of nucleation is greatly reduced by hexagonally packed oxygen atoms on sapphire C-planes. Nucleation and/or crystallization on sapphire A-planes require higher temperatures probably because of sparse surface atoms. Nevertheless, pseudomorphic growth of α-Ga2O3 having the same corundum structure is selected due to the template effect of the substrate at elevated temperatures. Deposition with H2O severely decelerated crystallization and promoted nucleation of γ-Ga2O3. Preferentially (311)-oriented γ-Ga2O3 was obtained on sapphire C-planes. While pseudomorphic growth of α-Ga2O3 occurred on sapphire A-planes above 800 °C, small γ-Ga2O3 crystallites coexisted with them. Conversion between the phases was not observed even at 800 °C, reflecting a high activation barrier for rearrangement of atoms that constitute crystalline lattices.
    No preview · Article · Jan 2016 · Vacuum
  • Housei Akazawa · Yuko Ueno
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    ABSTRACT: The wettability of hydroxyapatite (HAp) films can be controlled by process conditions and substrates. C-plane-terminated HAp films on Si(100) substrate were hydrophobic. Only when sputter deposition with Ar was carried out with H2O vapor, followed by solid-phase crystallization at 500-700 °C, did highly hydrophilic films with water contact angles between 10 and 20° form. The hydrophilic property was characterized by H2O bending mode signals at 1595 cm-1 in infrared spectroscopy. Preferentially c- or (310)-plane-oriented epitaxial HAp films on sapphire c-planes were hydrophobic, whereas randomly oriented polycrystalline HAp films on sapphire a-planes were moderately hydrophilic.
    No preview · Article · Oct 2015 · Applied Physics Express
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    Housei Akazawa · Hiroshi Fukuda
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    ABSTRACT: We produced slab waveguides consisting of a LiNbO3 (LN) core layer that was sandwiched with Al-doped ZnO cladding layers. The ZnO/LN/ZnO stacked layers were grown on sapphire C-planes by electron cyclotron resonance (ECR) plasma sputtering and were subjected to structural, electrical, and optical characterizations. X-ray diffraction confirmed that the ZnO and LN layers were epitaxial without containing misoriented crystallites. The presence of 60°-rotational variants of ZnO and LN crystalline domains were identified from X-ray pole figures. Cross-sectional transmission electron microscopy images revealed a c-axis orientated columnar texture for LN crystals, which ensured operation as electro-optic sensors based on optical anisotropy along longitudinal and transversal directions. The interfacial roughness between the LN core and ZnO bottom layers as well as that between the ZnO top and the LN core layers was less than 20 nm, which agreed with surface images observed with atomic force microscopy. Outgrowth of triangular LN crystalline domains produced large roughness at the LN film surface. The RMS roughness of the LN film surface was twice that of the same structure grown on sapphire A-planes. Vertical optical transmittance of the stacked films was higher than 85% within the visible and infrared wavelength range. Following the approach adopted by Teng and Man [Appl. Phys. Lett. 56, 1734 (1990)], ac Pockels coefficients of r33 = 24-28 pm/V were derived for c-axis oriented LN films grown on low-resistive Si substrates. Light propagation within a ZnO/LN/ZnO slab waveguide as well as within a ZnO single layer waveguide was confirmed. The birefringence of these waveguides was 0.11 for the former and 0.05 for the latter.
    Preview · Article · May 2015 · AIP Advances
  • Housei Akazawa · Hiroyuki Shinojima
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    ABSTRACT: We identified prerequisite conditions to obtain intense photoluminescence at 1.54 μm from Er3+ ions doped in ZnO host crystals. The epitaxial ZnO:Er films were grown on sapphire C-plane substrates by sputtering, and Er3+ ions were resonantly excited at a wavelength of 532 nm between energy levels of 4I15/2 and 2H11/2. There is a threshold deposition temperature between 500 and 550 °C, above which epitaxial ZnO films become free of miss-oriented domains. In this case, Er3+ ions are outside ZnO crystallites, having the same c-axis lattice parameters as those of undoped ZnO crystals. The improved crystallinity was correlated with enhanced emissions peaking at 1538 nm. Further elevating the deposition temperature up to 650 °C generated cracks in ZnO crystals to relax the lattice mismatch strains, and the emission intensities from cracked regions were three times as large as those from smooth regions. These results can be consistently explained if we assume that emission-active Er3+ ions are those existing at grain boundaries and bonded to single-crystalline ZnO crystallites. In contrast, ZnO:Er films deposited on a ZnO buffer layer exhibited very weak emissions because of their degraded crystallinity when most Er3+ ions were accommodated into ZnO crystals. Optimizing the degree of oxidization of ZnO crystals is another important factor because reduced films suffer from non-radiative decay of excited states. The optimum Er content to obtain intense emissions was between 2 and 4 at. %. When 4 at. % was exceeded, the emission intensity was severely attenuated because of concentration quenching as well as the degradation in crystallinity. Precipitation of Er2O3 crystals was clearly observed at 22 at. % for films deposited above 650 °C. Minimizing the number of defects and impurities in ZnO crystals prevents energy dissipation, thus exclusively utilizing the excitation energy to emissions from Er3+ ions.
    No preview · Article · Apr 2015 · Journal of Applied Physics
  • Housei Akazawa · Yuko Ueno
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    ABSTRACT: We investigated the crystallinity, orientation, and stability of hydroxyapatite (HAp) thin films on structural materials including SiO2 and metals (Ru, Ti, and Pt). The HAp films were deposited by electron cyclotron resonance (ECR) plasma sputtering with an Xe sputtering gas under simultaneous flow of H2O vapor. The HAp films as-crystallized during deposition and solid-phase crystallized by post annealing were analyzed with X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman scattering. The lowest solid-phase crystallization temperatures were 350 °C on Ru, 300 °C on Ti, and 300 °C on Pt. These crystallization temperatures are much lower than those on silicon and SiO2 substrates (550 °C). For HAp films as-crystallized at temperatures above 400 °C, the elements out diffused and reacted with substrates. Precipitation of CaO was observed for HAp films deposited on SiO2 and Ru substrates. Diffusion of PO43 − into SiO2 and Ru-catalyzed decomposition of HAp crystals are suggested for the mechanism. When deposited on Pt substrates, CaPt2O4 alloy byproducts were created for solid-phase crystallized films and CaPt2 alloy byproducts were created for as-crystallized films, indicating reaction of Ca atoms with Pt substrates.
    No preview · Article · Feb 2015 · Surface and Coatings Technology
  • Housei Akazawa · Hiroyuki Shinojima
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    ABSTRACT: We investigated how the crystallinity of ZnO host films deposited on Si(1 0 0) substrates by electron cyclotron resonance (ECR) plasma sputtering affects the optical activity of Er3+ ions doped in them under resonant excitation with 532-nm laser light. Photoluminescence from ZnO:Er films deposited at room temperature (RT) exhibited a main emission peak at 1538 nm with a sub-peak at 1553 nm. The emission intensity reached a steady state at Er content between 0.6 and 3 at.%, and attenuated above 3 at.%. This concentration quenching correlates with derivation of X-ray diffraction angles from the Vegard's law. Deposition at RT produced non-equilibrium state of ZnO:Er films possibly stabilized with vacancies to maintain charge neutrality. After post annealing of ZnO:Er films with Er content less than 1 at.%, the emission intensities were markedly reduced as the crystalline lattice was rearranged. For films in the concentration quenching range, however, post annealing increased the emission peak width while maintaining emission intensity, indicating that Er3+ ions migrated and populated various emission-active sites. In this case, the main peak wavelength blue shifted to 1536 nm and produced a new sub peak at 1546 nm. When the deposition temperature exceeded 500 °C, emissions from Er3+ ions were greatly enhanced. This will result from improvement in crystallinity and fixing Er3+ ions at six-fold coordinated C4v symmetry sites.
    No preview · Article · Nov 2014 · Materials Science and Engineering B
  • Housei Akazawa · Yuko Ueno
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    ABSTRACT: We report how the crystallinity and orientation of hydroxyapatite (HAp) films deposited on sapphire substrates depend on the crystallographic planes. Both solid-phase crystallization of amorphous HAp films and crystallization during sputter deposition at elevated temperatures were examined. The low-temperature epitaxial phase on C -plane sapphire substrates has c -axis orientated HAp crystals regardless of the crystallization route, whereas the preferred orientation switches to the (310) direction at higher temperatures. Only the symmetric stretching mode (ν 1) of PO43− units appears in the Raman scattering spectra, confirming well-ordered crystalline domains. In contrast, HAp crystals grown on A -plane sapphire substrates are always oriented toward random orientations. Exhibiting all vibrational modes (ν 1, ν 3, and ν 4) of PO43− units in the Raman scattering spectra reflects random orientation, violating the Raman selection rule. If we assume that Raman intensities of PO43− units represent the crystallinity of HAp films, crystallization terminating the surface with the C-plane is hindered by the presence of excess H2O and OH species in the film, whereas crystallization at random orientations on the A-plane sapphire is rather promoted by these species. Such contrasting behaviors between C-plane and A-plane substrates will reflect surface-plane dependent creation of crystalline seeds and eventually determine the orientation of resulting HAp films.
    No preview · Article · Oct 2014 · Journal of Crystal Growth
  • Housei Akazawa
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    ABSTRACT: The manner in which hydrogen atoms contribute to the electric conduction of undoped ZnO and Ga-doped ZnO (GZO) films was investigated. Hydrogen atoms were permeated into these films through annealing in an atmospheric H-2 ambient. Because the creation of hydrogen donors competes with the thermal annihilation of native donors at elevated temperatures, improvements to electric conduction from the initial state can be observed when insulating ZnO films are used as samples. While the resistivity of conductive ZnO films increases when annealing them in a vacuum, the degree of increase is mitigated when they are annealed in H-2. Hydrogenation of ZnO crystals was evidenced by the appearance of OH absorption signals around a wavelength of 2700 nm in the optical transmittance spectra. The lowest resistivity that was achieved by H-2 annealing was limited to 1-2 x 10(-2) Omega cm, which is one order of magnitude higher than that by native donors (2-3 x 10(-3) Omega cm). Hence, all native donors are converted to hydrogen donors. In contrast, GZO films that have resistivities yet to be improved become more conductive after annealing in H-2 ambient, which is in the opposite direction of GZO films that become more resistive after vacuum annealing. Hydrogen atoms incorporated into GZO crystals should assist in reactivating Ga3+ donors. (C) 2014 American Vacuum Society.
    No preview · Article · Sep 2014 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
  • Housei Akazawa
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    ABSTRACT: Li2O-Nb2O5 ternary compound thin films were deposited by dual sputtering from LiNbO3 and Li2O targets to cover a wide range of Li/Nb molar ratios. When all parts of the films were crystallized during sputtering at elevated temperatures, pyrochlore phase crystallites of Li3NbO4, LiNbO3, and LiNb3O8 were preferentially oriented toward their respective (222), (006), and (− 602) directions. These closely packed crystallographic planes having the lowest surface free energies were initially created under exposure to the plasma and the crystallization that followed proceeded exclusively on these seed planes. The volume fraction of each crystalline component as a function of the Li/Nb molar ratio obeyed the established phase diagram. In contrast, solid-phase crystallization of pre-deposited amorphous films through post annealing exhibited a non-equilibrium nature, and the volume fraction of each crystalline component depended on the crystallization rate rather than the Li/Nb molar ratio. Solid-phase crystallization decelerated when excess Li2O was contained in the film. Specifically, LiNbO3 crystallites with near-stoichiometric compositions were randomly oriented without the operating mechanism of preferential orientation toward c-planes. In certain situations, crystalline domains were spatially of inhomogeneous composition, resulting in the simultaneous coexistence of Li3NbO4, LiNbO3, and LiNb3O8.
    No preview · Article · Apr 2014 · Thin Solid Films
  • Housei Akazawa
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    ABSTRACT: This study evaluated the long-term stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films that had been left in an atmospheric ambient environment for 5 to 6 yr. When ZnO films are stored in a clean room with a controlled temperature and humidity of 23 °C and 45%, respectively, throughout the year, the increases in sheet resistance are less than 5% of their initial value. The ZnO films stored in a non-air-conditioned laboratory room, whose temperature varies between 5 and 35 °C and humidity varies between 30% and 70% per year, suffer from increases in the sheet resistance by almost 13%, which is associated with a slight rise in the near-infrared transmittance level. Postannealing of these degraded ZnO films at 150–200 °C recovers the initial conductance by removing the H2O molecules that have penetrated the film. One hour of irradiation with electron cyclotron resonance Ar plasma effectively restores the conductive surfaces while maintaining a temperature below 70 °C. The GZO films containing a few weight percent of Ga2O3 are stable even when stored in a non-air-conditioned laboratory room, with changes in the sheet resistance of less than 3%. The GZO films with a Ga2O3 content larger than 10 wt. %, however, exhibit serious degradation probably due to the strong affinity of segregated Ga2O3 domains with H2O vapor molecules. Neither postannealing nor Ar plasma irradiation can recover the initial sheet resistance of these GZO films.
    No preview · Article · Mar 2014 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
  • Housei Akazawa
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    ABSTRACT: Transparent conductive ZnO and Ga-doped ZnO (GZO) films were deposited on polyethylene naphtalate (PEN) sheet substrates using electron cyclotron resonance plasma sputtering. Both ZnO and GZO films were highly adhesive to the PEN substrates without inserting an intermediate layer in the interface. When compared at the same thickness, the transparent conductive properties of GZO films on PEN substrates were only slightly inferior to those on glass substrates. However, the carrier concentration of ZnO films on PEN substrates was 1.5 times that of those on glass substrates, whereas their Hall mobility was only 60% at a thickness of 300 nm. The depth profile of elements measured by secondary ion mass spectroscopy revealed the diffusion of hydrocarbons out of the PEN substrate into the ZnO film. Hence, doped carbons may act as donors to enhance carrier concentration, and the intermixing of elements at the interface may deteriorate the crystallinity, resulting in the lower Hall mobility. When the ZnO films were thicker than 400 nm, cracks became prevalent because of the lattice mismatch strain between the film and the substrate, whereas GZO films were free of cracks. The authors investigated how rolling the films around a cylindrical pipe surface affected their conductive properties. Degraded conductivity occurred at a threshold pipe radius of 10 mm when tensile stress was applied to the film, but it occurred at a pipe radius of 5 mm when compressive stress was applied. These values are guidelines for bending actual devices fabricated on PEN substrates.
    No preview · Article · Mar 2014 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
  • Housei Akazawa · Yuko Ueno
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    ABSTRACT: Hydroxyapatite (HAp) films were deposited by electron cyclotron resonance plasma sputtering under a simultaneous flow of H2O vapor gas. Crystallization during sputter-deposition at elevated temperatures and solid-phase crystallization of amorphous films were compared in terms of film properties. When HAp films were deposited with Ar sputtering gas at temperatures above 460 °C, CaO byproducts precipitated with HAp crystallites. Using Xe instead of Ar resolved the compositional problem, yielding a single HAp phase. Preferentially c-axis-oriented HAp films were obtained at substrate temperatures between 460 and 500 °C and H2O pressures higher than 1×10−2 Pa. The absorption signal of the asymmetric stretching mode of the PO43− unit (ν3) in the Fourier-transform infrared absorption (FT-IR) spectra was the narrowest for films as-crystallized during deposition with Xe, but widest for solid-phase crystallized films. While the symmetric stretching mode of PO43− (ν1) is theoretically IR-inactive, this signal emerged in the FT-IR spectra of solid-phase crystallized films, but was absent for as-crystallized films, indicating superior crystallinity for the latter. The Raman scattering signal corresponding to ν1 PO43− sensitively reflected this crystallinity. The surface hardness of as-crystallized films evaluated by a pencil hardness test was higher than that of solid-phase crystallized films.
    No preview · Article · Jan 2014 · Journal of Physics and Chemistry of Solids
  • Housei Akazawa · Hiroyuki Shinojima
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    ABSTRACT: We identified the conditions necessary for photoluminescence from Eu3+ ions doped in ZnO thin films to occur via indirect excitation of ZnO host crystal with a 325-nm laser light. The ZnO:Eu films were sputter-deposited on Si(100) substrates at room temperature with H2O vapor flowing as the oxygen source gas. A very narrow emission peak at 612 nm, assigned to 5D0-7F2 transition of Eu3+ ions, appeared after annealing at 200–300 °C in a vacuum or at 500 °C in an O2 ambient. At temperatures higher than these, the 612-nm emission peak attenuated and emission spectra exhibited weaker double peaks at 612 and 620 nm. This observation suggested that most Eu3+ ions withdrew from the most emission-active sites, and occupied two distinct chemical sites. With further increasing temperature, the double peaks merged to become broader, indicating that Eu3+ ions occupied sites more randomly. Emission intensity was the highest at an Eu content of 0.9 at. % and concentration quenching was observed as Eu content was increased. Substitution of Zn2+ cation sites with Eu3+ ions was confirmed from the primary X-ray diffraction peaks of ZnO(002); the ZnO lattice expanded as larger numbers of Eu3+ ions were incorporated. After the films were post-annealed at high temperatures, the diffraction angles approached those of undoped ZnO crystals, which reflected shifting Eu3+ ions into grain boundaries. The photoabsorption of OH species within a wavelength range of 2700–3000 nm and the Eu3+ emission intensity revealed correlated behavior. These results confirmed that the presence of OH and/or H species in the film is necessary for emissions from Eu3+ ions to appear. When OH and/or H species were diminished after high-temperature annealing, Eu3+ - ons existing at substitutional sites became unstable and their emissions were consequently deactivated. The origin of emission-active Eu3+ ions can be explained in terms of either the defect mediated energy transfer model or the substitutional model.
    No preview · Article · Oct 2013 · Journal of Applied Physics
  • Housei Akazawa · Hiroyuki Shinojima
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    ABSTRACT: We found that ZnO:Eu films sputter-deposited with H2O vapor gas produce intense photoluminescence from Eu3+ ions through excitation of ZnO host crystals with a 325-nm laser light, whereas those deposited with O2 gas do not. At optimum annealing temperatures of 300--500 °C, the primary luminescence from Eu3+ ions appeared at 612 nm, which was much stronger than the near-band edge and defect emissions from ZnO. After annealing at higher temperatures, the 612-nm peak attenuated, and two emission lines at 612 and 620 nm, corresponding to two distinct chemical sites, had comparable intensities. These observations suggest that incorporating H+ and/or OH- species in ZnO crystals are prerequisite for emissions from Eu3+ ions to occur. Characterization with infrared absorption spectroscopy and X-ray diffraction suggested that Eu3+ ions substitute Zn2+ sites when OH- (H+) species are contained in ZnO crystals. The role of H+ and/or OH- species may be either charge compensation when substituting Zn2+ sites with Eu3+ ions or creation of trapping centers of excited energies that mediate efficient energy transfer from ZnO to Eu3+ ions.
    No preview · Article · Jul 2013 · Japanese Journal of Applied Physics
  • Housei Akazawa · Yuko Ueno
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    ABSTRACT: We deposited hydroxyapatite (HAp) thin films on Si(1 0 0) substrates by means of electron cyclotron resonance (ECR) plasma sputtering from a HAp target and characterized their structural properties by X-ray diffraction (XRD) and Fourier transform infrared absorption spectroscopy. Deposition in the presence of an H2O vapor at room temperature incorporated H2O and OH species in the deposited films. Post-annealing in an O2 ambient self-organized OH− and PO43− functional groups in HAp crystals. The XRD patterns revealed randomly orientation when the annealing temperature ranged between 700 and 900 °C. In contrast, preferentially c-axis-oriented HAp crystals nucleated after prolonged annealing at 550–600 °C. The possible scenario for the preferred orientation is that C-plane terminated HAp crystallites were initially created in the near-surface region, and the following crystallization proceeded exclusively on the seed surface. After post annealing in a vacuum or in an Ar gas ambient at 900 °C, films were reduced into tricalcium phosphate, increasing photoabsorption in the infrared range.
    No preview · Article · Mar 2013 · Applied Surface Science
  • Housei Akazawa
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    ABSTRACT: Adding N2 gas during reactive sputtering of a Ti target prevented the target surface from being severely poisoned by oxygen atoms and sustained a high deposition rate for titanium oxynitride films under metal-mode-like sputtering conditions. With progress in the degree of oxidization, films deposited onto a glass substrate varied from TiO1−xNx having a face-centered cubic (fcc) structure to TiO2−xNx having an anatase structure. Titanium oxynitride films deposited on an Al2O3(0 0 0 1) substrate were epitaxial with major orientations toward the (1 1 1) and (2 0 0) directions for fcc-TiO1−xNx and (1 1 2) for anatase-TiO2−xNx. Intermediately oxidized films between TiO1−xNx and TiO2−xNx were amorphous on the glass substrate but crystallized into a Magneli phase, TinO(N)2n−1, on the Al2O3(0 0 0 1) substrate. Partially substituting oxygen in TiO2 with nitrogen as well as continuously irradiating the growing film surface with a Xe plasma stream preferentially formed anatase rather than rutile. However, the occupation of anion sites with enough oxygen rather than nitrogen was the required condition for anatase crystals to form. The transparent conductive properties of epitaxial TiO2−xNx films on Al2O3(0 0 0 1) were superior to those of microcrystalline films on the glass substrate. Since resistivity and optical transmittance of TiOxNy films vary continuously with changing N2 flow rate, their transparent conductive properties can be controlled more easily than TiOx. Nb5+ ions could be doped as donors in TiO2−xNx anatase crystals.
    No preview · Article · Dec 2012 · Applied Surface Science
  • Housei Akazawa
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    ABSTRACT: We investigated the transparent conductive properties of hetero-double layers: Ga-doped ZnO (GZO) overlaid on undoped ZnO. We prepared five samples for given unit thicknesses h: ZnO and GZO films with a thickness of h to characterize the hybrid structure, ZnO and GZO films with a thickness of 2h, and a GZO/ZnO double layer with a thickness of h for each layer (h = 50, 100, 150, and 200 nm). If we assumed that the upper and bottom half of the 2h-thick GZO films as well as those of GZO/ZnO were connected in parallel in terms of the equivalent electric circuit, the calculated sheet resistance of the upper GZO layer scarcely depended on whether the bottom layer was GZO or ZnO. Hence, the bottom layer played the role of providing a crystalline template for the upper layer that actually governed electrical transport. Also, the infrared transmittance of the upper GZO layer was immune to what the bottom layer consisted of. While GZO/ZnO had 1.1–1.5 times higher sheet resistance than 2h-thick GZO, the optical transmittance of GZO/ZnO in the near-infrared region was 20–40% higher, demonstrating that the GZO/ZnO double layer structure is suitable for transparent electrodes in solar cells.
    No preview · Article · Dec 2012 · Thin Solid Films
  • Housei Akazawa
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    ABSTRACT: Boron delta-doped structures in Si crystals were fabricated by means of photoexcited chemical vapor deposition (CVD). Core electronic excitation with high-energy photons ranging from vacuum ultraviolet to soft x rays decomposes B2H6 molecules into fragments. Combined with in situ monitoring by spectroscopic ellipsometry, limited number of boron hydrides can be delivered onto a Si(100) surface by using the incubation period before the formation of a solid boron film. The boron-covered surface is subsequently embedded in a Si cap layer by Si2H6 photo-excited CVD. The crystallinity of the Si cap layer depended on its thickness and the substrate temperature. The evaluation of the boron depth profile by secondary ion mass spectroscopy revealed that boron atoms were confined within the delta-doped layer at a concentration of 2.5 × 1020 cm-3 with a full width at half maximum of less than 9 nm, while the epitaxial growth of a 130-nm-thick Si cap layer was sustained at 420 °C.
    No preview · Article · Feb 2012 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
  • Housei Akazawa
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    ABSTRACT: We investigated the thermal stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films when they were annealed in a high vacuum with stepwise increasing temperature. The ZnO samples included VO-rich and Zn-rich ZnO films; the primary donors were respectively oxygen vacancies (VO) or Zn atoms highly unsaturated with oxygen atoms. VO-rich ZnO was the most unstable against annealing; resistivity initially within the 10−3Ωcm range diverged higher than 10Ωcm when a critical temperature was exceeded. The critical temperature between 350 and 450°C depended on the film thickness, which indicated that VO's were diminished through recombination with migrating interstitial oxygen atoms. In contrast, Zn-rich ZnO films remained highly conductive up to 550°C. They became more and more transparent and their crystallinity improved at higher annealing temperatures, which was the consequence of metallic-like Zn atoms being removed through desorption from the surface or being accommodated into the crystalline lattice. Comparatively, GZO films were more robust against annealing with their resistivities remaining unchanged up to 350°C.
    No preview · Article · Jan 2012 · Thin Solid Films
  • Housei Akazawa
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    ABSTRACT: We investigated the structural and transparent conductive properties of oxygen-deficient TiOx films that were deposited by metal-mode reactive electron cyclotron resonance plasma sputtering from a Ti target at 400 °C. Crystallites in a strongly reduced state (x≈ 1) had face centered cubic (fcc) structures with the resistivities ranging from 10-4 to 10-3 Ω cm, and the optical transmittance in the visible wavelength was between 25 and 55%. In a sufficiently oxidized state (x≈ 2), rutiles nucleated with resistivites higher than 10-2 Ω cm, and the optical transmittance was between 60 and 80%. The intermediate composition (1< x < 2) corresponded to fcc structures although the crystallinity approached an amorphous state with increasing x. Crystallization into magneli phases (TinO2n-1) was observed only for thick films at deposition temperatures higher than 500 °C. Carriers were n-type for rutile, but p-type for the fcc and magneli phases. Nb-doped TiOx films were produced by metal-mode sputtering of TiOx with co-sputtering Nb and O from an Nb2O5 target. The donor role of Nb5+ could be identified only in the oxidized rutile state, but the resistivity increased at higher Nb2O5 sputtering powers due to oxidation of Nb atoms that substituted Ti sites.
    No preview · Article · Jan 2012 · Japanese Journal of Applied Physics