Kei-ichiro Murai

Transparent conducting amorphous Zn–Sn–O films deposited by simultaneous dc sputtering

The films of ZnO–SnO2 system were deposited on glass substrates by simultaneous dc magnetron sputtering apparatus, in which ZnO and SnO2:Sb (Sb2O5 3 wt % doped) targets faced each other. The substrate temperatures were maintained at 150, 250, and 350 °C, respectively. As an experimental parameter, current ratio δ=IZn/(IZn+ISn), which corresponds to ZnO target current (IZn) divided by the sum of ZnO and SnO2:Sb target currents (IZn+ISn), was adopted. Amorphous transparent films appeared for 0.50⩽δ⩽0.73, which could be correlated to compositions as [Zn]/([Sn]+[Zn])=0.33–0.67 by x-ray fluorescent analysis. At [Zn]/([Sn]+[Zn])=1/2 (δ=0.62), 2/3 (δ=0.73) and all other ratios in as-deposited films, neither crystalline ZnSnO3 nor Zn2SnO4 was obtained. Minimum resistivity of 4–6×10−2 Ω cm was found at δ=0.50, whose composition was approximately SnO2⋅ZnSnO3. Resistivity increased linearly with an increase of the current ratio, until the composition reached Zn2SnO4. The amorphous phase showed a constant Hall mobili...

Local structure and mean-square relative displacement in SiO2 and GeO2 polymorphs

Extended X-ray absorption fine structure (EXAFS) spectra near the Si and Ge K-edge for SiO_2 and GeO_2 polymorphs were measured in transmission mode with synchrotron radiation at the Photon Factory, Tsukuba. The local structures and mean-square relative displacements were determined in \alpha-tridymite, \alpha-quartz and stishovite. In stishovite, Si is octahedrally coordinated and the four coplanar Si—O bonds [1.755 (8) A] are shorter than the other two axial bonds [1.813 (15) A]. The high-temperature phase tridymite [1.597 (3) A] has a smaller local bond distance than \alpha-quartz [1.618 (5) A]. The temperature variation of the local structural parameters for quartz-type GeO_2 (q-GeO_2) and rutile-type GeO_2 (r-GeO_2) have been determined in the temperature range 7–1000 K. The harmonic effective interatomic potential V(u)=\alpha{u}^2/2 was evaluated from the contribution to the thermal vibration, where u is the deviation of the bond distance from the location of the potential minimum. The potential coefficient \alpha for the Ge—O bond of the tetrahedron in q-GeO_2 is 24.6 eV A−2. The potential coefficients \alpha for the four coplanar Ge—O bonds and the two axial bonds of the octahedron in r-GeO_2 are 12.9 and 14.9 eV A−2, respectively. The potential coefficient \alpha for the second-nearest Ge—Ge distance in q-GeO_2 is 9.57 eV A−2. The potential coefficients \alpha for the second- and third-nearest Ge—Ge distances in r-GeO_2 are 11.6 and 7.18 eV A−2, respectively. The effective interatomic potential is largely influenced by the local structure, particularly by the coordination numbers. The phonon dispersion relations for q-GeO_2 and r-GeO_2 were estimated along [100] by calculating the dynamical matrix using the potential coefficients \alpha for the Ge—O and Ge—Ge motions. The quartz-type structure has a more complex structure with a wide gap between 103 and 141 meV and a highest energy of 149 meV, whereas the rutile-type structure has a continuous distribution and a highest energy of 126 meV.

LUMINESCENT PROPERTIES OF (Y,Gd)3Al5O12:Ce PHOSPHORS PREPARED BY CITRIC-GEL METHOD

Y3Al5O12:Ce and (Y,Gd)3Al5O12:Ce powder phosphors were prepared by a citric-gel method by using metals nitrates as starting materials and citric acid as a complexing agent. Single phase of Y3Al5O12:Ce was obtained by firing a precursor of the metal-citrate complexes at as low as 1000°C for 5 hours. All the Y3Al5O12:Ce phosphors showed broad emission peaks in the range of 480-650nm and had the maximum intensity at 529nm, which were stimulated by blue-LEDs. Excitation spectra of Y3Al5O12:Ce with different concentrations of Ce showed that the optimum amount of the doping was 1mol% for best luminescence. The photoluminescent wavelength of the (Y,Gd)3Al5O12:Ce (Ce:1mol%) increased monotonically with increasing the amount of Gd concentration (up to 560nm for Gd3Al5O12:Ce), in accordance with a linear increase of their lattice constant (e.g. 12.01A for Y3Al5O12:Ce and 12.11A for Gd3Al5O12:Ce).

XAFS study of GeO2 glass under pressure

Using a large-volume high-pressure apparatus, Li2O–4GeO2 glass and pure GeO2 gel have been compressed to 14 GPa at room temperature and their local structural changes have been investigated by an in situ XAFS (x-ray absorption fine-structure) method. On compression of Li2O–4GeO2 glass, the Ge–O distance gradually becomes short below 7 GPa, showing the conventional compression of the GeO4 tetrahedron. Abrupt increase in the Ge–O distance occurs between 8 and 10 GPa, which corresponds to the coordination number (CN) changing from 4 to 6. The CN change is completed at 10 GPa. On decompression, the reverse transition occurs gradually below 10 GPa. In contrast to the case for Li2O–4GeO2 glass, the Ge–O distance in GeO2 gel gradually increases over a pressure range from 2 to 12 GPa, indicating that continuous change in CN occurs. The Ge–O distance at 12 GPa is shorter than that of Li–4GeO2 indicating that the change in CN is not completed even at this pressure. On complete release of pressure, the Ge–O distance reverts to that of the starting gel.

Structural changes of quartz-type crystalline and vitreous GeO2 under pressure

Using a large volume high pressure apparatus, quartz-type crystalline GeO2 and Li2O-4GeO2 glass have been compressed up to 14 GPa at room temperature and their local structural changes have been investigated by an in-situ XAFS method. In quartz-type crystalline GeO2, the change of the coordination number from 4 to 6 begins above 8 GPa and finishes below 12 GPa. On decompression, reversal transition begins below 8 GPa and there is a large hysteresis. Almost no sixfold coordination of Ge is preserved after releasing pressure. Change of coordination number in vitreous Li2O-4GeO2 begins above 6 GPa and is completed below 10 GPa. Reversal transition begins below 10 GPa and the hysteresis is smaller than that of quartz-type GeO2. Change of coordination number in vitreous Li2O-4GeO2 is also reversal.

Amorphous transparent conductive oxide films of In2O3-ZnO with additional Al2O3 impurities

Al-doped In2O3-ZnO films were developed and the influence of Al doping in electrical and optical properties of In2O3-ZnO films was investigated. Amorphous and homologous phases appeared with increasing δ=Zn∕(Zn+In) ratios. The carrier generation mechanism is discussed for these films. Native oxygen defects are the primary donors for the In2O3-ZnO films. Then the low resistivities of 2–4×10−4Ωcm were attained within a narrow range of δ=Zn∕(Zn+In) in the amorphous phase film. Two wt % (percent by weight) Al2O3-doping decreased the resistivity to 1.5–2.1×10−4Ωcm. At doping of 3 and 4 wt % Al2O3, film resistivities of 2–4×10−4Ωcm were attained for a relatively wide range of δ in the amorphous phase due to an increase in carrier concentration at δ=0.3–0.6. However, Al2O3 doping in homologous phase In2O3-ZnO films decreased carrier concentration independently of the levels of Al2O3 doping. Al2O3 doped in the homologous ZnkIn2Ok+3 crystalline films did not act as a donor impurity. The optical band-gap energy for...

PREPARATION AND LUMINESCENCE PROPERTIES OF Eu2+-ACTIVATED Ba-Six-O-N PHOSPHORS

A series of oxonitridosilicates phosphors Ba-Six-O-N /Eu2+ (x=Si/Ba=1~8, 5atom% Eu2+) were synthesized using traditional solid-state reaction. For x = 2~8, the main phase of the obtained samples was Ba3Si6O12N2. All Ba-Six-O-N /Eu2+(x = 1~8) materials could be efficiently excited in the UV to visible region(310~450nm) and had a green emission at 508~522nm, making them attractive as conversion phosphors for white LED applications. With increasing x values, the emission peaks shifted to the longer wavelength region, while the emission intensity had a maximum at x = 6. The influence of the firing times was also discussed, after twice fired, there was a tendency of single-phased formed of the obtained materials and the emission intensity was greatly improved.

Effective Pair Potentials of NaCl- and CsCl-type KBr Determined by X-Ray Absorption Fine Structure under Pressure.

A difference in effective pair potentials between the NaCl- and CsCl-type KBr has been investigated using the X-ray absorption fine structure (EXAFS) technique under pressure. In the potential parameter fitting, we have directly carried out the numerical integration of the EXAFS function and evaluated the precise effective pair potential. The effective pair potential is influenced not only by pressure but also by coordination numbers. The high pressure CsCl-type phase has broader effective pair potentials than the NaCl-type phase.

Photoluminescence Properties of (Ba1-(x+y)SrxEuy)2Si6O12N2 Phosphors for White LED Applications

(Ba1-(x+y)SrxEuy)2Si6O12N2 oxynitride phosphors were successfully synthesized by the solid-state reaction method at 1200°C under a H2(5%) + N2(95%) atmosphere. The Sr2+ content (x) was varied in the range 0-0.6 and the Eu2+ content varied in the range 0.05-0.25, with the Si/(Ba+Sr+Eu) ratio fixed at 3. Results showed that the emission characteristics of (Ba1-(x+y)SrxEuy)2Si6O12N2 phosphors under UV or blue-light excitation was strongly dependent on the chemical composition. The phosphor (Ba0.95Eu0.05) Si6O12N2 showed an intense green emission peak at 520 nm, whilst the phosphor (Ba0.45Sr0.5Eu0.05)Si6O12N2 had a weaker emission maximum at 548 nm. Ba2+ substitution with Sr2+ decreased the lattice volume of the (Ba1-(x+y)SrxEuy)2Si6O12N2 phosphors and was responsible for the red-shift in the emission peak. Optimization of the Eu2+ concentration at a fixed Sr2+ content of 0.2 identified the phosphor (Ba0.65Sr0.2Eu0.15)2Si6O12N2 as a potential alternative to YAG:Ce yellow phosphors for white LED applications.

A-Site and B-Site Non-stoichiometry and Sintering Characteristics of (Sr1−xLax)1−yTi1−zO3 Perovskites

The A-Site and B-Site non-stoichiometry and sintering characteristics of (Sr1− x Lax )1− y Ti1− z O3 perovskites were studied using samples prepared by solid-state mixing. A-Site or B-Site deficiency was observed for La-doped SrTiO3 perovskites, and this increased with increasing La content. For A-Site deficient (Sr1 − x Lax )1 − y TiO3 perovskites, the cell volumes decreased although the reverse was observed for B-Site deficient (Sr1 − x Lax )Ti1 − z O3 perovskites. This may be related to number of B-Site vacancies in the perovskites through charge compensation. It was found that A-Site and B-Site deficiencies increase the density of (Sr1 − x Lax )1 − y Ti1 − z O3 perovskites. These characteristics may prove useful in the development of solid oxide fuel cell interconnect materials.