Showing papers by "Toshio Kamiya published in 2013"

Effects of Diffusion of Hydrogen and Oxygen on Electrical Properties of Amorphous Oxide Semiconductor, In-Ga-Zn-O

Abstract: We found that device-quality thin films of amorphous oxide semiconductor, a-In-Ga-Zn-O, unintentionally include high-density hydrogens over 1020 cm−3 but the electron concentration remains low at ~1015 cm−3 in as-deposited states. The hydrogens exist in the form of hydroxyl group –OH, but most of them are inactive as an electron donor, which is explained by charge compensation due to incorporation of excess oxygens. Although their diffusions are fast compared with those in crystalline ZnO, but oxygen can diffuse only 20 nm even at 400°C. Hydrogen diffusion is much faster, and easily hydrogenizes a thin channel of a thin-film transistor.

Hydrogen passivation of electron trap in amorphous In-Ga-Zn-O thin-film transistors

Abstract: We report an experimental evidence that some hydrogens passivate electron traps in an amorphous oxide semiconductor, a-In-Ga-Zn-O (a-IGZO). The a-IGZO thin-film transistors (TFTs) annealed at 300 °C exhibit good operation characteristics; while those annealed at ≥400 °C show deteriorated ones. Thermal desorption spectra (TDS) of H2O indicate that this threshold annealing temperature corresponds to depletion of H2O desorption from the a-IGZO layer. Hydrogen re-doping by wet oxygen annealing recovers the good TFT characteristic. The hydrogens responsible for this passivation have specific binding energies corresponding to the desorption temperatures of 300–430 °C. A plausible structural model is suggested.

Surface reactivity and oxygen migration in amorphous indium-gallium-zinc oxide films annealed in humid atmosphere

Abstract: An isotope tracer study, i.e., 18O/16O exchange using 18O2 and H218O, was performed to determine how post-deposition annealing (PDA) affected surface reactivity and oxygen diffusivity of amorphous indium–gallium–zinc oxide (a-IGZO) films. The oxygen tracer diffusivity was very high in the bulk even at low temperatures, e.g., 200 °C, regardless of PDA and exchange conditions. In contrast, the isotope exchange rate, dominated by surface reactivity, was much lower for 18O2 than for H218O. PDA in a humid atmosphere at 400 °C further suppressed the reactivity of O2 at the a-IGZO film surface, which is attributable to –OH-terminated surface formation.

Unusual pressure effects on the superconductivity of indirectly electron-doped (Ba 1-x La x )Fe 2 As 2 epitaxial films

Abstract: Applying an external pressure to indirectly electron-doped 122-type (Ba${}_{1\ensuremath{-}x}$La${}_{x}$)Fe${}_{2}$As${}_{2}$ epitaxial films enhances the superconducting critical temperature (${T}_{\mathrm{c}}$) up to 30.3 K. Different from the other family compounds, the ${T}_{\mathrm{c}}$ is enhanced not only in the underdoped region but also in the optimally doped and overdoped regions. Narrowing of the superconducting transition width and an increase in the carrier density take place simultaneously in the whole doping region, except at the heavily overdoped limit. This characteristic is unique to and is observed only in (Ba${}_{1\ensuremath{-}x}$La${}_{x}$)Fe${}_{2}$As${}_{2}$, in which the La doping is stabilized via nonequilibrium growth of the vapor phase epitaxy, among the 122-type iron-based superconductors, $A$Fe${}_{2}$As${}_{2}$ ($A$ $=$ Ba, Sr, and Ca).

Anomalous scaling behavior in a mixed-state Hall effect of a cobalt-doped BaFe 2 As 2 epitaxial film with a high critical current density over 1 MA/cm 2

Abstract: The mixed-state Hall effect was examined in a Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ epitaxial film with a high critical current density. The transverse resistivity ${\ensuremath{\rho}}_{xy}$ and the longitudinal resistivity ${\ensuremath{\rho}}_{xx}$ follow power law scaling ${\ensuremath{\rho}}_{xy}$ $=$ $A{\ensuremath{\rho}}_{xx}$${}^{\ensuremath{\beta}}$. In the temperature sweep with a fixed field ($T$ sweep), all of the \ensuremath{\beta} values are independent of magnetic field up to 9 T, and are lower than 2.0 (around 1.8). In contrast, the \ensuremath{\beta} values in the magnetic-field sweep with a fixed temperature ($H$ sweep) change from 1.8 to 2.0 as the temperature increases from 13 to 16 K even in the $T$/$H$ region that overlaps with the $T$ sweep measurements. These results indicate that the vortices introduced at low temperatures are trapped by strong pinning centers, but a portion of the vortices introduced at high temperatures are not strongly trapped by the pinning centers. The sign of ${\ensuremath{\rho}}_{xy}$ is negative, and a sign reversal is not detected. These distinct scaling behaviors, which sharply contrast cuprates and MgB${}_{2}$, are explained by high-density pinning centers in the Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ epitaxial film and are consistent with a wider vortex liquid phase.

Magnetic scattering and electron pair breaking by rare-earth-ion substitution in BaFe 2 As 2 epitaxial films

Abstract: The effect of electron doping by trivalent charge state rare-earth-ion (RE = La, Ce, Pr and Nd) substitutions on the superconductivity in BaFe2As2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with magnetic/structural phase transitions, leading to resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature (T onset c ) of 22.4K for La doping and 13.4K for Ce doping, while only broad resistivity drops were observed at 6.2K for Pr doping and 5.8K for Nd doping although zero resistivity and the distinct Meissner effect were not observed at least down to 2K. The decrease in T onset c with increasing the number of RE 4f electrons cannot be explained in terms of the crystalline quality or crystallographic structure parameters of BaFe2As2 films. It was clarified, based on resistivity-temperature analyses, that magnetic scattering became increasingly significant in the above

Magnetic scattering and electron pair breaking by rare-earth-ion substitution in BaFe2As2 epitaxial films

Abstract: The effect of electron doping by trivalent charge state rare-earth ion (RE = La, Ce, Pr, and Nd) substitutions on the superconductivity in BaFe2As2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with the magnetic/structural phase transitions, leading to the resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature (Tconset) of 22.4 K for La-doping and 13.4 K for Ce-doping, while only broad resistivity drops were observed at 6.2 K for Pr-doping and 5.8 K for Nd-doping but neither zero resistivity nor distinct Meissner effect were observed at least down to 2 K. The decrease in Tconset with increasing the number of RE 4f electrons cannot be explained in terms of the crystalline qualities or crystallographic structure parameters of the BaFe2As2 films. It was clarified, based on resistivity-temperature analyses, that magnetic scattering became increasingly significant in the above order of the RE dopants. The negative magnetoresistance was enhanced by the Ce- and Pr-doping, implying that the decrease in Tc originates from magnetic pair breaking by interaction of the localized 4f orbitals in the RE dopants with the itinerant Fe 3d orbitals.

Superconducting Properties and Phase Diagram of Indirectly Electron-Doped $(\hbox{Sr}_{1 - x}\hbox{La}_{x})\hbox{Fe}_{2}\hbox{As}_{2}$ Epitaxial Films Grown by Pulsed Laser Deposition

Abstract: A nonequilibrium phase (Sr1-xLax)Fe2As2 was formed by epitaxial film-growth. The resulting films emerged superconductivity along with suppression of the resistivity anomaly, which is associated with magnetic and structural phase transitions. The maximum critical temperature was 20.8 K, which is almost the same as that of directly electron-doped Sr(Fe1-xCox)2As2. Its electronic phase diagram is much similar to that of Sr( Fe1-xCox)2As2, indicating that the difference in the electron doping sites does not influence the superconducting properties of 122-type SrFe2As2.

Superconducting Properties and Phase Diagram of Indirectly Electron-Doped (Sr1-xLax)Fe2As2 Epitaxial Films Grown by Pulsed Laser Deposition

Abstract: A non-equilibrium phase (Sr1-xLax)Fe2As2 was formed by epitaxial film-growth. The resulting films emerged superconductivity along with suppression of the resistivity anomaly that is associated with magnetic and structural phase transitions. The maximum critical temperature was 20.8 K, which is almost the same as that of directly electron-doped Sr(Fe1-xCox)2As2. Its electronic phase diagram is much similar to that of Sr(Fe1-xCox)2As2, indicating that the difference in the electron doping sites does not influence the superconducting properties of 122-type SrFe2As2.

P.142L: Late-News Poster: Electron Injecting Material for OLEDs driven by Oxide TFTs: Amorphous C12A7 Electride

Abstract: Transparent thin film of amorphous 12CaO·7Al2O3 electride, sputter-deposited at RT, having work function of 2.9 − 3.1 eV, was found to act as a good electron injection layer for OLED, and was adequate in combination with n-channel IGZO-TFTs.

4.1: Invited Paper: Electronic Structure, Carrier Transport, Defects and Impurities in Amorphous Oxide Semiconductor

Abstract: This review paper provides the present status of amorphous oxide semiconductor technology along with knowledge obtained to date on their carrier transport, defects and impurities in relation to their stability issues.

P.3: 3‐D Stacked Complementary TFT Devices using n‐type α‐IGZO and p‐type F8T2 TFTs — Operation Confirmation of NOT and NAND Logic Circuits –

Abstract: We have developed 3-D stacked complementary TFT devices using n-type α-IGZO and p-type F8T2 TFTs on PET substrates. We confirmed proper I-V characteristics of both the α-IGZO and F8T2 TFTs and correct input-output characteristics of both NOT and NAND logic circuits. The operation confirmation of the NAND logic circuit suggests that any advanced logic circuit can be composed using these complementary TFT devices.