We investigated the structural and electrical properties of a-axis-oriented Sc3+-doped TiO2 (Ti0.99Sc0.01O2-delta) thin film with rutile structure prepared by RF magnetron sputtering. The prepared thin film had smaller lattice constant than that of the bulk crystal due to the stress from the substrate and a lot of oxygen vacancies. The Ti 2p photoemission (PES) spectrum exhibited the mixed valence states of Ti4+ and Ti3+. The electrical conductivity in in-plane exhibited the semiconducting-like behavior with the activation energy of similar to 100meV and electron-ion mixed conduction was obtained by oxygen partial pressure (P-O2) dependence of the electrical conductivity at room temperature (R.T.). The existence of OH- peak was also observed in the O 1s PES spectrum. These results indicate that the a-axis-oriented Ti0.99Sc0.01O2-delta thin film has the electron-proton mixed conduction on the surface at R.T.

1517701@ed.tus.ac.jp Pulsed-induced resistivity modulation of a Pt/Ti 0.99 Sc 0.01 O 2-  /Pt multilayer with a cross-point structure was investigated. This multilayer exhibits nonlinear current – voltage characteristic based on the Schottky barrier at the Pt/Ti 0.99 Sc 0.01 O 2-  interface. When the electrical pulses of 1.5 V were applied with short interval time of 10 s, the resistivity modulation corresponding to the long-term-memorization (LTM) were observed. X-ray photoemission spectroscopy showed O-H bond that contributes to electron-proton mixed conduction at the Pt/Ti 0.99 Sc 0.01 O 2-  interface. This LTM resistivity modulation is considered to be due to the local proton migration at the Pt/Ti 0.99 Sc 0.01 O 2- interface, and the operation voltage is lower than that

The structural and electrical properties of Nd0.6Sr0.4FeO3-δ (NSFO) have been investigated in the bulk ceramic and thin film forms. Although NSFO bulk ceramic exhibits p-type conduction, the NSFO thin film changes to n-type conduction. The NSFO thin film has larger lattice distortion than the bulk ceramic due to the lattice mismatch between NSFO and Al2O3 substrate. The low crystal symmetry of the thin film is reflected in the electronic structure measured by soft-X-ray spectroscopy. The electrical conductivity of the NSFO thin film with 112 nm thickness is higher than that of the bulk ceramic at above 850 K. This originates from the change of electron number for eg-subband in the Fe 3d valence band with the lattice distortion.

The surface electronic structure of as-deposited and post-annealed La0.67Sr0.33MnO3 (LSMO) thin films on SrTiO3 (100) substrates has been studied by soft-X-ray spectroscopy. The crystal lattice reduces by post-annealing in air at 1100 ℃. The electrical resistivity also decreases by the post-annealing. These thin films has the mixed valence states of Mn3+ and Mn4+. The valence band near the Fermi level (EF) consists of the eg↑ and t2g↑ states of Mn 3d. The density of state at EF is higher in the post-annealed LSMO thin film. These results directly indicate that the change of the Mn valence state is closely related to the electrical resistivity and Mn 3d electron number of LSMO thin film at the surface state.

The surface electronic structure of as-deposited and post-annealed La0.67Sr0.33MnO3 (LSMO) thin films on SrTiO3 (100) substrates has been studied by soft-X-ray spectroscopy. The crystal lattice reduces by post-annealing in air at 1100 C. The electrical resistivity also decreases by the postannealing. These thin films has the mixed valence states of Mn3+ and Mn4+. The valence band near the Fermi level (EF) consists of the eg and t2g states of Mn 3d. The density of state at EF is higher in the post-annealed LSMO thin film. These results directly indicate that the change of the Mn valence state is closely related to the electrical resistivity and Mn 3d electron number of LSMO thin film at the surface state.

An all-solid-state neuromorphic transistor composed of a WO3 thin film and a proton-conducting electrolyte was fabricated for application to next-generation information and communication technology including artificial neural networks. The drain current exhibited a 4-order-of-magnitude increment by redox reaction of the WO3 thin film owing to proton migration. Learning and forgetting characteristics were well tuned by the gate control of WO3 redox reactions owing to the separation of the current reading path and pulse application path in the transistor structure. This technique should lead to the development of versatile and low-power-consumption neuromorphic devices.

Amorphous WO3-x thin films with ～200 and ～600 nm thicknesses have been prepared on Pt/SiO2 substrates by RF magnetron sputtering. The mixed valence states of W6+ and W5+ are observed in the photoemission spectroscopy (PES) spectra of W 4d core level. The electrical conductivities exhibit the thermal activation-type behaviors in the temperature region of 100～200°C. The activation energies of 200 and 600 nm films are 0.1 and 0.6 eV, respectively. The band gaps (Eg) of 200 and 600 nm films are ～2.6 and ～2.0 eV, respectively. The expanded PES in the Eg region and X-ray absorption spectra exhibit the W 5d-DOS at the Fermi level and defect-induced state at the bottom of conduction band, respectively. The above results indicate that the conducting carrier of amorphous WO3-x thin film is closely related to the film thickness and oxygen vacancies.

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li-Fe3O4) with that for pristine Fe3O4 clearly demonstrated that the number of Fe2+ ions at octahedral B sites is increased by lithiation. The valence band RPES spectra of Li-Fe3O4 further showed that lithiation increases the density of states near the Fermi level originating Fe2+ ions at octahedral B sites. These findings agree well with the observed decrease in the saturation magnetization in the magnetization-magnetic field (M-H) loop of Li-Fe3O4 thin film, indicating that minority spins (down spins) increase (i.e., total spins decrease) due to lithiation. The variation in the number of Fe2+ ions at B sites is suggested to be an underlying operating mechanism of a nanoionics-based magnetic property tuning device. (C) 2017 The Japan Society of Applied Physics

Resistivity modulation behavior in Pt/TiO2-delta/Pt multilayer devices was investigated in terms of nanoionics-based neuromorphic function. The current relaxation behavior, which corresponds to short-term and long-term memorization in neuromorphic function, was analyzed using electrical pulses. In contrast to the huge difference in ionic conductivity for bulk crystal materials of TiO2-delta and WO3, the difference in the relaxation behavior was small. Rutherford backscattering spectrometry and hydrogen forward scattering spectrometry revealed that the TiO2-delta thin film contained 5.6 at.% of protons. This indicates that the neuromorphic function in TiO2-delta-based devices is caused by extrinsic proton transport, presumably through the grain boundary. (C) 2017 The Japan Society of Applied Physics

Polycrystalline Zr0.92Y0.08O2 (YSZ) thin films were prepared by RF magnetron sputtering. 80- and 160-nm thin films exhibited (111) orientation and a polycrystalline structure, respectively. The 80-nm thin film had larger distortion than the 160-nm thin film. While the 80-nm thin film and a 120-nm thin film exhibited proton conduction, the 160-nm thin film did not, indicating that surface proton conduction can depend on film thickness. The activation energy of the 80- and 120-nm thin films measured in a wet atmosphere (0.52 eV) was about half of that measured in a dry atmosphere. The enhancement of conductivity for the thin films may be attributed to distortion, which may change the structure around an oxygen vacancy at the YSZ grain surface, accompanied by possible enhancement of H2O adsorption. H2O-annealed thin film had a hydrogen-induced level in the band gap energy region. This is the first observation of hydrogen-induced level at the surface state of the YSZ thin film obtained by X-ray absorption spectroscopy.

We have prepared c-axis controlled α-Fe2O3 thin films on Al2O3 substrates by RF magnetron sputtering and studied their electronic structure by soft-X-ray spectroscopy. The lattice constant of c-axis increases with increasing film thickness due to the relaxation of lattice mismatch between α-Fe2O3 and Al2O3 and formation of oxygen vacancies. The electrical conductivity is higher in thicker thin film. The valence band consists of t2g- and eg-subbband of Fe 3d state hybridized with O 2p state. The band gaps of ∼25 and ∼95 nm thicknesses of Fe2O3 thin film are ∼1.8 and 1.4 eV, respectively, which correspond to the activation energy of electron conductivity. The above results indicate that the band gap and the conductivity of α-Fe2O3 thin film directly affect the change of the lattice constant of c-axis and formation of oxygen vacancies.

Anatase TiO2−δ thin film was prepared by RF magnetron sputtering using oxygen radical and Ti-metal target. Degrees of the TiO2−δ crystal orientation in the thin film depends of the oxygen gas pressure () in the radical gun. The (004)- and (112)-oriented TiO2−δ thin films crystallized without postannealing have the mixed valence Ti4+/Ti3+ state. The electrical conductivities, which corresponds to n-type oxide semiconductor, is higher in the case of (004)-oriented TiO2−δ thin film containing with high concentration of oxygen vacancy. The donor band of TiO2−δ thin film is observed at ∼1.0 eV from the Fermi level (EF). The density-of-state at EF is higher in (004)-oriented TiO2−δ thin film. The above results indicate that the oxygen vacancies can control by changing the of the oxygen radical.