Masaaki Kobata

Structural characteristics of GeTe-rich GeTe–Sb2Te3 pseudobinary metastable crystals

The (GeTe)1−γ–(Sb2Te3)γ pseudobinary system has, over almost its entire composition range, two kinds of crystalline phase: one is a metastable phase with a NaCl-type structure and the other is a spectrum of stable phases with homologous structures. In the metastable phase, Ge/Sb atoms and intrinsic vacancies occupy the Na sites; on the other hand, Te atoms are located at the Cl sites. These vacancies are produced by following γ/1+2γ to ensure the stoichiometry of the metastable pseudobinary compound. This metastable phase obstinately holds its NaCl-type structure and resists transformation to stable homologous structures, even at high temperatures on the GeTe-rich side of the system. In GeTe (γ=0), the NaCl-type atomic configuration itself is the stable structure. GeTe has, as is well known, a high-temperature cubic phase and a low-temperature rhombohedral phase. This GeTe and the pseudobinary compounds containing a small quantity of Sb2Te3 have their single-phase regions not on the GeTe–Sb2Te3 tie line b...

Fe3−xZnxO4 thin film as tunable high Curie temperature ferromagnetic semiconductor

Epitaxial ferri(ferro)magnetic Fe3−xZnxO4 thin films (x=0–0.9) were prepared using a pulsed-laser deposition technique. The electrical conductivity and magnetic properties of Fe3−xZnxO4 thin film were systematically modulated for the entire range of Zn substitution. Anomalous Hall coefficient measurements revealed the presence of spin-polarized carriers at room temperature. Valence band spectra obtained by hard x-ray photoemission spectroscopy revealed that the density of states near the Fermi level was reduced with an increasing Zn concentration of x. These results indicate that this system will serve as a tunable ferromagnetic semiconductor with a strong electron correlation.

In situ x-ray photoelectron spectroscopy for electrochemical reactions in ordinary solvents

In situ electrochemical X-ray photoelectron spectroscopy (XPS) apparatus, which allows XPS at solid/liquid interfaces under potential control, was constructed utilizing a microcell with an ultra-thin Si membrane, which separates vacuum and a solution. Hard X-rays from a synchrotron source penetrate into the Si membrane surface exposed to the solution. Electrons emitted at the Si/solution interface can pass through the membrane and be analyzed by an analyzer placed in vacuum. Its operation was demonstrated for potential-induced Si oxide growth in water. Effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined at sub-nanometer resolution.

Chemical insight into electroforming of resistive switching manganite heterostructures

We have investigated the role of the electroforming process in the establishment of resistive switching behaviour for Pt/Ti/Pr0.5Ca0.5MnO3/SrRuO3 layered heterostructures (Pt/Ti/PCMO/SRO) acting as non-volatile Resistance Random Access Memories (RRAMs). Electron spectroscopy measurements demonstrate that the higher resistance state resulting from electroforming of as-prepared devices is strictly correlated with the oxidation of the top electrode Ti layer through field-induced electromigration of oxygen ions. Conversely, PCMO exhibits oxygen depletion and downward change of the chemical potential for both resistive states. Impedance spectroscopy analysis, supported by the detailed knowledge of these effects, provides an accurate model description of the device resistive behaviour. The main contributions to the change of resistance from the as-prepared (low resistance) to the electroformed (high resistance) states are respectively due to reduced PCMO at the boundary with the Ti electrode and to the formation of an anisotropic n-p junction between the Ti and the PCMO layers.

Band offsets in complex-oxide thin films and heterostructures of SrTiO3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy

The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν > 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations.

Hard X-ray Photoemission Spectroscopy Combined with Magnetic Circular Dichroism: Application to Fe3-xZnxO4 Spinel Oxide Thin Films

We have performed hard X-ray photoemission spectroscopy combined with magnetic circular dichroism (MCD-HXPES) for 10-nm-thick Fe3-xZnxO4 (x = 0,0.5) thin films. For the Fe3-xZnxO4 (x = 0,0.5) thin films, a clear MCD signal was obtained in the Fe 2p core-level photoemission without any surface treatments. This result shows that MCD-HXPES has a large probing depth for both electronic and magnetic states, and indicates a potential application of MCD-HXPES to various magnetic materials and spintronics devices.

Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As.

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO3 films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.

Observation and simulation of hard x ray photoelectron diffraction to determine polarity of polycrystalline zinc oxide films with rotation domains

X ray photoelectron diffraction (XPD) patterns of polar zinc oxide (ZnO) surfaces were investigated experimentally using hard x rays and monochromatized Cr Kα radiation and theoretically using a cluster model approach and a dynamical Bloch wave approach. We focused on photoelectrons emitted from the Zn 2p3/2 and O 1s orbitals in the analysis. The obtained XPD patterns for the (0001) and (0001¯) surfaces of a ZnO single crystal were distinct for a given emitter and polarity. Polarity determination of c-axis-textured polycrystalline ZnO thin films was also achieved with the concept of XPD, even though the in-plane orientation of the columnar ZnO grains was random.

Hard-X-ray Photoelectron Diffraction from Si(001) Covered by a 0–7-nm-Thick SiO2 Layer

X-ray photoelectron diffraction has become a common method to determine element-specific local atomic surface structure. The use of hard X-rays makes this method bulk-sensitive and capable of studying the atomic structure of new materials such as multilayers and buried layers. We present the first Cr Kα-excited angle-resolved photoelectron diffraction from Si(001) covered by a 0–7-nm-thick SiO2 layer and demonstrate the information depth of this technique. The measured results are compared with a cluster model simulation.