Yoshiyuki Yamashita

Thermoelectric properties and electronic structure of substituted Heusler compounds: NiTi0.3-xScxZr0.35Hf0.35Sn

The effect of Ti substitution by Sc on the thermoelectric properties of the Heusler compounds NiTi0.3−xScxZr0.35Hf0.35Sn (where 0<x≤0.05) was studied. The thermoelectric properties were investigated by measuring the electrical conductivity, Seebeck coefficient, and thermal conductivity. A reduction of the thermal conductivity by a factor of 2 was obtained by substitution of Ti by Sc. The pure compound NiTi0.3Zr0.35Hf0.35Sn showed n-type conductivity with a Seebeck coefficient of −288u2002μV/K at 350 K, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230u2002μV/K (350 K) was obtained by 4% Sc substitution, which is the highest value for p-type thermoelectric compounds based on Heusler alloys. The electronic structure was studied by photoelectron spectroscopy excited by hard x-ray synchrotron radiation. Massive in gap states are observed for the parent compound. This proves that the electronic states close to the Fermi energy play a key role on the behavior of t...

Probing bulk electronic structure with hard X-ray angle-resolved photoemission

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.

Present Status of the NIMS Contract Beamline BL15XU at SPring-8

The revolver undulator beamline BL15XU at SPring‐8, which is the contract beamline of National Institute for Materials Science (NIMS), was established for materials science using soft‐and‐hard X‐ray photoelectron spectroscopy (XPS) and high‐resolution powder X‐ray diffraction (XRD). We have performed beamline reconstruction for further developments of the experiments in the research field of materials science. The flat double‐crystal monochromator (DCM) with liquid nitrogen cooling, X‐ray total reflection double‐mirror system with (+,−) geometry, and high‐energy‐resolution channel‐cut monochromator have been installed into the beamline. The refined beamline provides monochromatic X‐rays from 2 to 36 keV. The improvement of the photon flux density at the XRD and XPS experimental stations was confirmed. The photon flux was estimated to be several 1012u2009photons/sec with ΔE/E of ∼10−4.

Bulk electronic structure of the dilute magnetic semiconductor Ga1−xMnxAs through hard X-ray angle-resolved photoemission

A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400u2009meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.

Spectroscopic observation of interface states of ultrathin silicon oxide

Interface states in the Si band gap present at oxide/Si(100) interfaces for ∼3‐nm‐thick Pt/2.1∼3.6‐nm‐thick silicon oxide/n‐Si(100) metal–oxide–semiconductor devices are investigated by measurements of x‐ray photoelectron spectra under biases between the Pt layer and the Si substrate, and their energy distribution is obtained by analyzing the amount of the energy shift of the substrate Siu20092p3/2 peak measured as a function of the bias voltage. All the interface states observed using this new technique have discrete energy levels, showing that they are due to defect states. For the oxide layer formed in H2SO4+H2O2, the interface states have three density maxima at ∼0.3, ∼0.5, and ∼0.7 eV above the valence‐band maximum (VBM). For the oxide layer produced in HNO3, two density maxima appear at ∼0.3 and ∼0.7 eV above the VBM. The energy distribution for the oxide layer grown in HCl+H2O2 has one peak at ∼0.5 eV. The 0.5 eV interface state is attributed to the isolated Si dangling bond defect. The 0.3 and 0.7 eV ...

Observation of boron diffusion in an annealed Ta/CoFeB/MgO magnetic tunnel junction with standing-wave hard x-ray photoemission

The CoFeB/MgO system shows promise as a magnetic tunnel junction with perpendicular magnetization and low critical current densities for spin-torque driven magnetization switching. The distribution of B after annealing is believed to be critical to performance. We have studied the distribution of B in a Ta/Co0.2Fe0.6B0.2/MgO sample annealed at 300u2009°C for 1u2009h with standing-wave hard x-ray photoemission spectroscopy (SW-HXPS). Comparing experimental rocking curve data to x-ray optical calculations indicates diffusion of 19.5% of the B uniformly into the MgO and of 23.5% into a thin TaB interface layer. SW-HXPS is effective for probing depth distributions in such spintronic structures.

Depletion of the In2O3(001) and (111) surface electron accumulation by an oxygen plasma surface treatment

Using x-ray photoemission spectroscopy (XPS) and current-voltage (I-V) measurements of Hg contacts we show that the surface electron accumulation layer of In2O3 can be removed by an oxygen plasma treatment. For the untreated sample, XPS measured a downward band bending toward the surface and a conduction band peak, and the I-V curve was linear which indicated the presence of a surface accumulation layer. After the treatment an upward bending, the absence of the conduction band peak, and a nonlinear I-V curve indicated the absence of the surface accumulation layer. The sheet resistance of the surface accumulation layer of >45u2002kΩ was deduced from the increase of the total sheet resistance upon the treatment. The removal of the surface electron accumulation layer opens up the possibility to use Schottky contacts for electrical characterization and device applications of semiconducting In2O3.

Bias application hard x-ray photoelectron spectroscopy study of forming process of Cu/HfO2/Pt resistive random access memory structure

The forming process of Cu/HfO2/Pt, which is an oxide based resistive random access memory (ReRAM), structure that exhibited resistance switching behavior at a voltage of 1.3u2009V was investigated by hard x-ray photoelectron spectroscopy under bias operation. A bias application to the structure reduced the Cu2O bonding state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing the evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. We directly observed Cu diffusion at the Cu/HfO2 interface under device operation, which is the evidence of the metal filament formation in the oxide-based ReRAM.

Oxygen migration at Pt/HfO2/Pt interface under bias operation

The interfacial electronic states of a Pt/HfO2/Pt diode were investigated by using hard x-ray photoelectron spectroscopy under bias operation. The application of a forward bias to the Pt/HfO2/Pt diode increased the Pt–O bonding peak, providing evidence of Pt electrode oxidization and oxygen vacancy formation around the Pt/HfO2 interface. Under a reverse bias, hafnium was drawn to the Pt electrode, where it took part in Hf–Pt bonding. We achieved the direct observation of oxygen migration at a Pt/HfO2 interface under device operation, which is the key to controlling the electrical properties of metals on oxides.

Detection of the valence band in buried Co2MnSi-MgO tunnel junctions by means of photoemission spectroscopy

This work reports on the detection of the valence band of buried Heusler compounds by means of hard x-ray photoemission spectroscopy. The measurements have been performed on the so-called “half” tunnel junctions that are thin films of Co2MnSi underneath MgO. Starting from the substrate, the structure of the samples is MgO(buffer)–Co2MnSi–MgO(tMgO)–AlOx with a thickness tMgO of the upper MgO layer of 2 and 20nm. The valence band x-ray photoemission spectra have been excited by hard x rays of about 6keV energy. The valence band spectra have been used to estimate the mean free path of the electrons through the MgO layer to be 17nm at kinetic energies of about 6keV. In particular, it is shown that the buried Co2MnSi films exhibit the same valence density of states as in bulk samples.