Shohei Ogura

Two charged states of hydrogen on the SrTiO3(001) surface

The effects of hydrogen exposure on the electronic structure of two types of SrTiO3(001) surfaces, oxygen-deficient (OD) and nearly-vacancy-free (NVF) surfaces, were investigated with ultraviolet photoemission spectroscopy and nuclear reaction analysis. Upon molecular hydrogen exposure to the OD surface which reveals in-gap states at 1.3 eV below the Fermi level, the in-gap state intensity was reduced to half the initial value at a hydrogen coverage of 0.9 ± 0.7 × 10(14) cm(-2). On the NVF surface which has no in-gap state, on the other hand, atomic-hydrogen exposure induced in-gap states, and the hydrogen saturation coverage was evaluated to be 3.1 ± 0.8 × 10(14) cm(-2). We argue that H is positively charged as H(∼0.3 +) on the NVF surface by being coordinated to the O atom, whereas H is negatively charged as H(-) on the OD surface by occupying the oxygen vacancy site. The stability of H(-) at the oxygen vacancy site is discussed.

Knudsen layer formation in laser induced thermal desorption

Laser induced thermal desorption of Xe atoms into vacuum from a metal surface following the nano-second pulsed laser heating was investigated by the time-of-flight (TOF) measurement. The desorption flow was studied at a wide range of desorption flux by varying the initially prepared Xe coverage Θ (1 ML = 4.5 × 10(18) atoms/m(2)). At Θ = 0.3 ML, the TOF of Xe was well represented by a Maxwell-Boltzmann velocity distribution, which is in good agreement with thermal desorption followed by collision-free flow. At Θ > 0.3 ML, the peak positions of the TOF spectra were shifted towards the smaller values and became constant at large Θ, which were well fitted with a shifted Maxwell-Boltzmann velocity distribution with a temperature TD and a stream velocity u. With TD fixed at 165 K, u was found to increase from 80 to 125 m/s with increasing Θ from 1.2 to 4 ML. At Θ > 4 ML, the value of u becomes constant at 125 m/s. The converging feature of u was found to be consistent with analytical predictions and simulated results based on the Knudsen layer formation theory. We found that the Knudsen layer formation in laser desorption is completed at Knudsen number Kn <0.39.

Effects of Hydrogen on the Electronic State and Electric Conductivity of the Rutile TiO2(110) Surface

The adsorption of atomic hydrogen on a rutile TiO2(110) surface was investigated by nuclear reaction analysis (NRA), ultraviolet photoelectron spectroscopy (UPS), and conductivity measurements. The TiO2(110) surface was annealed in O2 of 1 × 10−4 Pa, which is regarded as a quasi-stoichiometric surface. After exposure to atomic hydrogen, UPS showed a localized in-gap state (IGS) at about 0.8 eV below the Fermi level and downward band bending with a decrease in the work function. Along with these changes, the conductivity was increased by 2.9 µS/□. Our results indicate that hydrogen donates electrons to the substrate. The amount of charge transfer and electric conductivity are discussed on the basis of the experimental data.

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant (1)H((15)N,αγ)(12)C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a (15)N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the (1)H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~10(13) cm(-2) (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~10(18) cm(-3) (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal (15)N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, (1)H((15)N,αγ)(12)C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of (15)N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Photostimulated desorption of Xe from Au(001) surfaces via transient Xe − formation

Photostimulated desorption (PSD) of Xe atoms from the Au(001) surface in thermal and nonthermal regimes was investigated by the time-of-flight measurement at photon energies of 6.4 and 2.3 eV. Xe was desorbed in a thermal way at high laser fluence, which was in good agreement with theoretical simulations. At a low laser fluence, on the other hand, desorption was induced only at a photon energy of 6.4 eV by a nonthermal one-photon process. We argue that the nonthermal PSD occurs via transient formation of Xe

Mechanism of gate dielectric degradation by hydrogen migration from the cathode interface

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Nuclear Dynamics and Electronic Effects of Hydrogen on Solid Surfaces.

on Au(001). The lifetime of Xe

Dynamical observation of H-induced gate dielectric degradation through improved nuclear reaction analysis system

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Photoinduced conversion of carbon dioxide and water molecules to methanol on the surface of molybdenum oxide MoO x (x < 2)

is estimated to be

Thermal Desorption Spectroscopy

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