Atomic bonding state of silicon oxide anodized in extremely diluted hydrofluoric solution

Arai, Taiki*; Yoshigoe, Akitaka; Motohashi, Mitsuya*

Zairyo No Kagaku To Kogaku, 60(5), p.153 - 158, 2023/10

Si oxide films are currently widely used as insulating materials in electronic devices and biomaterials. The atomic bonding state of these films significantly influences the properties of each device, thus it is particularly necessary to understand and control the chemical bonding state between Si and O in the films. In this study, the Si oxide films formed by anodic oxidation on Si substrate surfaces in extremely low concentrations of HF solutions were analyzed by X-ray photoelectron spectroscopy mainly focusing on Si2p and F1s spectra. Although the HF concentration is in the order of ppm, the films contain percent order of F atoms, suggesting the formation of Si-F and Si-O-F bonds in the films. It was also found that the different depth profiles for F and O atoms was observed, indicating that the surface reaction processes seem to be different depending on each element.

Application of vibrational sum frequency generation spectroscopy to studies of chemical reactions on water surface and actinide interface chemistry

Kusaka, Ryoji

Bunko Kenkyu, 72(4), p.155 - 162, 2023/08

Vibrational sum frequency generation (VSFG) spectroscopy is an optical second-order nonlinear vibrational spectroscopy using ultrashort pulse lasers. Because VSFG spectroscopy is a unique and powerful tool for studying molecular structures of interfaces, it has been widely used in many research fields. However, there still undoubtedly remains some VSFG research areas that have not studied well, partly because VSFG measurements are not so easily performed in comparison with relatively general spectroscopy methods. This review presented recent applications of VSFG spectroscopy to two research topics: (1) chemical reactions on water surfaces, and (2) actinide chemistry.

Study on chemical interaction between UO and Zr at precisely controlled high temperatures

Shirasu, Noriko; Sato, Takumi; Suzuki, Akihiro*; Nagae, Yuji; Kurata, Masaki

Journal of Nuclear Science and Technology, 60(6), p.697 - 714, 2023/06

https://doi.org/10.1080/00223131.2022.2138602

Interaction tests between UO and Zr were performed at precisely controlled high temperatures between 1840 and 2000 C to understand the interaction mechanism in detail. A Zr rod was inserted in a UO crucible and then heat-treated at a fixed temperature in Ar-gas flow for 10 min. After heating in the range of 1890 to 1930 C, the Zr rod was deformed to a round shape, in which the post-analysis detected the significant diffusion of U into the Zr region and the formation of a dominant -Zr(O) matrix and a small amount of U-Zr-O precipitates. The abrupt progress of liquefaction was observed in the sample heated at around 1940 C or higher. The higher oxygen concentration in the -Zr(O) matrix suppressed the liquefaction progress, due to the variation in the equilibrium state. The U-Zr-O melt formation progressed by the selective dissolution of Zr from the matrix, and the selective diffusion of U could occur via the U-Zr-O melt.

Work function lowering of LaB by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodes

Yamaguchi, Hisato*; Yusa, Ryunosuke*; Wang, G.*; Pettes, M. T.*; Liu, F.*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Abukawa, Tadashi*; Moody, N. A.*; Ogawa, Shuichi*

Applied Physics Letters, 122(14), p.141901_1 - 141901_7, 2023/04

https://doi.org/10.1063/5.0142591

A lowering of work function for LaB by monolayer hexagonal BN coating is reported. Photoemission electron microcopy (PEEM) and thermionic emission electron microscopy (TEEM) both revealed that the hBN coated region of a LaB(100) single crystal has lower work function compared to the bare (i.e., non-coated) and graphene coated regions. A larger decrease of work function for the hBN coated LaB(100) compared to graphene coated LaB(100) was qualitatively supported by our density functional theory (DFT) calculations. Adding an oxide layer in the calculations improved consistency between the calculation and experimental results. We followed up our calculations with synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS) and confirmed the presence of an oxide layer on our LaB.

Lanthanide and actinide ion complexes containing organic ligands investigated by surface-enhanced infrared absorption spectroscopy

Hirata, Sakiko*; Kusaka, Ryoji; Meiji, Shogo*; Tamekuni, Seita*; Okudera, Kosuke*; Hamada, Shoken*; Sakamoto, Chihiro*; Honda, Takumi*; Matsushita, Kosuke*; Muramatsu, Satoru*; et al.

Inorganic Chemistry, 62(1), p.474 - 486, 2023/01

https://doi.org/10.1021/acs.inorgchem.2c03618

Roles of excess minority carrier recombination and chemisorbed O species at SiO/Si interfaces in Si dry oxidation; Comparison between p-Si(001) and n-Si(001) surfaces

Tsuda, Yasutaka; Yoshigoe, Akitaka; Ogawa, Shuichi*; Sakamoto, Tetsuya*; Yamamoto, Yoshiki*; Yamamoto, Yukio*; Takakuwa, Yuji*

Journal of Chemical Physics, 157(23), p.234705_1 - 234705_21, 2022/12

https://doi.org/10.1063/5.0109558

Restoration of oxygen vacancies on an anatase TiO(001) surface with supersonic seeded oxygen molecular beam

Katsube, Daiki*; Ono, Shinya*; Inami, Eiichi*; Yoshigoe, Akitaka; Abe, Masayuki*

Vacuum and Surface Science, 65(11), p.526 - 530, 2022/11

https://doi.org/10.1380/vss.65.526

The oxidation of oxygen vacancies at the surface of anatase TiO (001) was investigated by synchrotron radiation photoelectron spectroscopy and supersonic O beam (SSMB). The oxygen vacancies at the top surface and subsurface could be eliminated by the supply of hyperthermal oxygen molecules. Oxygen vacancies are present on the surface of anatase TiO(001) when it is untreated before transfer to a vacuum chamber. These vacancies, which are stable in the ambient condition, could also be effectively eliminated by using oxygen SSMB. This result is promising as a surface processing for various functional oxides.

Analysis of the electric double layer structure formed in an ionic liquid using neutron reflectivity

Tamura, Kazuhisa; Akutsu-Suyama, Kazuhiro*; Cagnes, M.*; Darwish, T. A.*

ECS Advances (Internet), 1(2), p.020503_1 - 020503_5, 2022/06

https://doi.org/10.1149/2754-2734/ac6963

The ionic liquid/Si electrode interface was investigated using neutron reflectivity. We precisely elucidated the structure of the electrical double layer formed at 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM]TFSA)/Si(100) electrode interface with the orientation of the [BMIM]TFSA molecule using a partially deuterated [BMIM]TFSA. The results revealed that [BMIM]TFSA molecules form a layered structure. Cation and anion molecules are alternatingly stacked and molecules in the first three layers are horizontally oriented to the electrode surface at E = -1.2 V, i.e., on the negatively charged electrode surface. It was also revealed that the imidazole ring in [BMIM] cation is parallel to the electrode surface.

Preface for the special issue "Ambient pressure X-ray photoelectron spectroscopy for observation of surface reactions; Present status and future prospects"

Yoshigoe, Akitaka; Ogawa, Shuichi*; Takakuwa, Yuji*

Hoshako, 35(3), P. 157, 2022/05

Ambient Pressure-XPS [AP-XPS] has been in operation at synchrotron radiation facilities around the world since around 2000 to study gas-surface reactions. Furthermore, it is also applied to observe liquid and solid-liquid interfaces. Development and utilization research is underway as a tool to take the true picture of surface phenomena. A special issue on AP-XPS for the Journal of the Japanese Society for Synchrotron Radiation Research was designed and its purpose is explained.

Impact of nitridation on the reliability of 4H-SiC(110) MOS devices

Nakanuma, Takato*; Kobayashi, Takuma*; Hosoi, Takuji*; Sometani, Mitsuru*; Okamoto, Mitsuo*; Yoshigoe, Akitaka; Shimura, Takayoshi*; Watanabe, Heiji*

Applied Physics Express, 15(4), p.041002_1 - 041002_4, 2022/04

https://doi.org/10.35848/1882-0786/ac5ace

The leakage current and flat-band voltage (VFB) instability of NO-nitrided SiC (110) (a-face) MOS devices were systematically investigated. Although NO nitridation is effective in improving the interface properties, we found that it reduces the onset field of Fowler-Nordheim (F-N) current by about 1 MVcm, leading to pronounced leakage current. Synchrotron X-ray photoelectron spectroscopy revealed that the nitridation reduces the conduction band offset at the SiO/SiC interface, corroborating the above finding. Furthermore, systematical positive and negative bias stress tests clearly indicated the VFB instability of nitrided a-face MOS devices against electron and hole injection.