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Journal Articles

Evaluation of doped potassium concentrations in stacked two-Layer graphene using real-time XPS

Ogawa, Shuichi*; Tsuda, Yasutaka; Sakamoto, Tetsuya*; Okigawa, Yuki*; Masuzawa, Tomoaki*; Yoshigoe, Akitaka; Abukawa, Tadashi*; Yamada, Takatoshi*

Applied Surface Science, 605, p.154748_1 - 154748_6, 2022/12

 Times Cited Count:0 Percentile:0.01(Chemistry, Physical)

Immersion of graphene in KOH solution improves its mobility on SiO$$_{2}$$/Si wafers. This is thought to be due to electron doping by modification with K atoms, but the K atom concentration C$$_{K}$$ in the graphene has not been clarified yet. In this study, the C$$_{K}$$ was determined by XPS analysis using high-brilliance synchrotron radiation. The time evolution of C$$_{K}$$ was determined by real-time observation, and the C$$_{K}$$ before irradiation of synchrotron radiation was estimated to be 0.94%. The C 1s spectrum shifted to the low binding energy side with the desorption of K atoms. This indicates that the electron doping concentration into graphene is decreasing, and it is experimentally confirmed that K atoms inject electrons into graphene.

Journal Articles

Roles of excess minority carrier recombination and chemisorbed O$$_{2}$$ species at SiO$$_{2}$$/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

 Times Cited Count:0 Percentile:0.01(Chemistry, Physical)

Journal Articles

Restoration of oxygen vacancies on an anatase TiO$$_{2}$$(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

The oxidation of oxygen vacancies at the surface of anatase TiO$$_{2}$$ (001) was investigated by synchrotron radiation photoelectron spectroscopy and supersonic O$$_{2}$$ 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$$_{2}$$(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.

Journal Articles

Observation of chemisorbed O$$_2$$ molecule at SiO$$_2$$/Si(001) interface during Si dry oxidation

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

e-Journal of Surface Science and Nanotechnology (Internet), 21(1), p.30 - 39, 2022/11

Journal Articles

Electrical properties and energy band alignment of SiO$$_{2}$$/GaN metal-oxide-semiconductor structures fabricated on N-polar GaN(000$$bar{1}$$) substrates

Mizobata, Hidetoshi*; Tomigahara, Kazuki*; Nozaki, Mikito*; Kobayashi, Takuma*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*

Applied Physics Letters, 121(6), p.062104_1 - 062104_6, 2022/08

The interface properties and energy band alignment of SiO$$_{2}$$/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN(000$$bar{1}$$) substrates were investigated by electrical measurements and synchrotron-radiation X-ray photoelectron spectroscopy. They were then compared with those of SiO$$_{2}$$/GaN MOS structures on Ga-polar GaN(0001). Although the SiO$$_{2}$$/GaN(000$$bar{1}$$) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO$$_{2}$$/GaN(000$$bar{1}$$) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO$$_{2}$$/GaN(000$$bar{1}$$) was smaller than that for SiO$$_{2}$$/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN(000$$bar{1}$$) substrates for MOS device fabrication.

Journal Articles

Impact of post-nitridation annealing in CO$$_{2}$$ ambient on threshold voltage stability in 4H-SiC metal-oxide-semiconductor field-effect transistors

Hosoi, Takuji*; Osako, Momoe*; Moges, K.*; Ito, Koji*; Kimoto, Tsunenobu*; Sometani, Mitsuru*; Okamoto, Mitsuo*; Yoshigoe, Akitaka; Shimura, Takayoshi*; Watanabe, Heiji*

Applied Physics Express, 15(6), p.061003_1 - 061003_5, 2022/06

 Times Cited Count:0 Percentile:0(Physics, Applied)

The combination of NO annealing and subsequent post-nitridation annealing (PNA) in CO$$_{2}$$ ambient for SiO$$_{2}$$/SiC structures has been demonstrated to be effective in obtaining both high channel mobility and superior threshold voltage stability in SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs). N atoms on the SiO$$_{2}$$ side of the SiO$$_{2}$$/SiC interface incorporated by NO annealing, which are plausible cause of charge trapping sites, could be selectively removed by CO$$_{2}$$-PNA at 1300$$^{circ}$$C without oxidizing the SiC. CO$$_{2}$$-PNA was also effective in compensating oxygen vacancies in SiO$$_{2}$$, resulting high immunity against both positive and negative bias-temperature stresses.

Journal Articles

Hydrogen absorption and diffusion behaviors in cube-shaped palladium nanoparticles revealed by ambient-pressure X-ray photoelectron spectroscopy

Tang, J.*; Seo, O.*; Rivera Rocabado, D. S.*; Koitaya, Takanori*; Yamamoto, Susumu*; Namba, Yusuke*; Song, C.*; Kim, J.*; Yoshigoe, Akitaka; Koyama, Michihisa*; et al.

Applied Surface Science, 587, p.152797_1 - 152797_8, 2022/06

 Times Cited Count:4 Percentile:90.27(Chemistry, Physical)

The hydrogen absorption and diffusion mechanisms on cube-shaped Pd nanoparticles (NPs) which are important hydrogen-storage materials were studied using X-ray photoelectron spectroscopy and DFT calculations. In the surface region, hydrogen absorption showed almost similar behavior regardless of the NPs size. It was found that the octahedral sites are more favorable than the tetrahedral sites for hydrogen occupation. We also clarified that the hydrogen atoms absorbing on the smaller-sized Pd NPs diffuse to the subsurface more actively because of the weakened Pd-H bond by the surface disordering, which plays an important role in hydrogen adsorption at a low H$$_{2}$$ pressure.

Journal Articles

Comprehensive physical and electrical characterizations of NO nitrided SiO$$_{2}$$/4H-SiC(11$$overline{2}$$0) interfaces

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

Japanese Journal of Applied Physics, 61(SC), p.SC1065_1 - SC1065_8, 2022/05

 Times Cited Count:4 Percentile:90.27(Physics, Applied)

Nitridation of SiO$$_{2}$$/4H-SiC(11$$overline{2}$$0) interfaces with post-oxidation annealing in an NO ambient (NO-POA) and its impact on the electrical properties were investigated. Sub-nm-resolution nitrogen depth profiling at the interfaces was conducted by using a scanning X-ray photoelectron spectroscopy microprobe. The results showed that nitrogen atoms were incorporated just at the interface and that interface nitridation proceeded much faster than at SiO$$_{2}$$/SiC(0001) interfaces, resulting in a 2.3 times higher nitrogen concentration. Electrical characterizations of metal-oxide-semiconductor capacitors were conducted through capacitance-voltage ($$C-V$$) measurements in the dark and under illumination with ultraviolet light to evaluate the electrical defects near the conduction and valence band edges and those causing hysteresis and shifting of the $$C-V$$ curves. While all of these defects were passivated with the progress of the interface nitridation, excessive nitridation resulted in degradation of the MOS capacitors. The optimal conditions for NO-POA are discussed on the basis of these experimental findings.

Journal Articles

Impact of nitridation on the reliability of 4H-SiC(11$$bar{2}$$0) 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

 Times Cited Count:1 Percentile:55.51(Physics, Applied)

The leakage current and flat-band voltage (VFB) instability of NO-nitrided SiC (11$$bar{2}$$0) (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$$^{-1}$$, leading to pronounced leakage current. Synchrotron X-ray photoelectron spectroscopy revealed that the nitridation reduces the conduction band offset at the SiO$$_{2}$$/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.

Journal Articles

Oxidation mechanisms of hafnium overlayers deposited on an Si(111) substrate

Kakiuchi, Takuhiro*; Matoba, Tomoki*; Koyama, Daisuke*; Yamamoto, Yuki*; Yoshigoe, Akitaka

Langmuir, 38(8), p.2642 - 2650, 2022/03

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

0xidation processes at the interface and the surface of Si(111) substrate with thin Hf films were studied using photoelectron spectroscopy in conjunction with supersonic oxygen molecular beams (SOMB). The oxidation starts at the outermost Hf layers and produces stoichiometric HfO$$_{2}$$. Hf silicates (Hf-O-Si configuration) were generated in the vicinity of the HfO$$_{2}$$/Si interface in the case of the irradiation of 2.2 eV SOMB. The oxidation of the Si substrate takes place to generate SiO$$_{2}$$ compounds. Si atoms were emitted from the SiO$$_{2}$$/Si interface region underneath the HfO$$_{2}$$ overlayers to release the stress generated within the strained Si layers. The emitted Si atoms can pass through the HfO$$_{2}$$ overlayers and react with the impinging O$$_{2}$$ gas.

Journal Articles

High reactivity of H$$_{2}$$O vapor on GaN surfaces

Sumiya, Masatomo*; Sumita, Masato*; Tsuda, Yasutaka; Sakamoto, Tetsuya; Sang, L.*; Harada, Yoshitomo*; Yoshigoe, Akitaka

Science and Technology of Advanced Materials, 23(1), p.189 - 198, 2022/00

 Times Cited Count:2 Percentile:72.12(Materials Science, Multidisciplinary)

GaN is an attracting material for power-electronic devices. Understanding the oxidation at GaN surface is important for improving metal-oxide-semiconductor (MOS) devices. In this study, the oxidation at GaN surfaces depending on the GaN crystal planes (+c, -c, and m-plane) was investigated by real time XPS and DFT-MD simulation. We found that H$$_{2}$$O vapor has the highest reactivity due to the spin interaction between H$$_{2}$$O and GaN surfaces. The bond length between the Ga and N on the -c GaN surface was increased by OH attacking the back side of three-fold Ga atom. The chemisorption on the m-plane was dominant. The intense reactions of oxidation and Al$$_{x}$$Ga$$_{1-x}$$N formation for p-GaN were observed at the interface of the Al$$_{2}$$O$$_{3}$$ layer deposited by ALD using H$$_{2}$$O vapor. This study suggests that an oxidant gas other than H$$_{2}$$O and O$$_{2}$$ should be used to avoid unintentional oxidation during Al$$_{x}$$Ga$$_{1-x}$$N atomi layer deposition.

Journal Articles

Revisit of XPS studies of supersonic O$$_{2}$$ molecular adsorption on Cu(111); Copper oxides

Hayashida, Koki*; Tsuda, Yasutaka; Yamada, Takashi*; Yoshigoe, Akitaka; Okada, Michio*

ACS Omega (Internet), 6(40), p.26814 - 26820, 2021/10

 Times Cited Count:2 Percentile:28.42(Chemistry, Multidisciplinary)

We report the X-ray photoemission spectroscopy (XPS) characterization of the bulk Cu$$_{2}$$O(111) surface and 8-type and 29-type oxide structures on Cu(111) prepared by using 0.5 eV O$$_{2}$$ supersonic molecular beam (SSMB) source. We propose a new structural model for the 8-type oxide structure and also confirmed the previously proposed model for the [29] oxide structure on Cu(111), based on the O1s XPS spectra. The detection-angle dependence of the O 1s spectra supports that the nanopyramidal model is more preferable for the ($$sqrt{3}$$X$$sqrt{3}$$)R30$$^{circ}$$ Cu$$_{2}$$O(111). We also report the electronic excitations which O1s electrons suffer.

Journal Articles

Oxidation of anatase TiO$$_{2}$$(001) surface using supersonic seeded oxygen molecular beam

Katsube, Daiki*; Ono, Shinya*; Takayanagi, Shuhei*; Ojima, Shoki*; Maeda, Motoyasu*; Origuchi, Naoki*; Ogawa, Arata*; Ikeda, Natsuki*; Aoyagi, Yoshihide*; Kabutoya, Yuito*; et al.

Langmuir, 37(42), p.12313 - 12317, 2021/10

 Times Cited Count:1 Percentile:14.13(Chemistry, Multidisciplinary)

We investigated the oxidation of oxygen vacancies at the surface of anatase TiO$$_{2}$$(001) using supersonic seeded molecular beam (SSMB) of oxygen. The oxygen vacancies at the top-surface and sub-surface could be eliminated by the supply of oxygen using an SSMB. These results indicate that the interstitial vacancies can be mostly assigned to oxygen vacancies, which can be effectively eliminated by using an oxygen SSMB. Oxygen vacancies are present on the surface of anatase TiO$$_{2}$$(001) when it is untreated before transfer to a vacuum chamber. These vacancies, which are stable in the as-grown condition, could also be effectively eliminated using the oxygen SSMB.

Journal Articles

Two-step model for reduction reaction of ultrathin nickel oxide by hydrogen

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

Journal of Vacuum Science and Technology A, 39(4), p.043207_1 - 043207_9, 2021/07

 Times Cited Count:1 Percentile:16.47(Materials Science, Coatings & Films)

Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes were investigated by in situ time-resolved photoelectron spectroscopy. We propose a two-step reduction reaction model. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H$$_{2}$$ molecules.

Journal Articles

Oxidation reaction kinetics on transition metal surfaces observed by real-time photoelectron spectroscopy

Ogawa, Shuichi*; Zhang, B.*; Yoshigoe, Akitaka; Takakuwa, Yuji*

Vacuum and Surface Science, 64(5), p.218 - 223, 2021/05

The oxidation reaction kinetics on Ti(0001) and Ni(111) surfaces were observed by real-time photoelectron spectroscopy using synchrotron radiation to measure the oxidation state and oxide thickness. After the Ti(0001) surface was wholly covered by TiO with a thickness of 1.2 nm, the rapid growth of n-type TiO$$_{2}$$ proceeded through the diffusion of Ti$$^{4+}$$ ions to the TiO$$_{2}$$ surface at 400$$^{circ}$$C. A saturation of oxygen uptake on the TiO surface indicates that the O$$_{2}$$ sticking coefficient on the TiO surface is negligibly small and the segregation of Ti to the TiO surface is a trigger to initiate the TiO$$_{2}$$ growth. On the Ni(111) surface at 350$$^{circ}$$C, a thermally stable NiO$$_{x}$$ proceeded preferentially and then the growth of p-type NiO was initiated. The time evolution of NiO thickness was represented by a logarithmic growth model, where the NiO growth is governed by the electron tunneling to the NiO surface.

Journal Articles

Interface atom mobility and charge transfer effects on CuO and Cu$$_{2}$$O formation on Cu$$_{3}$$Pd(111) and Cu$$_{3}$$Pt(111)

Tsuda, Yasutaka; Gueriba, J. S.*; Makino, Takamasa*; Di$~n$o, W. A.*; Yoshigoe, Akitaka; Okada, Michio*

Scientific Reports (Internet), 11, p.3906_1 - 3906_8, 2021/02

 Times Cited Count:2 Percentile:9.42(Multidisciplinary Sciences)

Journal Articles

Development of ${it spatiotemporal}$ measurement and analysis techniques in X-ray photoelectron spectroscopy; From NAP-HARPES to 4D-XPS

Toyoda, Satoshi*; Yamamoto, Tomoki*; Yoshimura, Masashi*; Sumida, Hirosuke*; Mineoi, Susumu*; Machida, Masatake*; Yoshigoe, Akitaka; Suzuki, Satoru*; Yokoyama, Kazushi*; Ohashi, Yuji*; et al.

Vacuum and Surface Science, 64(2), p.86 - 91, 2021/02

We have developed ${it spatiotemporal}$ measurement and analysis techniques in X-ray photoelectron spectroscopy. To begin with, time-division depth profiles of gate stacked film interfaces have been achieved by NAP-HARPES (Near Ambient Pressure Hard X-ray Angle-Resolved Photo Emission Spectroscopy) data. We then have promoted our methods to quickly perform peak fittings and depth profiling from time-division ARPES data, which enables us to realize 4D-XPS analysis. It is found that the traditional maximum entropy method (MEM) combined with Jackknife averaging of sparse modeling in NAP-HARPES data is effective to perform dynamic measurement of depth profiles with high precision.

Journal Articles

Surface segregation effect for prevention of oxidation in Ni-X (X=Sn, Sb) alloy by in situ photoelectron spectroscopy

Doi, Takashi*; Yoshigoe, Akitaka

Surface and Interface Analysis, 52(12), p.1117 - 1121, 2020/12

 Times Cited Count:1 Percentile:4.05(Chemistry, Physical)

Ni-base alloys has been widely used for chemical plants because of their high strength and excellent oxidation resistance. In particular, the addition of Sn and Sb in Ni-base alloys significantly improves the metal dusting resistance. It is indicated that Sn and Sb are segregated on the alloy surface in the metal dusting environment; however, the details have not been clarified yet. The behavior of the Ni-Sn and Ni-Sb alloys under a high-temperature oxidation environment was investigated by in situ X-ray photoelectron spectroscopy. It was confirmed that Sn and Sb have been segregated at the surface of their alloys during oxidation in low oxygen potential environment. These results indicate that Sn and Sb segregation improves the metal dusting resistance.

Journal Articles

Dynamic observation and theoretical analysis of initial O$$_{2}$$ molecule adsorption on polar and $$m$$-plane surfaces of GaN

Sumiya, Masatomo*; Sumita, Masato*; Asai, Yuya*; Tamura, Ryo*; Uedono, Akira*; Yoshigoe, Akitaka

Journal of Physical Chemistry C, 124(46), p.25282 - 25290, 2020/11

 Times Cited Count:5 Percentile:29.29(Chemistry, Physical)

The initial oxidation of different GaN surfaces [the polar Ga-face (+c) and N-face (-c) and the nonpolar (10$$bar{1}$$0) ($$m$$)plane] under O$$_{2}$$ molecular beam irradiation was studied by real-time synchrotron radiation X-ray photoelectron spectroscopy and DFT molecular dynamics calculation. The results predict that triplet O$$_{2}$$ either dissociates or chemisorbs at the bridge position on the +c-surface, while on N-terminated -c-surface the O$$_{0}$$2 molecule only undergoes dissociative chemisorption. On the $$m$$-GaN surface, although the dissociation of O$$_{2}$$ is dominant, the bond length and angle were found to fluctuate from those of O$$_{2}$$ molecules adsorbed on the polar surfaces. The computational model including both the surface spin and polarity of GaN is useful for understanding the interface between GaN and oxide layers in metal-oxide electronic.

Journal Articles

Gas barrier properties of chemical vapor-deposited graphene to oxygen imparted with sub-electronvolt kinetic energy

Ogawa, Shuichi*; Yamaguchi, Hisato*; Holby, E. F.*; Yamada, Takatoshi*; Yoshigoe, Akitaka; Takakuwa, Yuji*

Journal of Physical Chemistry Letters (Internet), 11(21), p.9159 - 9164, 2020/11

 Times Cited Count:3 Percentile:26.62(Chemistry, Physical)

Atomically thin layers of graphene have been proposed to protect surfaces through the direct blocking of corrosion reactants such as oxygen with low added weight. The long term efficacy of such an approach, however, is unclear due to the long-term desired protection of decades and the presence of defects in as-synthesized materials. Here, we demonstrate catalytic permeation of oxygen molecules through previously-described impermeable graphene by imparting sub-eV kinetic energy to molecules. These molecules represent a small fraction of a thermal distribution thus this exposure serves as an accelerated stress test for understanding decades-long exposures. The permeation rate of the energized molecules increased 2 orders of magnitude compared to their non-energized counterpart. Graphene maintained its relative impermeability to non-energized oxygen molecules even after the permeation of energized molecules indicating that the process is non-destructive and a fundamental property of the exposed material.

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