Mekaru, Harutaka*; Yoshigoe, Akitaka; Nakamura, Michihiro*; Doura, Tomohiro*; Tamanoi, Fuyuhiko*
ACS Applied Nano Materials (Internet), 2(1), p.479 - 488, 2019/01
Organosilica nanoparticles are attractive for use as drug delivery systems for cancer therapy. Ideally, the nanoparticles need to degrade in the body after drug delivery to minimize their risk. However, the biodegradation of organosilica nanoparticles has not been thoroughly investigated. In this study, we used X-ray photoelectron spectroscopy and field-emission scanning electron microscopy to investigate the degradation of organosilica nanoparticles by glutathione, a peptide found in cells. Our results indirectly indicate that glutathione is able to reduce the disulfide bonds present in the network of one type of the nanoparticles, promoting dissociation of these nanoparticles. The dissociated nanoparticles form large maple leaf-shaped structures under certain conditions. We consider the formation mechanism of these symmetrical structures.
X-sen Kodenshi Bunkoho, p.271 - 282, 2018/12
Based on the series of author's studies, the author explains the time-resolved synchrotron radiation X-ray photoelectron spectroscopy and supersonic oxygen molecular beam study on the adsorption reaction dynamics of oxygen molecules at the Si single crystal surface in the section 5.10 of the new textbook "X-ray Photoelectron Spectroscopy" (Kodan-sha) edited by Yuji Takakuwa.
Watanabe, Kenta*; Terashima, Daiki*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Ishida, Masahiro*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; et al.
Japanese Journal of Applied Physics, 57(6S3), p.06KA03_1 - 06KA03_6, 2018/06
The advantage of SiO/AlON stacked gate dielectrics over SiO, AlON and AlO single dielectric layers was demonstrated. Our systematic research revealed that the optimized stacked structure with 3.3-nm-thick AlON interlayer is beneficial in terms of superior interface quality, reduced gate leakage current and C-V hysteresis for next-generation high frequency and high power AlGaN/GaN MOS-HFETs.
Nozaki, Mikito*; Watanabe, Kenta*; Yamada, Takahiro*; Shih, H.-A.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Japanese Journal of Applied Physics, 57(6S3), p.06KA02_1 - 06KA02_7, 2018/06
We fabricated AlON dielectric films by repeating thin AlN deposition and in situ O oxidation for AlGaN/GaN MOS-HFETs. Uniform nitrogen distribution is achievable by the proposed ALD-based process and that nitrogen concentration can be precisely controlled by changing AlN thickness (ALD cycle number) in each step. It was found that AlON films grown by ALD system offers significant advantages in terms of practical application while keeping superior Vth stability and electrical properties at the insulator/AlGaN interface in AlGaN/GaN MOS-HFETs.
Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Hisashi*; Takahashi, Tokio*; Shimizu, Mitsuaki*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 11(1), p.015701_1 - 015701_4, 2018/01
A simple and feasible method for fabricating high-quality and highly reliable GaN-based metal-oxide-semiconductor (MOS) devices was developed on the basis of systematic physical and electrical characterizations. Chemical vapor deposition of SiO films directly onto GaN substrates forming Ga-oxide interlayers was used to fabricate SiO/GaO/GaN stacked structures. Although well-behaved hysteresis-free GaN-MOS capacitors with extremely low interface state density below 10cmeV were obtained by post-deposition annealing, Ga diffusion into overlying SiO layers severely degraded the insulating property and dielectric breakdown characteristics of the MOS devices. However, this problem was found to be solved by employing rapid thermal processing, leading to superior performance of the GaN-MOS devices in terms of interface quality, insulating property and gate dielectric reliability.
Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Applied Physics Letters, 111(4), p.042102_1 - 042102_5, 2017/07
AlGaN/GaN HFET (hetero-junction field-effect transitor) has gained much attention as next-generation high frequency and high power devices. In this study, we systematically investigated the interface reaction between Al-based dielectrics (AlO and AlON) and AlGaN layer during deposition and post-deposition annealing (PDA), and revealed high thermal stability of AlON/AlGaN interface.
Yamada, Takahiro*; Ito, Joyo*; Asahara, Ryohei*; Watanabe, Kenta*; Nozaki, Mikito*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; et al.
Journal of Applied Physics, 121(3), p.035303_1 - 035303_9, 2017/01
Initial oxidation of GaN(0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of X-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy and X-ray diffraction measurements. It was found that, whereas initial oxide formation tends to saturate at temperatures below 800C, selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth leading to rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also revealed that polycrystalline - and -phase GaO grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. On the basis of these experimental findings, we also developed a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport.
Okada, Michio*; Tsuda, Yasutaka*; Yoshigoe, Akitaka; Dio, W. A.*
Do To Dogokin, 56(1), p.232 - 236, 2017/00
We reported the our studies on the surface temperature (Ts) dependence of oxidation on the CuAu(111) surface by a supersonic O molecular beam, using synchrotron radiation X-ray photoemission spectroscopy. Clean surface shows strong Au segregation to the top layer, i.e., Au surface enrichment of the clean surface. Complete Cu segregation to the surface occurs at 0.5 ML O surface coverage. The Au-rich second and third layers of the oxidized surface demonstrate the protective layer formation against oxidation deeper into the bulk. We found that CuO formation occurs. At Ts = 300K, the CuO growth is not so effective. The surface oxidation is less effective on CuAu(111) than on Cu(111). At Ts = 400K, the protection by the Au-rich layer against oxidation into bulk is effective. At Ts = 500K, the Au protective layer is broken due to effective Au diffusion and thus CuO grows deeper into bulk.
Asahara, Ryohei*; Nozaki, Mikito*; Yamada, Takahiro*; Ito, Joyo*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Applied Physics Express, 9(10), p.101002_1 - 101002_4, 2016/10
The superior physical and electrical properties of AlON gate dielectrics on AlGaN/GaN substrates in terms of thermal stability, reliability, and interface quality were demonstrated by direct AlON deposition and subsequent annealing. Nitrogen incorporation into alumina was proven to be beneficial both for suppressing intermixing at the insulator/AlGaN interface and reducing the number of electrical defects in AlO films. Consequently, we achieved high-quality AlON/AlGaN/GaN metal-oxide-semiconductor capacitors with improved stability against charge injection and a reduced interface state density as low as 1.210 cmeV. The impact of nitrogen incorporation into the insulator was discussed on the basis of experimental findings.
Nozaki, Mikito*; Ito, Joyo*; Asahara, Ryohei*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 9(10), p.105801_1 - 105801_4, 2016/10
Interface reactions between Ti-based electrodes and n-type GaN epilayers were investigated by synchrotron radiation X-ray photoelectron spectroscopy. Metallic Ga and thin TiN alloys were formed at the interface by subsequently depositing Al capping layers on ultrathin Ti layers even at room temperature. By comparing results from stacked Ti/Al and single Ti electrodes, the essential role of Al capping layers serving as an oxygen-scavenging element to produce reactive Ti underlayers was demonstrated. Further growth of the metallic interlayer during annealing was observed. A strategy for achieving low-resistance ohmic contacts to n-GaN with low-thermal-budget processing is discussed.
Ogawa, Shuichi*; Tang, J.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Takakuwa, Yuji*
Journal of Chemical Physics, 145(11), p.114701_1 - 114701_7, 2016/09
Enhancement of SiO/Si(001) interfacial oxidation induced by thermal strain during rapid thermal oxidation was revealed by real time photoelectron spectroscopy using high intensity and high energy-resolution synchrotron radiation. This experimental result indicates the usefulness of the unified Si oxidation reaction model mediated by point defect generation.
Okada, Michio*; Tsuda, Yasutaka*; Oka, Kohei*; Kojima, Kazuki*; Dio, W. A.*; Yoshigoe, Akitaka; Kasai, Hideaki*
Scientific Reports (Internet), 6, p.31101_1 - 31101_8, 2016/08
We report results of our experimental and theoretical studies on the oxidation of Cu-Au alloy surfaces, viz., CuAu(111), CuAu(111), and AuCu(111), using hyperthermal O molecular beam (HOMB). We observed strong Au segregation to the top layer of the corresponding clean (111) surfaces. This forms a protective layer that hinders further oxidation into the bulk. The higher the concentration of Au in the protective layer formed, the higher the protective efficacy. As a result, of the three Cu-Au surfaces studied, AuCu(111) is the most stable against dissociative adsorption of O, even with HOMB. We also found that this protective property breaks down for oxidations occurring at temperatures above 300 K.
Esaka, Fumitaka; Nojima, Takehiro; Udono, Haruhiko*; Magara, Masaaki; Yamamoto, Hiroyuki
Surface and Interface Analysis, 48(7), p.432 - 435, 2016/07
XPS is widely used for non-destructive chemical state analysis of solid materials. In this method, depth profiling can be carried out by a combination with ion beam sputtering. However, the sputtering often causes segregation and preferential sputtering of atoms and gives inaccurate information. The use of energy-tunable X-rays from synchrotron radiation (SR) enables us to perform non-destructive depth profiling in XPS. Here, the analytical depth can be changed by changing excitation X-ray energy. In the present study, we examined methods to perform depth profiling with XPS by changing excitation energy and XAS by changing electron energy for detection. These methods were then applied to the analysis of native surface oxide layers on MgSi crystals. In this XAS analysis, the peak at 1843.4 eV becomes dominant when the electron energy for detection increases, which implies that Si-O or Si-O-Mg structure is formed as the surface oxide layer on the MgSi.
Doi, Takashi*; Nishiyama, Yoshitaka*; Yoshigoe, Akitaka; Teraoka, Yuden
Surface and Interface Analysis, 48(7), p.685 - 688, 2016/07
Ni-based alloys has been widely used for plant application because of their high strength and excellent oxidation resistance. In particular, the addition of Cu in Ni-based alloys significantly improves the metal dusting resistance. It is indicated that Cu is segregated on the alloy surface in the metal dusting environment; however, the details have not been clarified yet. The behavior of Ni-2Cu alloy under a high temperature oxidation environment was investigated using X-ray photoelectron spectroscopy. It was confirmed that Cu have been segregated at the surface of Ni-2Cu alloy during oxidation. These results propose that the Cu segregation improves the metal dusting resistance.
Tsuda, Yasutaka*; Yoshigoe, Akitaka; Teraoka, Yuden; Okada, Michio*
Materials Research Express (Internet), 3(3), p.035014_1 - 035014_8, 2016/03
We report a study on the surface-temperature () dependence of oxidation process at CuAu(111) by using a hyperthermal oxygen molecular beam and synchrotron-radiation X-ray photoemission spectroscopy. The O-1s spectra and the corresponding O-uptake curves demonstrate that CuO domains grow effectively at high of 400 and 500 K. The simple analysis of the O distribution suggests that the temperature-induced atomic diffusion causes the CuO domains growing thicker at 500 K. The oxidation of CuAu(111) is less efficient at = 300-500 K than that of Cu(111), demonstrating that the protective nature of CuAu against oxidation, in comparison to Cu, remains even at high .
Fan, M.*; Xu, Y.*; Sakurai, Junya*; Demura, Masahiko*; Hirano, Toshiyuki*; Teraoka, Yuden; Yoshigoe, Akitaka
International Journal of Hydrogen Energy, 40(37), p.12663 - 12673, 2015/10
The catalytic properties of single-phase NiSn powder in the production of hydrogen via the decomposition of methanol were investigated in isothermal tests at 713, 793, and 873 K. The catalytic activity of NiSn significantly increased with time at 793 and 873 K, but not at 713 K, suggesting that NiSn is spontaneously activated at temperatures above 793 K. At these temperatures, NiSn showed high selectivity for H and CO production and low selectivity for CH, CO, and HO production, indicating that methanol decomposition was the main reaction, and that side reactions such as methanation and water-gas shift reaction were suppressed. Surface analysis revealed that fine NiSn particles were formed during the reaction, accompanied by a small amount of deposited carbon. The formation of these particles was suggested to be the cause for the spontaneous activation of NiSn.
Hasegawa, Mika*; Sugawara, Kenta*; Suto, Ryota*; Sambonsuge, Shota*; Teraoka, Yuden; Yoshigoe, Akitaka; Filimonov, S.*; Fukidome, Hirokazu*; Suemitsu, Maki*
Nanoscale Research Letters, 10, p.421_1 - 421_6, 2015/10
Graphene has attracted much attention as a promising material in electronics and photonics. The graphitization temperature of 1473 K or higher of graphene-on-silicon(GOS), however, is still too high to be fully compatible with the Si technology. Here, the first application of Ni-assisted formation of graphene to the GOS method was reported. We demonstrate that the graphene formation temperature can be reduced by more than 200 K by this method. Moreover, solid-phase reactions during heating/annealing/cooling procedures have been investigated in detail by using synchrotron-radiation X-ray photoelectron spectroscopy. As a result, we clarify the role of Ni/SiC reactions, in which not only Ni silicidation and but also Ni carbonization is suggested as a key process in the formation of graphene.
Asahara, Ryohei*; Hideshima, Iori*; Oka, Hiroshi*; Minoura, Yuya*; Ogawa, Shingo*; Yoshigoe, Akitaka; Teraoka, Yuden; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Letters, 106(23), p.233503_1 - 233503_4, 2015/06
Advanced metal/high-/Ge gate stacks with a sub-nm equivalent oxide thickness and improved interface properties were demonstrated by controlling interface reactions using ultrathin aluminum oxide (AlOx) interlayers. Comprehensive study by means of physical and electrical characterizations revealed distinct impacts of AlOx interlayers, plasma oxidation.
Okada, Ryuta; Yoshigoe, Akitaka; Teraoka, Yuden; Yamada, Yoichi*; Sasaki, Masahiro*
Applied Physics Express, 8(2), p.025701_1 - 025701_4, 2015/02
We studied the surface oxidation on a Ge(111)-c(28) surface at room temperature using supersonic oxygen beams as a function of the translational energy of the incident oxygen molecules ranging from 26 meV to 2.3 eV. In situ synchrotron photoemission spectroscopy performed during the oxidation revealed that the surface oxidation terminated with the formation of a sub-monolayer oxide of at most 0.52 ML, for all the beam energies examined. In addition, the oxidation state of the surface oxides was found to depend on the translational energy. These results demonstrate the precise chemical control of the ultrathin surface oxides of Ge(111)-c(28).
Mori, Takeo*; Kitayama, Satoshi*; Kanai, Yuina*; Naimen, Sho*; Fujiwara, Hidenori*; Higashiya, Atsushi*; Tamasaku, Kenji*; Tanaka, Arata*; Terashima, Kensei*; Imada, Shin*; et al.
Journal of the Physical Society of Japan, 83(12), p.123702_1 - 123702_5, 2014/12
We show that the strongly correlated 4-orbital symmetry of the ground state is revealed by linear dichroism in core-level photoemission spectra, as we have discovered for YbRhSi and YbCuSi. Theoretical analysis shows us that the linear dichroism reflects the anisotropic charge distributions resulting from a crystalline electric field. We have successfully determined the ground-state 4 symmetry for both compounds from the polarization-dependent angle resolved core-level spectra at a temperature well below the first excitation energy. The excited-state symmetry is also probed by temperature dependence of the linear dichroism where the high measurement temperatures are on the order of the crystal-field-splitting energies.