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Kawasuso, Atsuo; Fukaya, Yuki*; Hayashi, Kazuhiko; Hashimoto, Mie; Ichimiya, Ayahiko
Physica Status Solidi (C), 4(10), p.3924 - 3927, 2007/09
Well-focused and highly parallel positron beam is totally reflected at the first surface layer when grazing incidence at a small enough angles. Therefore, the reflection high-energy positron diffraction (RHEPD) is suited to determine the surface structures without any disturbance from the bulk. Using the electro-magnetic lenses we developed a positron beam which has more coherence than before. We found that the RHEPD pattern from the Si-7
7 surface is considerably different from that of electron diffraction. The difference is reasonably explained as a strong thermal diffuse scattering of positrons. As a further application, we studied the phase transition of Si-Ag ultra-thin film. We found that the surface-sensitive diffraction spots exhibit significant temperature dependences. Based on the dynamical diffraction theory, the temperature dependences are reproduced considering the order-disorder phase transition. This will terminate the long debates in the surface science community. Furthermore, we recently found that the positron-surface interaction is different from the electron-surface interaction, especially in the elastic processes. We present the above recent advances of surface studies using RHEPD.
Yu, R.; Maekawa, Masaki; Miwa, Yukio; Hirade, Tetsuya; Nishimura, Akihiko; Kawasuso, Atsuo
Physica Status Solidi (C), 4(10), p.3577 - 3580, 2007/09
Times Cited Count：4 Percentile：73.32(Physics, Atomic, Molecular & Chemical)Employing a positron beam with lateral resolution of about 30 micometer, we performed one and two dimensional scan of crack tips in mechanically fatigued as well as stress corrosion cracked (SCC) stainless steels. Results suggest the existence of quite different crack failure mechanisms in the two types of specimen. In agreement with normal crack propagation principal, creation of dislocations and vacancies close to a crack in the mechanically fatigued sample lead to an expected narrowing of the positron annihilation 511 keV Doppler broadening 
-ray peak (S(crack)/S(bulk)=1.01). Contrarily, widening of the Doppler broadening peak was observed for the SCC specimen (S(crack)/S(bulk)=0.97). Such phenomenon was ascribed to the micro-compositional and micro-structural differences between the studied samples.
Maekawa, Masaki; Yu, R.; Kawasuso, Atsuo
Physica Status Solidi (C), 4(10), p.4016 - 4019, 2007/09
Times Cited Count：4 Percentile：73.32(Physics, Atomic, Molecular & Chemical)no abstracts in English
Hirade, Tetsuya; Suzuki, Naoki*; Saito, Fuminori*; Hyodo, Toshio*
Physica Status Solidi (C), 4(10), p.3714 - 3717, 2007/09
Times Cited Count：6 Percentile：82.43(Physics, Atomic, Molecular & Chemical)Positronium (Ps) can be formed by a reaction of trapped electrons and free positrons at low temperatures in molecular solids. While Ps formation by Ore and spur process must be fast, Ps formation by trapped electrons and free positrons is possible even at later positron age. The evidence of this delayed Ps formation was obtained by age-momentum correlation measurement of electron-positron pair annihilation -rays. In this work the 
parameter curves measured with trapped electrons and no trapped electrons in polyethylene are interpreted by introducing possible positron reactions. Positrons that escape from Ps formation by the Ore and the spur processes have chance to form Ps with trapped electrons. This Ps formation competes with the positron localization resulting from the trapping between molecules or the weakly binding on molecules, ions, or radicals. Assuming the possibility of both the delayed Ps formation and delayed positron trapping and that S-parameters for ortho-Ps and free positrons are the same, we obtain a good fit to the observed time dependent S-parameter curves .
Chen, Z. Q.*; Maekawa, Masaki; Kawasuso, Atsuo; Naramoto, Hiroshi
Physica Status Solidi (C), 4(10), p.3646 - 3649, 2007/09
Times Cited Count：2 Percentile：56.91(Physics, Atomic, Molecular & Chemical)We implanted B
, O, Al, and P ions into ZnO (Dose: 4E+15/cm
). The thermal recovery of implantation-induced defects were studied using a slow positron beam. S parameters show much increase after ion implantation, indication introduction of vacancy defects. The thermal recovery of these vacancies induced by different ions shows much difference. For O implantation, vacancy clusters disappear rapidly after annealing up to about 700
C. For the lighter ion B implantation, however, vacancy clusters grow to a larger size at 500C. The vacancies begin to recover and disappear after further annealing up to 900C. For the Al implantation, the vacancies grow into even a much larger size after annealing at 600C, and they are annealed out at 900C. However, for the P -implanted sample, it was shown that a much weaker agglomeration process of the vacancy clusters compared with Al implantation. A higher annealing temperature of 1100C is needed to fully remove these vacancies.
Hori, Fuminobu*; Fukuzumi, Masafumi*; Kawasuso, Atsuo; Zushi, Yoshihiro*; Chimi, Yasuhiro; Ishikawa, Norito; Iwase, Akihiro*
Physica Status Solidi (C), 4(10), p.3530 - 3533, 2007/09
Times Cited Count：0 Percentile：0.07(Physics, Atomic, Molecular & Chemical)no abstracts in English
Kawasuso, Atsuo; Maekawa, Masaki; Fujinami, Masanori*; Oguma, Koichi*; Akahane, Takashi*; Watanabe, Kazuya*; Matsukawa, Kazuto*
no journal, ,
Transition-metals are ubiquitous in Si devices to degrade their properties. Gettering has proved effective to trap contaminants and it is important to study their interaction. Extended defects such as stacking faults and precipitates have been investigated by TEM and impurity profile is analyzed by SIMS. However, such a approach cannot afford us the interaction of them. PAS is a powerful tool to analyze the defects and their coupled elements. A Cz-Si wafer was implanted with 3MeV Si ions to dose of 1E+14/cm and Cu ion implantation was done into the back side (200 keV, 1E+14/cm). Variable-energy positron beam was employed and coincidence Doppler broadening spectra were taken at 15 keV after sample annealing. Up to 500C, no differences between with and without Cu ion implantation are found and the clustering of vacancies takes place. At 600C, the profile is very similar to that for the bulk Cu. At 700C, the trapped Cu are released and oxygen atoms are coupled with vacancy clusters.
