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Journal Articles

Suppression of vacancy aggregation by silicon-doping in low-temperature-grown Ga$$_{1-x}$$Cr$$_{x}$$N

Yabuuchi, Atsushi*; Maekawa, Masaki; Kawasuso, Atsuo; Zhou, Y.-K.*; Hasegawa, Shigehiko*; Asahi, Hajime*

Applied Physics Letters, 102(14), p.142406_1 - 142406_4, 2013/04

 Times Cited Count:1 Percentile:5.34(Physics, Applied)

Journal Articles

Composition analysis of high-stable transparent conductive zinc oxide by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy

Kuchiyama, Takashi*; Hasegawa, Shigehiko*; Yamamoto, Kenji*; Teraoka, Yuden; Asahi, Hajime*

Japanese Journal of Applied Physics, 50(12), p.121101_1 - 121101_4, 2011/12

 Times Cited Count:1 Percentile:5.37(Physics, Applied)

Journal Articles

Positron beam study on vacancy defects in GaCrN grown by molecular beam epitaxy

Kawasuso, Atsuo; Yabuuchi, Atsushi; Maekawa, Masaki; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*

JAEA-Review 2010-065, JAEA Takasaki Annual Report 2009, P. 148, 2011/01

Vacancy defects in GaCrN grown by molecular beam epitaxy have been characterized by energy variable positron beam. Both positron lifetime and the Doppler broadening of annihilation $$gamma$$ ray (DBAR) measurements show that the GaCrN film grown at low 540$$^{circ}$$C contains vacancy defects. The observed vacancy defects are identified as eight-vacancy clusters. Although the Si doping reduces such vacancy clusters probably due to the occupation of Ga sites, another type of vacancy defects still survives. From the detailed theoretical calculation, the residual vacancy defects are attributable to SiGa-VN complexes.

Journal Articles

Defect structure of MBE-grown GaCrN diluted magnetic semiconductor films

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y. K.*; Asahi, Hajime*

Journal of Physics; Conference Series, 262(1), p.012066_1 - 012066_4, 2011/01

 Times Cited Count:0 Percentile:0.06

no abstracts in English

Journal Articles

Electronic structure of Ga$$_{1-x}$$Cr$$_{x}$$N and Si-doping effects studied by photoemission and X-ray absorption spectroscopy

Song, G.*; Kobayashi, Masaki*; Hwang, J. I.*; Kataoka, Takashi*; Takizawa, Masaru*; Fujimori, Atsushi; Okochi, Takuo; Takeda, Yukiharu; Okane, Tetsuo; Saito, Yuji; et al.

Physical Review B, 78(3), p.033304_1 - 033304_4, 2008/07

 Times Cited Count:8 Percentile:39.26(Materials Science, Multidisciplinary)

Oral presentation

Positron annihilation study on vacancy-type defects in MBE-grown GaCrN films

Yabuuchi, Atsushi; Maekawa, Masaki; Hasegawa, Shigehiko*; Asahi, Hajime*; Kawasuso, Atsuo

no journal, , 

In the crystalline growth of the GaCrN dilute magnetic semiconductor, the growth is carried out in a relatively low temperature to increase Cr concentration. However, it is expected that vacancy-type defects are introduced into the low temperature growth films. It is considered that the magnetized state of the dilute magnetic semiconductor is influenced from the existence of the vacancy-type defects. Thus, it is important to understand the relation between the growth conditions and the vacancy-type defects concentration. In the present study, we investigated the growth temperature dependency and the effect of Si doping on vacancy-type defects introduction in MBE-grown GaCrN films. The result of our experiment suggested that the vacancy-type defect exists in low temperature growth GaCrN film. Furthermore, the defect concentration was decreased with Si doping.

Oral presentation

Positron annihilation in MBE-grown GaCrN diluted magnetic semiconductors

Kawasuso, Atsuo; Yabuuchi, Atsushi; Maekawa, Masaki; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*

no journal, , 

no abstracts in English

Oral presentation

Positron annihilation study on vacancy-type defects in MBE-grown GaCrN films, 2

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.*; Asahi, Hajime*

no journal, , 

no abstracts in English

Oral presentation

Growth temperature dependence and Si-doping effect on vacancy-type defect structure in MBE-grown GaCrN films

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.*; Asahi, Hajime*

no journal, , 

In the MBE growth of the GaCrN dilute magnetic semiconductor films, low temperature (LT) growth is important in order to dope Cr atoms to high concentration. However, the problem with LT growth is the formation of lattice defects in the films. In this study, we have investigated growth temperature dependence and Si-doping effect on vacancy-type defect structure in MBE-grown GaCrN films probed by a slow positron beam. As a result, LT-grown undoped GaCrN film indicates a high value of positron annihilation parameter which implies the existence of vacancy-type defects. Furthermore, the positron annihilation parameter value was decreased by Si doping. First principle calculations of annihilation $$gamma$$-ray spectra revealed that both of the detected defects from the undoped LT-GaCrN and from the Si-doped LT-GaCrN film were vacancy clusters (V$$_{6}$$-V$$_{12}$$) and VN-related defects, respectively.

Oral presentation

Positron annihilation in irradiation-induced defects in GaN

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*

no journal, , 

The Doppler broadening of the positron annihilation radiation of the electron irradiated GaN sample was measured. The purpose of this study was to clarify the positron annihilation characteristics at point defects in GaN. 2 MeV electrons were irradiated to the MOCVD-GaN (2 $$mu$$m)/Sapphire substrate with a total fluence of 6.5$$times$$10$$^{17}$$ e$$^{-}$$/cm$$^2$$ at 60 $$^{circ}$$C. The irradiated GaN was measured by using slow positron beam. As a result, the observed spectrum showed the existence of not monovacancies but vacancy clusters. This means vacancy clusters existed intrinsically in MOCVD-GaN film.

Oral presentation

Effects of growth conditions on formation of vacancy-type defects in MBE-grown GaN

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*

no journal, , 

In the MBE growth of dilute magnetic semiconductors, the low-temperature growth has been attempted to suppress the precipitation of secondary phases. A current problem is a control of vacancy-type defects. Thus in this study, effects of growth conditions on formation of vacancy-type defects in MBE-grown GaN were investigated by positron annihilation spectroscopy. GaN buffer layers with the thickness of 30 nm were grown on sapphire substrates at 700$$^{circ}$$C. Furthermore, GaN-cap (20 nm)/GaCrN (200 nm) layers were grown at different temperatures (540$$^{circ}$$C, 300$$^{circ}$$C and room temperature). For these samples, positron annihilation $$gamma$$-ray peak intensity measurements were performed. As a result, the $$gamma$$-ray peak intensity increased with decreasing the growth temperature. This result shows that the concentration or size of vacancy-type defects contained in the GaCrN layer increases with decreasing growth temperature.

Oral presentation

Irradiation-induced defects recovery and magnetic property of the Gd$$^{+}$$ ion implanted GaN

Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*

no journal, , 

Recent GaN-based DMS studies have reported that Gd-doped GaN tend to indicate large magnetization in defective films. In addition, recent theoretical calculation study has also reported that a presence of vacancies affects the magnetic properties in DMS. In this study, Gd$$^+$$ ions were implanted into MOCVD-GaN film by using ion implantation techniques. Furthermore, we have attempted to clarify the correlation between the magnetic properties and presence of vacancy-type defects. As a result, irradiation-induced defects have decreased greatly at 1000 $$^{circ}$$C annealing. However, vacancy-type defects were still remained after 1300 $$^{circ}$$C annealing. Alternating gradient magnetometer (AGM) measurements were also performed at room temperature for the unimplanted, as-implanted and post-implanted 1300 $$^{circ}$$C annealed GaN films. However, clear magnetic hystereses were not observed even in the as-implanted film. In further study, high temperature ion implantation experiments are needed.

Oral presentation

Positron annihilation study of magnetic elements doped GaN grown by molecular beam epitaxy

Yabuuchi, Atsushi*; Maekawa, Masaki; Kawasuso, Atsuo; Zhou, Y.*; Hasegawa, Shigehiko*; Asahi, Hajime*

no journal, , 

Chromium-doped gallium nitride (GaCrN) has attracted much attention as a candidate material for spintronics devices. Two approaches have been utilized in improving the magnetization. Si-doping of GaCrN to enhance carrier-induced ferromagnetism and low temperature molecular beam epitaxy (MBE) growth to increase Cr atom concentration without formation of CrN precipitates. In this study, GaCrN films grown at high and low temperatures with and without Si-doping have been probed by a slow positron beam. As a result, no vacancy defects, which are larger than Ga vacancies, were detected in either the undoped or doped GaCrN films grown at 700$$^{circ}$$C. In contrast, vacancy clusters with sizes of $$V_6$$ - $$V_{12}$$ were found to be responsible for positron trapping in undoped GaCrN films grown at 540$$^{circ}$$C. Divacancies ($$V_{textrm{Ga}}V_{textrm{N}}$$) were present in Si-doped GaCrN films grown at 540$$^{circ}$$C, although vacancy clusters were eliminated by Si-doping.

Oral presentation

Full-f gyrokinetic simulation including kinetic electrons

Idomura, Yasuhiro; Asahi, Yuichi*; Hayashi, Nobuhiko*; Urano, Hajime*

no journal, , 

Full-f gyrokinetic simulations are important tools for analyzing nonlocal turbulent transport, plasma profiles, and the confinement time in fusion plasmas. However, the conventional full-f simulations were limited to ion turbulence with adiabatic electrons. In order to analyze ITER relevant electron turbulence, in this work, we develop a new hybrid electron model for full-f simulations, and verify its accuracy. In the model, passing electrons responses, which induce high frequency noises, are approximated by analytic solutions, and long time scale full-f simulations are enabled by eliminating the high frequency noises. Numerical experiments of electron turbulence using this model clarify new mechanisms for turbulence suppression and momentum transport related to electron turbulence transport. In a validation study, an experimental observation on plasma rotation changes induced by electron heating is successfully reproduced.

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