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Cr
NYabuuchi, 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：4.63(Physics, Applied)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：4.79(Physics, Applied)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 
ray (DBAR) measurements show that the GaCrN film grown at low 540
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.
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(Physics, Applied)no abstracts in English
CrN and Si-doping effects studied by photoemission and X-ray absorption spectroscopySong, 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：37.3(Materials Science, Multidisciplinary)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.
Kawasuso, Atsuo; Yabuuchi, Atsushi; Maekawa, Masaki; Hasegawa, Shigehiko*; Zhou, Y.-K.*; Asahi, Hajime*
no journal, ,
no abstracts in English
Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Hasegawa, Shigehiko*; Zhou, Y.*; Asahi, Hajime*
no journal, ,
no abstracts in English
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 -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
-V
) and VN-related defects, respectively.
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 
m)/Sapphire substrate with a total fluence of 6.5
10
e
/cm
at 60 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.
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 700C. Furthermore, GaN-cap (20 nm)/GaCrN (200 nm) layers were grown at different temperatures (540C, 300C and room temperature). For these samples, positron annihilation -ray peak intensity measurements were performed. As a result, the -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.
ion implanted GaNYabuuchi, 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 C annealing. However, vacancy-type defects were still remained after 1300 C annealing. Alternating gradient magnetometer (AGM) measurements were also performed at room temperature for the unimplanted, as-implanted and post-implanted 1300 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.
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 700C. In contrast, vacancy clusters with sizes of 
- 
were found to be responsible for positron trapping in undoped GaCrN films grown at 540C. Divacancies (
) were present in Si-doped GaCrN films grown at 540C, although vacancy clusters were eliminated by Si-doping.
