1. RIAM Department of Material Science and Engineering Seoul National University Kwanakro-1, Kwanakgu Seoul 08826 Korea
2. SK Hynix Research Center 2091 Gyeongchungdaero, Bubuleup Icheonsi Gyeonggido 17419 Korea
3. Radiation Dosimetry Center Korea Atomic Energy Research Institute P.O. Box 105, Yuseong Daejon 34057 Korea