1. a Memory R&D Division, Hynix Semiconductor, Inc., San 136-1, Ami-ri , Bubal-eup, , Ichon-si Kyoungki-do, 476-701, Korea
2. b Department of Materials Science & Engineering , Chung Jun National University, 123, Geomdan-ri Iryu-myeon , Chungju-si, Chungbuk, 380-720, Korea