1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan R.O.C., National Nano Device Laboratories, Hsinchu, 30078, Taiwan R.O.C., Department of Electronics Engineering, National Chiao Tung University, Hsinchu, 30050, Taiwan R.O.C., School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245