[1] Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
[2] Electronics and Optoelectronics Research Laboratories (EOL), Industrial Technology Research Institute (ITRI), Hsinchu 310, Taiwan
[3] Nano Technology Research Center, ITRI, Hsinchu 310, Taiwan
[4] Department of Physics, National Chung Cheng University, Chiayi 621, Taiwan
[5] Institute of Physics, Academia Sinica, Taipei 115, Taiwan