1. Department of Electrophyics, National Chiao Tung University, Hsinchu 30010, Taiwan
2. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Department of Physics, National Chung Cheng University, Chiayi 62102, Taiwan
4. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan