1. Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan
2. Department of Mathematics and Physics, University of Santo Tomas, Manila 1008, Philippines
3. Department of Electronics Engineering & Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan
4. National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
5. Taiwan Semiconductor Research Institute, National Applied Research Laboratories, Hsinchu 300, Taiwan
6. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan