1. Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
2. Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
3. Department of Materials and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
4. National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
5. Department
of Physics, National Tsing Hua University, Hsinchu 300, Taiwan