1. Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
2. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
3. Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
4. Department of Physics, Tamkang University, Tamsui 251, Taiwan