1. Institute of Electronics Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Department of Physics, National Chung Hsing University, Taichung 40227, Taiwan
3. Department of Materials Science & Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
4. Center for Nanotechnology, Materials Science and Microsystem, National Tsing Hua University, Hsinchu 30013, Taiwan
5. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
6. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan