1. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
2. Institute of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan
4. Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan
5. Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
6. Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
7. Institute of Physics, Academia Sinica, 128 Sec. 2, Academia Road, Nankang, Taipei, 11529, Taiwan