1. Department of Electrical Engineering, National Tsing Hua University, 30013 Hsinchu, Taiwan
2. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
4. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
5. Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
6. Department of Physics, Durham University, Durham DH1 3LE, United Kingdom
7. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
8. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan