1. Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Department of Physics, National Central University, Zhongli 32001, Taiwan
3. Taiwan Semiconductor Research Institute, Hsinchu 30078, Taiwan
4. School of Physics and Technology, Wuhan University, Wuhan, Hubei 430072, China
5. Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan