1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
3. Department of Materials Science and Engineering, Institute for Advanced Study, City University of Hong Kong, Kowloon, Hong Kong
4. School of Technical Physics, Xidian University, Xi'an 710071, People's Republic of China
5. Key Laboratory of Wide Band Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China
6. Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan