1. School of Electronic and Computer Engineering, Peking University, Shenzhen Graduate School, Shenzhen 518055, China
2. Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan
3. Department of Physics, National Sun Yat-sen University, Kaohsiung 804, Taiwan
4. Institute of Materials and Optoelectronics, National Sun Yat-sen University, Kaohsiung 804, Taiwan