1. Institute of Fundamental and Frontier Sciences; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
2. School of Microelectronics and Solid-State Electronics; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
3. Department of Materials Science and Engineering; National Tsing Hua University; Hsinchu 30013 Taiwan
4. Department of Photonics and Institute of Electro-Optical Engineering; National Chiao Tung University; Hsinchu 30010 Taiwan
5. Department of Chemical Engineering; National Tsing-Hua University; Hsinchu 30013 Taiwan
6. State Key Laboratory for Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Science; Beijing 100083 China