1. School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China
2. Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, P. R. China
3. State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-Sen University, Guangzhou 510275, Guangdong, P. R. China
4. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
5. Institute of Semiconductors, South China Normal University, Guangzhou 510631, P. R. China