1. School of Materials and EnergyGuangdong University of Technology Guangzhou 510006 China
2. Institute of SemiconductorsSouth China Normal University Guangzhou 510631 P. R. China
3. State Key Laboratory for Superlattices and MicrostructuresInstitute of SemiconductorsChinese Academy of Sciences P.O. Box 912 Beijing 100083 China
4. School of Physics and Optoelectronic EngineeringGuangdong University of Technology Guangzhou 510006 China