1. State Key Laboratory for Superlattices and MicrostructuresInstitute of SemiconductorsChinese Academy of Sciences and Center of Materials Science and Optoelectronic EngineeringUniversity of Chinese Academy of Sciences Beijing 100083 China
2. State Key Laboratory on Integrated OptoelectronicsCollege of Electronic Science and EngineeringJilin University Changchun 130012 P. R. China