1. Division of NanophotonicsCAS Key Laboratory of Standardization and Measurement for NanotechnologyCAS Center for Excellence in NanoscienceNational Center for Nanoscience and Technology Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences 19 A Yuquan Rd, Shijingshan District Beijing 100049 P. R. China
3. Department of Materials Science and EngineeringCollege of EngineeringPeking University Beijing 100871 P. R. China
4. Research Center for Wide Gap SemiconductorPeking University Beijing 100871 P. R. China