[1] Chinese Acad Sci, Inst Semicond, State Lab Surface Phys, Beijing 100083, Peoples R China
[2] Chinese Acad Sci, Ctr Condensed Matter Phys, Beijing 100083, Peoples R China
[3] Chinese Acad Sci, Natl Lab Superlattices & Microstruct, Beijing 100083, Peoples R China
[4] Chinese Acad Sci, Inst Semicond, Beijing 100083, Peoples R China