[1] Peking Univ, Sch Mat Sci & Engn, Beijing 100871, Peoples R China
[2] Nanyang Technol Univ, Div Phys & Appl Phys, Sch Phys & Math Sci, Singapore 637371, Singapore
[3] CAS Ctr Excellence Nanosci, Natl Ctr Nanosci & Technol, CAS Key Lab Standardizat & Measurement Nanotechno, Beijing 100190, Peoples R China
[4] Peking Univ, Int Ctr Quantum Mat, Sch Phys, Electron Microscopy Lab, Beijing 100871, Peoples R China
[5] Hunan Univ, Changsha Semicond Technol & Applicat Innovat Res, Coll Semicond, Coll Integrated Circuits, Changsha 410082, Peoples R China