[1] Henan Normal Univ, Dept Phys, Xinxiang 453007, Henan, Peoples R China
[2] Peking Univ, Sch Phys, State Key Lab Artificial Microstruct & Mesoscop P, Beijing 100871, Peoples R China
[3] Henan Normal Univ, Coll Elect & Elect Engn, Xinxiang 453007, Henan, Peoples R China
[4] Chinese Acad Sci, Inst Semicond, Beijing 100083, Peoples R China