Key Laboratory of Semiconductor Materials Science;
Institute of Semiconductors;
Chinese Academy of Sciences;
Beijing 100083;
College of Materials Science and Opto-Electronic Technology;
University of Chinese Academy of Sciences;
Beijing 100049;
Department of Applied Physics and Materials Science;
California Institute of Technology;
California 91125;
USA;
State Key Laboratory for Mesoscopic Physics;
School of Physics;
Peking University;
Beijing 100871;