1. Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
2. CNRS-CRHEA, Centre de Recherche sur L'Hétéro-Epitaxie et ses Applications-Centre National de la Recherche Scientifique, Valbonne 06560, France
3. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China