1. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100083, China
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
4. Beijing Academy of Quantum Information Sciences, Beijing 100193, China