1. School of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, Inner Mongolia, P. R. China
2. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
3. College of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot 010022, Inner Mongolia, P. R. China