1. Department of Physics, Shantou University 1 , 243 Daxue Road, Shantou 515063, Guangdong, China
2. Max Planck Institute for Intelligent Systems (Formerly MPI for Metals Research) 2 , Heisenbergstrasse 3, D-70569 Stuttgart, Germany
3. Center of Semiconductor Materials and Devices, Shantou University 3 , 243 Daxue Road, Shantou 515063, Guangdong, China