1. National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 P. R. China
2. College of Electronic Science and Engineering; Nanjing University of Posts and Telecommunications; Nanjing 210003 P. R. China
3. National Laboratory of Solid State Microstructures, Department of Physics; Nanjing University; Nanjing 210093 P. R. China