1. School of Science, Shandong Jianzhu University, Jinan 250101, P. R. China
2. Institute of Novel Semiconductors, State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, P. R. China
3. National Center for International Joint Research of Electronic Materials and Systems, Henan Key Laboratory of Laser and Opto-electric Information Technology, School of Electrical and Information Engineering, Zhengzhou University, Zhengzhou 450052, Henan, P. R. China