1. Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Shaanxi Key Lab of Information Photonic Technique and Ministry of Education Key Laboratory for Multifunctional Materials and Structures and Institute of Wide Bandgap Semiconductors Xi'an Jiaotong University Xi'an 710049 P. R. China
2. Institute of Nano/Photon Materials and Application and International Joint Research Laboratory of New Energy Materials and Devices of Henan Province, School of Physics and Electronics Henan University Kaifeng 475004 P. R. China
3. School of Mathematics and Science Nanyang Institute of Technology No. 80 Changjiang Road Nanyang Henan 473004 P. R. China