1. Institute of Nano/Photon Materials and Application & 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
2. Key Laboratory for Physical Electronics and Devices of the Ministry of Education & School of Science & Shaanxi Key Lab of Information Photonic Technique & Institute of Wide Bandgap Semiconductors Xi'an Jiaotong University Xi'an 710049 PR China