1. Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Key Laboratory of Photonics Technology for Information of Shaanxi Province, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China
2. Wide Band Gap Semiconductor Research Center, Xi’an Jiaotong University, Xi’an 710049, PR China
3. ISCAS-XJTU Joint Laboratory of Functional Materials and Devices for Informatics, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China