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. 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